📘 Hess's Law
The total enthalpy change for a reaction is independent of the pathway between reactants and products, provided the initial and final conditions are the same. This is a consequence of enthalpy being a state function.
Using Formation Data (ΔHf⦵)
\( \Delta H_r^\ominus = \sum \Delta H_f^\ominus (\text{products}) - \sum \Delta H_f^\ominus (\text{reactants}) \)
The cycle goes: Reactants → Elements → Products
Using Combustion Data (ΔHc⦵)
\( \Delta H_r^\ominus = \sum \Delta H_c^\ominus (\text{reactants}) - \sum \Delta H_c^\ominus (\text{products}) \)
Note: reversed compared to formation. Reactants minus products
⚠️ Examiner Traps
- Stoichiometry: Multiply ΔHf⦵ by the coefficient in the balanced equation
- Elements: ΔHf⦵ of elements in standard state = 0 (don't forget!)
- Combustion products: CO₂ and H₂O are combustion products. Their ΔHc⦵ = 0 in the combustion cycle
- Arrow direction: In energy cycle diagrams, if you travel against an arrow, reverse the sign
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