1. Using and Controlling Reactions 1

2.  Most chemical reactions don’t go to completion.  Instead with the right conditions they will reach a balance between reactants and products.  This is called Equilibrium 2

3.  Forward reaction: Reactants  Products  Back reaction: Products  Reactants  Overall: Reactants  Products  Not a static state, but a dynamic (moving) state (forward and back reactions are occurring)  Rate of forward reaction = Rate of back reaction 3

4.  Concentration of reactants and products is constant  No changes in macroscopic properties (colour intensity, pressure, pH) CONDITIONS FOR EQUILIBRIUM  Closed system. No gain or loss of reactants or products to or from the surroundings eg. Solutions, Sealed containers  Constant Temperature 4

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6.  Initially the rate of forward reaction is high because the concentration of reactants is high.  Rate decreases as [reactants] decreases  Rate of back reaction is zero until some products form.  Rate increases as [products] increases  Until at equilibrium rate of forward reaction = rate of back reaction 6

7.  If reactants A and B reacted to form products C and D this could be represented by the following reaction. ( a, b, c and d refer to mole ratio in which they react) aA + bB  cC + dD  At equilibrium the concentrations of these species are constant and can be used to calculate the equilibrium constant for the reaction. 7

8. 8  K c has no units  Constant value at constant temperature  [A] [B] etc represent concentrations in molL -1

9.  The size of K c indicates the yield of the products relative to the amount of reactants at equilibrium.  High K c values > 10 indicate a high yield of products while K c values < 0.1 indicate a low yield of products.  K c is only affected by temperature changes not changes in pressure or concentrations. 9

10.  Henry Le Châtelier was a French chemist.  Observed in 1888 how concentration changes, pressure changes and heat changes altered the position of the equilibrium. 10

11.  If the conditions of equilibrium are changed then the system will respond in such a way as to counteract the introduced changes, if that is possible.  “Reaction proceeds to partially oppose the change” 11

12.  Conditions: Constant volume, Constant temperature  If a reactant or product is added or removed, then the reaction will proceed in the direction to decrease/ increase the species that was altered. 12

13. Reactants  Products  [Reactant]  equilibrium moves to the right  [Product]  equilibrium moves to the left  [Reactant]  equilibrium moves to left  [Product]  equilibrium moves to right 13

14. 14 N 2 O 4  2NO 2 ΔH= +58kJmol -1

15.  Pressure is  to the total number of molecules in the gas phase.  Reduction in the total number of molecules lowers the internal pressure.  Increase in the total number of molecules raises the internal pressure.  The pressure of an equilibrium system can be changed by changing the volume of the system. 15

16. INCREASE PRESSURE  Decrease volume. Concentration of all species increases. Reaction proceeds to decrease pressure. Moves to reduce moles. DECREASE PRESSURE  Increase volume. Concentration of all species decreases. Reaction proceeds to increase pressure. Moves to increase moles. 16

17. 17 N 2 O 4  2NO 2 ΔH= +58kJmol -1

18.  If there are equal number of molecules on both sides then an equilibrium can not adjust to pressure changes. 18

19.  Quoted  H value always refers to the forward reaction.  If the forward reaction of an equilibrium system is exothermic, then the back reaction is endothermic (same value). 19

20.  Temperature increase causes equilibrium to shift in endothermic direction to absorb heat.  Temperature decrease causes equilibrium to shift in exothermic direction to release heat. 20

21. 21 N 2 O 4  2NO 2 ΔH= +58kJmol -1

22.  Temperature changes do affect K c EXOTHERMIC (Forward reaction)  If Temp increases K c decreases  If Temp decreases K c increases ENDOTHERMIC (Forward reaction)  If Temp increases K c increases  If Temp decreases K c decreases This means that calculated K c values are specific for a given temperature.

23.  Increase the rate of both forward and back reactions.  Equilibrium is reached more quickly.  Do not affect the position of the equilibrium. 23

24.  For the reaction at 273 o C; CO2(g) + H2(g)  H2O(g) + CO(g)  0.750 moles of H2 and 1.20 moles of CO2 were allowed to come to equilibrium in a 2.00L flask. At equilibrium 0.600 moles of CO was present.  Calculate Kc 24

25.  A 2.50L reaction flask containing 2.95 moles of nitrogen and 4.16 moles of hydrogen was allowed to come to equilibrium according to: N2(g) + 3H2(g)  2NH3(g)  At equilibrium 0.240 moles of ammonia was present. Verify that the equilibrium constant is 0.00232 (3 sf) under these conditions. 25