FACTORS AFFECTING EQUILIBRIUM

4 factors : Concentration Pressure Temperature Catalyst

LE CHATELIER’S PRINCIPLE

Definition :

if a system at equilibrium is disturbed, the equilibrium moves in the direction which tend to reduce the disturbance ,

( and as a result of that , achieves a new equilibrium position )

Effect of concentration

1. A + B C + D 2. If A or B is increased , Rate of forward reaction increases Concentration of C and D increases Rate of reverse reaction increases Rate of forward reaction = rate of reverse

reaction New equilibrium achieved

3.Position of equilibrium changes

Equilibrium constant unchanged

4. Examples : a.Fe3+(aq) + SCN-(aq) [FeSCN]2+(aq) Yellow deep red At equilibrium ,colour of solution = red i) [Fe3+] increased , Equilibrium shifts right , solution more red ii) [SCN-] decreased, Equilibrium shifts left , red solution lightens

b.2CrO42-(aq) + 2H+(aq)Cr2O7

2-(aq) + H2O

Yellow orange At equilibrium , colour of solution = orange i) Add H+ , Concentration of H+ increases Equilibrium shifts right, solution more orange

ii) Add OH- , OH- neutralises H+ , [H+] decreased , Equilibrium shifts left , solution turns yellow

Effect of pressure 1.Pressure exerted depends on no. of moles of

gases ( no of moles ↑ , pressure ↑ ) 2.Changes in pressure : a. pressure ↑ Equilibrium will shift so as to decrease pressure Equilibrium will shift to a direction which produces

fewer no of moles of gases b. pressure ↓ , equilibrium will shift so as to increase

pressure Equilibrium will shift to a direction which produces

more no of moles of gases

3.Example 1 N2O4 (g) 2 NO2 (g) pale yellow brown Colour of mixture : brown Forward reaction is accompanied by an increase in no

of moles of gases (1) If pressure ↑ , Equilibrium will shift to a direction which produces less

no of moles of gases equilibrium shifts left Observation : mixture becomes less brown

4. Effect of changes in pressure : Little effect on equilibrium where overall

difference in no of moles of gases is small Eg 4NH3(g) + 5O2(g) 4NO(g) +6H2O(g) 9 moles 10 moles

small difference

5.No effect on equilibrium if no overall change in no of moles of gases :

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

6.Position of equilibrium changes

Equilibrium constant is unchanged

Effect of temperature

1.Changes in temperature : a.Temperature ↑ , equilibrium shifts to a

direction to cool system down Absorbs heat ( endothermic rxn favored) b.Temperature ↓ , equilibrium shifts to a

direction to increase temperature Evolves heat ( exothermic rxn favored )

2.Example

N2O4(g) 2NO2 (g) H= +58 kJmol-1

pale yellow brown Colour of mixture : brown Forward reaction is endothermic (1) (Therefore reverse reaction is exothermic)

If temperature increases , Equilibrium shifts to a direction which

absorbs heat ( endothermic reaction ) Equilibrium shifts right More NO2 forms Observation : mixture becomes more brown

3.Changes in temperature has no effect on equilibrium where H for the reaction is zero

4.Position of equilibrium and equilibrium constant changes

5. Note :Temperature increase , rate of forward and reverse reaction increase

However rate of endothermic reaction increases more

Effect of catalyst (catalysis)

1.Positive catalyst increases rate of reaction by providing an alternative route with a lower activation energy ( Ea )

(With a different mechanism) Definition : Activation energy is the minimum

energy required for a reaction to occur

Energy

Reaction pathway

H

Reactants

Products

Ea reverse

(catalysed)

Ea reverse

(uncatalysed)

Ea forward

(catalysed)

Ea forward

(uncatalysed)

2. Enthalpy diagram : eg reversible exothermic reaction

Decrease in Ea

3.Ea for both forward and reverse reactions lowered by the same amount

Rate of both reactions increased by the same extent

Equilibrium position unchanged Equilibrium constant unchanged 4.Catalyst only results in equilibrium

achieved more quickly

SummaryFactors affecting equilibrium

Influence Equilibrium

constant

Equilibrium position

Change in conc

No change Changes

Change in pressure

No change Changes

Addition of catalyst

No change No change

Change in temp

Changes Changes

Questions

Br2 + H2O 2H+ + Br- + OBr-

brown / orange colourless Colour of solution : brown / orange Add HCl : [H+] increase (1) equilibrium shifts left (1) Solution becomes more brown (1)

Add NaOH, OH- neutralises H+ , [H+] decrease (1) Equilibrium shifts right (1) Solution becomes less brown (1)

2H2 + CO CH3OH H positive Temperature decreased, Forward rxn is endothermic (1) When temp↓ , equilibrium shifts left (1) Formation of methanol decreases (1)

Pressure increased Forward rxn is accompanied by a decrease in

the no of moles of gases (1) When pressure ↑, equilibrium shifts right (1) Formation of methanol increases (1)