AP Chemistry : Thermochemistry and Kinetics

Study concepts, example questions & explanations for AP Chemistry

varsity tutors app store varsity tutors android store

Example Questions

Example Question #3 : Entropy

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

Possible Answers:

Building a skyscraper

Osmosis

A clean room becomes cluttered

An igloo melting

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 #4 : 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 #71 : Thermochemistry And Kinetics

              

                

               

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 #6 : 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 #7 : 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 #1 : 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 #1 : Entropy

                    

                    

                    

                    

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 #10 : 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.

Example Question #11 : Entropy

A reaction will reach equilibrium when the system's entropy __________.

Possible Answers:

is at a maximum

is at a minimum

is zero

entropy does not influence equilibrium at all

Correct answer:

is at a maximum

Explanation:

Equilibrium is the basis of entropy, which can essentially be thought of as the distribution of energy in the system. For a reaction at a given temperature, the system's entropy will be at a maximum when it reaches equilbrium because this is the ideal energy state, and the forward and reverse reactions are occurring at a steady rate.

Example Question #12 : Entropy

Consider the following reaction:

Below is a list of the standard entropy of formation for the compounds in the reaction:

What is the entropy of the reaction when one mole of ammonia is formed?

Possible Answers:

Correct answer:

Explanation:

Since we have the entropies of formation, we can find the entropy of the reaction using the following equation:

We will need to use the coefficients from the balanced equation to calculate the entropy.

Since this is the entropy for the reaction when two moles of ammonia are formed, we must divide this entropy by two in order to solve for the entropy of one mole.

Learning Tools by Varsity Tutors