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Le Châtelier

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StandardStandard

Know how to use Le Châtelier’sKnow how to use Le Châtelier’s principle to predict the effect of principle to predict the effect of changes in concentration, changes in concentration, pressure, and temperature.pressure, and temperature.

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18.2 Le Châtelier’s Principle

Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system adjusts in a way to reduce the change.

Stress causes a change in a system at equilibrium

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II.A. 3 Types of StressII.A. 3 Types of Stress Three kinds of stress:

changes in the concentrations of reactants or products

changes in pressure changes in temperature

When a stress is first applied to a system, equilibrium is disturbed and the rates of the forward and backward reactions are no longer equal.

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II.A.1. Changes in II.A.1. Changes in ConcentrationConcentration

If more of a substance is added to a If more of a substance is added to a reaction at equilibrium, the system will reaction at equilibrium, the system will shift to remove the excess amount of that shift to remove the excess amount of that substancesubstance

If a substance is taken out of a reaction at If a substance is taken out of a reaction at equilibrium, the system will shift to equilibrium, the system will shift to produce more of the substance that was produce more of the substance that was removedremoved

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Pencil’s down

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II.A.1. Changes in Concentration

aq) aq aq l2 22 4 3 3 4 2[Cu(H O) ] ( 4NH ( ) [Cu(NH ) ] ( ) 4H O( )

• In a reaction of two colored complex ions:

pale blue blue-purple

• When the reaction mixture in a beaker is pale blue, we know that chemical equilibrium favors the formation of reactants.

**If ammonia is added, the system responds by forming more product and the solution becomes blue-purple.

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Predicting the Effect of a Change in Concentration on the Equil Position

Which way would the following equilibria shift in order to re-establish equilibrium:

1. CO(g) + Cl2(g) COCl2(g)

a. Remove COCl2

Answer: Shift to RIGHT

b. Add chlorine gas

Answer: Shift to RIGHT

2. 2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g)

a. Remove O2

Answer: Shift to LEFT

b. Add PbS

Answer: NO Change

c. Add SO2

Answer: Shift to LEFT

3. 2H 2S (g) + O2(g) 2S(s) + 2H2O (g)

a. Add sulfur

Answer: No Change

b. Remove water vapor

Answer: Shift to RIGHT

c. Remove H2S

Answer: Shift to LEFT

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II.A.2. Pressure and Volume Changes

A (g) + B (g) C (g)

Change Shifts the Equilibrium

↑ pressure Side with fewest moles of gas

↓ pressure Side with most moles of gas

↓ volume

↑ volume Side with most moles of gas

Side with fewest moles of gas

Reaction shifts to change the equilibrium Reaction shifts to change the equilibrium position (concentrations), not the equilibrium position (concentrations), not the equilibrium constant.constant.

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II.A.2. Pressure Changes cont’dII.A.2. Pressure Changes cont’d

NN2(g)2(g) +3 H +3 H2(g)2(g) <=> 2 NH <=> 2 NH3(g)3(g) (a) A mixture of gaseous N(a) A mixture of gaseous N22, H, H22, and NH, and NH33 at equilibrium. at equilibrium. (b) Predict what happens when the pressure is increased.(b) Predict what happens when the pressure is increased.(c) Reaction occurs from left to right, (c) Reaction occurs from left to right, decreasing the total decreasing the total

number of gaseous moleculesnumber of gaseous molecules until equilibrium is re- until equilibrium is re-establishedestablished

(d) Which molecule would end up with the highest (d) Which molecule would end up with the highest concentration?concentration?

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An equilibrium reaction that has the same # of moles of gas onboth sides will not be affected by changes in pressure.

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SOLUTION:

II.A.2. Predicting the Effect of a Change in Pressure on the Equilibrium Position

PROBLEM: How would you change the pressure of each of the following reactions to increase the yield of products?

(a) CaCO3(s) CaO(s) + CO2(g)

(b) S(s) + 3F2(g) SF6(g)

(c) Cl2(g) + I2(g) 2ICl(g)

PLAN: When gases are present a change in pressure will affect the concentration of the gas. If the pressure increases, the reaction will shift to fewer moles of gas and vice versa.

(a) CO2 is the only gas present. To increase its yield, we should decrease the pressure.

(b) There are more moles of gaseous reactants than products, so we should increase the pressure to shift the reaction to the right.

(c) There are an equal number of moles of gases on both sides of the reaction, therefore a change in volume will have no effect.

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II.A.3. Temperature ChangesII.A.3. Temperature Changes

Change Exothermic Rx

↑ temperature Keq ↓ (less products)

↓ temperature Keq ↑ (more products)

Endothermic Rx

Keq ↑

Keq ↓

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

***Temp—only factor that affects the value of Keq

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SOLUTION:

II.A.3. Predicting the Effect of a Change in Temperature on the Equilibrium Position

PROBLEM: How does an increase in temperature affect the concentration of the underlined substance and Keq for the following reactions?

(a) CaO(s) + H2O(l) Ca(OH)2(aq) H0 = -82kJ

(b) SO2(g) S(s) + O2(g) H0 = 297kJ

PLAN: Express the heat of reaction as a reactant or a product. Then consider the increase in temperature and its effect on Keq.

(a) CaO(s) + H2O(l) Ca(OH)2(aq) heat

An increase in temperature will shift the reaction to the left, decrease [Ca(OH)2], and decrease Keq.

(b) SO2(g) + heat S(s) + O2(g)

The reaction will shift right resulting in an decrease in [SO2] and increase in Keq.

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Le Châtelier’s Principle

Change Shift Equilibrium

Change Equilibrium Constant

Concentration yes no

Pressure yes no

Volume yes no

Temperature yes yes

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QUIZ 3-5

Which way will the Which way will the equilibrium shift with the equilibrium shift with the following changes?following changes?

2PbS(s)+3O2(g)↔2PbO(s)+2SO2(g)

a. Remove O2

b. Add PbS c. Add SO2

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QUIZ 3-6

S (s) + 3FS (s) + 3F22(g) (g) ↔ ↔ SFSF66(g) (g) ΔΔH = -35 kJH = -35 kJ

Which way would the equilibrium Which way would the equilibrium shift if:shift if:

a.a. Increase pressure.Increase pressure.

b.b. Increase temperatureIncrease temperature

c.c. Increase the amount of SIncrease the amount of S

d.d. Increase the amount of FIncrease the amount of F22

e.e. Increase the amount of SFIncrease the amount of SF66

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QUIZ 3-7

ClCl2 2 (g) + I(g) + I2 2 (g) (g) ↔ ↔ 2ICl (g) 2ICl (g) ΔΔH = +35 kJH = +35 kJ

Which way would the equilibrium Which way would the equilibrium shift if:shift if:

a.a. Increase pressure.Increase pressure.

b.b. Increase temperatureIncrease temperature

c.c. Increase the amount of ClIncrease the amount of Cl22

d.d. Increase the amount of IIncrease the amount of I22

e.e. Increase the amount of IClIncrease the amount of ICl

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QUIZ 3-8

2H2H22SS(g)(g)+ O+ O2(g) 2(g) ↔ ↔ 2S2S(s)(s)+ 2H+ 2H22OO(g)(g) ΔΔH = 80kJH = 80kJ

1.1. How would you adjust How would you adjust concentration to get more of the concentration to get more of the products?products?

2.2. How would you adjust pressure to How would you adjust pressure to get more of the products?get more of the products?

3.3. How would you adjust temperature How would you adjust temperature to get more products?to get more products?

4.4. Write an equilibrium expression.Write an equilibrium expression.

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The Haber Process

‘‘During peace time a During peace time a scientist belongs to the scientist belongs to the world, but during war world, but during war time a scientist time a scientist belongs to his country.’belongs to his country.’

Allows nitrogen compounds to Allows nitrogen compounds to be produced from nitrogen in be produced from nitrogen in the air.the air.

- - freed Germany from freed Germany from having to having to import gun import gun powder and fertilizerpowder and fertilizer

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The Haber Process

NN22(g) + 3H(g) + 3H22(g) (g) ↔↔ 2NH 2NH33(g) (g) ΔΔH=-90kJH=-90kJ

Write an equilibrium expression Write an equilibrium expression for this reaction.for this reaction.

Calculate KCalculate Keqeq if at equilibrium: if at equilibrium:

[N[N22] = 2.0 M ; [H] = 2.0 M ; [H22] = 2.0 M ;] = 2.0 M ;

and [NHand [NH33] = 6.0 M] = 6.0 M

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The Haber Process

NN22(g) + 3H(g) + 3H22(g) (g) ↔↔ 2NH 2NH33(g) (g) ΔΔH=-90kJ H=-90kJ

List 5 ways to increase the yield List 5 ways to increase the yield of ammonia in this process. (Be of ammonia in this process. (Be precise.)precise.)

The Haber process is carried out The Haber process is carried out at high temperature even at high temperature even though the yield is lower. Why?though the yield is lower. Why?