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