What does Hess's Law state?

• A. The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.
• B. Enthalpy change depends on the path.
• C. Enthalpy change is always negative for exothermic reactions.
• D. Enthalpy change can be determined only by calorimetry.

Answer: (A)

Hess's Law hinges on enthalpy being a state function: the overall enthalpy change of a reaction depends only on the starting and ending states, not on the path taken. If a reaction can be broken into steps, the total enthalpy change is simply the sum of the enthalpy changes of those steps. This means you can determine the enthalpy of a reaction by combining known enthalpies of formation or bond enthalpies for the steps that connect reactants to products.

This idea is powerful because it lets you compute unknown enthalpies by reassembling reactions you know, as long as all the initial and final states match. The statement that enthalpy depends on the route would contradict enthalpy being a state function, so that option isn’t correct. Not all reactions release heat; some absorb heat, so the enthalpy change isn’t always negative for exothermic processes. And while calorimetry can measure enthalpy directly, Hess’s Law provides a way to find it without calorimetry by using tabulated data for the steps involved.