AP Chemistry 9.5: Free Energy and Equilibrium

Practice AP Chemistry questions on relating ΔG°, K, and temperature to product or reactant favorability and equilibrium values.

Exam points

Relate the sign of ΔG° to whether products or reactants are favored and whether K is above or below 1.
Use ΔG° = −RT ln K or K = e^(−ΔG°/RT) to calculate or interpret equilibrium constants.

[Maximum number: 1]

A student investigates the reactions of nitrogen oxides. One of the reactions in the investigation requires an equimolar mixture of NO(g) and $\mathrm{NO}_{2}(g)$ , which the student produces by using the reaction represented above.

The student begins with an equimolar mixture of NO(g) and $\mathrm{NO}_{2}(g)$ in a rigid reaction vessel and the mixture reaches equilibrium at 298 K.

Calculate the value of the equilibrium constant, K, for the reaction at 298 K.

\begin{aligned} & \Delta G^{\circ}=-R T \ln K \\ & K=e^{-\Delta G^{\circ} / R T} \\ & K=e^{-\frac{870 \mathrm{~J} / \mathrm{mol}}{\left(8.314 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}\right)(298 \mathrm{~K})}} \\ & K=0.70 \end{aligned}

point is earned for a correct calculation of K.

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9.5.A—Explain whether a process is thermodynamically favored using the relationships between K, ΔGo, and T question 1