Topic 23: Chemical Energetics

Question 1

Definitions I:
(a) Enthalpy change of atomisation
(b) Lattice energy
(c) First electron affinity

Answer 1

Definitions I:
(a) Enthalpy to form 1 mol gaseous atoms, e.g., Na(s) → Na(g).
(b) Enthalpy to form solid lattice from gas ions, e.g., Na⁺(g) + Cl⁻(g) → NaCl(s).
(c) Enthalpy when 1 mol electrons added to gaseous atoms, e.g., Cl(g) + e⁻ → Cl⁻(g).

Question 2

Definitions II:
(a) Enthalpy change of hydration
(b) Entropy
(c) Gibbs free energy

Answer 2

Definitions II:
(a) Enthalpy when 1 mol gaseous ions hydrate, e.g., Na⁺(g) → Na⁺(aq).
(b) Measure of particle/energy arrangements in a system, S.
(c) Energy for reaction feasibility, ΔG = ΔH - TΔS.

Question 3

Construct a Born-Haber cycle for NaCl and label ΔHₐₜ and ΔHₗₐₜₜ. [3 Marks]

Answer 3

Na(s) → Na(g) (ΔHₐₜ), Cl₂(g) → 2Cl(g) (ΔHₐₜ) (1 mark)
Na(g) → Na⁺(g) + e⁻, Cl(g) + e⁻ → Cl⁻(g) (1 mark)
Na⁺(g) + Cl⁻(g) → NaCl(s) (ΔHₗₐₜₜ) (1 mark)

Question 4

Calculate ΔHₗₐₜₜ for MgO given: Mg ΔHₐₜ = +148, O ΔHₐₜ = +249, Mg²⁺ EA₁+₂ = +2188, ΔHᵢ = -602 kJ/mol. [2 Marks]

Answer 4

ΔHᵢ = ΔHₐₜ(Mg) + ΔHₐₜ(O) + IE₁+₂ + EA₁+₂ + ΔHₗₐₜₜ (1 mark)
-602 = 148 + 249 + 2188 + (-141 - 352) + ΔHₗₐₜₜ, ΔHₗₐₜₜ = -2694 kJ/mol (1 mark)

Question 5

Predict the sign of ΔS for:
(a) H₂O(l) → H₂O(g) [1 Mark]
(b) 2H₂(g) + O₂(g) → 2H₂O(l) [1 Mark]

Answer 5

(a) Positive, liquid to gas, more disorder (1 mark)
(b) Negative, 3 gas → 2 liquid, less disorder (1 mark)

Question 6

Calculate ΔG for N₂ + 3H₂ → 2NH₃ at 298 K: ΔH = -92, ΔS = -199 J/mol·K. Is it feasible? [3 Marks]

Answer 6

ΔG = -92 - 298 × (-0.199) = -92 + 59.3 = -32.7 kJ/mol (1 mark)
ΔG < 0, feasible (1 mark)
Units: kJ/mol (ΔS to kJ) (1 mark)

Question 6

In a lab, NaCl dissolves with ΔHₛₒₗ = +4, ΔHₗₐₜₜ = +787 kJ/mol. Calculate ΔHₕᵧₔ for Na⁺ + Cl⁻. [2 Marks]

Answer 6

ΔHₛₒₗ = ΔHₗₐₜₜ + ΔHₕᵧₔ (1 mark)
4 = 787 + ΔHₕᵧₔ, ΔHₕᵧₔ = -783 kJ/mol (1 mark)

Question 7

How does ionic radius affect ΔHₕᵧₔ for Group 1 ions like Na⁺ vs. Cs⁺? [2 Marks]

Answer 7

Smaller Na⁺, stronger ion-water attraction (1 mark)
More negative ΔHₕᵧₔ than larger Cs⁺ (1 mark)

Question 7

Why is ΔHₗₐₜₜ more negative for MgO than NaCl? [2 Marks]

Answer 7

Mg²⁺ vs. Na⁺: higher charge, stronger attraction (1 mark)
O²⁻ vs. Cl⁻: smaller radius, greater lattice energy (1 mark)

Question 8

Calculate ΔS for CH₄ + 2O₂ → CO₂ + 2H₂O given S°: CH₄ 186, O₂ 205, CO₂ 214, H₂O 70 J/mol·K. [2 Marks]

Answer 8

ΔS = [214 + 2×70] - [186 + 2×205] (1 mark)
ΔS = 354 - 596 = -242 J/mol·K (1 mark)

Question 9

Predict the effect of increasing T on feasibility of 2SO₂ + O₂ → 2SO₃ (ΔH -ve, ΔS -ve). [2 Marks]

Answer 9

ΔG = ΔH - TΔS, both negative (1 mark)
Higher T increases -TΔS, less feasible (1 mark)

Question 10

Explain the trend in EA₁ for Group 16: O (-141) vs. S (-200 kJ/mol). [2 Marks]

Answer 10

O smaller, more repulsion in O⁻, less negative EA (1 mark)
S larger, less repulsion, more negative EA (1 mark)