AP Chemistry : AP Chemistry

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #2 : Chemical Equilibrium

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

Possible Answers:

There will be no change

Decreased pH

Increased pH

Decreased pKa

Increased pKa

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 #1 : Chemical Equilibrium

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 #231 : Ap Chemistry

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 #4 : Chemical 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.

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 #1 : Chemical Equilibrium

  1.  
  2.  

What is ? Is reaction 3 spontaneous?

Possible Answers:

, nonspontaneous

, spontaneous

, spontaneous

, nonspontaneous

Correct answer:

, nonspontaneous

Explanation:

There are two concepts to consider in this problem.  First, the question asks for the .  Reaction 3 has  being reduced so the potential for the half-reaction becomes negative.  half-reaction appears the same in reaction 3 so the potential is the same. Second, negative voltages indicate non-spontaneity and positive voltages are spontaneous.

Example Question #1 : Equilibrium Constant And Reaction Quotient

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

Possible Answers:

The concentration of reactants

The concentration of products

The equilibrium constant expression

The rate law

Correct answer:

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 #3 : Equilibrium

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

Possible Answers:

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.

There is no difference between the two.

The reaction quotient is equivalent to the rate law.

Correct answer:

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 #11 : Chemical Equilibrium

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

Possible Answers:

pressure

physical state

volume

temperature

Correct answer:

temperature

Explanation:

Equilibrium expressions are specific for certain temperatures. Physical state, pressure, and volume do not factor into the expression. 

Example Question #12 : Chemical Equilibrium

Why do the concentrations of pure liquids and solids not appear in the equilibrium constant expression?

Possible Answers:

Their concentrations don't change over the course of the reaction.

They are completely consumed before the reaction reaches equilibrium.

Their concentrations change too quickly to be accounted for.

They are not present in the mixture at equilibrium.

Correct answer:

Their concentrations don't change over the course of the reaction.

Explanation:

This is one of the properties of the law of mass action—the concentrations of pure solids and liquids don't change over the course of the reaction. 

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