Chapter 15: Chemical Equilibrium

Question 1

What does K measure?

Answer 1

The value of K is a measure of how far a reaction proceeds–the larger the value of K, the moe the reaction proceeds towards the products.

Question 2

What is the dynamic equilibrium for a chemical reaction?

Answer 2

The condition in which the rate of the forward reaction equals the rate of the reverse reaction.

Question 3

How is the equilibrium constant (K) defined?

Answer 3

K is defined for a reaction as the ratio–at equilibrium–of the concentrations of the produces raised to their stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients.

Question 4

What is the law of mass action?

Answer 4

The relationship between the balanced chemical equation and the expression of the equilibrium constant.

Question 5

If the equation is reversed, what happens to the equilibrium constant?

Answer 5

It is inverted.

Question 6

If we multiply the coefficients in the equation by a factor, what happens to the equilibrium constant?

Answer 6

We raise the equilibrium constant to the same factor.

Question 7

If we add two or more individual chemical equations to obtain an overall equation, what happens to the equilibrium constants?

Answer 7

We multiply the corresponding equilibrium constants by each other to obtain the overall equilibrium constant.

Question 8

What is Kc?

Answer 8

The equilibrium constant with respect to concentration in molarity.

Question 9

What is Kp?

Answer 9

The equilibrium constant with respect to partial pressures in atmospheres.

Question 10

How is the expression for Kp related to the expression for Kc?

Answer 10

The expression for Kp takes the form of the expression for Kc, except that we use the partial pressure of each gas in place of its concentration.

Question 11

Are Kp and Kc equal? Why?

Answer 11

Because the partial pressure of a gas in atm in not the same as its concentration in molarity, the value of Kp for a reaction is not necessarily equal to the value of Kc. However, as long the gases are behaving ideally, we can derive a relationship between the two constants.

Question 12

Give the expression that relates Kp to Kc.

Answer 12

Kp = Kc(RT)^∆n

Question 13

What is the reaction quotient?

Answer 13

The ratio–at any point in the reaction–of the concentrations of the products raised to their stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients.

Question 14

What is the difference between K and Q?

Answer 14

At a given T, K has only one value and it specifies the relative amounts of reactants and products at equilibrium. Q, by contrast, depends on the current state of the reaction and has many different values as the reaction proceeds.

Question 15

Why is the reaction quotient (Q) useful?

Answer 15

The value of Q relative to K is a measure of the progress of the reaction toward equilibrium.

Question 16

When is Q equal to K?

Answer 16

At equilibrium.

Question 17

How will Q change when it is less than K?

Answer 17

It must get larger as the reaction proceeds toward equilibrium. Q becomes larger as the reactant concentration decreases and the product concentration increases–the reaction proceeds to the right.

Question 18

How will Q change when it is greater than K?

Answer 18

It must get smaller as the reaction proceeds toward equilibrium. Q becomes smaller as the reactant concentration increases and the product concentration decreases–the reaction proceeds to the left.

Question 19

How do you check the validity of the x is small approximation?

Answer 19

By comparing the calculated value of x to the number from which it was subtracted. The ratio of x to the number it is subtracted from should be less than 0.05 for the approximation to be valid.

Question 20

What is Le Chatlier’s principle?

Answer 20

When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance.

Question 21

What happens when the concentration of a reactant is lowered?

Answer 21

Lowing the concentration of a reactant (which makes Q > K) causes the system to shift in the direction of the reactants to minimize the disturbance.

Question 22

What happens when the concentration of a product is lowered?

Answer 22

Lowering the concentration of a product (which makes Q < K) causes the system to shift in the direction of products.

Question 23

What effect does decreasing the volume of a reaction have?

Answer 23

Decreasing the volume causes the reaction to shift in the direction that has the fewer moles of gas particles.

Question 24

What effect does increasing the volume of a reaction have?

Answer 24

Increasing the volume causes the reaction to shift in the direction that has the greater number of moles of gas particles.

Question 25

What effect does adding an inert gas to a mixture have?

Answer 25

At a fixed volume, this has no effect on the equilibrium.

Question 26

Under what circumstance does a change in volume not produce an effect on equilibrium?

Answer 26

When a reaction has an equal number of moles of gas on both side of the chemical equation.

Question 27

When the temperature of a reaction at equilibrium is increased, what happens?

Answer 27

The reaction shifts in the direction that tends to decrease the temperature and vice versa.

Question 28

How can heat be considered in a chemical equation?

Answer 28

For exothermic reactions, heat can be considered a product, and for endothermic reactions, a reactant.

Question 29

In an exothermic reaction, what effect does increasing the temperature have on K?

Answer 29

The value of K decreases and the reaction shifts to the left.

Question 30

In an exothermic reaction, what effect does decreasing the temperature have on K?

Answer 30

The value of K increases and the reaction shifts to the right.

Question 31

In an endothermic reaction, what effect does increasing the temperature have on K?

Answer 31

The value of K increases and the reaction shifts to the right.

Question 32

In an endothermic reaction, what effect does decreasing the temperature have on K?

Answer 32

The value of K decreases and the reaction shifts to the left.