A Level Chemistry OCR Practice Exam

What happens to energy during a reaction that follows Hess’ law?

1. Energy is neither created nor destroyed, it is conserved.

2. Energy levels increase with the number of pathways.

3. Energy changes are dependent solely on the reactants used.

4. Energy is released as heat regardless of the reaction.

The correct answer is: Energy is neither created nor destroyed, it is conserved.

The concept that energy is neither created nor destroyed, but rather conserved, aligns with the principle behind Hess’s law. Hess’s law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps the reaction occurs in or the pathway taken from reactants to products. This means that the overall change in energy is consistent, reinforcing the idea that energy remains constant in a closed system. In the context of a chemical reaction, this principle implies that even if a reaction can occur through multiple intermediate steps, the total enthalpy change will be the same as if the reaction had taken place in one single step. Consequently, this conservation of energy highlights the predictable nature of energy changes in chemical reactions. While some of the other responses may seem plausible, they do not accurately encapsulate the fundamental nature of energy changes described by Hess's law. For example, the idea that energy levels increase with pathways disregards the concept of energy conservation and specifically how enthalpy changes function. Similarly, stating that energy changes are solely dependent on the reactants does not consider the roles of products and reaction pathways, and claiming that energy is released as heat in all cases overlooks the possibility of endothermic reactions where energy is absorbed