FACTORS AFFECTING EQUILIBRIUM l l l 4 factors

FACTORS AFFECTING EQUILIBRIUM l l l 4 factors : Concentration Pressure Temperature Catalyst

LE CHATELIER’S PRINCIPLE l 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 l l l l 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

l 3. Position of equilibrium changes l Equilibrium constant unchanged

l l l l 4. Examples : a. Fe 3+(aq) + SCN-(aq) [Fe. SCN]2+(aq) Yellow deep red At equilibrium , colour of solution = red i) [Fe 3+] increased , Equilibrium shifts right , solution more red ii) [SCN-] decreased, Equilibrium shifts left , red solution lightens

b. 2 Cr. O 42 -(aq) + 2 H+(aq) Cr 2 O 72 -(aq) + H 2 O Yellow orange l At equilibrium , colour of solution = orange l i) Add H+ , l Concentration of H+ increases l Equilibrium shifts right, solution more orange l

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

Effect of pressure l l l l 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 l l l l 1 N 2 O 4 (g) 2 NO 2 (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

l l 4. Effect of changes in pressure : Little effect on equilibrium where overall difference in no of moles of gases is small Eg 4 NH 3(g) + 5 O 2(g) 4 NO(g) +6 H 2 O(g) 9 moles 10 moles l l small difference

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

Effect of temperature l l l 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 l l l N 2 O 4(g) 2 NO 2 (g) H= +58 k. Jmol-1 pale yellow brown Colour of mixture : brown Forward reaction is endothermic (1) (Therefore reverse reaction is exothermic)

l l l If temperature increases , Equilibrium shifts to a direction which absorbs heat ( endothermic reaction ) Equilibrium shifts right More NO 2 forms Observation : mixture becomes more brown

l l 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) l l l 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

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

l l l 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

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

Questions l l l l Br 2 + H 2 O 2 H+ + Br- + OBrbrown / orange colourless Colour of solution : brown / orange Add HCl : [H+] increase (1) equilibrium shifts left (1) Solution becomes more brown (1)

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

l l l 2 H 2 + CO CH 3 OH H positive Temperature decreased, Forward rxn is endothermic (1) When temp↓ , equilibrium shifts left (1) Formation of methanol decreases (1)

l l 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)