Unit 6Equilibrium

Equilibrium is NOT when all things are equal.

Equilibrium is signaled by no net change in the concentrations of reactants or products.

What is equilibrium?

At equilibrium, the concentrations of reactants and products no longer change with time.

For equilibrium to occur, neither reactants nor products can escape from the system.

At equilibrium a particular ratio or concentration terms equals a constant.

Important notes about equilibrium

Consider the reaction: aA + bB cC + dD At equilibrium the equilibrium constant (keq)

is calculated as…

The Equilibrium Constant

Write the equilibrium constant expressions for the following equations:

2 O3(g) 3 O2(g)

2 NO(g) + Cl2(g) 2 NOCl(g)

Ag+(aq) + 2 NH3(aq) Ag(NH3)2+(aq)

Examples

Experiment

Initial [NO]

Initial [NO2]

Equilibrium [N2O4]

Equilibrium [NO2]

Keq

1 0.0 0.02 0.0014 0.0172 0.21

2 0.0 0.03 0.0028 0.0243 0.21

3 0.0 0.04 0.00452 0.031 0.21

4 0.02 0.0 0.00452 0.031 0.21

Calculating Keq

When all of the reactants and products of a chemical reaction are gasses we can write the equilibrium constant expression in terms of each components partial pressure.

Equilibrium Constant in Terms of Pressure kp

keq and kp are often times numerically different even for the same reaction.

We can however use one of the two to calculate the other.

Keq vs kp

In the synthesis of ammonia from nitrogen and hydrogen,

N2(g) + 3 H2(g) 2 NH3(g)

keq = 9.60 at 300oC. Calculate kp for this reaction at 300oC.

Example

keq values for different reactions can range from very large numbers to very small numbers.

Consider the reaction:

If keq >> 1 the equilibrium lies to the right and products predominate.

If keq << 1 the equilibrium lies to the left and the reactants predominate.

Working With Equilibrium Constants

Equilibrium can be approached from either direction.

Consider:

The Direction of the Chemical Equation and keq

We have already seen cases where the overall reaction is actually a series of other reactions.

Consider the following two reactions:

Multi-Step Reactions

A heterogeneous equilibrium is one where not all of the chemical species are in the same physical state.

Example:

Whenever a pure liquid or a pure solid is involved in a heterogeneous equilibrium, its concentration is not included in the keq expression.

Heterogeneous Equilibria

Write the keq expression for the following reactions:

CO2(g) + H2(g) CO(g) + H2O(l)

SnO2(s) + 2 CO(g) Sn(s) + 2 CO2(g)

Example

The Haber process is a reaction that was first put to use by Fritz Haber as a way to produce ammonia.

One problem he had was that this reaction was an equilibrium reaction.

Calculating Equilibrium Constants

A chemists performs the Haber reaction at 472oC. The equilibrium mixture of the gases is found to contain 7.38 atm H2, 2.46 atm N2, and 0.166 atm of NH3. Calculate kp for the Haber process at this temperature.

Example

Often times we do not know the equilibrium concentrations of all of the chemical species in a reaction.

But if we know the equilibrium concentration of at least one species we can use stoichiometry to find the others (and eventually keq or kp)

Follow these steps:1. Tabulate all the known initial and equilibrium

concentrations. 2. Use these to calculate the change in concentration

of that chemical species.3. Use stoichiometry to calculate the change in the

other chemical species.4. Use the initial concentrations with the change in

concentration to calculate the equilibrium concentration.

A closed system initially containing 1.0 x 10-

3 M H2 and 2.0 x 10-3 M I2 at 448oC is allowed to reach equilibrium. Analysis of the equilibrium mixture shows that [HI] = 1.87 x 10-3 M. Calculate keq at 448oC.

Example

We have seen that the magnitude of keq tells us to which side the equilibrium lies.

We can also use keq to tell us which way a reaction will proceed if it isn’t already at equilibrium.

If Q = keq the reaction is at equilibrium. If Q > keq the concentration of products is too large and

the reaction will proceed to the left. If Q < keq the concentration of reactants is too large

and the reaction will proceed to the right.

Applications of Equilibrium Constants

At 448oC the equilibrium for the reaction: H2(g) + I2(g) 2 HI(g)

is 50.5. Predict the direction the reaction will proceed if we start with 0.02 mol of HI, 0.01 mol of H2 and 0.03 mol of I2 in a 2.00-L container.

Example

For the Haber process at 500oC the equilibrium constant is 1.45 x 10-5. An analysis of an equilibrium mixture of the reaction shows that the partial pressures of the reactants are H2 = 0.928 atm, N2 = 0.432 atm. Calculate the partial pressure of NH3.

Calculating Equilibrium Concentrations

Many times we will not know the equilibrium concentrations of at least one species.

Example: A 1.0 L flask is filled with 1.0 mol H2 and 2.0

mol I2. The keq for this reaction at 448oC is 50.5. What are the equilibrium concentrations of H2, I2, and HI?

Calculating Equilibrium Concentrations using Initial Concentrations

The Chatelier’s Principle states that: If a system at equilibrium is disturbed by a

change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position to counteract the change.

In other words, keq , must remain constant.

Le Chatelier’s Principle

N2(g) + H2(g) 2 NH3(g)

Changes in Reactant or Product Concentration.

If the volume of a gaseous system is decreased what would happen to the pressure of the system?

Reducing the volume of a gaseous equilibrium will cause the system to shift in the direction that decreases the total moles of gas.

Consider: N2O4(g) 2 NO2(g)

Effects of Volume and Pressure Changes

Changes in concentrations, volume, or pressure cause equilibrium shifts without changing the value of keq.

However changing the temperature of an equilibrium system will change the numeric value of keq.

When the temperature of an equilibrium system is increase, the system reacts as if we added a reactant to the endothermic reaction. The equilibrium will shift a such to consume that reactant.

The Effect of Temperature Change

If the forward reaction of an equilibrium is endothermic increasing the temperature increases the value of k.

If the forward reaction is exothermic increasing the temperature decreases the value of k.

How keq changes with Temperature

Consider the equilibrium:N2O4(g) 2 NO2(g)

In which direction will the equilibrium shift when (a) N2O4 is added. (b) NO2 is removed. (c) The total pressure of the system is increased. (d) The volume is increased. (e) The temperature is decreased.

Example