7.5 Equilibrium

Suppose that every time a car passes by a toll booth in one direction, another car passes through in the opposite direction. Then, the number of cars on either side of the toll plaza is constant.

7.5 Equilibrium

Under what conditions do physical and chemical equilibria occur?

Types of Equilibria

When a physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes.

7.5 Equilibrium

Equilibrium is a state in which the forward and reverse paths of a change take place at the same rate.

Balanced traffic at a toll bridge is a model for a system in equilibrium.

When opposing physical changes take place at the same rate, a physical equilibrium is reached.

When opposing chemical changes take place at the same rate, a chemical equilibrium is reached.

Types of Equilibria

7.5 Equilibrium

Physical Equilibrium

Liquid water left in a closed container eventually reaches equilibrium with its vapor when the rate of evaporation equals the rate of condensation.

Types of Equilibria

7.5 Equilibrium

Liquid water in equilibrium with water vapor can be described by this equation.

(l) stands for liquid and (g) stands for gas.

The pair of arrows in this equation indicates that the forward change and the reverse change happen and at the same rate.

Types of Equilibria

7.5 Equilibrium

When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions.

Most reactions are reversible to some extent.

Types of Equilibria

A reversible reaction is a reaction in which the conversion of reactants into products and the conversion of products into reactants can happen simultaneously.

Chemical Equilibrium

7.5 Equilibrium

In the synthesis of sulfur trioxide, the forward reaction produces sulfur trioxide. Some molecules of sulfur trioxide change back into the reactants by the reverse reaction.

During chemical equilibrium, the reactants change into products just as fast as the products change back into reactants.

Types of Equilibria

7.5 Equilibrium

How do equilibrium systems respond to change?

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change.

This rule was first observed by Henri Le Châtelier. Today, the rule is known as Le Châtelier’s principle.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

The making of ammonia is an example of a process in which chemists apply Le Châtelier’s principle.

If a system containing nitrogen, hydrogen, and ammonia is in equilibrium, you can predict how the system will be affected by changes in temperature, pressure, and concentration.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

Temperature

In the equation for the synthesis of ammonia, heat is written as a product.

• The forward reaction is exothermic. • In the reverse reaction, heat is a reactant, so

the decomposition of ammonia is endothermic.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

According to Le Châtelier’s principle, if you added heat to the system, the equilibrium would shift in the direction that removes heat from the system.

The system would favor the reverse reaction, which is endothermic. By increasing the temperature, you would decrease the amount of ammonia.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

Pressure

According to Le Châtelier’s principle, if you increased the pressure, the equilibrium would shift in the direction that decreases the pressure of the system.

In order to decrease pressure, the system would favor the reaction that produces fewer gas molecules. By increasing the pressure, you would shift the equilibrium to the right, producing more ammonia.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

Concentration

Le Châtelier’s principle tells you that if you decreased the ammonia concentration by removing ammonia from the system, the equilibrium would shift in the direction that produces ammonia.

In order to produce ammonia, the system would favor the forward reaction.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

Operating an ammonia plant at relatively low temperature, high pressure, and low ammonia concentration maximizes the amount of ammonia produced.

Factors Affecting Chemical Equilibrium

7.5 Equilibrium

Assessment Questions

1. In a chemical reaction that is at equilibrium, which of the following is true? a. All of the reactants have been converted to products.

b. The reaction stops even though some reactants remain.

c. Forward and reverse reactions occur at the same rate.

d. All of the products are gases.

7.5 Equilibrium

Assessment Questions

1. In a chemical reaction that is at equilibrium, which of the following is true? a. All of the reactants have been converted to products.

b. The reaction stops even though some reactants remain.

c. Forward and reverse reactions occur at the same rate.

d. All of the products are gases.

ANS: C

7.5 Equilibrium

Assessment Questions

2. In the chemical reaction below, how could you shift the equilibrium toward water?

a. increase the temperatureb. increase the pressurec. decrease the amount of oxygend. add steam to the mixture

7.5 Equilibrium

Assessment Questions

2. In the chemical reaction below, how could you shift the equilibrium toward water?

a. increase the temperatureb. increase the pressurec. decrease the amount of oxygend. add steam to the mixture

ANS: B