Hess Law

Hess’s law says the enthalpy change for an overall reaction depends only on the initial and final states, not on the route taken between them. That works because enthalpy is a state function, so if you split a reaction into several intermediate steps, the energy changes for those steps add algebraically to give the same total. This lets you calculate enthalpy changes for reactions that are hard to measure directly by combining easier reactions with known H values.

Use this when you need the enthalpy change for a reaction that is not measured directly, especially in thermochemistry problems involving formation equations, combustion equations, or multiple reaction cycles. It is also useful for building energy cycles from tabulated enthalpies of formation or combustion.

• Forgetting to reverse the sign of H when an equation is reversed
• Adding equations correctly but not cancelling species that appear on both sides
• Using coefficients inconsistently, especially when scaling reactions without scaling H by the same factor