The statement that the overall enthalpy of a reaction equals the sum of the enthalpies of its steps is known as which law?

• A. Le Chatelier's Principle
• B. Dalton's Law
• C. Kirchhoff's Law
• D. Hess's Law

Answer: (D)

Enthalpy is a state function, so the energy change depends only on the starting and ending states, not on the path taken. Because of this, you can break a reaction into a sequence of steps, find the enthalpy change for each step, and add them together to get the overall enthalpy change. This is Hess's Law. It’s particularly useful for calculating the heat of a reaction by combining known step enthalpies, such as formation or combustion values, to obtain the total ΔH. The other laws describe different ideas: Le Châtelier's Principle explains how equilibria respond to stresses, Dalton's Law deals with partial pressures in gas mixtures, and Kirchhoff's Law concerns how enthalpy changes vary with temperature rather than summing steps.