Hess's Law: The enthalpy change for a reaction is the same whether it occurs in one step or multiple steps, as shown by the consistent ΔH° values for the indirect and direct pathways from 2A + B to 2D.

Diagram illustrating Hess's Law, showing two pathways (indirect and direct) from reactants 2A + B to product 2D, with enthalpy changes labeled as +50 kJ, -30 kJ, and -80 kJ, demonstrating that the total enthalpy change is the same regardless of the path.