1. Chemical equilibrium L.O.:  Use Kc (the equilibrium constant ) to work out the composition of an equilibrium mixture.

2. for an equilibrium reaction of the form... aA + bB cC + dD then (at constant temperature) [C] c. [D] d = a constant, (K c ) [A] a. [B] b where [ ] denotes the equilibrium concentration in mol dm -3 K c is known as the Equilibrium Constant THE EQUILIBRIUM CONSTANT K c

3. VALUE OF K c AFFECTED bya change of temperature NOT AFFECTED bya change in concentration of reactants or products a change of pressure adding a catalyst for an equilibrium reaction of the form... aA + bB cC + dD then (at constant temperature) [C] c. [D] d = a constant, (K c ) [A] a. [B] b where [ ] denotes the equilibrium concentration in mol dm -3 K c is known as the Equilibrium Constant THE EQUILIBRIUM CONSTANT K c

4. 2A(g) + B(g) 2C (g) At equilibrium there were found to be 3 × 10 -3 mol of A, 2 × 10 -3 mol of B and 6.2 × 10 -3 mol of C with a total volume of 2 dm 3. Calculate Kc

5. Example 2 Consider the equilibrium P + 2Q R + S (all species are aqueous) One mole of P and one mole of Q are mixed. Once equilibrium has been achieved 0.6 moles of P are present. How many moles of Q, R and S are present at equilibrium ? P + 2Q R + S Initial moles 1 1 0 0 At equilibrium 0·6 0·2 0·4 0·4 (0·4 reacted) (2 x 0·4 reacted) (get 1 R and 1 S for every P that reacts) 1- 0·6 remain 1- 0·8 remain Explanation if 0.6 mol of P remain of the original 1 mol, 0.4 mol have reacted the equation states that 2 moles of Q react with every 1 mol of P this means that 0.8 (2 x 0.4) mol of Q have reacted, leaving 0.2 mol one mol of R and S are produced from every mol of P that reacts this means 0.4 mol of R and 0.4 mol of S are present at equilibrium CALCULATIONS INVOLVING K c

6. Calculating amounts at equilibrium A2.CHEM4.2.001.doc A2.CHEM4.2.002.doc

7. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units CALCULATIONS INVOLVING K c

8. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CALCULATIONS INVOLVING K c

9. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CH 3 COOH(l) + C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l) + H 2 O(l) CALCULATIONS INVOLVING K c

10. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CH 3 COOH(l) + C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l) + H 2 O(l) moles (initially) 1 1 0 0 moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 Initial moles of CH 3 COOH = 1 moles reacted = 2/3 equilibrium moles of CH 3 COOH = 1/3 For every CH 3 COOH that reacts; a similar number of C 2 H 5 OH’s react (equil moles = 1 - 2/3) a similar number of CH 3 COOC 2 H 5 ’s are produced a similar number of H 2 O’s are produced CALCULATIONS INVOLVING K c

11. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CH 3 COOH(l) + C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l) + H 2 O(l) moles (initially) 1 1 0 0 moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V V = volume (dm 3 ) of the equilibrium mixture CALCULATIONS INVOLVING K c

12. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CH 3 COOH(l) + C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l) + H 2 O(l) moles (initially) 1 1 0 0 moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V V = volume (dm 3 ) of the equilibrium mixture K c = [CH 3 COOC 2 H 5 ] [H 2 O] [CH 3 COOH] [C 2 H 5 OH] CALCULATIONS INVOLVING K c

13. construct the balanced equation, including state symbols (aq), (g) etc. determine the number of moles of each species at equilibrium divide moles by volume (in dm 3 ) to get the equilibrium concentrations in mol dm -3 (If no volume is quoted, use a V; it will probably cancel out) from the equation constructed in the first step, write out an expression for K c. substitute values from third step and calculate the value of K c with any units Example 1 One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc. CH 3 COOH(l) + C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l) + H 2 O(l) moles (initially) 1 1 0 0 moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V V = volume (dm 3 ) of the equilibrium mixture K c = [CH 3 COOC 2 H 5 ] [H 2 O] = 2/3 / V. 2/3 / V = 4 [CH 3 COOH] [C 2 H 5 OH] 1/3 / V. 1/3 / V CALCULATIONS INVOLVING K c

14. A(g) + B(g) C(g) 0.50 mol of A and 0.40 mol of B were mixed in a 10 dm 3 container. At equilibrium there were 0.2 mol of A. What is the value of Kc?

15. Finding equilibrium concentrations from Kc values For the following equilibrium system, Kc = 0.1 X(g) Y(g) If 10 moles of X were allowed to reach equilibrium, how many moles of Y would there be in the equilibrium mixture?

17. 4.2 Exercise 1 - Kc Q 4-11 Note: you can find similar examples in Q4 of chem factsheet no 21

18. Chemical equilibrium L.O.:  Use Kc (the equilibrium constant ) to work out the composition of an equilibrium mixture.