Which statement best describes Hess's law?

• A. The enthalpy change of a reaction depends on the path taken.
• B. The enthalpy change depends on the catalyst used.
• C. The enthalpy change is independent of the path and depends only on the initial and final states.
• D. The enthalpy change is the same as the total entropy change.

Answer: (C)

The main idea is that enthalpy is a state function. This means the energy change when going from reactants to products depends only on those initial and final states, not on the path taken. So whether the reaction happens in one step or through multiple intermediate steps, the total enthalpy change is the same. This is Hess’s law: you can break a reaction into steps, add their enthalpy changes, and get the same overall ΔH as for the direct reaction.

Catalysts don’t alter the energy difference between reactants and products; they only speed up the rate, so they don’t change the enthalpy change. Entropy is a different thermodynamic property, and enthalpy changes aren’t the same as entropy changes.