Q.What is the equilibrium constant $K_{eq}$ for the following reaction at 400K. $2NOCl(g) \rightleftharpoons 2NO(g) + Cl_2(g)$, given that $\Delta H^0 = 77.2$ kJ mol$^{-1}$; and $\Delta S^0 = 122$ JK$^{-1}$mol$^{-1}$.

Step 1. $\Delta G^0=\Delta H^0-T\Delta S^0=77{,}200-400\times122=77{,}200-48{,}800=28{,}400$ J mol$^{-1}$ (positive, since $T\Delta S^0<\Delta H^0$ at 400 K).

Step 2. From the Van't Hoff equation, $\ln K_{eq}=-\Delta G^0/(RT)=-28{,}400/(8.314\times400)=-28{,}400/3325.6\approx-8.541$. …

Find $\Delta G^0$ from $\Delta H^0-T\Delta S^0$ first, then convert to $K_{eq} …

• Using $\log$ (base 10) with the natural-log form of the Van't Hoff equation, or vice versa, withou …