Question:

For a chemical reaction \(\mathrm{A + B \rightleftharpoons C + D}\)

(\(\Delta_r H^\Theta = 80\ \mathrm{kJ\ mol^{-1}}\)) the entropy change \(\Delta_r S^\Theta\) depends on the temperature T (in K) as \(\Delta_r S^\Theta = 2T\ (\mathrm{J\ K^{-1}\ mol^{-1}})\).

Minimum temperature at which it will become spontaneous is ___________ K. (Integer)

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Spontaneous when \(\Delta G = \Delta H - T\Delta S \lt 0\); substitute \(\Delta S = 2T\) and solve for the threshold T.
Updated On: Nov 4, 2026
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Correct Answer: 200

Solution and Explanation

\(\Delta_r G = \Delta_r H - T\Delta_r S\), with \(\Delta_r H = 80\ \mathrm{kJ\ mol^{-1}} = 80000\ \mathrm{J\ mol^{-1}}\) and \(\Delta_r S = 2T\ \mathrm{J\ K^{-1}\ mol^{-1}}\).
\(\Delta_r G = 80000 - T(2T) = 80000 - 2T^2\).
The reaction is spontaneous when \(\Delta_r G \lt 0\): \(2T^2 \gt 80000\), so \(T^2 \gt 40000\) and \(T \gt 200\ \mathrm{K}\).
The minimum temperature (at which \(\Delta G = 0\)) is 200 K.
Answer: 200
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