Enthalpy Changes In Reactions
Easychemistry
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:
Select the correct option:
Solution
Incorrect! Answer:
222.5 kJ
- Calculate Moles of CH₄: n=164.0=0.25 mol.
- Relate to Combustion Enthalpy: The standard enthalpy of combustion is given per mole. For 0.25 mol: q=0.25×890=222.5 kJ (magnitude of heat released).
- Sign Convention: ΔH_comb is negative (exothermic), so 222.5 kJ of heat is released to the surroundings.
- 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).
- Sanity Check: Burning a small lump of methane (≈4 g) releases about 222.5 kJ, which is physically reasonable.
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About This Question
- Subject
- chemistry
- Chapter
- chemical thermodynamics
- Topic
- enthalpy changes in reactions
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
222.5 kJ
- Calculate Moles of CH₄: n=164.0=0.25 mol.
- Relate to Combustion Enthalpy: The standard enthalpy of combustion is given per mole. For 0.25 mol: q=0.25×890=222.5 kJ (magnitude of heat released).
- Sign Convention: ΔH_comb is negative (exothermic), so 222.5 kJ of heat is released to the surroundings.
- 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).
- Sanity Check: Burning a small lump of methane (≈4 g) releases about 222.5 kJ, which is physically reasonable.
This easy difficulty chemistry question is from the chapter chemical thermodynamics, covering the topic of enthalpy changes in reactions. It appeared in the 2025 exam.
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