U6.7 · Bond EnthalpiesPractice · 键焓
Lessons covered:U6.7 Bond Enthalpies
- 1Multiple choice
A gas-phase reaction is exothermic. In terms of chemical bonds, which statement is correct?
ExplanationBreaking bonds always absorbs energy and forming bonds always releases it. ΔH ≈ Σbroken − Σformed < 0 means more energy comes out from forming bonds, so overall the product bonds are stronger.
Review: U6.7 Bond Enthalpies
- 2Numeric
Use average bond enthalpies to estimate ΔH for H₂(g) + F₂(g) → 2HF(g). Average bond enthalpies (kJ/mol): H–H 436, F–F 155, H–F 567.
kJ/molExplanationBroken: one H–H and one F–F, 436 + 155 = 591 kJ. Formed: two H–F, 2 × 567 = 1134 kJ. ΔH ≈ 591 − 1134 = −543 kJ/mol. The F–F bond is very weak and the H–F bond very strong, so the reaction is strongly exothermic.
Review: U6.7 Bond Enthalpies
- 3Numeric
Hydrazine is a rocket fuel: N₂H₄(g) + O₂(g) → N₂(g) + 2H₂O(g), where hydrazine has the structure . Use average bond enthalpies to estimate ΔH. Average bond enthalpies (kJ/mol): N–N 163, N–H 391, O=O 495, N≡N 941, O–H 463.
kJ/molExplanationBroken: in N₂H₄ one N–N and four N–H, plus one O=O in O₂: 163 + 4 × 391 + 495 = 2222 kJ. Formed: one N≡N in N₂ and four O–H in the two H₂O: 941 + 4 × 463 = 2793 kJ. ΔH ≈ 2222 − 2793 = −571 kJ/mol. Forming the very strong N≡N bond is the main reason so much heat is released.
Review: U6.7 Bond Enthalpies
- 4Numeric
From enthalpies of formation, ΔH = −1266.4 kJ/mol for 4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(g). Average bond enthalpies (kJ/mol): N–H 391, O=O 495, O–H 463. Use these data to estimate the N≡N bond enthalpy.
kJ/molExplanationBroken: 12 N–H in 4 NH₃ and 3 O=O, 12 × 391 + 3 × 495 = 6177 kJ. Formed: 2 N≡N (x each) and 12 O–H in 6 H₂O (5556 kJ). −1266.4 = 6177 − (2x + 5556), so 2x = 6177 − 5556 + 1266.4 = 1887.4 and x ≈ 944 kJ/mol, close to the table value of 941.
Review: U6.7 Bond Enthalpies
- 5Multiple choice
The H–F bond enthalpy equals ΔH (per mole) for which process?
ExplanationA bond enthalpy is the energy absorbed to break 1 mol of a bond in gaseous molecules into gaseous atoms, so it is always positive. Forming the bond from atoms has the opposite sign; decomposing into H₂ and F₂ also forms H–H and F–F bonds; and ionizing in water involves hydration. None of those is the bond enthalpy.
Review: U6.7 Bond Enthalpies
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