Chemistry Quiz: Interpret Chemical Energy Diagrams
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Interpret Chemical Energy DiagramsQuestion 1 of 20

A reaction energy diagram for a single-step reaction shows a curve that rises from the reactants to a highest point, then falls to the products. What does the highest point (the peak) represent?

The products, because they form after the reaction
The transition state (activated complex), the highest-energy point
The reactants, because the reaction starts there
The overall energy change ΔH\Delta H
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Chemistry Quiz

Chemistry Quiz: Interpret Chemical Energy Diagrams

Practice Interpret Chemical Energy Diagrams in Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Interpret Chemical Energy Diagrams, giving you a quick way to practice the rules, question types, and explanations that matter most for Chemistry.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A reaction energy diagram for a single-step reaction shows a curve that rises from the reactants to a highest point, then falls to the products. What does the highest point (the peak) represent?

  1. The products, because they form after the reaction
  2. The transition state (activated complex), the highest-energy point (correct answer)
  3. The reactants, because the reaction starts there
  4. The overall energy change ΔH\Delta H
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! The highest point (peak) on the curve represents the TRANSITION STATE or ACTIVATED COMPLEX—this is the unstable, high-energy arrangement where old bonds are partially broken and new bonds are partially formed, like a molecular "halfway point" during the reaction. Choice B correctly identifies the peak as the transition state (activated complex), the highest-energy point where the reacting molecules are in their most unstable configuration as bonds rearrange. Choice A incorrectly identifies the products as the highest point—products are at the END of the reaction (right side) and can be either higher or lower than reactants, but never at the peak. Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 2

A single-step energy profile is drawn with Energy on the y-axis and Reaction progress on the x-axis. The curve starts at the reactants, rises to one peak, then falls to the products. Which diagram feature shows the overall energy change (ΔH\Delta H)?

  1. The vertical difference between the reactant energy level and the product energy level (correct answer)
  2. The vertical difference between the reactant energy level and the peak
  3. The height of the peak above the x-axis
  4. The horizontal distance from reactants to products
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! The overall energy change ΔH is shown by the vertical difference between reactant and product levels, regardless of the peak. Choice A correctly interprets the energy diagram by identifying the reactant-product difference as ΔH. Choice D fails by using horizontal distance—that's reaction progress, not energy; energy is vertical! Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 3

A single-step energy diagram shows reactants at 90 kJ, a peak at 160 kJ, and products at 50 kJ. Which statement correctly describes the overall energy change ΔH\Delta H?

  1. ΔH\Delta H is positive because the peak is above the reactants
  2. ΔH\Delta H is negative because products are lower in energy than reactants (correct answer)
  3. ΔH\Delta H is the vertical distance from reactants to the peak
  4. ΔH\Delta H must be zero because energy is conserved
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! Here, products at 50 kJ are lower than reactants at 90 kJ, so ΔH = 50 - 90 = -40 kJ, negative. Choice B correctly interprets the energy diagram by measuring from reactants to products and noting the negative ΔH. Choice A fails by focusing on the peak being above reactants—that's activation energy, not ΔH; remember to compare endpoints for overall change. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume 'upward curve' means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors. You're on a roll—fantastic!

Question 4

A single-step reaction energy diagram shows reactants at 50 kJ and products at 90 kJ (with one peak in between). Which statement about the overall energy change (ΔH\Delta H) is correct?

  1. The reaction is exothermic; ΔH\Delta H is negative.
  2. The reaction is endothermic; ΔH\Delta H is positive. (correct answer)
  3. The reaction is exothermic because it must have a peak.
  4. The reaction has ΔH=0\Delta H = 0 because only activation energy matters.
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 50 kJ and products at 90 kJ, the overall energy change ΔH = products - reactants = 90 - 50 = +40 kJ, which is POSITIVE, indicating an ENDOTHERMIC reaction where energy is absorbed from the surroundings to raise products to a higher energy level than reactants. Choice B correctly identifies this as endothermic with positive ΔH because products (90 kJ) are at a higher energy than reactants (50 kJ), meaning energy must be absorbed during the reaction. Choice A incorrectly calls it exothermic, Choice C wrongly links the presence of a peak to being exothermic (all reactions have peaks!), and Choice D incorrectly claims ΔH = 0. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. For this problem: products (90 kJ) > reactants (50 kJ), so ENDOTHERMIC!

Question 5

An energy profile for a single-step reaction is shown (Energy on the y-axis, Reaction progress on the x-axis). The curve starts at the reactants at about 90 kJ, rises to a peak at about 150 kJ (transition state), then drops to the products at about 50 kJ. Based on this diagram, is the reaction exothermic or endothermic?

  1. Endothermic, because the curve rises to the peak
  2. Exothermic, because the products are at lower energy than the reactants (correct answer)
  3. Endothermic, because the activation energy is positive
  4. Neither; energy diagrams cannot show whether energy is released or absorbed
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, the reactants are at 90 kJ, the peak at 150 kJ, and products at 50 kJ, so ΔH = 50 - 90 = -40 kJ (negative, exothermic) since products are lower. Choice B correctly interprets the energy diagram by noting the products are at lower energy than reactants, indicating an exothermic reaction. Choice A fails by confusing the initial rise (activation) with the overall change—remember, all reactions rise to a peak, but exo/endo depends on endpoints! Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 6

A single-step reaction energy diagram has reactants at 100 kJ, a peak at 160 kJ, and products at 140 kJ. Based on the diagram, which statement is correct?

  1. The reaction is exothermic because products are formed after the peak
  2. The reaction is endothermic because products are at higher energy than reactants (correct answer)
  3. The activation energy is the vertical drop from the peak to the products
  4. The overall energy change equals the peak energy minus zero
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, reactants are at 100 kJ and products at 140 kJ, so products are higher, making the reaction endothermic. Choice B correctly interprets the energy diagram by stating the reaction is endothermic because products are at higher energy than reactants. Choice C fails because the activation energy is from reactants to peak (60 kJ), not the drop from peak to products (20 kJ). Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 7

Two single-step reaction energy diagrams have the same reactant and product energy levels, but Diagram 1 has a higher peak than Diagram 2. Which statement is true?

  1. Diagram 1 has a larger activation energy than Diagram 2 (correct answer)
  2. Diagram 1 has a larger (more positive) ΔH\Delta H than Diagram 2
  3. Diagram 2 must be endothermic while Diagram 1 must be exothermic
  4. Diagram 1 has no transition state because the peak is higher
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! Since both diagrams have the same reactant and product levels, ΔH is identical, but Diagram 1's higher peak means larger activation energy. Choice A correctly interprets the energy diagrams by noting that Diagram 1 has a larger activation energy due to its higher peak. Choice B fails because ΔH is the same for both since reactant and product levels match—only the peak differs. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 8

A single-step reaction energy diagram shows reactants at 50 kJ, a peak at 120 kJ, and products at 30 kJ (Energy on y-axis, Reaction progress on x-axis). Which part of the diagram represents the activation energy EaE_a?

  1. The vertical distance from the reactant energy level up to the peak (correct answer)
  2. The vertical distance from the product energy level up to the peak
  3. The vertical distance from the x-axis up to the peak
  4. The horizontal distance from reactants to products
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, reactants are at 50 kJ and the peak at 120 kJ, so the activation energy Ea is the vertical distance from reactants up to the peak (70 kJ). Choice A correctly interprets the energy diagram by measuring from the reactant energy level up to the peak for activation energy. Choice C fails because measuring from the x-axis to the peak ignores the starting reactant level and overestimates the barrier. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 9

A single-step energy profile (Energy vs Reaction progress) shows reactants at 70 kJ, a peak at 130 kJ, and products at 20 kJ. Which vertical difference on the diagram corresponds to the overall energy change ΔH\Delta H?

  1. From the reactant level to the peak
  2. From the peak down to the product level
  3. From the reactant level to the product level (correct answer)
  4. From the x-axis up to the reactant level
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, the vertical difference for ΔH is from reactants at 70 kJ to products at 20 kJ, giving ΔH = -50 kJ. Choice C correctly interprets the energy diagram by measuring the overall energy change ΔH from the reactant level to the product level. Choice A fails because from reactant to peak is activation energy (60 kJ), not ΔH. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 10

In a single-step reaction energy diagram (Energy vs Reaction progress), the products are drawn at a higher energy level than the reactants. What does this tell you about the overall energy change ΔH\Delta H?

  1. ΔH\Delta H is negative (energy released)
  2. ΔH\Delta H is zero (no energy change)
  3. ΔH\Delta H is positive (energy absorbed) (correct answer)
  4. ΔH\Delta H equals the activation energy
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, products are at a higher energy level than reactants, so ΔH is positive, meaning the reaction is endothermic and energy is absorbed. Choice C correctly interprets the energy diagram by recognizing that higher product energy means positive ΔH (energy absorbed). Choice A fails because if products are higher, ΔH is positive, not negative—remember, exothermic means energy released (products lower). Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 11

A single-step reaction energy diagram is shown.

Diagram description: Energy (y-axis) vs Reaction coordinate (x-axis). Reactants start at a lower energy level, the curve rises to one peak, and the products end at a higher energy level than the reactants.

Which statement is true?

  1. The reaction is exothermic because it has a transition state
  2. The reaction is endothermic because the products are at higher energy than the reactants (correct answer)
  3. The activation energy is measured from the products down to the reactants
  4. The peak represents the overall energy change (ΔH\Delta H)
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, reactants start lower, peak in between, products end higher, so it's endothermic (products > reactants, positive ΔH). Choice B correctly interprets the energy diagram by stating it's endothermic due to higher product energy. Choice D fails by saying the peak is ΔH—the peak is for Ea, ΔH is endpoints! Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 12

The reaction energy diagram shows reactants at 50 kJ and products at 80 kJ (with a single peak in between). What does the diagram indicate about energy for the reaction overall?

  1. Energy is released overall because the curve goes up and then down.
  2. Energy is absorbed overall because the products are at higher energy than the reactants. (correct answer)
  3. No energy change occurs overall because there is a peak.
  4. Energy is absorbed overall because the activation energy is measured from the x-axis to the peak.
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 50 kJ and products at 80 kJ, we calculate ΔH = products - reactants = 80 - 50 = +30 kJ; the positive value means energy is absorbed (endothermic), as the products (80 kJ) are at a HIGHER energy level than reactants (50 kJ), requiring 30 kJ of energy input from surroundings. Choice B correctly states that energy is absorbed overall because the products are at higher energy than the reactants—this is the defining characteristic of an endothermic reaction where the system takes in energy from its surroundings. Choice A incorrectly concludes energy is released based on the curve shape (up then down) rather than comparing endpoint levels; Choice D incorrectly measures activation energy from the x-axis instead of focusing on the overall energy change between reactants and products. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything!

Question 13

In an energy diagram for a single-step reaction, the products are drawn higher on the energy (y) axis than the reactants. What does this mean?

  1. The reaction is endothermic (energy absorbed overall) (correct answer)
  2. The reaction is exothermic (energy released overall)
  3. The activation energy is zero
  4. The peak represents the products
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! When products are drawn HIGHER on the energy axis than reactants, this means the products have MORE energy than the reactants—energy must have been absorbed from the surroundings to reach this higher energy state, making the reaction ENDOTHERMIC. Choice A correctly identifies that products higher than reactants means the reaction is endothermic, with energy absorbed overall to lift the system to a higher energy state. Choice B incorrectly calls it exothermic (that would require products LOWER than reactants), while C and D make false claims about activation energy and peak identity. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. The vertical positions of the endpoints tell the thermodynamic story!

Question 14

In a single-step reaction energy diagram (energy vs reaction progress), the reactants are at 60 kJ and the products are at 90 kJ. What does this indicate about the overall energy change ΔH\Delta H?

  1. ΔH\Delta H is negative; the reaction is exothermic
  2. ΔH\Delta H is zero; reactants and products have the same energy
  3. ΔH\Delta H is positive; the reaction is endothermic (correct answer)
  4. ΔH\Delta H equals the activation energy
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 60 kJ and products at 90 kJ, the overall energy change ΔH = 90 - 60 = +30 kJ (positive), meaning products are HIGHER than reactants—energy must be absorbed from the surroundings, making this ENDOTHERMIC. Choice C correctly identifies that ΔH is positive and the reaction is endothermic because the products (90 kJ) have more energy than the reactants (60 kJ), requiring energy input overall. Choice B incorrectly states that reactants and products have the same energy—the diagram clearly shows products at 90 kJ versus reactants at 60 kJ, a 30 kJ difference. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything!

Question 15

A single-step reaction energy diagram shows reactants at 100 kJ, a peak at 160 kJ, and products at 70 kJ. Which statement correctly describes both EaE_a and ΔH\Delta H?

  1. EaE_a is measured from products to the peak; ΔH\Delta H is measured from peak to reactants
  2. EaE_a is measured from reactants to the peak; ΔH\Delta H is measured from reactants to products (correct answer)
  3. EaE_a is measured from reactants to products; ΔH\Delta H is measured from reactants to the peak
  4. EaE_a and ΔH\Delta H are both measured from the x-axis to the peak
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 100 kJ, peak at 160 kJ, and products at 70 kJ: Ea = 160 - 100 = 60 kJ (reactants to peak), and ΔH = 70 - 100 = -30 kJ (reactants to products, negative means exothermic). Choice B correctly states that Ea is measured from reactants to the peak (the energy barrier) and ΔH is measured from reactants to products (the overall energy change). Choice C incorrectly reverses these measurements—Ea is NOT from reactants to products (that's ΔH), and ΔH is NOT from reactants to peak (that's Ea). Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything!

Question 16

A reaction energy diagram (single-step) shows products at a lower energy level than reactants. What does this indicate about energy during the reaction?

  1. Energy is absorbed overall (endothermic).
  2. No energy is involved because the curve returns downward.
  3. Energy is released overall (exothermic). (correct answer)
  4. The activation energy must be zero.
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! When products are at a LOWER energy level than reactants, the overall energy change (ΔH) is negative, meaning energy has been released to the surroundings—this is an EXOTHERMIC reaction, like combustion releasing heat or a ball rolling downhill releasing potential energy. Choice C correctly identifies this as exothermic (energy released overall) because when products end up lower than reactants, the "extra" energy that reactants had must go somewhere—it's released to the surroundings as heat. Choice A incorrectly calls it endothermic (that would require products HIGHER than reactants), Choice B wrongly suggests no energy is involved, and Choice D incorrectly claims zero activation energy (all reactions need some activation energy to proceed). Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. Common diagram-reading mistakes to avoid: DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants!

Question 17

Two single-step reactions, A and B, have the same reactant and product energy levels, but reaction A has a higher peak than reaction B. Which statement is correct?

  1. Reaction A has a smaller activation energy than reaction B.
  2. Reaction A has a larger activation energy than reaction B. (correct answer)
  3. Reaction A must be more exothermic than reaction B.
  4. Reaction A must have a different ΔH\Delta H than reaction B.
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! If reactions A and B have the same reactant and product energy levels but A has a HIGHER peak, then A has a LARGER activation energy (bigger climb from reactants to peak), making it harder to start—like comparing a steep mountain pass to a gentle hill between the same two valleys. Choice B correctly states that reaction A has a larger activation energy than B because activation energy is measured from reactants UP to the peak, and A's peak is higher (same starting point, higher summit = bigger climb). Choice A reverses this relationship, while Choices C and D incorrectly focus on ΔH, which must be the SAME for both reactions since they have identical reactant and product levels. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. When comparing reactions: if reactants and products are at the same levels, ΔH is identical, but different peak heights mean different activation energies—higher peak = larger Ea = harder to start!

Question 18

A reaction coordinate diagram (Energy vs Reaction progress) for a single-step reaction starts at 80 units (reactants), rises to a peak at 140 units, and ends at 30 units (products). Which statement is correct?

  1. The reaction is endothermic because it must climb to the peak
  2. The activation energy is the difference between the peak and the products
  3. The reaction is exothermic because the products are at lower energy than the reactants (correct answer)
  4. The peak represents the products forming and releasing energy
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 80 units, peak at 140, and products at 30, the reaction is exothermic since products (30) are lower than reactants (80), ΔH negative. Choice C correctly interprets the energy diagram by focusing on the reactant-product energy difference for exo/endo. Choice A fails by calling it endothermic due to the climb to the peak—that's activation, not overall change! Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 19

A single-step reaction energy diagram shows reactants at 90 kJ, a peak at 140 kJ, and products at 60 kJ. Which statement correctly interprets both EaE_a and ΔH\Delta H from the diagram?

  1. EaE_a is measured from products to the peak, and ΔH\Delta H is measured from reactants to the peak
  2. EaE_a is measured from reactants to the peak, and ΔH\Delta H is measured from reactants to products (correct answer)
  3. EaE_a is measured from the x-axis to the peak, and ΔH\Delta H is measured from the x-axis to products
  4. EaE_a equals ΔH\Delta H because both are vertical distances on the diagram
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! In this diagram, Ea is from reactants (90 kJ) to peak (140 kJ, so 50 kJ), and ΔH is from reactants to products (60 kJ, so -30 kJ). Choice B correctly interprets the energy diagram by measuring Ea from reactants to peak and ΔH from reactants to products. Choice A fails by swapping the measurements—Ea is not from products to peak. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything! Common diagram-reading mistakes to avoid: (1) DON'T measure activation energy from the x-axis to peak—measure from REACTANT LEVEL to peak! The absolute height doesn't matter; it's the climb from where you start. (2) DON'T confuse activation energy with overall energy change—activation is about the barrier (reactants to peak), overall is about net change (reactants to products). A reaction can have HIGH activation energy (hard to start, tall peak) but be EXOTHERMIC overall (products lower, releases energy)—common for combustion! (3) DON'T assume "upward curve" means endothermic—ALL reactions go up to the peak first (activation), then what matters is whether products end up higher or lower than reactants. Check the endpoints, not just the middle! These three checks prevent most diagram errors.

Question 20

A single-step reaction energy diagram shows reactants at 60 kJ and products at 95 kJ (with one peak in between). What does the diagram indicate about the overall energy change, ΔH\Delta H?

  1. ΔH\Delta H is negative because the curve has a peak
  2. ΔH\Delta H is positive because the products are higher in energy than the reactants (correct answer)
  3. ΔH\Delta H is zero because reactants and products are connected by one curve
  4. ΔH\Delta H is negative because the products are higher in energy than the reactants
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 60 kJ and products at 95 kJ, the overall energy change ΔH = products - reactants = 95 - 60 = +35 kJ—positive because products are HIGHER, meaning energy was absorbed overall (endothermic). Choice B correctly states that ΔH is positive because the products (95 kJ) are higher in energy than the reactants (60 kJ), indicating net energy absorption. Choice A incorrectly relates ΔH to having a peak (all reactions have peaks for activation), while D contradicts itself by saying ΔH is negative when products are higher. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Determine exo/endo: products LOWER than reactants (energy drops, released) = EXOTHERMIC. Products HIGHER than reactants (energy rises, absorbed) = ENDOTHERMIC. This three-level reading (reactants, peak, products) gives you everything!