AP Chemistry : Entropy

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #72 : Thermochemistry And Kinetics

Which of the following does not represent an increase in entropy?

Possible Answers:

NaCl dissolving in water

Boiling water

Melting of mercury

Condensation of water on glass

Sublimation of CO2

Correct answer:

Condensation of water on glass

Explanation:

Of phases, from greatest to least entropy is gas>liquid>solid. Dissolving a salt into solution is an increase in entropy because the salt has greater space to move. The only decrease in entropy is condensation of water on glass (gas to liquid)

Example Question #75 : Thermochemistry And Kinetics

Which of the following reactions represents a negative change in entropy?

Possible Answers:

Salt dissolving

Sublimation of dry ice

Water heating

Ice melting

None of these represent a negative change in entropy

Correct answer:

None of these represent a negative change in entropy

Explanation:

Entropy for phases are Gas>Liquid>Solid. A salt dissolving into solution has a positive entropy change (the solution becomes more chaotic). Therefore all of the listed reactions are a positive change in entropy, making E the correct answer.

Example Question #76 : Thermochemistry And Kinetics

Which of the following scenarios describes a process with a negative entropy change?

Possible Answers:

Building a skyscraper

An igloo melting

Osmosis

A clean room becomes cluttered

Correct answer:

Building a skyscraper

Explanation:

Entropy can be thought of as the tendency for a system to become disordered, or random. Osmosis, melting ice, and a clean room becoming dirty are all examples of entropy at work. Each of these processes will occur spontaneously without the input of outside energy or work.

If you make a skyscraper, you are incorporating a series of compounds and building products into a single, distinct formation. Think of it this way: what are the odds that mother nature would spontaneously build a building? This process is very ordered, which means that entropy of the universe has decreased.

Example Question #1 : Entropy

              

                

                  

What is the change in entropy for the following reaction? 

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

              

                

                  

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #2 : Entropy

              

                

               

What is the change in entropy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

              

                

               

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #3 : Entropy

               

               

               

What is the change in entropy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

               

               

               

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #2 : Entropy

                     

                       

                 

What is the change in entropy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

                     

                       

                 

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #3 : Entropy

                             

                                    

                                

                                

What is the change in entropy for the following equation?

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

                             

                                    

                                

                                

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #84 : Thermochemistry And Kinetics

                    

                    

                    

                    

What is the entropy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in entropy is calculated by:

When  cannot be measured, it can be calculated from known entropies of formation.

                    

                    

                    

                    

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

Example Question #4 : Entropy

Which reaction will result in a negative entropy?

Possible Answers:

Correct answer:

Explanation:

A reaction will have a positive change in entropy when the disorder of the system is increasing. This can happen when a liquid transitions to a gas, or more gas molecules are formed in the reaction.

Since two molecules of gaseous nitrogen dioxide can combine to form one molecule of dinitrogen tetroxide, the amount of gaseous molecules is halved. This increases order, which decreases the entropy of the system.

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