Bond energies and enthalpy change

Break, then make

Every reaction breaks old bonds and makes new ones. Breaking a bond always takes energy in (endothermic). Making a bond always gives energy out (exothermic).

For 2H₂ + O₂ making 2H₂O, first the H–H and O=O bonds break into 4 H atoms and 2 O atoms. The book's values: 435 kJ for each mole of H₂ and 498 kJ for O₂, so 2 × 435 + 498 = 1368 kJ is absorbed.

Then the atoms join into 4 O–H bonds at 484 kJ each, giving out 4 × 484 = 1936 kJ. More comes out than went in.

ΔH = energy absorbed − energy released = 1368 − 1936 = −568 kJ for 2 moles of water, or −284 kJ per mole. Weak bonds broken and strong bonds formed make a reaction exothermic; the reverse makes it endothermic.

Count the bonds first. Each bond broken adds its energy; each bond made subtracts it.

Key terms

Bond dissociation energy

The energy absorbed side.

Bond formation energy

The energy released side.

Energy absorbed

First half of the sum.

ΔH

The overall heat of reaction.

Short questions with model answers

1. Q1. H₂(g) + I₂(g) → 2HI(g). Bond energies: H₂ 436, I₂ 151, HI 299 kJ mol⁻¹. Find ΔH. (Sample problem, p. 74)

   • 436 + 151 = 587 kJ
   • 2 × 299 = 598 kJ
   • ΔH = 587 − 598 = −11 kJ

   ΔH = −11 kJ (slightly exothermic)

2. Q2. Using the book's values, find ΔH for one mole of gaseous water. (pp. 73–74)

   • 2 × 435 + 498 = 1368 kJ
   • 4 × 484 = 1936 kJ
   • 1368 − 1936 = −568 kJ
   • −568 ÷ 2 = −284 kJ mol⁻¹

   ΔH = −284 kJ mol⁻¹

Common mistakes

• ✗ Breaking a bond releases energy.

  ✓ Breaking always absorbs energy; making releases it (p. 73).

• ✗ For 2H₂, count H–H once: 435 kJ.

  ✓ Two moles of H₂: 2 × 435 = 870 kJ.

• ✗ ΔH = released − absorbed.

  ✓ ΔH = absorbed − released, so more released gives a negative ΔH.

MCQs

1. 1. Breaking a chemical bond is:

   1. (a) Exothermic
   2. (b) Endothermic
   3. (c) Neither
   4. (d) Exothermic only for H₂
   Show answer

   (b) Energy must be put in to pull atoms apart (p. 73).

2. 2. An exothermic reaction shows that the bonds broken are:

   1. (a) Weaker than those formed
   2. (b) Stronger than those formed
   3. (c) Not related to the heat
   4. (d) Exactly as strong as those formed
   Show answer

   (a) MCQ (vi), p. 80, and key point 6.

3. 3. Average C–H bond energy is 412 kJ mol⁻¹. Which change has ΔH close to +412 kJ mol⁻¹?

   1. (a) CH₄ → C + 4H
   2. (b) CH₄ → CH₃ + H
   3. (c) C + 2H₂ → CH₄
   4. (d) CH₄ → C + 2H₂
   Show answer

   (b) Only one C–H bond breaks (MCQ, p. 80).

4. 4. N₂ + O₂ → 2NO. Book data: N₂ 958.38, O₂ 498, NO 626 kJ/mol. ΔH is about:

   1. (a) +830 kJ
   2. (b) +204 kJ
   3. (c) −204 kJ
   4. (d) −626 kJ
   Show answer

   (b) Absorbed 958.38 + 498 = 1456.38; released 2 × 626 = 1252; ΔH = +204.38 kJ (Descriptive Q i, p. 82).

5. 5. Absorbed 1368 kJ, released 1936 kJ. ΔH =

   1. (a) +568 kJ
   2. (b) −568 kJ
   3. (c) +3304 kJ
   4. (d) −3304 kJ
   Show answer

   (b) Absorbed − released = −568 kJ (p. 74).

Quick revision

• Bond breaking absorbs energy; bond making releases it.
• ΔH = energy absorbed (bonds broken) − energy released (bonds made).
• Multiply each bond energy by how many moles of that bond break or form.