# AP Chemistry : Chemical Equilibrium

## Example Questions

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### Example Question #1 : Chemical Equilibrium

Consider the following gasesous, reversible, exothermic reaction: What could be done to increase the equilibrium concentration of species C?

Increase the amount of species A

Decrease the amount of species A

Increase the amount of species E

Increase the reaction temperature

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 #2 : Chemical Equilibrium

The reaction is at equilibrium. You have measured the concentration of and to be  What is the equilibrium constant, ?      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 #1 : Chemical 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?

Increasing the volume

Removing Decreasing the pressure

Adding 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 #1 : Chemical Equilibrium

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

Increased pH

There will be no change

Increased pKa

Decreased pH

Decreased pKa

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 #1 : Chemical Equilibrium

For a given chemical reaction, . What is the for this reaction at ?   There is not enough information  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 #1 : Chemical Equilibrium

A reaction at equilibrium releases 25 Joules of heat energy at . What is the entropy change for this same reaction?    There is not enough information to answer the question 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 #1 : Chemical Equilibrium Which of the following is the correct equilibrium expression for the given reaction?     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.

### Example Question #1 : Chemical Equilibrium

Which of the following can always be determined based on the equation itself?

The concentration of reactants

The concentration of products

The equilibrium constant expression

The rate law

The equilibrium constant expression

Explanation:

According to the law of mass action, the equilibrium constant expression can always be written given the equation of the reaction itself. The equilibrium-constant expression will be written as the products over the reactants, each raised to their respectively stoichiometric coefficient. The rate law and concentrations can only be determined if there is additional data given.

### Example Question #1 : Equilibrium

What is the difference between the reaction quotient and the equilibrium constant?

There is no difference between the two.

The reaction quotient is equivalent to the rate law.

The equilibrium constant is valid only at equilibrium, and the reaction quotient can be calculated at any given moment in the reaction.

The reaction quotient  is valid only at equilibrium, and the equilibrium constant can be calculated at any given moment in the reaction.

The equilibrium constant is valid only at equilibrium, and the reaction quotient can be calculated at any given moment in the reaction.

Explanation:

The correct answer gives the accurate definition of both the equilibrium constant and the reaction quotient.

### Example Question #1 : Chemical Equilibrium

The law of mass action gives an expression that is specific for a certain __________.

volume

temperature

pressure

physical state 