Bond Enthalpies: Quiz

Bond Enthalpies: Quiz

7 questions

1. Question 1EasyWhich statement is correct?

   ABreaking bonds releases energyBBreaking bonds requires energyCForming bonds requires energyDBond enthalpies are negative
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   Answer: Breaking bonds requires energy

   Energy must be supplied to pull atoms apart, so bond breaking is endothermic; forming bonds releases the same energy. Bond enthalpies are positive.

2. Question 2EasyWhich bond is the shortest and strongest?

   AC–CBC=CCC≡CDThey are all equal
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   Answer: C≡C

   Higher bond order means a shorter, stronger bond: C≡C (120 pm, 839 kJ/mol) beats C=C and C–C.

3. Question 3MediumEstimate ΔH for H₂ + Br₂ → 2HBr (all gases). H–H 436, Br–Br 193, H–Br 366 kJ/mol.

   A+103 kJ/molB−103 kJ/molC−263 kJ/molD+263 kJ/mol
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   Answer: −103 kJ/mol

   Broken: 436 + 193 = 629 kJ/mol. Formed: 2 × 366 = 732 kJ/mol. ΔH ≈ 629 − 732 = −103 kJ/mol. −263 forgets the factor of 2.

4. Question 4MediumEstimate ΔH for 2H₂ + O₂ → 2H₂O (gases). H–H 436, O=O 498, O–H 463 kJ/mol.

   A−482 kJ/molB+482 kJ/molC−471 kJ/molD−9 kJ/mol
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   Answer: −482 kJ/mol

   Broken: 2 × 436 + 498 = 1370 kJ/mol. Formed: 4 O–H = 4 × 463 = 1852 kJ/mol. ΔH ≈ 1370 − 1852 = −482 kJ/mol.

5. Question 5MediumWhy is the ΔH from bond enthalpies only an estimate?

   ABond enthalpies change with pressureBMost bond enthalpies are averages, and apply to gases onlyCBonds cannot be brokenDIt ignores the coefficients
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   Answer: Most bond enthalpies are averages, and apply to gases only

   A C–H bond is slightly different in every molecule, so tables give averages; changes of state are also left out.

6. Question 6HardFor H₂ + I₂ → 2HI, ΔH = −9 kJ/mol. With H–H 436 and I–I 151 kJ/mol, what is the H–I bond enthalpy?

   A289 kJ/molB298 kJ/molC596 kJ/molD587 kJ/mol
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   Answer: 298 kJ/mol

   −9 = (436 + 151) − 2x, so 2x = 596 and x = 298 kJ/mol. 596 kJ/mol is the energy for two H–I bonds.

7. Question 7HardIn terms of bonds, why is burning methane exothermic?

   AMethane has no bondsBThe bonds in CO₂ and H₂O are stronger in total than those in CH₄ and O₂CBreaking C–H bonds releases energyDOxygen has a weak bond
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   Answer: The bonds in CO₂ and H₂O are stronger in total than those in CH₄ and O₂

   Bonds broken total 2648 kJ/mol; bonds formed total 3450 kJ/mol. More energy is released than absorbed: ΔH ≈ −802 kJ/mol.