Hess's law states which of the following about the enthalpy change of a reaction?

• A. The enthalpy change depends on the route taken between reactants and products.
• B. Enthalpy changes depend on the path taken arbitrarily; no standard path independence.
• C. The enthalpy change of a reaction is zero if the reaction is reversible.
• D. The total enthalpy change equals the sum of the enthalpy changes of its steps and is independent of the path.

Answer: (D)

Enthalpy change is a state function, meaning it depends only on the initial and final states of the system, not on how the system got there. Because of this, if a reaction occurs in steps, the total enthalpy change is the sum of the enthalpy changes of each step, and the route taken does not affect the overall value. In other words, ΔHtotal = ΔH1 + ΔH2 + …, no matter how you sequence the steps, so the overall enthalpy change is independent of the path.

This is why you can compute the enthalpy of a complex transformation by adding the enthalpies of its constituent steps or by using formation enthalpies, and you’ll get the same result. Reversing a reaction simply changes the sign of the enthalpy change, not its magnitude being zero. So the statement that the total enthalpy change equals the sum of the step changes and is independent of the path is the correct description.