AP Chemistry : Equilibrium and Thermodynamics

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Equilibrium And Thermodynamics

Consider the following gasesous, reversible, exothermic reaction:

What could be done to increase the equilibrium concentration of species C?

Possible Answers:

Decrease the amount of species A

Increase the amount of species A

Increase the amount of species E

Increase the reaction temperature

Correct answer:

Increase the amount of species A

Explanation:

According to Le Chatelier's principle, any changes in concentration or pressure to a system at equilibrium will cause the system to readjust to a new equilbrium. The addition of a reactant will cause the reaction to shift to the right, increasing the equilibirum concentration of the products. Thus, adding additional reactant A would increase the equlibrium concentration of product C.

Removing species A or adding species E will drive the reaction to the left, reducing the amount of species C. Since the reaction is exothermic, heat can be considered a product. Thus, increasing the temperature will also shift the reaction to the left.

Example Question #1 : Chemical Equilibrium

The reaction  is at equilibrium. You have measured the concentration of  and  to be

What is the equilibrium constant, ?

Possible Answers:

Correct answer:

Explanation:

Recall that the rate constant for the equilibrium reaction

is , always remembering that it is products over reactants. 

Since we are at equilibrium, for every mol of  we have, we have 3 mols of . Therefore,

Likewise, for every 2 mols of  we have, we have 2 mols of . Therefore,

Plugging all of this into the above equation we get

Example Question #41 : Reactions And Equilibrium

Consider the following chemical reaction.

This reaction is allowed to equilibrate at some volume and pressure.

Which of the following actions will not shift the equilibrium toward the products?

Possible Answers:

Increasing the volume

Decreasing the pressure

Adding an inert gas

Removing 

Adding 

Correct answer:

Adding an inert gas

Explanation:

Adding an inert gas will not shift the equilibrium toward the products, but neither will it shift the equilibrium toward the reactants. It has no effect because the partial pressure of each species, , , and , will still remain constant. Since the partial pressures are constant, we know from the ideal gas law that the concentration of each species remains constant. For example, for

Example Question #42 : Reactions And Equilibrium

If formate is added to a solution of formic acid in water, which of the following changes would occur?

Possible Answers:

Decreased pH

Increased pH

Decreased pKa

Increased pKa

There will be no change

Correct answer:

Increased pH

Explanation:

Because formic acid is a weak acid, it will exist in equilibrium with its conjugate base, formate, while in water according to the following reaction:

If formate is added to this solution, then according to Le Chatlier's principle, the reaction will shift towards the left. Thus, the  in solution will decrease and the pH will consequently increase. Furthermore, the addition of formate has no effect on the pKa.

Example Question #2 : Equilibrium And Thermodynamics

For a given chemical reaction, . What is the  for this reaction at ?

Possible Answers:

There is not enough information

Correct answer:

Explanation:

To answer this question, it is necessary to use following equation:

 

 We can solve for  by rearranging the equation to the following:

Example Question #5 : Chemical Equilibrium

A reaction at equilibrium releases 25 Joules of heat energy at . What is the entropy change for this same reaction?

Possible Answers:

There is not enough information to answer the question

Correct answer:

Explanation:

To solve this problem, we must make use of the following equation:

 

At equilibrium, the value of  is 0. Therefore, we can simplify the equation.

 

We must also convert temperature from degress Celsius into Kelvin.

Example Question #44 : Reactions And Equilibrium

Which of the following is the correct equilibrium expression for the given reaction?

Possible Answers:

Correct answer:

Explanation:

For this question, we're asked to determine the correct equilibrium expression for a given reaction that is in equilibrium.

To begin with, we need to remember that pure solids and liquids are omitted from the equilibrium expression. Only gases and aqueous components of a reaction are included. The reason for this is because in a reaction at equilibrium, a pure solid or liquid will not have any appreciable change in concentration. Because their concentrations remain essentially unchanged, their value gets incorporated into the constant.

Also remember that an equilibrium expression is represented by the products divided by the reactants, each of which is raised to its stoichiometric coefficient.

In the case of the reaction given, we can see that it is already balanced. This means we are ready to set up the expression.

Here, the  term is being squared because it has a stoichiometric coefficient of  in the equilibrium reaction. Also notice that  does not show up in the expression because it is a solid.

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