1. Unit 3 Redox reactions

2. Go to question 1 2 3 4 5 6 7 8 How many moles of I 2 are reduced by 1 mole of Cr 2 O 7 2- ions? Which of the following is not a redox reaction? In order to balance this ion-equation I – to IO 3 –. You need to ……. The balanced redox equation for the two ion-equations below is (you will need to look a you data book, page 11) What colour change takes place during this reaction? 25 cm 3 of 0.200 mol –1 potassium permanganate was titrated against a solution of Fe 2+ ions. The concentration of Fe 2+ ions was? The production of Aluminium during the electrolysis of aluminium oxide can be represented by ………. A solution of HCl is electrolysed. What current is needed to produce 4.8 l of H 2 gas in 3 min 13 sec? 1 mole of gas occupies 24 l.

3. a. 4 b. 3 c. 2 d. 1 Cr 2 O 7 2- (aq) + 14H + (aq) + 6e2Cr 3+ (aq) + 7H 2 O (l) I 2 (aq) + 2e - 2I - (aq) How many moles of I 2 are reduced by 1 mole of Cr 2 O 7 2- ions?

4. 1 st hint The number of electrons on both sides of the redox equation must balance. a hint!!!! 2 nd hint How many more moles of iodine are now needed?

5. Ans: b. 3 How many moles of I 2 are reduced by 1 mole of Cr 2 O 7 2- ions? Correct because….. The number of electrons lost and gained must be equal. Cr 2 O 7 2- (aq) + 14H + (aq) + 6e2Cr 3+ (aq) + 7H 2 O (l) I 2 (aq) + 2e - 2I - (aq) X 3 Cr 2 O 7 2- (aq) + 14H + (aq) + 3I 2 (aq)2Cr 3+ (aq) + 7H 2 O (l) + 6I - (aq)

6. Which of the following is not a redox reaction. a. Mg (s) + 2HCl (aq) MgCl 2 (aq) + H 2 (g) b. CuCO 3 (s) + H 2 SO 4 (aq) CuSO 4 (aq) + CO 2 (g) + H 2 O (l) c. Zn(NO 3 ) 2 (aq) + 2Ag (s) 2AgNO 3 (aq) + Zn (s) d. H 2 (g) + Cl 2 (g) 2HCl (g)

7. Which reaction does not involve the formation of ions.? a hint!!!!

8. Which of the following is not a redox reaction. Correct because…… A redox reaction involves the transfer of electrons between reactants. a. Mg (s) + 2HCl (aq) MgCl 2 (aq) + H 2 (g) Mg + 2H +  Mg 2+ + H 2 b. CuCO 3 (s) + H 2 SO 4 (aq) CuSO 4 (aq) + CO 2 (g) + H 2 O (l) CO 3 2- + 2H +  CO 2 + H 2 O c. Zn(NO 3 ) 2 (aq) + 2Ag(s) 2AgNO 3 (aq) + Zn (s) Zn 2+ + 2Ag  2Ag + + Zn Ans: d. H 2 (g) + Cl 2 (g) HCl (g)

9. In order to balance this ion-equation I – to IO 3 – You need to a. b. c. d. Number H + Number of e - 63 36 33 66

10. 1 st hint Balance the oxygen by adding water molecules. a hint!!!! 2 nd hint Balance the hydrogen by adding H + ions. 3 rd hint Balance the charges by adding electrons

11. In order to balance this ion-equation I – to IO 3 – Correct because……… 1 st balance the O using water molecules I – + 3H 2 O  IO 3 – 2 nd balance the H using H + ions. I – + 3H 2 O  IO 3 – + 6H + 3 rd balance the charges by adding electrons I – + 3H 2 O  IO 3 – + 6H + + 6e -

12. The balanced redox equation for the two ion-equations below is (you will need to look a your data book, page 11) a. 2Fe (s) + 3SO 4 2- (aq) 2Fe 3+ (aq) + 3SO 3 2- (aq) b. 2Fe (s) + 3SO 4 2- (aq) + 6H + (aq) 2Fe 3+ (aq) + 3SO 3 2- (aq) + 3H 2 (g) c. 2Fe (s) + 3SO 4 2- (aq) + 6H + (aq) 2Fe 3+ (aq) + 3SO 3 2- (aq) + 3H 2 O (l) d. 3Fe (s) + SO 4 2- (aq) + 6H + (aq) Fe 3+ (aq) + SO 3 2- (aq) + 3H 2 O (l) SO 4 2- (aq) SO 3 2- (aq) Fe (s) Fe 3+ (aq)

13. a hint!!!! 1 st hint SO 4 2- (aq) + 2H + (aq) + 2e - SO 3 2- (aq) + H 2 O (l) 2 nd hint Fe (s) Fe 3+ (aq) + 3e -

14. The balanced redox equation for the two ion-equations below is (you will need to look a your data book, page 11) SO 4 2- (aq) SO 3 2- (aq) Fe (s) Fe 3+ (aq) SO 4 2- (aq) + 2H + (aq) + 2e - SO 3 2- (aq) + H 2 O (l) Correct because…… Fe (s) Fe 3+ (aq) + 3e - X 2 3SO 4 2- (aq) + 6H + (aq) + 2Fe (s) 3SO 3 2- (aq) + 3H 2 O (l) + 2Fe 3+ (aq) X 3 Ans: C

15. What colour change takes place during this reaction? a. Blue to colourless b. Colourless to purple c. Colourless to blue d. Purple to colourless 5 Fe 2+ (aq) + 8H + (aq) + MnO 4 - (aq)  5 Fe 3+ (aq) + Mn 2+ (aq) + 4H 2 O(l)

16. a hint!!!! The permanganate ion is purple.

17. What colour change takes place during this reaction? Correct because……….. The MnO 4 - (aq) ion is purple, while the Mn 2+ (aq) ion is colourless. 5 Fe 2+ (aq) + 8H + (aq) + MnO 4 - (aq)  5 Fe 3+ (aq) + Mn 2+ (aq) + 4H 2 O(l)

18. 25 cm 3 of 0.200 mol l –1 potassium permanganate was titrated against a solution of Fe 2+ ions. What was the concentration of Fe 2+ ions? 5 Fe 2+ (aq) + 8H + (aq) + MnO 4 - (aq)  5 Fe 3+ (aq) + Mn 2+ (aq) + 4H 2 O(l) Titration results, 20.5 cm 3, 20.1 cm 3 and 20.0 cm 3 a. 1.25 mol l -1 b. 1.24 mol l -1 c. 0.250 mol l -1 d. 1.00 mol l -1

19. Have you used concordant titration results? a hint!!!!

20. n = V x C and a 5:1 ratio of moles of Fe 2+ to MnO 4 - a hint!!!!

21. V x x C x n x = V y x C y n y

22. 25 cm 3 of 0.200 mol l –1 potassium permanganate was titrated against a solution of Fe 2+ ions. The concentration of Fe 2+ ions was? 5 Fe 2+ (aq) + 8H + (aq) + MnO 4 - (aq)  5 Fe 3+ (aq) + Mn 2+ (aq) + 4H 2 O(l) Titration results, 20.5 cm 3, 20.1 cm 3 and 20.0 cm 3 V x x C x n x = V y x C y n y X = MnO 4 - Y = Fe 3+ (20.1+20.0)/2 + Y 25 x 0.200 1 5 = Ans : b. 1.24 mol l -1

23. The production of aluminium during the electrolysis of aluminium oxide can be represented by a. 96,500 C b. 19300 C c. 57,900 C d. 193,000 C What is the quantity of electricity needed to produce 0.2 moles of Al? Al 3+ + 3e Al

24. 1 st hint How many moles of electrons are needed to deposit 1 mol of Al? a hint!!!! 2 nd hint The charge on a mole of electrons is 96,500 C?

25. The production of aluminium during the electrolysis of aluminium oxide can be represented by What is the quantity of electricity needed to produce 0.2 moles of Al? Correct because…….. To produce 1 mole of aluminium, 3 moles of electrons are needed. i.e. 96,500 C x 3 = 289,500 C needed. Al 3+ + 3e Al 1 mole Al = 289,500 C 0.2 mole Al = 289,500 x 0.2 = 57,900C

26. A solution of HCl is electrolysed. What current is needed to produce 4.8 litres of H 2 gas in 3 min 13 sec? 1 mole of gas occupies 24 litres. a. 200 A b. 50 A c. 100 A d. 20 A

27. 1 st hint How many moles of hydrogen does 4.8 litres represent? a hint!!!! 2 nd hint How many moles of electrons do you need to deposit 1 mole of hydrogen gas? Q = I x t

28. A solution of HCl is electrolysed. What current is needed to produce 4.8 l of H 2 gas in 3 min 13 sec? 1 mole of gas occupies 24 l ? Correct because…….. 2H + (aq) + 2e -  H 2 (g) So to produce a mole of hydrogen gas, 2 moles of electrons are needed. No of moles of H 2 gas porduced = 4.8/24 mol = 0.2 mol of gas produced. 1 mol of gas would need 96500 x 2 C 0.2 mol would need (96500 x 2) x 0.2 C = 38,600 C Quantity of charge = current x time Q = I x t So I = Q/t = 38600 / 193 = 200 A