High School Chemistry : Help with Endergonic and Exergonic Reactions

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

Example Question #8 : Thermochemistry And Energetics

Which of the following is considered an exothermic reaction?

Possible Answers:

Decomposition reactions

Breaking a bond

Synthesis reactions

Forming a bond

Single-replacement reactions

Correct answer:

Forming a bond

Explanation:

An exothermic reaction releases energy into the environment. In contrast, endothermic reactions require an input of energy to initiate the reaction.

Forming a bond is always an exothermic reaction because it releases energy. Breaking a bond always requires energy, and is thus an endothermic process. Synthesis, decomposition, and single-replacement reactions can be either exothermic or endothermic, and cannot be determined without more information.

Example Question #1 : Help With Endergonic And Exergonic Reactions

Which type of reaction will happen spontaneously?

Possible Answers:

Endothermic reactions

Endergonic reactions

Exergonic reactions

Exothermic reactions

Reactions that increase entropy

Correct answer:

Exergonic reactions

Explanation:

Spontaneity is determined by Gibbs free energy. When Gibbs free energy is less than zero, the reaction is considered exergonic and will occur spontaneously. When Gibbs free energy is greater than zero, the reaction is considered endergonic and will not occur spontaneously.

Exothermic reactions cause a release of enthalpy (heat) from the system and endothermic reactions require and input of energy to initiate the reaction. Gibbs free energy is determined by enthalpy, entropy, and temperature. A negative enthalpy, high temperature, and high entropy will cause the reaction to be more spontaneous, but must all come together to contribute. Simply because a reaction is exothermic does not meant that is increases entropy enough to be spontaneous.

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