AP Chemistry : Reactions and Equilibrium
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Example Questions
Example Question #51 : Reactions And Equilibrium
Why do the concentrations of pure liquids and solids not appear in the equilibrium constant expression?
Their concentrations change too quickly to be accounted for.
They are not present in the mixture at equilibrium.
They are completely consumed before the reaction reaches equilibrium.
Their concentrations don't change over the course of the reaction.
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 #11 : Chemical Equilibrium
Which of the following is not a property of the law of mass action?
A system shifts to reduce applied "stress."
The larger the equilibrium constant (K), the farther to the right equilibrium lies.
The equlibrium constant is characteristic of a particular reaction at a certain temperature.
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 #12 : Chemical Equilibrium
If the equilibrium constant lies farther to the right, this indicates that the reaction __________.
does not have a bearing on the reaction
includes a catalyst
is less complete
is more complete
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 #5 : Equilibrium Constant And Reaction Quotient
What is the equilbrium constant for a reaction written in reverse if the forward reaction has constant K?
2K
4K
1/K
K/2
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 #2 : Equilibrium
Which of the following factors will change the equilibrium constant, Keq?
Introducing additional reactants
Introducing additional products
Change in temperature
Change in solvent volume
Adding a chemical that will cause side reactions
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 #51 : Reactions And 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 #13 : 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.
Example Question #14 : Chemical Equilibrium
What is the correct equilibrium expression for this reaction?
Solids are not included in any equilibrium expression. The equilibrium expression will contain only aqueous solutions and gases. In addition, the coefficients of a balanced chemical equation correspond to exponents, not coefficients, in the equilibrium expression.
For this reaction, the only relevant product is oxygen gas and the only relevant reactant is fluorine gas.
Example Question #12 : Equilibrium Constant And Reaction Quotient
Consider the following balanced reaction.
Write the equilibrium constant expression for this reaction.
A key point to remember is that the equilibrium constant expression only includes compounds that are in aqueous solution or in the gaseous phase. Pure solids and liquids are omitted from the expression. As a result, solid barium sulfate will not be included in the expression.
The equilibrium constant is found by the concentration of the products over the concentration of the reactants, each raised to the power of their coefficients.
In our reaction, the only reactant is a pure solid, so only the products are used in the expression.
Example Question #21 : Chemical Equilibrium
Consider the following (arbitrary) reaction:
At equilibrium, ![[A_2O_4]=0.25\hspace{1 mm}M](https://vt-vtwa-assets.varsitytutors.com/vt-vtwa/uploads/formula_image/image/20263/gif.latex)
![[AO_2]=0.04\hspace{1 mm}M](https://vt-vtwa-assets.varsitytutors.com/vt-vtwa/uploads/formula_image/image/20264/gif.latex)
What is the value for the equilibrium constant, 
First, consider the formula for the equilibrium constant of arbitrary reaction:
![K_{eq}=\frac{[C]^c[D]^d}{[A]^a[B]^b}](https://vt-vtwa-assets.varsitytutors.com/vt-vtwa/uploads/formula_image/image/20267/gif.latex)
The formula for the equilibrium constant for this reaction is:
![K_{eq}=\frac{[AO_2]^2}{[A_2O_4]}=\frac{[0.04]^2}{[0.25]}=6.4\times10^{-3}](https://vt-vtwa-assets.varsitytutors.com/vt-vtwa/uploads/formula_image/image/20268/gif.latex)
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