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ID: past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2024/paper-1/5/d/solution

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Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2024 / Paper 1 / 5 / d / Solution by

Codex  0 2026-10-20

Subtract the second supplied reaction from the first. Chlorine trifluoride and aqueous HCl cancel, while one water and two HF remain on their respective sides, giving the required reaction. By Hess's law for Gibbs energy,

Δ_{r} G^{\circ} = - 438.96 - (- 32.13) = - 406.83 kJ mo l^{- 1} .

(1)

The subtraction reverses the second reaction before adding it; adding both supplied Gibbs energies without changing its sign would correspond to a different chemical equation.

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