Topic 5: Chemical Energetics

Question 1

Definitions I:
(a) Enthalpy change
(b) Standard conditions
(c) Activation energy

Answer 1

Definitions I:
(a) Heat energy change in a reaction at constant pressure, ΔH.
(b) 298 K and 101 kPa, shown as ⦵ for standard enthalpy changes.
(c) Minimum energy needed to start a reaction by breaking bonds.

Question 2

Definitions II:
(a) Enthalpy change of reaction
(b) Enthalpy change of formation
(c) Enthalpy change of combustion

Answer 2

Definitions II:
(a) Enthalpy change for a reaction as written, ΔHᵣ⦵.
(b) Enthalpy change when 1 mol of a compound forms from elements, ΔHᵢ⦵.
(c) Enthalpy change when 1 mol of a substance burns completely, ΔHᵪ⦵.

Question 3

Definitions III:
(a) Enthalpy change of neutralisation
(b) Bond energy
(c) Hess’s law

Answer 3

Definitions III:
(a) Enthalpy change when 1 mol H₂O forms from acid-base reaction, ΔHₙₑᵤₜ⦵.
(b) Energy to break 1 mol of a covalent bond in gaseous state.
(c) Total enthalpy change is independent of reaction pathway.

Question 4

Draw a reaction pathway diagram for an exothermic reaction with labels. [3 Marks]

Answer 4

Diagram: reactants higher, products lower, ΔH negative (1 mark)
Activation energy as peak above reactants (1 mark)
Labelled axes: enthalpy vs. reaction progress (1 mark)

Question 5

Calculate ΔHᵣ for N₂ + 3H₂ → 2NH₃ using bond energies: N≡N 945, H-H 436, N-H 391 kJ/mol. [3 Marks]

Answer 5

Bonds broken: 1 N≡N (945), 3 H-H (1308), total 2253 kJ (1 mark)
Bonds made: 6 N-H (2346 kJ) (1 mark)
ΔHᵣ = 2253 - 2346 = -93 kJ/mol (1 mark)

Question 6

Is the reaction 2H₂ + O₂ → 2H₂O exothermic or endothermic, and why? [2 Marks]

Answer 6

Exothermic, ΔH negative (1 mark)
More energy released making H-O bonds than breaking H-H, O=O (1 mark)

Question 7

What is the enthalpy change when 50 g water at 25°C rises to 35°C (c = 4.18 J/g°C)? [2 Marks]

Answer 7

q = mcΔT = 50 × 4.18 × 10 (1 mark)
q = 2090 J = 2.09 kJ (1 mark)

Question 8

Use Hess’s law to find ΔH for:
(a) C + O₂ → CO₂ given CO steps [1 Mark]
(b) CH₄ + 2O₂ → CO₂ + 2H₂O [1 Mark]

Answer 8

(a) C + ½O₂ → CO (-111), CO + ½O₂ → CO₂ (-283), ΔH = -394 kJ/mol (1 mark)
(b) Uses ΔHᵪ⦵ directly, ΔH = -890 kJ/mol (1 mark)

Question 9

Why are some bond energies averages and not exact? [2 Marks]

Answer 9

Averages from various compounds (1 mark)
Exact for specific molecules, e.g., H₂ (1 mark)

Question 10

Calculate ΔH for neutralising 0.1 mol HCl with NaOH if q = 5.58 kJ. [2 Marks]

Answer 10

ΔH = -q/n = -5.58 / 0.1 (1 mark)
ΔH = -55.8 kJ/mol (1 mark)

Question 11

How do bond breaking and making affect enthalpy changes? [2 Marks]

Answer 11

Breaking bonds absorbs energy, endothermic (1 mark)
Making bonds releases energy, exothermic (1 mark)

Question 12

Find ΔHᵣ for 2C + 2H₂ → C₂H₄ using ΔHᵢ: C₂H₄ -52, H₂O -286, CO₂ -394 kJ/mol. [2 Marks]

Answer 12

2C + 2O₂ → 2CO₂ (-788), 2H₂ + O₂ → 2H₂O (-572), C₂H₄ + 3O₂ → 2CO₂ + 2H₂O (+1410) (1 mark)
ΔHᵣ = -788 - 572 + 1410 = +50 kJ/mol (1 mark)