CHEMISTRY COMMUNITY

Can somebody explain Hess' Law and its use to me?

Hess's Law states that enthalpy changes are additive; the change of enthalpy at each step of a multi-step reaction can be added to determine the total change in enthalpy. Here is an example for how to use Hess's Law:
1. The first reaction shows one of the reactants from the overall reaction as a reactant in a different chemical equation. The reaction enthalpy for this reaction is -110.5 kJ.
2. The second reaction shows one of the products from the overall reaction as a product in a different chemical equation. The reaction enthalpy for this reaction is -283.0 kJ.
3. The third reaction is the overall reaction; add the two equations above and cancel out species that appear on both sides of the equation. Once this series is complete, combine the reaction enthalpies of the first two reactions to find the reaction enthalpy of the overall reaction.

Additionally, you can manipulate equations and add them together to get the final desired equation. When you reverse an equation, the delta H changes signs, and when you multiply an equation by a constant, the delta H is multiplied by that constant. Then, you can add all the new enthalpy values together to get the final enthalpy value.

Conceptually, you can sum all the changes using Hess's Law because enthalpy is a state function. Think of these problems as puzzles where you manipulate the steps and cancel out compounds on opposite sides of the reaction to get to the desired equation.