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FACTORS AFFECTING EQUILIBRIUM

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 ,

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FACTORS AFFECTING EQUILIBRIUM

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  1. FACTORS AFFECTING EQUILIBRIUM • 4 factors : • 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 10 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 2 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 loweractivation 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 sameamount • 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. SummaryFactors affecting equilibrium

  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. 2H2 + 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)

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