Physical Chemistry : Endothermic and Exothermic Reactions

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Endothermic And Exothermic Reactions

Which of the following are endothermic?

Possible Answers:

More than one of these are endothermic processes

Condensation

Freezing

Melting

Correct answer:

Melting

Explanation:

Endothermic processes involve a positive change in enthalpy. This means that the enthalpy of products is higher than the enthalpy of reactants and net heat energy is consumed. Phase changes that involve increasing the distance between particles (meaning conversion of solid to liquid (melting), liquid to gas (evaporation) and solid to gas (sublimation)) require an input of energy and are considered endothermic processes. On the other hand, phase changes that decrease the distance between particles (such as gas to liquid (condensation), liquid to solid (freezing), and gas to solid (deposition)) release energy and are considered exothermic processes.

Example Question #2 : Endothermic And Exothermic Reactions

Which of the following is true regarding an exothermic reaction?

I. Entropy always increases

II. It is always spontaneous

III. It can facilitate other energy-consuming reactions

Possible Answers:

I and III

II only

I and II

III only

Correct answer:

III only

Explanation:

Exothermic reactions involve release of heat energy. This means that the energy of the products is lower than the energy of the reactants. Entropy is the measure of disorder in a system. It does not depend on the enthalpy and can increase or decrease during an exothermic process. Spontaneity is determined by looking at the change in Gibbs free energy, . Negative  corresponds to a spontaneous process and positive  corresponds to a nonspontaneous process. While the equation for Gibbs free energy, , involves change in enthalpy, the spontaneity also depends on temperature and change in entropy. Exothermic reactions release energy in the form of heat. This heat energy can be used to power other processes that require energy; therefore, exothermic reactions can facilitate active processes that consume energy.

Example Question #3 : Endothermic And Exothermic Reactions

In an __________ reaction, the products are more stable than the reactants; in an __________ reaction the reactants are more stable than the products.

Possible Answers:

endergonic . . . exergonic

endergonic . . . endergonic

exergonic . . . endergonic

exergonic . . . exergonic

Correct answer:

exergonic . . . endergonic

Explanation:

Exergonic reactions release energy; therefore, the energy of products is lower than that of the reactants. Endergonic reactions consume energy; therefore, the energy of products is greater than that of the reactants. In other words, exergonic reactions are spontaneous, while endergonic reactions are nonspontaneous, and require the net input of energy to drive the reaction.

Example Question #4 : Endothermic And Exothermic Reactions

Consider the following spontaneous reaction:

What can you conclude about the enthalpy change in this reaction?

Possible Answers:

It is negative

It is positive

Cannot be determined from the given information

It is zero

Correct answer:

It is negative

Explanation:

We need to use the following equation to answer this question:

Above,  is change in Gibbs free energy,  is change in enthalpy,  is temperature, and  is change in entropy. The question states that the reaction is spontaneous; therefore,  is negative. We can also determine the  for this reaction by looking at the phases of the products and reactants. Recall that entropy is a measure of disorder. When it comes to phases, gases have the highest entropy and solids have the lowest entropy. This is because in gases the molecules are spread out and have more room for disorder while solids are compressed and well packaged, decreasing the disorder of the atoms/molecules. Liquids are intermediate in entropy. In this reaction, we are creating a liquid from two molecules of gas; therefore, we are decreasing the entropy of the system (going to a more ordered, liquid state). The change in entropy for this reaction is negative.

Rearranging and solving the equation above for  gives us:

Since both  and  are negative,  will always be negative (regardless of temperature). This is an exothermic reaction.

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