R1.4 Exam Practice

(a) Define entropy. [1]

(b) Predict the sign of ΔS for: (i) CaCO₃(s) → CaO(s) + CO₂(g), (ii) 2SO₂(g) + O₂(g) → 2SO₃(g). Justify. [2]

(c) Calculate ΔS° for CaCO₃ decomposition given S°: CaCO₃ = 93, CaO = 40, CO₂ = 214 J K⁻¹ mol⁻¹. [2]

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(a) A measure of the disorder/number of possible arrangements of particles in a system [1]

(b)(i) ΔS positive: solid → solid + gas (more moles of gas) [1]; (ii) ΔS negative: 3 moles gas → 2 moles gas [1]

(c) ΔS° = (40 + 214) − 93 = +161 J K⁻¹ mol⁻¹ [1][1]

For CaCO₃ decomposition: ΔH = +178 kJ mol⁻¹, ΔS = +161 J K⁻¹ mol⁻¹

(a) State the equation relating ΔG, ΔH and ΔS. [1]

(b) Calculate ΔG at 298 K and state whether the reaction is spontaneous. [2]

(c) Calculate the minimum temperature for spontaneity. [2]

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(a) ΔG = ΔH − TΔS [1]

(b) ΔG = 178 − (298 × 0.161) = 178 − 48.0 = +130 kJ mol⁻¹; not spontaneous (ΔG > 0) [1][1]

(c) T = ΔH/ΔS = 178/0.161 = 1106 K (833 °C) [1][1]