FACTORS AFFECTING EQUILIBRIUM 4 factors : Concentration Pressure Temperature Catalyst
Jan 30, 2016
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)