Hess's law — common mistakes

Pay close attention to the formula when using combustion data! It's Reactants - Products, the opposite of the formation data formula. A common mistake is to mix these up. Drawing the cycle with arrows pointing DOWN to combustion products helps to derive this correctly every time.

In an exam, always draw the full Hess's cycle, even if you use the formula. Marks are often awarded for a correctly drawn and labelled cycle. It also acts as a visual check to ensure you have the signs and stoichiometry correct.

The standard enthalpy of formation is the energy change to form one mole of a substance from its elements in their standard states. To 'form' an element like O₂(g) from its standard state (which is O₂(g)), there is no change, so the enthalpy change is zero by definition.

For formation data, the cycle's intermediate step is the constituent elements, and the arrows point 'up' from the elements to the reactants and products. For combustion data, the intermediate step is the combustion products (CO₂ and H₂O), and the arrows point 'down' from the reactants and products to these products.

While theoretically possible if you construct a very complex cycle, it is not a standard method and is highly prone to error. For A-Level Chemistry, you will always be given a consistent set of data – either all formation or all combustion values needed for the calculation.

When using formation data ($\Delta H_f^\ominus$), the value for the element (e.g., H₂(g)) is zero. When using combustion data ($\Delta H_c^\ominus$), the element will have a non-zero value (e.g., the enthalpy of combustion of hydrogen is -286 kJ mol⁻¹), which you must include in your calculation.