Chapter 17. Equilibrium: The Extent of Chemical Reactions

What is the gas constant (R)?

0.0821 atmL/molK

What is the reaction quotient always for all pure liquids and solids?

1

What are the three most common ways of disturbing a system at equilibrium?

1) Change concentrations of reactants and/or products 2) Change the temperature 3) Change the pressure of the system

What are the three ways pressure changes can occur?

1) Changing the concentration of a gaseous component 2) Adding an inert gas that does not take part in the reaction 3) Changing the volume of the reaction vessel

Why does this apparent cessation of chemical change occur?

All reactions are reversible and reach a state of equilibrium.

Why is this false: If an inert gas is added, the reaction shifts in the direction that produces fewer moles of gas.

An inert gas will have no effect on the equilibrium position because, although the total pressure in the system changes, the partial pressures remain unchanged.

How does adding an inert gas cause a pressure change?

As long as the volume of the system is constant, adding an inert gas has no effect on the equilibrium position because all concentrations, and thus partial pressures, remain the same. Moreover, the inert gas does not appear in Q, so it cannot have an effect.

When does K = Q?

At equilibrium at a given temperature

What does an intermediate K indicate?

Both reactants and products will be present in significant amounts at equilibrium (constant is close to 1)

What causes the value of Q to change?

Changing concentrations of products and reactants (specifically their ratio) as the reaction goes forward

The subscript "c" used in the symbols Qc and Kc indicate that these quantities are based on the ______________ of products and reactants.

Concentrations

The equilibrium constant K for a system at equilibrium expresses a particular ratio of equilibrium _______________ of products and reactants at a particular _____________. The value of K is also equal to the ratio of the rate _____________ for the forward and reverse reactions.

Concentrations, temperature, constants

How will a temperature decrease affect Kc for a system with a positive (Delta)Hrxn?

Decrease Kc

How will a temperature rise affect Kc for a system with a negative (Delta)Hrxn?

Decrease Kc

How does an increase in volume change pressure?

Decrease in pressure

What does a large K indicate?

Equilibrium favors the products; there will be more products than reactants at equilibrium

What does a small K indicate?

Equilibrium favors the reactants; there will be more reactants than products at equilibrium.

True or false: If the volume is increased, the value of K increases.

False

On which of the three states of matter does changes in pressure have a large effect on the equilibrium systems of?

Gas

Given sufficient time, what happens to concentrations of reactants and products?

Given sufficient time, the concentrations of the reactants and products attain certain values that no longer change.

What does the magnitude of K indicate?

How far a reaction proceeds toward product at a given temperature

Why is this true: If the total number of moles of gas in the products and reactants is the same, a change in volume will not affect the equilibrium position.

If (changing moles of gas) = 0, changing the volume will not affect the equilibrium position although the concentrations of all species will change.

How will a temperature decrease affect Kc for a system with a negative (Delta)Hrxn?

Increase Kc

How will a temperature rise affect Kc for a system with a positive (Delta)Hrxn?

Increase Kc

How does adding a gaseous reactant change pressure?

Increase in pressure

How does adding an inert gas change pressure?

Increase in total pressure, but no change in partial pressures of gases

As a reaction proceeds, the value of Q ___________.

Increases or decreases, as necessary, until it reaches the value of K

As a reaction is moving from reactants to products toward equilibrium, the value of Q is ____________.

Increasing

How does adding a solid change pressure?

No effect on pressure

Does a temperature decrease favor the products or reactants in an exothermic reaction?

Products

Does a temperature increase favor the products or reactants in an endothermic reaction?

Products

The equilibrium constant is a special value of the reaction _________ that occurs when reactant and product concentrations are at their _____________ values.

Quotient, equilibrium

Does a temperature decrease favor the products or reactants in an endothermic reaction?

Reactants

Does a temperature increase favor the products or reactants in an exothermic reaction?

Reactants

What is Qc?

Reaction quotient based on concentrations

The law of mass action reflects the observation that an equilibrium state is reached by a chemical system regardless of the ______________ concentrations of reacting substances. In other words, a chemical system can reach equilibrium either from the reactants or from the __________.

Starting, products

Why are the terms for pure solids and liquids eliminated in the expressions for Q and K?

The expressions are only concerned with concentrations that change as they approach equilibrium, and the concentrations, like the densities, of pure solids and liquids are constant

Why is this correct: The expression for Q is the same as the expression for K.

The expressions for Q and K for a given reaction are the same. The values of Q and K differ because K is calculated using equilibrium concentrations, whereas Q is calculated using concentrations at any time in the reaction.

The value of Kp for a given reaction is the equilibrium constant based on _____________.

The partial pressures of the reactants and products

What is Q?

The particular ratio of concentration terms that is written for a given reaction called the reaction quotient AKA mass-action expression)

If Q < K, then...

The reaction proceeds toward the products

If Q > K, then...

The reaction proceeds toward the reactants

Why is this correct: If the value of K for the forward reaction is less than 1, the value of K for the reverse reaction is greater than 1.

The value of K for the forward reaction is the reciprocal of the value of K for the reverse reaction.

Why does pressure changes not affect the equilibrium systems of solids and liquids?

They are nearly incompressible

True or False: If an inert gas is added, there will be no effect on the equilibrium position.

True

When Q < K, the reaction will proceed so that the concentrations of the _________ decrease and the concentrations of the ________ increase.

reactants; products