Hess's Law and Enthalpy Diagrams

Hess's law says the enthalpy change of a reaction is the same whether it happens in one step or several. Enthalpy is a state function — it depends only on the start and end points, not the path — so you can add reactions together to reach one you can't measure directly.

The two rules for manipulating steps

• Reverse a reaction → change the sign of ΔH. If forming a compound releases 286 kJ, breaking it apart absorbs 286 kJ.
• Multiply a reaction by a number → multiply ΔH by the same number. Doubling the amounts doubles the heat.

Arrange the given equations so that, when added, the unwanted species cancel and only your target reaction remains. The sum of the adjusted ΔH values is the answer.

The shortcut: enthalpies of formation

When you have a table of standard enthalpies of formation (ΔH°f), you can skip the bookkeeping:

Every element in its standard state has ΔH°f = 0. Multiply each substance's ΔH°f by its coefficient, sum the products, sum the reactants, and subtract.

Worked example (thermite)

Fe₂O₃(s) + 2 Al(s) → Al₂O₃(s) + 2 Fe(s). Using ΔH°f values (Al₂O₃ −1676, Fe₂O₃ −824.2, elements 0):

The large negative value shows why the thermite reaction is intensely exothermic.

Reading an enthalpy diagram

On an energy profile, products below the reactants mean an exothermic reaction (ΔH negative); products above mean endothermic (ΔH positive). The activation energy is the height of the hump from reactants to the transition state, and a catalyst lowers that hump without changing ΔH.

Measure heat in the lab with the Calorimetry Calculator (q = mcΔT), which is how ΔH values are determined experimentally.