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Enthalpy Change with Bond Energies
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Enthalpy Change with Bond Energies
📚 Part 1: Concept Check (Multiple Choice)
1. Which statement is correct?
Breaking bonds releases energy; forming bonds absorbs energy.Breaking bonds absorbs energy; forming bonds releases energy.Both breaking and forming bonds always release energy.
2. When estimating ΔH using average bond energies, the result is best described as:
An exact value valid for all physical states and molecules.A reasonable approximation that ignores molecular environment and state.A measurement that requires experimental enthalpy tables to confirm.
3. Which reaction is likely to be most exothermic (release most energy) using bond-energy reasoning?
A reaction that forms several strong bonds (large bond energies) and breaks few weak bonds.A reaction that breaks many strong bonds and forms few weak bonds.A reaction with equal numbers of bonds broken and formed.
✏️ Part 2: Calculations (Use bond energies below)
Use the following average bond energies (kJ mol−1): H–H 436; C–H 413; O–H 467; O=O 498; C=O (double) 745; C–C 347; C=C 614; H–Cl 431; Cl–Cl 242.
4. Estimate ΔH (kJ mol−1) for: H2(g) + 1/2 O2(g) → H2O(l). Show your working.
5. Estimate ΔH (kJ mol−1) for: H2(g) + Cl2(g) → 2 HCl(g). Show your working.
6. Estimate ΔH (kJ mol−1) for: C(s) + O2(g) → CO2(g). Show your working.
7. Estimate ΔH (kJ mol−1) for the combustion of methane: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l). Show your working.
8. Estimate ΔH (kJ mol−1) for hydrogenation: C2H4(g) + H2(g) → C2H6(g). Show your working and give the sign of ΔH.
9. In one or two sentences: State one important limitation of using average bond energies to predict reaction enthalpies (use an everyday example if helpful).
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