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Chemistry Help: Interpret Chemical Energy Diagrams

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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?

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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?

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

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 ( $\Delta H$ )?

The vertical difference between the reactant energy level and the product energy level (correct answer)
The vertical difference between the reactant energy level and the peak
The height of the peak above the x-axis
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 $\Delta H$ ?

$\Delta H$ is positive because the peak is above the reactants
$\Delta H$ is negative because products are lower in energy than reactants (correct answer)
$\Delta H$ is the vertical distance from reactants to the peak
$\Delta H$ 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 ( $\Delta H$ ) is correct?

The reaction is exothermic; $\Delta H$ is negative.
The reaction is endothermic; $\Delta H$ is positive. (correct answer)
The reaction is exothermic because it must have a peak.
The reaction has $\Delta H = 0$ 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?

Endothermic, because the curve rises to the peak
Exothermic, because the products are at lower energy than the reactants (correct answer)
Endothermic, because the activation energy is positive
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?

The reaction is exothermic because products are formed after the peak
The reaction is endothermic because products are at higher energy than reactants (correct answer)
The activation energy is the vertical drop from the peak to the products
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?

Diagram 1 has a larger activation energy than Diagram 2 (correct answer)
Diagram 1 has a larger (more positive) $\Delta H$ than Diagram 2
Diagram 2 must be endothermic while Diagram 1 must be exothermic
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 $E_a$ ?

The vertical distance from the reactant energy level up to the peak (correct answer)
The vertical distance from the product energy level up to the peak
The vertical distance from the x-axis up to the peak
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 $\Delta H$ ?

From the reactant level to the peak
From the peak down to the product level
From the reactant level to the product level (correct answer)
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 $\Delta H$ ?

$\Delta H$ is negative (energy released)
$\Delta H$ is zero (no energy change)
$\Delta H$ is positive (energy absorbed) (correct answer)
$\Delta H$ 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.