The principle that the enthalpy change of a reaction can be determined by summing the enthalpies of its steps is known as which law?

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

Answer: (D)

The principle being tested is Hess’s Law: the enthalpy change of a reaction depends only on the initial and final states, not on the path taken. Because enthalpy is a state function, you can break a reaction into a sequence of steps, assign an enthalpy change to each step, and add them to obtain the overall ΔH. This is why we can compute the enthalpy of a reaction by combining known enthalpies of formation for the products and reactants: ΔHrxn = sum of (n × ΔHf of products) minus sum of (m × ΔHf of reactants). In practice, this lets us determine the heat involved in a reaction even when we don’t observe it directly, simply by using the enthalpies of the steps that lead from reactants to products.

The other names refer to different ideas: Le Chatelier’s principle describes how equilibria shift to counteract disturbances; Dalton’s law concerns the total pressure as the sum of partial pressures; and Charles’s law relates gas volume to temperature at constant pressure.