2. Oxidation and Reduction Click to start

3. Question 1 In the reaction MnO 2 (s) + 4HCl(aq)  Cl 2 (g) + MnCl 2 (aq) + 2H 2 O(l) HCl is the oxidising agent Cl 2 is the oxidation product H 2 O is the reduction product MnO 2 is the reducing agent

4. Wrong Answer! The formation of Cl 2 from HCl is an oxidation process. The oxidation number changes from 0 to -1.

5. Question 2 When MnO 4 - (aq) reacts in an acidic solution it produces MnO 2 MnO 4 2- Mn 2+ Mn 3+

6. Wrong Answer! The products of the reaction of manganate(VII) ion depend on pH. However, under acidic conditions, manganate(II) ions are produced. MnO 4 - + 8H + + 5e -  Mn 2+ + 4H 2 O

7. Question 3 Bromide ions are oxidized to bromine by all of the following except K 2 Cr 2 O 7 / H + Cl 2 KMnO 4 I2I2

8. Wrong Answer! I 2 is below Br 2 in group 7 and hence not sufficiently powerful to oxidise bromide ions to bromine molecules.

9. Question 4 All the following equations represent oxidation- reduction reactions except 2C 3 H 7 OH + 9O 2  6CO 2 +8H 2 O Ni + 4CO  Ni(CO) 4 Cl 2 + 2NaBr  2NaCl + Br 2 2Na + 2H 2 O  2NaOH + H 2

10. Wrong Answer! Ni complex is neutral and consists of a Ni atom and 4 CO molecules.

11. SO 3 (g) + 2H 2 O(l)  HSO 4 - (aq) + H 3 O + (aq) Question 5 Which of the following represents an oxidation- reduction reaction? I 2 (s) + 2OH - (aq)  I - (aq) + OI - (aq) + 8H 2 O(l) PO 4 3- (aq) + H 2 O(l)  HPO 4 2- (aq) + OH - (aq) Cu 2+ (aq) + H 2 S(aq)  CuS (s) + 2H + (aq)

12. Wrong Answer! The iodine undergoes a change in oxidation number from 0 to -1.

13. Question 6 Magnesium is a more reactive metal than copper. Which is the strongest oxidizing agent? Mg Mg 2+ Cu 2+ Cu

14. Wrong Answer! Metals that are weak reducing agents (electron donors) form ions that are strong oxidizing agents (electron acceptor).

15. Question 7 The following information is given about reactions involving the metals X, Y and Z and solutions of their sulfates. X(s) + YSO 4 (aq)  No reaction Z(s) + YSO 4 (aq)  Y(s) + ZSO 4 (aq) When the metals are listed in decreasing order of reactivity (most reactive first), what is the correct order? X > Y > Z Z > Y > X Y > Z > X Y > X > Z

16. Wrong Answer! X is less reactive than Y. Z is more reactive than Y.

17. Question 8 In which one of the following species does chlorine exhibit the highest oxidation number? Cl 2 O Cl 2 PCl 5 HClO 3

18. Wrong Answer! The oxidation number is +5

19. Question 9 Which statement is correct for the electrolysis of molten sodium chloride? Sodium ions move towards the positive electrode. A gas is produced at the negative electrode. Both sodium and chloride ions move toward the electrodes. Only electrons move in the electrolyte.

20. Wrong Answer! Ions move to electrodes opposite in sign to their charges.

21. Question 10 When a direct current of electricity is conducted by an aqueous solution of an electrolyte, which one of the followig statements is false? The movements of ions accounts for the current flow through the solution. During electrolysis, the solution remains electrically neutral. The number of positive ions moving toward one electrode is always equal to the number of negative ions moving toward the other electrode. Electrons flow from the current source to the solution at one electrode, and an equal number of electrons flows away from the solution at the other electrode.

22. Wrong Answer! Eectrons consumed at the cathode by reduction are supplied by the anode by the action of oxidation.

23. Question 11 Which of the following represents a redox reaction? Cu(NO 3 ) 2 (aq) + Na 2 S(aq)  CuS(s) +2NaNO 3 (aq) 2Na(s) + I 2 (s)  2NaI(s) H 2 SO 4 (aq) + 2HBr(g)  SO 2 (g) + 2H 2 O(l) + Br 2 (l) KH(s) + H 2 O(l)  KOH(aq) +H 2 (g)

24. Wrong Answer! Precipitation reactions do not involve change in oxidation number.

25. Question 12 In which of the following does the metal undergo a change in the oxidation state? I.2MnO 4 2- + F 2  2MnO 4 - + 2F - II.2CrO 4 2- + 2H +  Cr 2 O 7 2- + H 2 O III.[Fe(H 2 O) 6 ] 2+ + 6CN -  [Fe(CN) 6 ] 3- + 6H 2 O I only II only I and III only I and II only

26. Wrong Answer! The manganese changes oxidation number from +6 to +7. The chromium remains in oxidation number +6. The iron changes from +2 to +3.

27. Question 13 In acid solution, manganate(VII) ions, MnO 4 - (aq), undergo reduction to manganate(II) ions, Mn 2+ (aq). What amount of MnO 4 - (aq) is required to convert 5.36 x 10 -3 moles of the ion Y 2+ (aq) to YO 3 - (aq)? 8.93 x 10 -3 mol 5.36 x 10 -3 mol 1.07 x 10 -3 mol 3.22 x 10 -3 mol

28. Wrong Answer! MnO 4 - + 8H + + 5e -  Mn 2+ + 4H 2 O (multiply by 3) 3H 2 O + Y 2+  YO 3 - + 6H + + 3e - (multiply by 5) 3MnO 4 - + 24H + +15e - + 15H 2 O + 5Y 2+  3Mn 2+ +12H 2 O+ 5YO 3 - + 30H + + 15e - 3MnO 4 - +3H 2 O + 5Y 2+  3Mn 2+ + 5YO 3 - + 6H + Amount of MnO 4 - = 3/5 x amount of Y 2+ = 3.216 x 10 -3 mol

29. Question 14 The following reaction occurs in acid solution: __H + (aq) +__NO 3 - (aq) + I - (aq)  IO 3 - (aq)+ __NO 2 (g)+__H 2 O The equation is not balanced. What is the coefficient of NO 3 - in the balanced equation? 6 5 2 4

30. Wrong Answer! 6H + (aq) +6NO 3 - (aq) + I - (aq)  IO 3 - (aq)+6NO 2 (g)+6H 2 O

31. Question 15 Which one of the following equations represents the half-equation that occurs at the anode during the electrolysis of potassium iodide? K + + e -  K 2H 2 O  O 2 + 4H + + 4e - 2I -  I 2 + 2e - 2H 2 O + 2e -  H 2 +2OH -

32. Wrong Answer! The anode is positive and attracts negative ions. Oxidation occurs at the anode of an electrolytic cell.

33. Question 16 In the reaction 2MnO 2 + 4KOH + O 2 + Cl 2  2KMnO 4 + 4KCl + 2H 2 O the oxidizing agent is/are KMnO 4 only MnO 2 only O 2 and Cl 2 MnO 2 and O 2

34. Wrong Answer! Oxidizing agents undergo reduction during a redox reaction. The oxidation number of oxygen changes frm 0 to -2, the oxidation number of chlorine changes from 0 to -1.

35. Congratulations!