High School Chemistry : Thermochemistry and Energetics

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

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Example Question #1 : Help With Gibb's Free Energy

Which of the following best describes the change in Gibbs free energy when a sample of ice is melting at a temperature of 21oC?

Possible Answers:

Always negative

Sometimes negative

Always positive

Positive or negative depending on the volume of the ice

Sometimes positive

Correct answer:

Always negative

Explanation:

Gibbs free energy is used to determine if a reaction will occur spontaneously. If the Gibbs free energy is negative for the given process, then the reaction will occur spontaneously.

In the given scenario, ice will always melt spontaneously when the temperature is above 0oC. As a result, we can state that the Gibbs free energy will always be negative for this process.

Example Question #3 : Help With Gibb's Free Energy

If a process is endothermic and involves a decrease in entropy, when will it be spontaneous?

Possible Answers:

At low temperatures

Upon addition of a catalyst

Never

In zero gravity

At high temperatures

Correct answer:

Never

Explanation:

.

If a process is endothermic, is positive.

If a process involves a decrease in entropy, is negative.

Under these conditions, the only way can be negative is if is below absolute zero, which is impossible. Catalysts do not effect , rather they only affect reaction kinetics, not thermodynamics.

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