Endergonic and Exergonic Reactions - GRE Subject Test: Chemistry

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Question

The equation for the change in Gibbs free energy is given below.

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

ΔH = change in enthalpy

ΔS = change in entropy

Which of the following scenarios guarantees a nonspontaneous reaction?

Answer

A positive value for ΔG (Gibbs free energy) will guarantee a nonspontaneous reaction. When ΔH (enthalpy) is postive and ΔS (entropy) is negative, the change in Gibbs free energy must be positive and, therefore, nonspontaneous.

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

$\Delta G=(+)-T(-)=(+)+(+)=+$

Because T (temperature) will always have a positive value, a negative entropy and positive enthalpy will always result in a positive Gibbs free energy.

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Question

The equation for the change in Gibbs free energy is given below.

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

ΔH = change in enthalpy

ΔS = change in entropy

In which of the scenarios will the reaction be spontaneous?

Answer

All of the following scenarios would lead to spontaneous reaction, since each scenario would result in a negative Gibbs free energy (-ΔG).

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

Negative enthalpy, positive entropy:

$\Delta G=(-)-T(+)=(-)+(-)=-$

Positive enthalpy and entropy with high temperature:

$\Delta G=(+)-(+++)(+)=(+)+(---)=-$

Negative enthalpy and entropy with low temperature:

$\Delta G=(-)-(+)(-)=(-)+(-)=-$

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Question

Which of the following scenarios describes a reaction in equilibrium?

Answer

According to the Gibbs free energy equation, a system is at equilibrium when $\Delta G$ is equal to 0.

Since $\Delta G = \Delta H - T\Delta S$ , a system is in equilibrium when $\Delta H = T\Delta S$ . A negative Gibbs free energy means that the reaction will be spontaneous. At equilibrium, the forward reaction rate equals the reverse reaction rate, though the net rate is zero.

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