Hess's Law

If the overall equation for a reaction can be expressed as the algebraic sum of the equations of two or more other reactions, then the $\Delta H$ of that reaction is the algebraic sum of the $\Delta H$ for those reactions

Example

Nitrogen Dioxide can be created in one step Or in two steps If you add the two equations Then you’ll get the one step equation at the top

Energy State Diagram

Rules of Enthalpy Changes

1. If you reverse a chemical reaction, you must also reverse the sign for $\Delta H$
2. If the moles of every reactant and product is multiplied, then $\Delta H$ is multiplied by that same number