Factors Affecting Equilibrium

Chapters 18

When a system is at equilibrium, it will stay that way until something changes this condition.

Le Chatelier’s Principal

when a change (“stress”) is applied to a system at equilibrium, the system will shift its equilibrium position to counter act the effect of the disturbance.

Factors affecting equilibrium include changes in:

Concentrations of reactants or products

Temperature

Pressure (gases)

Changes in Concentration:Consider this reaction at equilibrium:

H2(g) + I2(g) 2HI(g)

What will happen to the equilibrium if we:add some H2?

Reaction shifts to the right

(forms more product)

Changes in Concentration:Consider this reaction at equilibrium:

H2(g) + I2(g) 2HI(g)

What will happen to the equilibrium if we:remove some H2?

Reaction shifts to the left

(forms more reactants)

Changes in Concentration:When a substance is added, the stress is relieved by shifting equilibrium in the direction that consumes some of the added substance.

When a substance is removed, the reaction that produces that substance occurs to a greater extent.

Changes in Temperature:Consider this reaction at equilibrium:

2SO2(g) + O2(g) 2SO3(g) + 198 kJ

What will happen to the equilibrium if we:increase the temperature?

Reaction shifts to the left

(forms more reactants)

Changes in Temperature:Consider this reaction at equilibrium:

2SO2(g) + O2(g) 2SO3(g) + 198 kJ

What will happen to the equilibrium if we:decrease the temperature?

Reaction shifts to the right

(forms more products)

Changes in Temperature:

Increasing the temperature always favors the reaction that consumes heat, and vice versa.

Changes in Pressure:Consider this reaction at equilibrium:

2NO2(g) N2O4(g)

What will happen to the equilibrium if we:increase the pressure?

Reaction shifts to the right

(forms more product)

Changes in Pressure:Consider this reaction at equilibrium:

2NO2(g) N2O4(g)

What will happen to the equilibrium if we:decrease the pressure?

Reaction shifts to the left

(forms more reactant)

Changes in Pressure:

Increasing the pressure favors the reaction that produces the fewer moles of gas, and vice-versa.

Example: consider the rxn at equilibrium: N2(g) + 3H2(g) 2NH3(g) + 94 kJ

How would the equilibrium be influenced by: Increasing the temp:Decreasing the temp: Increasing the pressure:Decreasing the pressure:Adding more H2:

Removing some NH3:Adding a catalyst:

rxn shifts to the left

rxn shifts to the right

rxn shifts to the left

rxn shifts to the right

rxn shifts to the right

rxn shifts to the right

no change in equilibrium position

Catalysts

A catalyst increases the rate at which equilibrium is reached, but it does not change the composition of the equilibrium mixture.

Action of a Catalyst

Activation Energy

Without a catalyst

Action of a Catalyst

Lower Activation Energy

With a catalyst: A catalyst lowers the activation energy.

Example: How will an increase in pressure affect the equilibrium in the following reactions:

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

RXN SHIFTS LEFT

2H2(g) + O2(g) 2H2O(g)

RXN SHIFTS RIGHT

Example: How will an increase in temperature affect the equilibrium in the following reactions:

2NO2(g) N2O4(g) + heatRXN SHIFTS LEFT

H2(g) + Cl2(g) 2HCl(g) + 92 KJRXN SHIFTS LEFT

H2(g) + I2(g) + 25 kJ 2HI(g)

RXN SHIFTS RIGHT

How does this affect K??Concentration stress– no change

Temperature stress– K will go up or down depending on the shift

Pressure stress-- K will go up or down depending on the shift

SHIFT TOWARDS PRODUCT– K gets BIGGER SHIFT TOWARD REACTANT– K get SMALLER

To close, let’s talk about Fritz Haber and the “process” that made him famous…

Let’s say you want to manufacture ammonia gas (NH3). What are the optimum conditions?N2 + 3H2 2 NH3 + Heat

We wish to favor the forward reaction, thus producing more NH3 gas.

For example

N2 + 3H2 2 NH3 + Heat

By cooling the reaction, the reactions counters by producing heat. It does this by shifting to the right, producing heat, and more NH3 gas.

For example

N2 + 3H2 2 NH3 + Heat

By increasing the pressure, the reaction tries to reduce the pressure. It does this by shifting to the side with the fewest moles of gas. This is the product side with 2 moles of gas. Thus the reaction shifts to the right reducing pressure, and producing more NH3 gas.

4 moles of gas 2 moles of gas

For example

N2 + 3H2 2 NH3 + Heat

By adding additional N2 and H2, the reactions tries to use them up. In doing so, the reaction shifts to the right.

By removing NH3 as soon as its formed, the reactions tries to produce more. Shifting the reaction to the right.

For example

N2 + 3H2 2 NH3 + Heat

So if you want to produce maximum ammonia gas you should

Cool the reaction

Conduct the reaction under high pressure

Add N2 and H2

Remove NH3

con

cen

trati

on

time

H2 added here NH3 removed here

equilibrium equilibrium equilibrium

Example: N2(g) + 3H2(g) 2NH3(g) ∆H=-93 kJ mol-1

NH3

N2

H2