Apply Hess's law and bond enthalpy data to calculate enthalpy changes for reactions.

What this dot point is asking

You must use Hess's law with enthalpy cycles and standard enthalpies, and estimate enthalpy changes from average bond enthalpies.

Hess's law

Because enthalpy is a state function, the energy change depends only on the start and end points, not the path between them.

This means we can construct an energy cycle: if a direct reaction is hard to measure, we go via intermediate steps whose enthalpies are known, and add them up. Reversing a step changes the sign of its $\Delta H$; multiplying a step multiplies its $\Delta H$.

Using enthalpies of formation

The standard enthalpy of formation $\Delta H_f^\circ$ is the enthalpy change when one mole of a compound forms from its elements in their standard states. Elements in their standard state have $\Delta H_f^\circ = 0$. For any reaction:

Using bond enthalpies

A reaction breaks bonds in the reactants (requiring energy in) and makes bonds in the products (releasing energy). The estimated enthalpy change is:

Bond enthalpies are averages taken over many molecules, so this method gives an estimate rather than an exact value, and it only applies to gaseous species.

In the exam, write the balanced equation first, label which method you are using, multiply every term by its coefficient, and check the sign against whether you expect the reaction to be exothermic or endothermic.