Hess's law states that the enthalpy change of a chemical process is independent of the path taken.

• A. The enthalpy change of a chemical process is independent of the path taken.
• B. The entropy change is always positive for spontaneous processes.
• C. Enthalpy changes depend on the rate of reaction.
• D. The Gibbs free energy is constant for all reactions.

Answer: (A)

The main idea being tested is that enthalpy is a state function. Hess’s law follows from the fact that the enthalpy change for a chemical process depends only on the initial and final states, not on the path taken between them. Because enthalpy is determined by the state of the system, you can add up enthalpy changes of individual steps to get the same total as for a direct reaction, so long as the initial and final conditions are the same. At constant pressure, this enthalpy change corresponds to the heat exchanged, which is why breaking a reaction into steps still gives the same total heat evolved or absorbed.

Entropy changes, however, are not constrained to be positive for spontaneous processes in a simple way—spontaneity involves the total entropy change of the universe, and the system’s entropy can decrease if the surroundings’ entropy increases enough. Enthalpy changes are not set by the rate of reaction; kinetics tell you how fast a reaction proceeds, not how much energy is involved. Finally, Gibbs free energy is not constant for all reactions; it varies with temperature, pressure, and composition, and at equilibrium the change in G is zero, but it’s not universally constant.