Enthalpy Changes In Reactions

Question

The standard enthalpy of combustion of methane is −890 kJ mol⁻¹. If 4.0 g of methane (CH₄, molar mass = 16 g mol⁻¹) is burned completely, the heat released is:

RankGuru · Accepted Answer

1. **Calculate Moles of CH₄**: $n = \frac{4.0}{16} = 0.25$ mol.
2. **Relate to Combustion Enthalpy**: The standard enthalpy of combustion is given per mole. For 0.25 mol:
   $q = 0.25 	imes 890 = 222.5$ kJ (magnitude of heat released).
3. **Sign Convention**: ΔH_comb is negative (exothermic), so 222.5 kJ of heat is *released* to the surroundings.
4. **Why Not the Others?**
   - 890 kJ would be correct only for 1 mol (16 g) of CH₄.
   - 445 kJ corresponds to 0.5 mol (8 g).
   - 3560 kJ corresponds to 4 mol (64 g).
5. **Sanity Check**: Burning a small lump of methane (≈4 g) releases about 222.5 kJ, which is physically reasonable.

Enthalpy Changes In Reactions

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