H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 16 Dynamic Equilibrium 16.1Irreversible and Reversible Reactions 16.2Dynamic Nature of Chemical Equilibrium 16.3Characteristics of Chemical Equilibrium 16.4Equilibrium Law 16.5Determination of Equilibrium Constants

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Equilibrium Constant in Terms of Partial Pressures Pressures 16.7 Equilibrium Position 16.8 Significances of Equilibrium Constant 16.9Factors Affecting Equilibria

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 3 Reversible Reactions A + B C + D Remarks 1. A reversible reaction is one which does not proceed to completion. 2. A + B  C + D is called the forward reaction while C + D  A + B is called the backward reaction. 3. A and B are called the ‘reactants’ while C and D are called the ‘products’ Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 4 Br 2 (l) 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 5 Br 2 (l) 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 6 Br 2 (l) 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 7 Br 2 (l) 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 8 Br 2 (l) 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 9 Br 2 (l)Br 2 (g) An equilibrium is reached when forward rate = backward rate Br 2 (l)Br 2 (g)Br 2 (l)Br 2 (g) net Remarks 1. Note that there is a net change to the right from the start to the equilibrium. 2. Note that the equilibrium is a dynamic one. An Example of Physical Equilibrium 16.1 Irreversible and Reversible Reactions (SB p.90)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 10 Thermal Decomposition of Calcium Carbonate 16.1 Irreversible and Reversible Reactions (SB p.91) heating cooling CaCO 3 (s) CaO(s) + CO 2 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 11 Esterification 16.1 Irreversible and Reversible Reactions (SB p.91) CH 3 COOH(l) + CH 3 CH 2 OH(l) CH 3 COOCH 2 CH 3 (l) + H 2 O(l)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 12 Dynamic Equilibrium H 2 (g) + I 2 (g) \ == \ 2HI(g) 16.2 Dynamic Nature of Chemical Equilibrium (SB p.92)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 13 The changes in the rates of forward and backward reactions 16.2 Dynamic Nature of Chemical Equilibrium (SB p.93)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 14 A chemical equilibrium can only be achieved in a closed system. A chemical equilibrium can be attained from either side. A chemical equilibrium is dynamic in nature. The concentration of all species remain constant after equilibrium is reached (unless the equilibrium is distributed). A catalyst can shorten the time to reach equilibrium but it cannot increase the yield of the product Characteristics of Chemical Equilibrium (SB p.94)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Characteristics of Chemical Equilibrium (SB p.94)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 16 The Equilibrium system (Constant at a fixed temperature) Equilibrium constant Remark: Sometimes the expression ‘eqm’ is omitted for simplicity Equilibrium Law (SB p.97) aA + bB cC + dD

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 17 K c is independent of initial concentrations. It is a constant at a fixed temperature. The Equilibrium System of Hydrogen, Iodine and Hydrogen Iodide 16.4 Equilibrium Law (SB p.98) H 2 (g) + I 2 (g) 2HI(g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 18 The Equilibrium System of Ethanoic Acid, Ethanol, Ethyl Ethanoate and Water CH 3 COOH(aq) + CH 3 CH 2 OH(aq)  CH 3 COOCH 2 CH 3 (l) + H 2 O(l) Experimental determination of K C 16.5 Determination of Equilibrium Constants (SB p.100)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 19 Procedures: 1. Known amounts of CH 3 COOH & CH 3 CH 2 OH are mixed with a small of conc. H 2 SO 4 (catalyst). 2. The mixture is heated under reflux for hrs. until eqm. is reached. 3. By titrating with standard NaOH(aq), the amount of CH 3 COOH left (and hence the amount of other reactants and products) can be determined. CH 3 COOH(aq) + CH 3 CH 2 OH(aq)  CH 3 COOCH 2 CH 3 (l) + H 2 O(l) NaOH(aq) Amount of CH 3 COOH in eqm. mixture can be found. Eqm. mixture after reflux 16.5 Determination of Equilibrium Constants (SB p.100)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 20 CH 3 COOH(aq) + CH 3 CH 2 OH(aq)  CH 3 COOCH 2 CH 3 (l) + H 2 O(l) 16.5 Determination of Equilibrium Constants (SB p.100) Reactant/ Product Experiment 1 Experiment 2 Amount before reaction (mol) Amount after reaction (mol) Amount before reaction (mol) Amount after reaction (mol) CH 3 COOH CH 3 CH 2 OH H 2 SO 4 CH 3 COOCH 2 CH 3 H 2 O Results

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Determination of Equilibrium Constants (SB p.100) CH 3 COOH(l) + CH 3 CH 2 OH(l) CH 3 COOCH 2 CH 3 (l) + H 2 O(l) Using the equilibrium expression: Kc = For experiment 1:

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Determination of Equilibrium Constants (SB p.100) For experiment 2: Average K c = = = ( no unit)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 23 Determination of the Equilibrium Constant for a Redox Reaction Write K c for the system. Note that the concentration of a pure solid (or a pure liquid) like Ag(s) is a constant. related to the density of silver Thus we have 16.5 Determination of Equilibrium Constants (SB p.102) Fe 2+ (aq) + Ag + (aq) Fe 3+ (aq) + Ag(s)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 24 Step 1 To prepare some Fe(NO 3 ) 2 solution. FeSO 4 (aq) + Ba(NO 3 ) 2 (aq)  Fe (NO 3 ) 2 (aq) + BaSO 4 (s) 16.5 Determination of Equilibrium Constants (SB p.102)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 25 Step 2 To set up the equilibrium system. [Ag + (aq)] eqm = ? Fe 2+ (aq) + Ag + (aq) \ == \ Fe 3+ (aq) + Ag(s) 16.5 Determination of Equilibrium Constants (SB p.102) 25 cm 3

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 26 Step 3 Find out [Ag + (aq)] eqm by titration. Fe 3+ in the mixture serves as an indicator When all Ag + has been reacted, excess CNS - reacts with Fe 3+ to form red complex FeCNS Determination of Equilibrium Constants (SB p.102) Ag + (aq) + CNS - (aq) AgCNS(aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Determination of Equilibrium Constants (SB p.103) Calculations: Number of moles of KCNS(aq) = 0.10 mol dm -3 x 6.80 x dm 3 = 6.8 x mol ∴ Number of moles of Ag + (aq) in 25 cm 3 of reaction mixture at equilibrium = 6.8 x mol Since Fe 2+ (aq) and Ag + (aq) are consumed at the same rate, we have [Fe 2+ (aq)] eqm = [Ag + (aq)] initial = mol dm -3 Ag + (aq) + CNS - (aq)  AgCNS(aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 28 Fe 2+ (aq) + Ag + (aq) \ == \ Fe 3+ (aq) + Ag(s) [Fe 3+ (aq)] eqm = [Fe 2+ (aq)] initial – [Fe 2+ (aq)] eqm = 0.05 – = mol dm Determination of Equilibrium Constants (SB p.103) Kc = = 30.8 mol -1 dm 3

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 29 PV = nRT P/RT = n/V P = RT[gas] ( or P  [gas] at constant temp ) aA(g) + bB(g) \ === \ cC(g) + dD(g) Consider an equilibrium system involving gases Do you think the value of K p is equal to that of K C.? They may not be equal Equilibrium Constant in Terms of Partial Pressures (SB p.106) Ideal Gas Equation

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 30 Equilibrium Position Consider N 2 (g) + 3H 2 (g) \ === \ 2NH 3 (g) K c is fixed at a fixed temp Equilibrium Position (SB p.109) Expt.Initial concentration (mol dm -3 ) Equilibrium concentration (mol dm -3 ) K c at 500 o C (dm 6 mol -2 ) [N 2 (g)][H 2 (g)][NH 3 (g)][N 2 (g)][H 2 (g)][NH 3 (g)] Kc

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 31 K c (or K p ) can give information on the extent of reaction (how far a chemical reaction has gone towards completion). Large K c  eqm. position mainly lies towards product side Small K c  eqm. position mainly lies towards reactant side (‘Irreversible reactions’ are just reactions with extremely large K c values.) 16.8 Significances of Equilibrium Constant (SB p.111) aA + bB cC + dD Extent of Reaction

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 32 Factors Affecting Equilibrium Equilibrium System Something done on it (Eqm. disturbed) net change New Equilibrium System Has a different eqm. position when compared with the original eqm. Conc. change Pressure change Temp. change Adding catalyst 16.9 Factors Affecting Equilibria (SB p.113)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 33 Suddenly the conc. of O 2 is increased (eqm. disturbed) The Effects of Changes in Concentration on Equilibrium Original eqm. (forward rate = backward rate) forward rate > backward rate net r’x to the right New eqm. (forward rate = backward rate again) The equilibrium position has shifted to the right (i.e. there is an increase in the proportion of the products in the new equilibrium) Factors Affecting Equilibria (SB p.113) 2SO 2 (g) + O 2 (g) 2SO 3 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Factors Affecting Equilibria (SB p.114) original eqm net change to the right new eqm

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 35 Example of Predicting equilibrium Position Change by Le Chatelier’s Principle If a system in equilibrium is subjected to a change (concentration, pressure or temperature), the equilibrium position of the system will shift in a way to minimize the effect of the change. Suddenly the conc. of O 2 is increased (eqm. disturbed) net r’x to the right (until a new eqm is reached) 16.9 Factors Affecting Equilibria (SB p.113) Le Chatelier’s Principle 2SO 2 (g) + O 2 (g) 2SO 3 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 36 Volume kept constant Effect of Changes in Pressure on Equilibrium 3 ways to increase the pressure of a gaseous system: 1. Addition of a gaseous reactant or product when the volume of the container is kept constant. 1. Addition of a gaseous reactant or product when the volume of the container is kept constant. reactant / product 16.9 Factors Affecting Equilibria (SB p.115)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 37 Volume kept constant Effect of Changes in Pressure on Equilibrium 3 ways to increase the pressure of a gaseous system: 2. Addition of an ‘inert gas’ when the volume of the container is kept constant. An ‘inert’ gas 16.9 Factors Affecting Equilibria (SB p.116)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Changing the volume of the container. 3 ways to increase the pressure of a gaseous system: 16.9 Factors Affecting Equilibria (SB p.116) Effect of Changes in Pressure on Equilibrium

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Changing the volume of the container Factors Affecting Equilibria (SB p.116) Effect of Changes in Pressure on Equilibrium

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Factors Affecting Equilibria (SB p.116) 3. Changing the volume of the container. Effect of Changes in Pressure on Equilibrium

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Changing the volume of the container Factors Affecting Equilibria (SB p.116) Effect of Changes in Pressure on Equilibrium

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increasing the pressure by the addition of a gaseous reactant or product aA(g) + bB(g) cC(g) + dD(g) Example Increasing the pressure by the addition of A(g) is equivalent to increasing the concentration of reactant A. Hence the equilibrium position will shift (to the right in this case). But the is NO change in the K p value as there is no change in the temperature. (Work by yourself for the effect of decreasing the pressure by the removal of a gaseous reactant or product.) Effect of Changes in Pressure on Equilibrium 16.9 Factors Affecting Equilibria (SB p.116)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increasing the pressure by the addition of an inert gas Increasing the pressure by the addition of an inert gas has no effect on the partial pressure (and hence the concentration) of any reactant gas or any product gas. Hence there is NO change in the equilibrium position even. Effect of Changes in Pressure on Equilibrium 16.9 Factors Affecting Equilibria (SB p.116)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increasing the pressure by decreasing the volume of the container N 2 (g) + 3H 2 (g) 2NH 3 (g) Case 1 (the total no. of gas molecules at the ‘reactant’ side is higher) By Le Chatelier’s principle, increasing the pressure (by decreasing the volume) will cause the equilibrium position shifted to the right (so as to reduce the pressure). 1 mole 3 mole 2 moles There is a contraction in volume for the forward reaction & an expansion in volume for the backward reaction. (Work by yourself for the effect of decreasing the pressure by increasing the volume of the container.) 16.9 Factors Affecting Equilibria (SB p.116) Effect of Changes in Pressure on Equilibrium

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increasing the pressure by decreasing the volume of the container N 2 O 4 (g) 2NO 2 (g) Case 2 (the total no. of gas molecules at the ‘reactant’ side is lower) By Le Chatelier’s principle, increasing the pressure (by decreasing the volume) will cause the equilibrium position shifted to the left (so as to reduce the pressure). 1 mole 2 moles There is an expansion in volume for the forward reaction & a contraction in volume for the backward reaction. (Work by yourself for the effect of decreasing the pressure by increasing the volume of the container.) Remark N 2 O 4 is yellow NO 2 is brown 16.9 Factors Affecting Equilibria (SB p.116)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increasing the pressure by decreasing the volume of the container H 2 (g) + I 2 (g) 2HI (g) Case 3 (the total no. of gas molecules at the two sides are equal) By Le Chatelier’s principle, increasing the pressure (by decreasing the volume) will not cause any change in the equilibrium position. 1 mole 1 mole 2 moles There is no contraction or expansion in volume for both the forward reaction or the backward reaction. (Work by yourself for the effect of decreasing the pressure by increasing the volume of the container.) 16.9 Factors Affecting Equilibria (SB p.116)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 47 Effect of Changes in Temperature on Equilibrium 16.9 Factors Affecting Equilibria (SB p.120) Exothermic (  H=-ve) Endothermic (  H=+ve) In K = constant In which K is the eqilibrium constant, ΔH is the enthalpy change of reaction, R is the gas constant (R = 8.31 J K -1 mol -1 ), T is the temperature in Kelvin Even K c (or K p ) will vary with the variation in the temperature.

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 48 For an exothermic reaction, T   ln K  (and thus K  ) Thus T   eqm position shifted to the left reactants \ ==== \ products  H = -ve T   Both forward & backward r’x rate  (backward rate  more) As both forward & backward rate , eqm can be attained in a shorter time Factors Affecting Equilibria (SB p.120) Exothermic (  H = -ve)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 49 For an enothermic reaction, T   ln K  (and thus K  ) Thus T   eqm position shifted to the right T   Both forward & backward r’x rate  (forward  more) As both forward & backward rate , eqm can be attained in a shorter time Factors Affecting Equilibria (SB p.120) Endothermic (  H = +ve) reactants products  H = +ve

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 50 Can this be predicted by the Le Chatelier’s principle? For an exothermic reaction, T   eqm position shifted to the left For an endothermic reaction, T   eqm position shifted to the right An increase in temperature will cause the equilibrium position to shift a direction to absorb heat (i.e. to the left). (Work by yourself for the effect of increasing the temperature on the equilibrium position of an endothermic reaction.) 16.9 Factors Affecting Equilibria (SB p.120) reactants products  H = -ve For an exothermic reaction

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 51 Summary on Various Effect on Equilibrium 16.9 Factors Affecting Equilibria (SB p.123) FactorEquilibrium positionEquilibrium constant Increase in concentration of A or B Shifts to rightNo change Increase in concentration of C or D Shifts to leftNo change Increase in pressure by reducing the volume of the container Shifts to right if (c + d) < (a + b) Shifts to left if (a + b) < (c + d) No change if a + b = c + d No change aA + bB cC + dD

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Factors Affecting Equilibria (SB p.123) Increase in temperatureShifts to right if the forward reaction is endothermic Shifts to left if the forward reaction is exothermic Endothermic reaction: increased Exothermic reaction: decreased Addition of a catalystNo change aA + bB cC + dD Summary on Various Effect on Equilibrium FactorEquilibrium positionEquilibrium constant

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 53 The END