📘 Hess's Law
The total enthalpy change for a reaction is independent of the route taken, provided initial and final conditions are the same. Enthalpy is a state function.
Formation Cycle
\( \Delta H_r^\ominus = \sum \Delta H_f^\ominus (\text{products}) - \sum \Delta H_f^\ominus (\text{reactants}) \)
Route: Reactants ← Elements → Products
Combustion Cycle
\( \Delta H_r^\ominus = \sum \Delta H_c^\ominus (\text{reactants}) - \sum \Delta H_c^\ominus (\text{products}) \)
⚠️ Note: Reversed order compared to formation!
Bond Enthalpy Method
\( \Delta H = \sum(\text{bonds broken}) - \sum(\text{bonds formed}) \)
Only applies to gaseous species. Uses average values → approximate answers.
⚠️ Examiner Traps
- Stoichiometry: Multiply each ΔHf⦵ or ΔHc⦵ by the coefficient from the balanced equation
- Arrow direction: Travelling against an arrow in a cycle = reverse the sign
- Elements: ΔHf⦵ of any element in its standard state = 0
- Combustion products: CO₂ and H₂O cannot burn further → ΔHc⦵ = 0 for these