Bond Enthalpy Approximation

Question

Estimate ΔH (kJ mol⁻¹) for CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) using average bond enthalpies. (Data approximate: C–H: 413, O=O: 498, C=O in CO2: 799, O–H: 463).

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1. **Formula**: $\Delta H = \sum BH_{	ext{broken}} - \sum BH_{	ext{formed}}$.
2. **Bonds Broken (Reactants)**:
   - $4 	imes C-H$ = $4 	imes 413 = 1652$ kJ.
   - $2 	imes O=O$ = $2 	imes 498 = 996$ kJ.
   - Total reactant energy = $2648$ kJ.
3. **Bonds Formed (Products)**:
   - $2 	imes C=O$ (in $CO_2$) = $2 	imes 799 = 1598$ kJ.
   - $2 	imes 2(O-H)$ (in $2H_2O$) = $4 	imes 463 = 1852$ kJ.
   - Total product energy = $3450$ kJ.
4. **Calculation**: $\Delta H = 2648 - 3450 = -802$ kJ.
5. **Result**: Roughly **$-800$ kJ/mol**.

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