Question

question 13 of 30
the reaction below shows how the potential energy changes during a
chemical reaction. how is the change in enthalpy shown on the graph?
graph: potential energy vs reaction progress with a curve and a red arrow

a. the tallest point on the graph shows that enthalpy is small,
indicating an exothermic reaction.

b. the red arrow shows that enthalpy is negative, indicating an
exothermic reaction.

c. the red arrow shows that enthalpy is positive, indicating an
endothermic reaction.

d. the tallest point on the graph shows that enthalpy is large,
indicating an endothermic reaction.

Explanation:

1. Recall the concepts of enthalpy change (\(\Delta H\)) in chemical reactions:

• For an exothermic reaction, the products have lower potential energy than the reactants, so \(\Delta H = H_{products}-H_{reactants}<0\) (negative enthalpy change).
• For an endothermic reaction, the products have higher potential energy than the reactants, so \(\Delta H>0\) (positive enthalpy change).

1. Analyze the graph: The reactants' potential energy is at the initial horizontal dashed line, and the products' potential energy is lower (at the final horizontal line). The red arrow shows the difference between reactants' and products' potential energy (enthalpy change).
2. Evaluate each option:

• Option A: The tallest point is the activation energy, not related to enthalpy change magnitude for exothermicity. Eliminate A.
• Option B: The red arrow (enthalpy change) is negative (products have less energy than reactants), which means \(\Delta H < 0\), indicating an exothermic reaction. This matches the graph.
• Option C: The arrow shows a decrease in energy (products lower than reactants), so enthalpy change is negative, not positive. Eliminate C.
• Option D: The tallest point is activation energy, and the reaction is exothermic (not endothermic) as products have less energy. Eliminate D.

Answer:

B. The red arrow shows that enthalpy is negative, indicating an exothermic reaction.