⚡ Bond Energy Calculations
15 Questions · Sections A–C · ΔH = Energy(Break) − Energy(Make)
Formula & Setup
State the formula used to calculate ΔH from bond energies. Explain what each term represents.
Explain why bond breaking is endothermic and bond making is exothermic. Use particle theory.
The bond energy of H−H is 436 kJ/mol and Cl−Cl is 243 kJ/mol. The H−Cl bond energy is 432 kJ/mol. Set up (but do not solve) the ΔH calculation for: H₂ + Cl₂ → 2HCl
A student calculates ΔH = +150 kJ/mol for a reaction. What does the positive sign tell us? Which term was larger — energy to break bonds or energy to make bonds?
Bond energies are given in kJ/mol. What does this mean? Why are the values always positive?
Simple Calculations
Calculate ΔH for: H₂ + Br₂ → 2HBr Given: H−H = 436 kJ/mol, Br−Br = 193 kJ/mol, H−Br = 366 kJ/mol
Calculate ΔH for: N₂ + 3H₂ → 2NH₃ Given: N≡N = 944 kJ/mol, H−H = 436 kJ/mol, N−H = 391 kJ/mol
Calculate ΔH for: H₂ + F₂ → 2HF Given: H−H = 436 kJ/mol, F−F = 158 kJ/mol, H−F = 562 kJ/mol
Calculate ΔH for: Cl₂ + 2HBr → 2HCl + Br₂ Given: Cl−Cl = 243 kJ/mol, H−Br = 366 kJ/mol, H−Cl = 432 kJ/mol, Br−Br = 193 kJ/mol
Calculate ΔH for the decomposition of HCl: 2HCl → H₂ + Cl₂ Given: H−Cl = 432 kJ/mol, H−H = 436 kJ/mol, Cl−Cl = 243 kJ/mol
Complex Calculations
Calculate ΔH for the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O Given: C−H = 412 kJ/mol, O=O = 496 kJ/mol, C=O = 805 kJ/mol, O−H = 463 kJ/mol
Calculate ΔH for the combustion of ethane: C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O Given: C−C = 347 kJ/mol, C−H = 412 kJ/mol, O=O = 496 kJ/mol, C=O = 805 kJ/mol, O−H = 463 kJ/mol
Calculate ΔH for the formation of ethanol from ethene: CH₂=CH₂ + H₂O → CH₃CH₂OH Given: C=C = 614 kJ/mol, C−C = 347 kJ/mol, O−H = 463 kJ/mol, C−H = 412 kJ/mol, C−O = 358 kJ/mol
A student calculates ΔH for a reaction as −450 kJ/mol using bond energies, but the data book value is −560 kJ/mol. Suggest TWO reasons why bond energy calculations may not give exact ΔH values.
Calculate ΔH for the reaction of nitrogen with oxygen: N₂ + O₂ → 2NO Given: N≡N = 944 kJ/mol, O=O = 496 kJ/mol, N=O = 631 kJ/mol Then state whether lightning could supply enough energy to drive this reaction and explain why this reaction matters environmentally.
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