2012 topic 7.2

Topic 7.2

IB Chemistry Power Points

Topic 7

Equilibriumwww.pedagogics.ca

The Position of EquilibriumConsider

A BA + B C + D

A reacts with B in a sealed flask.

What happens to [A], [B], [C], and [D]?

The Position of Equilibrium

As the reaction progresses–[A] & [B] decrease to constant values,

–[C] & [D] increase from zero to constant values

–When [A], [B], [C] and [D] are all constant values, equilibrium is achieved.

A BA + B C + D

N2O4 (g) 2NO2 (g)

Start with NO2 Start with N2O4 Start with NO2 & N2O4

equilibrium

equilibrium

equilibrium

3 scenarios for the reactionsee simulation

= 4.63 x 10-3KC = [NO2]2

[N2O4]

Data for these (and other) scenarios might look like

The Equilibrium ConstantBIG IDEA : At a given temperature, no matter

the starting composition of reactants and products, the same ratio of concentrations is achieved at equilibrium.

• For a general reaction

aA bB cC dD

The Equilibrium Constant

the equilibrium constant expression is

where Kc is the equilibrium constant

and is dependent on temperature.

[ ] [ ][ ] [ ]

c d

C a b

C DK

A B

Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.N2O4 (g) 2NO2 (g)

Kc = [NO2]2

[N2O4]

CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)

Kc =‘[CH3COO-][H3O+][CH3COOH][H2O]

Kc = [CH3COO-][H3O+]

[CH3COOH]

[H2O] = does not change significantly so

is omitted

Practice – homogeneous equilibrium• Write the equilibrium expressions for

the following reactions:

H2 (g) + I2 (g) Ý 2 HI (g)

HF (aq) Ý H+ (aq) + F- (aq)

Practice – homogeneous equilibrium• Write the equilibrium expressions for

the following reactions:

H2 (g) + I2 (g) Ý 2 HI (g)

HF (aq) Ý H+ (aq) + F- (aq)

2

2 2

[ ]

[ ][ ]eq

HIk

H I

[ ][ ]

[ ]eq

H Fk

HF

Practice:The equilibrium concentrations for the reaction between carbon monoxide and molecular chlorine to form COCl2 (g) at 740C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] = 0.14 M. Calculate Kc

CO (g) + Cl2 (g) COCl2 (g)

220 M-1

Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.

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

Kc =‘[CaO][CO2]

[CaCO3][CaCO3] = constant[CaO] = constant

Kc = [CO2]

The concentration of solids and pure liquids do not change hence they are not included in the expression for the equilibrium constant.

[CO2] = Kc

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

[CO2]does not depend on the amount of CaCO3 or CaO

Practice – heterogeneous equilibrium• Write the equilibrium expression for the

following reaction:

PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)

Practice – heterogeneous equilibrium• Write the equilibrium expression for the

following reaction:

PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)

2 2[ ][ ]eqk Pb Cl

aA + bB cC + dD

K = [C]c[D]d

[A]a[B]b

K >> 1K << 1

Lie to the right Favor productsLie to the

leftFavor reactants

Equilibrium Will

Recall: for the reaction

What does the Kc value mean?

Summary Writing Equilibrium Expressions

• The concentrations of the reacting species in the condensed phase are expressed in M (mol dm3) In the gaseous phase, the concentrations can be expressed in M or in atm.

• The concentrations of pure solids, pure liquids and other solvents do not appear in the equilibrium constant expressions.

• In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature.

Le Chatelier’s Principle: if you disturb an equilibrium, it will shift to undo the disturbance.

Remember, in a system at equilibrium, the concentrations will always change to restore the same value for Kc (as long as there is a constant temperature).

7.2 Le Châtelier’s Principle

Changes in Concentration

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

AddNH3

Equilibrium shifts left to offset stress

Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the Equilibrium

Increase concentration of product(s) left

aA + bB cC + dD

Add

Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the Equilibrium

Increase concentration of product(s) leftDecrease concentration of product(s) right

aA + bB cC + dD

Remove

Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the Equilibrium

Increase concentration of product(s) leftDecrease concentration of product(s) rightIncrease concentration of reactant(s) right

aA + bB cC + dD

Add

Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the Equilibrium

Increase concentration of product(s) leftDecrease concentration of product(s) right

Decrease concentration of reactant(s)Increase concentration of reactant(s) right

left

aA + bB cC + dD

Remove

Le Châtelier’s Principle

Changes in Pressure

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

Change Shifts the Equilibrium

Increase pressureDecrease pressure Side with most moles of gas

Side with fewest moles of gas

Le Châtelier’s Principle Changes in Temperature

Change Response

Increase temperature forward reaction favored - remove heat

Decrease temperaturereverse reaction favored – absorb heat

colder hotter

N2O

4(g) 2 NO

2(g) ΔH is +ive

Room temperature. NO2 is brown

Le Châtelier’s Principle Changes in Temperature

Change Response

Increase temperature forward reaction favored - remove heat

Decrease temperaturereverse reaction favored – absorb heat

colder hotter

N2O

4(g) + heat 2 NO

2(g)

Room temperature. NO2 is brown

Le Châtelier’s Principle

ONLY changes in temperature affect Kc

Change Exothermic Rx

Increase temperature K decreasesDecrease temperature K increases

Endothermic Rx

K increasesK decreases

colder hotter

N2O

4(g) 2 NO

2(g) endothermic

Room temperature. NO2 is brown

uncatalyzed catalyzed

Catalyst lowers Ea for both forward and reverse reactions.

Catalyst does not change equilibrium constant or shift equilibrium.

• does not change the value of Kc

• does not shift the position of an equilibrium system• system will reach equilibrium sooner

Le Châtelier’s Principle Catalysts

Example

Example

a) shifts left to favor products

b) water vaporizes – shift right to favor reactants

c) shifts right to favor reactants

d) no effect (solids not part of equilibrium expression)

Chemistry In Action: The Haber Process – see fact sheet and video

N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol

Le Châtelier’s Principle (summary)

Change Shift EquilibriumChange Equilibrium

Constant

Concentration yes no

Pressure (g) yes no

Volume (g) yes no

Temperature yes yes

Catalyst no no