GRE Subject Test: Chemistry : Gibbs Free Energy

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #5 : Physical Chemistry

At , 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?

Possible Answers:

Correct answer:


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

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:

Example Question #6 : Physical Chemistry

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

Possible Answers:

All of these

Correct answer:


The sign of  indicates the direction of a chemical reaction and determines if a reaction is spontaneous or not. A chemical reaction in which  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, . A reaction is considered to be favorable when  is negative. The second law of thermodynamics indicates that the entropy of the universe is spontaneously increasing. This corresponds to a positive .

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