AP Chemistry : Equilibrium Constant and Reaction Quotient
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Example Question #1 : Equilibrium Constant And Reaction Quotient
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
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?
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.
The equilibrium constant is valid only at equilibrium, and the reaction quotient can be calculated at any given moment in the reaction.
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 __________.
pressure
physical state
volume
temperature
temperature
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?
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.
Their concentrations don't change over the course of the reaction.
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.
Example Question #13 : Chemical Equilibrium
Which of the following is not a property of the law of mass action?
The equlibrium constant is characteristic of a particular reaction at a certain temperature.
The larger the equilibrium constant (K), the farther to the right equilibrium lies.
A system shifts to reduce applied "stress."
The concentrations of pure solids and liquids don't appear in the equilbrium expression.
A system shifts to reduce applied "stress."
All of the statements are properties of the law of mass action, except for the answer, which is a statement of Le Chatelier's Principle.
Example Question #14 : Chemical Equilibrium
If the equilibrium constant lies farther to the right, this indicates that the reaction __________.
is more complete
is less complete
does not have a bearing on the reaction
includes a catalyst
is more complete
The equilibrium constant is given by the concentraton of products over the concentration of reactants. If it lies to the right, it means that it favors the forward reaction, and thus the reaction is "more complete" or closer to completion.
Example Question #4 : Equilibrium
What is the equilbrium constant for a reaction written in reverse if the forward reaction has constant K?
4K
K/2
2K
1/K
1/K
This is one of the properties of the law of mass action. The equilbrium constant for a reaction written in reverse is 1/K.
Example Question #5 : Equilibrium
Which of the following factors will change the equilibrium constant, Keq?
Adding a chemical that will cause side reactions
Change in solvent volume
Introducing additional products
Change in temperature
Introducing additional reactants
Change in temperature
The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.
Example Question #13 : Equilibrium
Consider the following reaction.
What is the reaction quotient if 
There is not enough information to answer this question
The reaction quotient, or Q, of the above reaction is equal to the products over the reactants. Q is calculated in the same manner as Keq, but does not require that the reaction be at equilibrium.
Example Question #16 : Chemical Equilibrium
Consider the following balanced reaction.
Write the equilibrium constant expression for this reaction.
When writing an equilibrium constant expression, remember that products are on the top, and reactants are on the bottom of the expression. The coefficients for the compounds in the balanced reaction become the exponents for the compounds seen in the expression.
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