An energy cycle is a diagram that shows alternative routes between reactants and products, allowing us to apply Hess's Law to find unknown enthalpy changes.
Hess's Law Recap
The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This is because enthalpy is a state function.
Formation Cycle
When enthalpies of formation are known for all reactants and products, we can use a formation cycle:
Formation Energy Cycle
\( \Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants}) \)
Combustion Cycle
When enthalpies of combustion are known, the cycle direction reverses:
Combustion Energy Cycle
\( \Delta H_{rxn} = \sum \Delta H_c(\text{reactants}) - \sum \Delta H_c(\text{products}) \)
Remembering the Formulas
Formation: Products − Reactants (P − R)
Combustion: Reactants − Products (R − P)
The "flip" occurs because in a combustion cycle, the arrows point downward from reactants and products to the same combustion products, so the subtraction reverses.
Think About It
Why can we not use a combustion cycle for every reaction?
Because not all substances can be burned. For example, CO₂ and H₂O are already combustion products — they have no enthalpy of combustion. Similarly, ionic compounds typically do not combust.