1. FACTORS AFFECTING EQUILIBRIUM
Concentration
Pressure
Temperature
Catalyst

2. 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 )

3. 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

4. 3.Position of equilibrium changes
Equilibrium constant unchanged

5. 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

6. b.2CrO42-(aq) + 2H+(aq)Cr2O72-(aq) + H2O
Yellow orange
At equilibrium , colour of solution = orange
i) Add H+ ,
Concentration of H+ increases
Equilibrium shifts right, solution more orange

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

8. 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

9. 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

10. 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 moles
small difference

11. 5.No effect on equilibrium if no overall change in no of moles of gases :
Eg H2(g) + I2(g)  2HI (g)
2 moles moles
6.Position of equilibrium changes
Equilibrium constant is unchanged

12. 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 )

13. 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)

14. If temperature increases ,
Equilibrium shifts to a direction which absorbs heat ( endothermic reaction )
Equilibrium shifts right
More NO2 forms
Observation : mixture becomes more brown

15. 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

16. 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

17. 2. Enthalpy diagram : eg reversible exothermic reaction
Energy
Decrease in Ea
Ea forward
(uncatalysed)
Ea forward
(catalysed)
Ea reverse
(uncatalysed)
Ea reverse
(catalysed)
Reactants H
Products
Reaction pathway

18. 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

19. Summary Factors affecting equilibrium
Influence
Equilibrium
constant
Equilibrium position
Change in conc
No change
Changes
Change in pressure
Addition of catalyst
Change in temp

20. 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)

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

22. 2H CO  CH3OH H positive
Temperature decreased,
Forward rxn is endothermic (1)
When temp↓ , equilibrium shifts left (1)
Formation of methanol decreases (1)

23. 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)