Which statement best describes the application of Hess's law when constructing overall enthalpy changes from steps that may be reversed?

• A. The total enthalpy change equals the sum of the step enthalpies, regardless of direction.
• B. The total enthalpy change equals the product of the step enthalpies.
• C. The total enthalpy change equals the sum of the step enthalpies, using the same sign for all steps.
• D. The total enthalpy change equals the sum of the step enthalpies with signs adjusted for reversed steps.

Answer: (D)

Hess's law is about adding up the enthalpy changes of steps to get the overall enthalpy change, but you must pay attention to the direction of each step. If a step is written in reverse, its enthalpy change changes sign. So the overall enthalpy is the sum of the signed enthalpy changes, with signs adjusted for any reversed steps.

This makes sense because enthalpy is a state function, and the net energy change depends only on the final state, not on the path taken. Reversing a step inverts the energy change, so you don’t keep the same sign for that step—you flip it and then add everything together.

In practice, you take each step’s enthalpy change as given, flip the sign for any reversed steps, and then sum all of them. This is why the correct approach is to add step enthalpies with signs adjusted for reversed steps.

The other ideas don’t fit because: using the same sign for all steps ignores reversals; multiplying step enthalpies would not reflect how enthalpy adds; and simply taking a product of step enthalpies isn’t how enthalpy changes combine.