GRE Subject Test: Chemistry - Gibbs Free Energy

At $25^{\circ}C$ , a reaction has a Gibb's free energy change of . If the enthalpy change of the reaction is , what is the entropy change of the reaction?

$-33.6\frac{J}{K}$

$33.6\frac{J}{K}$

$-400\frac{J}{K}$

$-10\frac{J}{K}$

Explanation

We can relate enthalpy, entropy, and Gibb's free energy using the Gibb's free energy equation:

$\Delta G = \Delta H-T\Delta S$

Keep in mind that temperature must be in Kelvin. Since we know enthalpy and Gibb's free energy for the reaction, we can solve for the entropy change:

$45,000J = 35,000J - (298K)(\Delta S)$

$\Delta S = -33.6\frac{J}{K}$

Which of the following options indicates that a chemical reaction is unfavorable?

$\Delta S\ is\ negative$

$\Delta H\ is\ negative$

All of these

$\Delta G\ is\ negative$

Explanation

The sign of $\Delta G$ indicates the direction of a chemical reaction and determines if a reaction is spontaneous or not. A chemical reaction in which $\Delta G< 0$ is considered to be a spontaneous reaction because it will proceed without requiring any outside energy. To approach this problem, we must consider the equation for the Gibbs free energy of a reaction, $\Delta G= \Delta H -T \Delta S$ . A reaction is considered to be favorable when $\Delta G$ is negative. The second law of thermodynamics indicates that the entropy of the universe is spontaneously increasing. This corresponds to a positive $\Delta S$ .

Gibbs Free Energy

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