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GATE 2021 CY – Question 32

Physical Chemistry · Equilibrium: Laws of thermodynamics. Standard states. Thermochemistry. Thermodynamic functions and their relationships: Gibbs-Helmholtz and Maxwell relations, Gibbs-Duhem equation, Van’t Hoff equation. Criteria of spontaneity and equilibrium. Absolute entropy. Partial molar quantities. Thermodynamics of mixing. Chemical potential. Fugacity, Activity and Activity coefficient. Chemical equilibria. Dependence of equilibrium constant on temperature and pressure. · 1 mark · Numerical answer

For $2P(s)+3Br_2(l)\to2PBr_3(l)$, $\Delta H=-243$ kJ. If 3.1 g phosphorus reacts, the enthalpy change is ______ kJ (two decimal places). Atomic weight of P is 31.

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Correct answer: -12.16 to -12.14

Explanation

The equation consumes 2 mol P for ΔH=−243 kJ. The actual amount is 3.1/31=0.10 mol P, corresponding to reaction extent 0.10/2=0.05 mol.

Therefore ΔH=−243×0.05=**−12.15 kJ**. The negative sign indicates heat release.