Interpret Chemical Energy Diagrams - Chemistry

1. Which label corresponds to energy released to surroundings in an exothermic energy diagram?

   $|\Delta H|$ (magnitude of negative $\Delta H$). The magnitude of negative $\Delta H$ equals energy released.

2. What effect does a catalyst have on the energies of reactants and products?

   No change to reactant or product energies. Catalysts only lower the transition state energy, not endpoint energies.

3. What does the peak of a reaction energy diagram represent?

   Transition state (activated complex). The highest energy point where bonds are breaking and forming simultaneously.

4. Identify the sign of $\Delta H$ if $E_{a,reverse}=120\,\text{kJ}$ and $E_{a,forward}=80\,\text{kJ}$.

   $\Delta H < 0$. Reverse barrier is larger, so products are lower than reactants.

5. What is the relationship between $E_{a,forward}$ and $E_{a,reverse}$ using $H$?

   $E_{a,reverse} = E_{a,forward} - \Delta H$. The reverse barrier equals forward barrier minus the enthalpy change.

6. Identify $\Delta H$ if the diagram shows reactants at $10\,\text{kJ}$ and products at $-15\,\text{kJ}$.

   $\Delta H = -25\,\text{kJ}$. $\Delta H = -15 - 10 = -25\,\text{kJ}$

7. Find $\Delta H$ if $E_{a,forward}=65\,\text{kJ}$ and $E_{a,reverse}=95\,\text{kJ}$.

   $\Delta H = -30\,\text{kJ}$. $\Delta H = 65 - 95 = -30\,\text{kJ}$

8. What does the energy difference between reactants and products determine about equilibrium?

   Thermodynamic favorability (relative stability), not rate. Energy difference determines equilibrium position, not how fast equilibrium is reached.

9. What does the vertical axis represent on a chemical energy diagram labeled Energy (kJ)?

   Potential energy (enthalpy) of the system. Energy represents the potential energy stored in chemical bonds and molecular arrangements.

10. What does each valley between peaks represent in a multistep energy diagram?

    A reaction intermediate. Valleys represent unstable species formed between elementary steps.

11. Which feature on an energy diagram indicates a multistep reaction mechanism?

    More than one peak (multiple transition states). Each peak represents a separate elementary step in the mechanism.

12. What is the relationship between forward and reverse enthalpy changes?

    $\Delta H_{reverse} = -\Delta H_{forward}$. Forward and reverse reactions have equal but opposite energy changes.

13. Choose the correct comparison for a catalyzed vs uncatalyzed diagram with same endpoints.

    Same $\Delta H$, lower $E_a$ with catalyst. Catalysts change kinetics but not thermodynamics.

14. What does it mean if the diagram shows products much lower than reactants but a very high peak?

    Thermodynamically favorable but kinetically slow. Thermodynamics favors products, but kinetics prevents rapid formation.

15. What effect does a catalyst have on $H$ on an energy diagram?

    No change to $\Delta H$. Catalysts only affect the pathway, not the energy difference between states.

16. Find $E_{a,reverse}$ if $E_{a,forward}=90\,\text{kJ}$ and $\Delta H=+20\,\text{kJ}$.

    $E_{a,reverse} = 70\,\text{kJ}$. $E_{a,reverse} = 90 - 20 = 70\,\text{kJ}$

17. Identify the quantity shown by the vertical gap from reactants to products on an energy diagram.

    $\Delta H$. The overall energy change from reactants to products.

18. Identify $E_a$ (forward) if reactants are at $-5\,\text{kJ}$ and the peak is at $60\,\text{kJ}$.

    $E_a = 65\,\text{kJ}$. $E_a = 60 - (-5) = 65\,\text{kJ}$

19. Identify the quantity shown by the vertical gap from products to the peak on an energy diagram.

    $E_a$ (reverse). The energy barrier from products back to the transition state.

20. What is the correct expression for $E_{a,forward}$ using energies $E_{TS}$ and $E_R$?

    $E_{a,forward} = E_{TS} - E_R$. Activation energy is the difference between transition state and reactant energies.

21. In a 2-step diagram, step 1 has $E_a=30\,\text{kJ}$ and step 2 has $E_a=65\,\text{kJ}$. Which step is RDS?

    Step 2. The rate-determining step has the largest activation energy.

22. What is true about product energy relative to reactant energy in an exothermic diagram?

    Products are at lower energy than reactants. Energy is released when products form, making them more stable than reactants.

23. What does it mean if the diagram shows two curves with the same start and end energies?

    Same $\Delta H$, different $E_a$ (often catalyzed vs uncatalyzed). Same thermodynamic change but different kinetic pathways are shown.

24. Classify the reaction if products lie $45\,\text{kJ}$ below reactants on the diagram.

    Exothermic. Products are lower than reactants, so energy is released.

25. Find $E_a$ (forward) if reactants are at $30\,\text{kJ}$ and the peak is at $110\,\text{kJ}$.

    $E_a = 80\,\text{kJ}$. $E_a = 110 - 30 = 80\,\text{kJ}$

26. What does it mean if $E_{a,reverse}$ is much larger than $E_{a,forward}$ on the same diagram?

    Forward reaction is kinetically favored relative to reverse. The reverse reaction has a much higher energy barrier to overcome.

27. What does a very small $E_a$ suggest about the reaction at room temperature?

    It can occur rapidly at room temperature. Low barriers allow reactions to proceed easily at moderate temperatures.

28. Classify the reaction if products lie $25\,\text{kJ}$ above reactants on the diagram.

    Endothermic. Products are higher than reactants, so energy is absorbed.

29. Which sign of $H$ indicates an exothermic reaction on an energy diagram?

    $\Delta H < 0$. Products have lower energy than reactants, releasing energy to surroundings.

30. Which reaction has the larger rate at the same temperature: $E_a=55\,\text{kJ}$ or $E_a=95\,\text{kJ}$?

    The reaction with $E_a=55\,\text{kJ}$. Lower activation energy corresponds to higher reaction rate.

31. Identify the more stable state on an energy diagram: higher energy or lower energy?

    Lower energy is more stable. Systems naturally move toward states of minimum potential energy.

32. Identify whether $\Delta H$ is positive or negative if $E_{a,reverse} < E_{a,forward}$.

    $\Delta H > 0$. If reverse barrier is smaller, products are higher than reactants.

33. In a 3-step diagram, peaks are at $50, 90, 70\,\text{kJ}$ above their preceding valleys. Which step is RDS?

    The step with $90\,\text{kJ}$ activation energy. The highest barrier determines the overall reaction rate.

34. What effect does a catalyst have on $E_a$ on an energy diagram?

    It lowers $E_a$. Catalysts provide alternative pathways with reduced energy barriers.

35. Identify whether $\Delta H$ is positive or negative if $E_{TS}-E_R < E_{TS}-E_P$.

    $\Delta H < 0$. If forward barrier is larger, products are lower than reactants.

36. Which sign of $\Delta H$ indicates an exothermic reaction on an energy diagram?

    $\Delta H < 0$. Products have lower energy than reactants, releasing energy to surroundings.

37. What is the correct expression for $H$ using reactant and product energies $E_R$ and $E_P$?

    $\Delta H = E_P - E_R$. Enthalpy change is the difference between product and reactant energies.

38. What is the sign of $H$ if reactants are at $50\,\text{kJ}$ and products are at $20\,\text{kJ}$?

    $\Delta H < 0$. Products are lower in energy than reactants.

39. What is the relationship between $E_{a,forward}$ and $E_{a,reverse}$ using $\Delta H$?

    $E_{a,reverse} = E_{a,forward} - \Delta H$. The reverse barrier equals forward barrier minus the enthalpy change.

40. Which pathway is faster if one curve has $E_a=40\,\text{kJ}$ and another has $E_a=75\,\text{kJ}$?

    The pathway with $E_a=40\,\text{kJ}$. Lower activation energy means faster reaction rate.

41. Find $H$ if $H_{reactants}=150\,\text{kJ}$ and $H_{products}=90\,\text{kJ}$.

    $\Delta H = -60\,\text{kJ}$. $\Delta H = 90 - 150 = -60\,\text{kJ}$

42. Which sign of $\Delta H$ indicates an endothermic reaction on an energy diagram?

    $\Delta H > 0$. Products have higher energy than reactants, absorbing energy from surroundings.

43. What does the term 'reaction coordinate' mean on an energy diagram?

    Progress from reactants through transition states to products. The pathway describing molecular changes during the reaction process.

44. What is the definition of activation energy, $E_a$, on an energy diagram?

    Energy from reactants to the peak (transition state). The energy barrier that must be overcome for the reaction to proceed.

45. Identify the quantity shown by the vertical gap from reactants to the peak on an energy diagram.

    $E_a$ (forward). The energy barrier from reactants to the transition state.

46. What does it mean if the diagram shows products higher than reactants but a low peak?

    Thermodynamically unfavorable but can be kinetically fast. Thermodynamics opposes products, but kinetics allows rapid formation.

47. Which label corresponds to energy absorbed from surroundings in an endothermic energy diagram?

    $\Delta H$ (positive). Positive $\Delta H$ represents energy absorbed from surroundings.

48. What do the left and right plateaus usually represent on an energy diagram?

    Reactants (left) and products (right). Starting materials are on the left, final products are on the right.

49. What is the correct expression for $E_{a,reverse}$ using energies $E_{TS}$ and $E_P$?

    $E_{a,reverse} = E_{TS} - E_P$. Reverse activation energy uses product energy instead of reactant energy.

50. What does a larger $E_a$ imply about reaction rate at the same temperature?

    Slower reaction rate. Higher energy barriers require more thermal energy to overcome at given temperature.

51. What does the horizontal axis typically represent on a reaction energy diagram?

    Reaction progress (reaction coordinate). Shows the pathway from reactants to products through intermediate states.

52. Find $\Delta H$ if $E_{a,forward}=85\,\text{kJ}$ and $E_{a,reverse}=60\,\text{kJ}$.

    $\Delta H = +25\,\text{kJ}$. $\Delta H = 85 - 60 = +25\,\text{kJ}$

53. What is true about product energy relative to reactant energy in an endothermic diagram?

    Products are at higher energy than reactants. Energy must be absorbed to form products, making them less stable than reactants.

54. What does a lower peak for a catalyzed pathway indicate compared with the uncatalyzed pathway?

    Lower activation energy for the catalyzed pathway. Catalysts reduce the energy barrier, increasing reaction rate.

55. Find $E_a$ (reverse) if products are at $20\,\text{kJ}$ and the peak is at $95\,\text{kJ}$.

    $E_a = 75\,\text{kJ}$. $E_a = 95 - 20 = 75\,\text{kJ}$

56. What is the definition of enthalpy change, $H$, from an energy diagram?

    $\Delta H = H_{products} - H_{reactants}$. The difference in energy between final and initial states.

57. Find $H$ if $H_{reactants}=80\,\text{kJ}$ and $H_{products}=120\,\text{kJ}$.

    $\Delta H = +40\,\text{kJ}$. $\Delta H = 120 - 80 = +40\,\text{kJ}$

58. Which statement is correct if a catalyst creates an alternate pathway with a lower peak?

    The catalyzed pathway has a lower $E_a$. Catalysts reduce activation energy by providing alternate pathways.

59. In a multistep diagram, which step is the rate-determining step?

    The step with the largest $E_a$ (tallest barrier). The slowest step has the highest activation energy barrier.

60. Which quantity changes when temperature increases: $\Delta H$, reactant energy levels, or effective reaction rate?

    Effective reaction rate increases. Higher temperature provides more thermal energy to overcome barriers.

61. Find $E_{a,reverse}$ if $E_{a,forward}=70\,\text{kJ}$ and $\Delta H=-30\,\text{kJ}$.

    $E_{a,reverse} = 100\,\text{kJ}$. $E_{a,reverse} = 70 - (-30) = 100\,\text{kJ}$

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