Enthalpy of vaporisation of Benzene (ΔvapH) = 30.67 kJ/mol and ΔSvap = 87 Jk–1mol

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Le Chateliers Principle

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Question

Enthalpy of vaporisation of Benzene (Δ_vapH) = 30.67 kJ/mol and ΔS_vap = 87 Jk^–1mol^–1. Identify the correct statement(s)

At 70°C vaporisation of benzene will be non-spontaneous.

Normal boiling point of benzene will be 79.5°C .

At 90°C vaporisation of benzene will be spontaneous.

At 70°C vaporisation of benzene will be spontaneous.

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${ΔS}_{surr} = - \frac{{ΔH}_{vap}}{T_{surr}}$

(a) ΔS_surr = – 89.4 JK^–1mol^–1 , ΔS_total = – 2.4 JK^–1mol^–1 (– ve so nonspontaneous)

At 70°C ΔS_surr = – $\frac{30 .67 \times 10^{3}}{343}$ = – 89.4 J/K-mol

(d) T_normal = $\frac{{ΔH}_{vap}}{{ΔS}_{vap}} = \frac{30 .67 \times 1000}{87}$ = 352.5 K = 79.5°C