Page 1

Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Skills- Balancing half equations of reduction and oxidation 1. Part A: Balance the following half equations (Hint: e- on LHS)

Steps for balancing half equations: 1. 2. 3. 4.

write down the product on RHS balance O by adding H2O balance back H by adding H+ balance the charge by adding echeck for balance or not: charge & element

(a)

MnO4-(aq)  Mn2+(aq)

(b)

Cl2(aq)  Cl-(aq)

(c)

Cr2O72-(aq)  Cr3+(aq)

(d)

Br2(aq)  Br-(aq)

(a)

H2(g)  H+(aq)

(e)

Ag+(aq)  Ag(s)

(b)

SO32-(aq)  SO42-(aq)

(f)

NO3-(aq)  NO2(g)

(c)

SO2(aq)  SO42-(aq)

(g)

Fe3+(aq)  Fe2+(aq)

(d)

Cu(s)  Cu2+(aq)

(h)

I2(aq)  I-(aq)

*(e)

*(i)

O2(g) + H2O(l)  4OH-(aq)

5.

Part B: Balance the following half equations (Hint: e- on RHS)

4OH-(aq)  O2(g) + H2O(l)

(f)

I-(aq)  I2(g)

(j)

Cu2+(aq)  Cu(s)

(g)

Fe2+(aq)  Fe3+(aq)

(k)

SO42-(aq)  SO2(g)

(h)

Br-(aq)  Br2(g)

(l)

H+(aq)  H2(g)

(i)

Cl-(aq) 

Cl2(g)

Information: Possible chemicals containing the ions (reactants side) above:

(k)

Acidified potassium permanganate KMnO4/H+ Chlorine gas or solution Cl2 Acidified potassium dichromate K2Cr2O7/H+ Bromine solution Br2 Silver nitrate solution AgNO3 Concentrated nitric acid HNO3 Iron(III) sulphate solution Fe2(SO4)3 Iodine solution I2 Oxygen fuel gas O2 Copper(II) sulphate solution CuSO4 Concentrated sulphuric acid H2SO4

(l)

Water / any acids

(a) (b) (c) (d) (e) (f) (g) (h) (i) (j)

(d)

Hydrogen fuel gas H2 Sodium sulphite solution Na2SO3 Sulphur dioxide gas SO2 Copper metal Cu

(e)

Water / any alkalis

(a) (b) (c)

(f) (g) (h) (i)

Potassium iodide solution KI Iron(II) sulphate solution FeSO4 Lead(II) bromide liquid PbBr2 Concentrated sodium chloride solution NaCl

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 1


Topic VII – Redox, Chemical cell & Electrolysis (Part A) 2.

by Ricky Tsui

Write down a half ionic equation for the following chemicals: (It may lose e- or gain e-)

Lost electrons  + ee- on RHS

(a) Acidified potassium permanganate solution 

Gain electron + e-  e- on LHS

(b) Copper solid  (c) Acidified sodium dichromate solution  (d) Hydroxide ions  (e) Hydrogen ions  (f) Iron(II) sulphate solution  (g) Silver nitrate solution 

More about Rule (4) and (5)… Find the O.N. of the underlined species.

e.g. Rule (4) H2S x = -2 3. NH3 x=

2(+1) + x = 0  O.N. of S is -2  O.N. of N is ___

e.g. Rule (5) HCO3x = +4 4. Cr2O72x=

(+1) + x + 3(-1) = -1  O.N. of C is +4  O.N. of Cr is ___

Skills- Assigning the oxidation number (O.N.) of an atom

Rules for assigning oxidation numbers: (Exception: hydride and O in peroxide) Rule (1): O.N. of free element = 0

Rule (2): some elements have fixed O.N. in their compounds

Rule (3): simple ion, O.N. equals to its charge

Na, Mg, N2, S8, Ar

= 0

All Gp. I All Gp. II F H O

= = = = =

Na+ Cr3+ ClO2-

= =

+1 +2 -1 +1 -2

+1 +3 = -1 = -2

Rule (4): sum of O.N.in a neutral compound = 0

NaCl H2O

(+1)+(-1) = 0 2(+1)+(-2) = 0

Rule (5): sum of O.N. in a polyatomic ion = charge of that ion

CO32NH4+

(+4)+3(-2) = -2 (-3)+4(+1) = -1

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

5. What is the oxidation number of carbon in CaCO3? A. 4 B. 0 C. +4 D. +5

6. Manganese has an oxidation number of +6 in

7. Which of the following underlined species have an oxidation number of +2?

8. Which of the following statements concerning oxidation number are correct?

(1) PbO (2) Mg (3) [CuCl4]2 A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

9. In which of the following substances does sulphur exhibit the highest oxidation number? A. B. C. D.

Na2SO4 SO2 Na2SO3 S8

11. In which of the following compounds is the oxidation number of oxygen NOT equal to 2? A. B. C. D.

MgO H2O CO F2O

A.MnO2. B. Mn3O4. C. MnO42. D.MnO4.

(1) The oxidation number of a free element is zero. (2) The oxidation numbers of Group I elements in all of their compounds are +1. (3) The oxidation number of oxygen in all of its compounds is 2. A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

10. In which of the following substances does nitrogen exhibit the lowest oxidation number? A. B. C. D.

NaNO2 HNO3 NH4Cl N2

12. What are the oxidation numbers of Cr in the following three chromium-containing species? Cr2O72, CrO42, CrO3 A. B. C. D.

+7, +4, +3 +6, +6, +6 2, 2, 0 +7, +3, +3

13. What is the oxidation number of Mn in KMnO4? A. +4 B. +5 C. +6 D. +7

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Topic VII – Redox, Chemical cell & Electrolysis (Part A)

Some elements have variable oxidation no.

[12DSE/MC#6] (hint: NH3: no charge, Cl-) 14. What is the oxidation number of Cu in Cu(NH3)4Cl2?

N NO3-

A. B. C. D.

0 +2 +4 +6

by Ricky Tsui

NO2 NO2NO N2 NH3

S H2S2O7 SO42SO3 SO32SO2 S8 H2S

+5 +4 +3 +2 0 -3

Fe +6 +6 +6 +4 +4 0 -2

FeCl3 FeCl2 Fe

+3 +2 0

[10CE/MC#11] 15. In which of the following species does the underlined element have an oxidation number of +3? A. B. C. D.

O3 H3PO4 NH4+ NO2-

[09CE/MC#2] 16. In which of the following reactions does nitrogen exhibit three different oxidation numbers in the species involved?

A. B. C. D.

NH4NO3 → N2O + 2H2O 8NH3 + 3 Cl2 → 6NH4Cl + N2 Mg + 4HNO3 → Mg(NO3)2 + 2NO2 + 2H2O 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O

A. B. C. D.

[08CE/MC#5] 18. In which of the following compounds is the oxidation number of nitrogen lowest? A. B. C. D.

NH4Cl NaNO2 NH2OH Pb(NO3)2

[01CE/MC#29] (hint: OH-) 20. The oxidation number of lead in [Pb(OH)4]2- is A. B. C. D.

[04CE/MC#7] 17. In which of the following compounds does sulphur exhibit the lowest oxidation number?

-2 +2 +4 +6

Na2S2O3 MgSO4 KHSO3 H2S2O7

[06CE/MC#3] (hint: NH3: no charge, Cl-) 19. What is the oxidation number of cobalt in Co(NH3)4Cl2? 2 0 +2 +6

A. B. C. D.

[00CE/MC#30] 21. In which of the following substances does nitrogen has the smallest oxidation number? A. B. C. D.

NH3 NO N2O N2

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Skills- To identify a reaction is REDOX reaction or not? (a) (b) (c) (d)

MnO4-(aq) + 8H+(aq) + 5e-  Mn2+(aq) + 4H2O(l) Cl2(g) + 2e-

Try to find out the oxidation number for the underline atoms. Lost electrons  + ee- on RHS

2Cl-(aq)

Cr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+(aq) + 7H2O(l) Br2(aq) + 2e-

2Br-(aq)

Gain electron + e-  e- on LHS

(a)

H2(g)  2H+(aq) + 2e-

(e)

Ag+(aq) + e-  Ag(s)

(b)

SO32-(aq) + H2O(l)  SO42-(aq) + 2H+(aq) + 2e-

(f)

NO3-(aq) + 2H+(aq) + e- NO2(g) + H2O(l)

(c)

SO2(aq) + 2H2O(l) 

(g)

Fe3+(aq) +e-  Fe2+(aq)

(d)

(h)

I2(aq) + 2e-  2I-(aq)

*(e)

*(i)

O2(g) + 2H2O(l) + 4e-  4OH-(aq)

(f)

2I-(aq)  I2(g) + 2e-

(j)

Cu2+(aq) + 2e-  Cu(s)

(g)

Fe2+(aq)  Fe3+(aq + e-)z

(h)

2Br-(aq)  Br2(g) + 2e-

(i)

2Cl-(aq) 

(k) (l)

SO42-(aq) + 4H+(aq) + 2e-  SO2(g) + 2H2O(l) 2H+(aq) + 2e-  H2(g)

SO42-(aq) + 4H+(aq) + 2e-

Cu(s)  Cu2+(aq) + 2e4OH-(aq)  O2(g) + 2H2O(l) + 4e-

Cl2(g) + 2e-

In terms of oxidation no.

Reduction Gain / accept Decrease

Oxidation Loss Increase

In terms of O

Removal

Addition

In terms of e-

 Neutralization is not a REDOX reaction (H+ + OH-  H2O)  Common REDOX reaction in M.C. [99CE/MC#36]  displacement reaction p.8 Q.43  metal extraction  disproportionation (a species is reduced and oxidized at the same time)  reaction involving ‘+ Oxygen’ or ‘- Oxygen’  avoid O.N. calculation for H and O  Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 5


Topic VII – Redox, Chemical cell & Electrolysis (Part A) 22. Which of the following indicates reduction? Oxidation number Decrease Decrease Decrease Increase

A. B. C. D.

Electrons

Oxygen

Lose Lose Gain Gain

Removal Addition Removal Addition

24. An element is oxidized if (1) oxygen atom(s) is/are added to it. (2) its oxidation number decreases. (3) it loses electron(s). A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

26. In which of the following reactions is the underlined reactant being oxidized? A. B. C. D.

SO2 + 2Mg  2MgO + S CuO + CO  Cu + CO2 Fe2+ + 2OH  Fe(OH)2 Cu + 2AgNO3  Cu(NO3)2 + 2Ag

by Ricky Tsui

23. Which of the following represents reduction? A. B. C. D.

Hydrogen ion changes to hydrogen. Iron(II) ion changes to iron(III) ion. Lead changes to lead(II) oxide. Calcium ion reacts with sulphate ion to form calcium sulphate.

25. In which of the following changes is chlorine reduced? A. B. C. D.

HCl to Cl2O HCl to HClO3 Cl to Cl2 Cl2 to HCl

27. Consider the following reaction, which is being reduced? Cl2 + 2I  2Cl + I2 A. B. C. D.

Cl2 I Cl I2

*28. Which of the following changes is an example of oxidation? A. B. C. D.

Iodide ions to iodine molecules Zinc ions to zinc atoms Iron(III) ions to iron(II) ions Sulphur dioxide to sulphurous acid

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Topic VII – Redox, Chemical cell & Electrolysis (Part A) 29. Which of the following is a redox reaction? A.CO2(g) + Ca(OH)2(aq)  CaCO3(s) + H2O(l) B. HNO3(aq) + KOH(aq)  KNO3(aq) + H2O(l) C. Na2CO3(aq) + 2HCl(aq)  2NaCl(aq) + CO2(g) + H2O(l) D. 2AgNO3(s)  2Ag(s) + 2NO2(g) + O2(g)

31. Which of the following are redox reactions? (1) ZnO(s) + H2SO4(aq)  ZnSO4(aq) + H2O(l) (2) Fe(s) + 2HCl(aq)  FeCl2(aq) + H2(g) (3) 2MnO2(s) + H2(g)  Mn2O3(s) + H2O(l) A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

by Ricky Tsui

30. Which of the following equations represents a redox reaction? A. B. C. D.

Na2SO4 + BaCl2  BaSO4 + 2NaCl CaCO3  CaO + CO2 CH4 + 2O2  CO2 + 2H2O Ca(OH)2 + SO2  CaSO3 + H2O

32. Which of the following equations represents a redox reaction? A. B. C. D.

2KNO3(s)  2KNO2(s) + O2(g) MgCO3(s)  MgO(s) + CO2(g) Ca(HCO3)2(aq)  CaCO3(s) + H2O(l) + CO2(g) 2 + 2CrO4 (aq) + 2H (aq)  Cr2O72(aq) + H2O(l)

33. Which of the following changes is NOT a redox reaction?

34. Which of the following equations does NOT represent a redox reaction?

A. 2NaHCO3  Na2CO3 + H2O + CO2 B. Fe2(SO4)3 + 2HI  2FeSO4 + I2 + H2SO4 C. 2NaClO3  2NaCl + 3O2 D. Zn + 2HCl  ZnCl2 + H2

A. Cl2(g) + 2Br(aq)  2Cl(aq) + Br2(aq) B. 2Fe3+(aq) + 2I(aq)  2Fe2+(aq) + I2(aq) C. Ca2+(aq) + CO32(aq)  CaCO3(s) D. 2Ag+(aq) + Cu(s)  2Ag(s) + Cu2+(aq)

[03CE/MC#16] 35. Which of the following conversions is NOT a reduction? A. B. C. D.

Fe2O3 → Fe Cu(OH)2 → CuO CH3CO2H → CH3CH2OH H2SO4 → SO2

[10CE/MC#24] 37.

[99CE/MC#9] 36. In which of the following reactions is the underlined reactant reduced? A. B. C. D.

Cu2+ + 2OH- → Cu(OH)2 SO2 + 2Mg → 2MgO + S 2NH3 + 3CuO → 3Cu + N2 + 3H2O Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

[05CE/MC#8] 38. Which of the following is NOT a redox reaction? A. B. C. D.

NH4NO3 → N2O + 2H2O NH3 + HNO3 → NH4NO3 N2O4 + H2O → HNO2 + HNO3 4HNO3 → 2H2O + 4NO2 + O2

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Topic VII – Redox, Chemical cell & Electrolysis (Part A) [05CE/MC#27] 39. Which of the following reactions involve oxidation and reduction? (1) 2KClO3(s)  2KCl(s) + 3O2(g) (2) Pb(s) + PbO2(s) + 2H2SO4(aq)  2PbSO4(s) + 2H2O(l) (3) H2O2(aq) + H2SO4(aq) + 2KI(aq)  K2SO4(aq) + I2(aq) + 2H2O(l) A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

[01CE/MC#22] 41. Which of the following equations represents a redox reaction? A. NH4+(aq) + OH-(aq) → NH3(g) + H2O(l) B. 2CrO42-(aq) + 2H+(aq) → Cr2O72-(aq) + H2O(l) C. 2FeSO4(s) → Fe2O3(s) + SO3(g) + SO2(g) D. 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g) [99CE/MC#36] 43. The equation below represents the reaction of chlorine with hot concentrated potassium hydroxide solution:

by Ricky Tsui

[00CE/MC#28] 40. Which of the following changes is NOT a redox reaction? A. Fe2(SO4)3 + H2S → 2FeSO4 + S + H2SO4 B. 2Al + 6HCl → 2AlCl3 + 3H2 C. CaCO3 + CO2 + H2O → Ca(HCO3)2 D. 2KClO3 → 2KCl + 3O2

[02CE/MC#13] 42. Which of the following reaction represents a redox reaction? A. Ca(HCO3)2 + 2HCl → CaCl2 + 2CO2 + 2H2O B. PCl3 + Cl2 → PCl5 C. Fe3+ + 3OH- → Fe(OH)3 D. Al2O3 + 2NaOH → 2NaAlO2 + H2O [07CE/MC#24] 44. Consider the redox reaction represented by the equation below: 2S2O32-(aq) + I2(aq)  S4O62-(aq) + 2I‾(aq)

3Cl2(g) + 6KOH(aq) → 5KCl(aq) + KClO3(aq) + 3H2O(l) Which of the following statements concerning this reaction is/are correct? (1) Potassium hydroxide acts as a reducing agent. (2) The oxidation number of chlorine changes from 0 to -1. (3) The oxidation number of chlorine changes from 0 to +5. A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

Which of the following statements is/are correct? (1) The oxidation number of sulphur in S2O32-(aq) is +3. (2) One of the half equations of this reaction is 2S2O32-(aq)  S4O62-(aq) + 2e‾ . (3) I2(aq) is oxidized by S2O32-(aq) in the reaction. A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 8


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Oxidizing agent (O.A.) & Reducing agent (R.A.) It can help another reactant carry out oxidation!

?

“Agent” = He / She / It helps you do something!

It can help another reactant carry out reduction! Ag+ itself is reduced. And it causes Zn(s) oxidized.

Ag+(aq) Worked example:

+1 +2 0 0 Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s) Zn(s)

Zn itself is oxidized. And it causes Ag+ in AgNO3 reduced.

Oxidizing agent

Reducing agent

AgNO3(aq) or Ag+(aq)

Zn(s)

Identify the O.A. and R.A for the following reactions:

Oxidizing agent(s)

Reducing agent(s)

45. SO2(g) + 2Mg(s) → 2MgO(s) + S(s) 46. 2S2O32-(aq) + I2(aq)

S4O62-(aq) + 2I‾(aq)

47. 2KNO3(s) + S(s) + 3C(s)  K2S(s) + N2(g) + 3CO2(g) 48. 3CuO(s) + 2NH3(g)  3Cu(s) + 3H2O(g) + N2(g) 49. Refer to Stage 1 – 3 individually: Stage 1 : SiO2(s)

C(s) heat

Stage 2 : Si(s)

Cl2(g) heat

Stage 3 : SiCl4(l)

H2(g) heat

Stage 1:

Stage 1:

Stage 2:

Stage 2:

Stage 3:

Stage 3:

Si(s)

SiCl 4(l) Si(s)

50. * 3Cl2(g) + 6KOH(aq) → 5KCl(aq) + KClO3(aq) + 3H2O(l)

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 9


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

SET A

Some common oxidizing agents (O.A.) Oxidizing agent

Acidified potassium permanganate

KMnO4 / H+ Acidified potassium dichromate

K2Cr2O7 / H+

Half ionic equation and change in O.N.

MnO4-(aq) + 8H+(aq) + 5e+7 purple

+2 colorless

acidified by dil. H2SO4 ONLY / a strong O.A. if acidified, weaker in alkaline condition

Cr2O72-(aq) + 14H+(aq) + 6e-

2Cr3+(aq) + 7H2O(l)

+6 orange

+3 green

acidified by dil. H2SO4 ONLY / a weaker O.A. compared with KMnO4 / H+

SO42-(aq) + 4H+(aq) + 2eConcentrated sulphuric acid

H2SO4(l) Concentrated nitric acid

HNO3

+6 colorless

reacts with metals (  Cu in MRS) to produce SO2(g) rather than H2(g)

NO3-(aq) + 2H+(aq) + e+5 colorless

Cl2 Bromine

NO2(g) + H2O(l) +4 brown gas

reacts with metals (  Cu in MRS) to produce NO2(g) rather than H2(g)

Dilute nitric acid

Chlorine

SO2(g) + 2H2O(l) +4 colorless gas

NO3-(aq) + 4H+(aq) + 3e-

HNO3

Mn2+(aq) + 4H2O(l)

+5 colorless

NO(g) + 2H2O(l) +2 colorless

reacts with metals (  Cu in MRS) to produce NO(g) rather than H2(g) / NO(g) will be further oxidized by the air to become NO2(g) 2NO(g) + O2(g)  2NO2(g)

Cl2(g) + 2e0 greenish-yellow gas

Br2(aq) + 2e-

2Cl-(aq) -1 colorless

2Br-(aq)

Br2

0 reddish-brown liquid

-1 colorless

Iodine

I2(aq) + 2e-

2I-(aq)

0 reddish-brown liquid

-1 colorless

I2

Fe3+(aq) + e-

Iron (III) ion

Fe3+

+3 yellow

Stronger O.A.

Weaker O.A.

Fe2+(aq) +2 green

Curriculum guide: Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 10


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

SET B

Some common reducing agents (R.A.) Reducing agent

Sulphur dioxide

SO2(g)

Half ionic equation and change in O.N.

SO42-(aq) + 4H+(aq) + 2e-

SO2(aq) + 2H2O(l) +4 colorless

+6 colorless

SO32-(aq) + H2O(l) Sulphite ion

SO32-

+4 colorless

+6 colorless SO2(g) and SO32- sometimes can act as oxidizing agent 2Mg(s) + SO2(g)  2MgO(s) + S(s)

2I-(aq)

Iodide ion

0 brown

Br-(aq)

Bromide ion

0 brown

Cl-(aq)

Chloride ion

Cl2(g) + 2e-

-1 colorless

-

0 greenish-yellow gas

Fe2+(aq)

Iron (II) ion

Carbon

+2 yellow

CO2(g) (no

C (s) + O2

C Carbon monoxide gas

CO

CO(g) + O2

Hydrogen gas

CO2(g) H2(g)

H2 For metal extraction, e.g. carbon reduction

Weaker R.A.

Fe3+(aq) + e-

+3 green

2+

Stronger R.A.

Br2(aq) + 2e-

-1 colorless

Br-

Fe

I2(aq) + 2e-

-1 colorless

I-

Cl

SO42-(aq) + 2H+(aq) + 2e-

half ionic equation)

(no half ionic equation)

2H+(aq) + 2eAnother method: by H2(g) or CO(g)

[09CE/2(b)/3 marks]

b. Carbon can be used to extract metals from certain metal oxides. (i) Suggest how copper can be extracted from copper (II) oxide using carbon. State the expected observation.

CuO + H2  Cu + H2O

[05CE/2(b)/4 marks]

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 11


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

SET A + SET B

When O.A. and R.A. come together… Set A - Common Oxidizing agent

Acidified potassium permanganate

KMnO4 / H+ Acidified potassium dichromate

K2Cr2O7 / H+ Concentrated sulphuric acid

H2SO4(l) Concentrated nitric acid

HNO3

+

[10CE/MC#47] Set B - Common 51. Which of the following reagents can convert Reducing agent iron(II) ions to iron(III) ions? (1) chlorine water Level 1 Sulphur dioxide (2) dilute nitric acid (3) potassium bromide solution

SET A

SO2(g)

Sulphite ion

SO32Iodide ion

I-

A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

[04CE/MC#32] 52. Which of the following substances can decolourise acidified potassium permanganate solution? (1) sodium nitrate solution (2) sodium sulphite solution (3) chlorine water

Bromide ion

Br-

Dilute nitric acid

Chloride ion

HNO3

Cl-

Chlorine

Iron (II) ion

Cl2

Fe2+

Bromine

Carbon

Br2

C

Iodine

Carbon monoxide gas

I2

by Ricky Tsui

CO

Iron (III) ion

Hydrogen gas

Fe3+

H2

A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

[99CE/MC#32] 53. Which of the following substances can react with acidified potassium permanganate solution? (1) Ethane (2) copper(II) sulphate solution (3) iron(II) sulphate solution A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

*[12DSE/MC#30] (hint: in page 4 –variable O.N. ) 54. Which of the following ions can act as both an oxidizing agent and a reducing agent? A. B. C. D.

Fe2+(aq) Cu2+(aq) Cr2O72-(aq) MnO4-(aq)

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 12


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

When O.A. and R.A. come together‌ Level 1 [06CE/MC#21] 55. When substance X is treated with an aqueous solution of iron(II) sulphate, the iron(II) ions act as oxidising agent. X may be A. B. C. D.

concentrated hydrochloric acid. aqueous ammonia. acidified potassium dichromate solution. zinc granules.

[03CE/MC#21] 57. Which of the following solutions would NOT undergo a colour change when mixed with sodium sulphite solution? A. B. C. D.

iodine solution acidified potassium permanganate solution chromium(III) sulphate solution iron(III) sulphate solution

[04CE/MC#39] 56. Which of the following gases can act as reducing agents? (1) ammonia (2) hydrogen (3) fluorine A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

[99CE/MC#24] 58. In an experiment, sulphur dioxide is passed into an iodine solution which is prepared by dissolving some iodine in potassium iodide solution. Which of the following statements concerning this experiment is correct? A. The colour of the iodine solution changes from purple to colourless. B. A brown solid is formed. C. A displacement reaction occurs. D. Sulphur dioxide is oxidized to sulphate ions.

Level 1**

[99CE/MC#15]* 59. Potassium permanganate solution acidified with dilute sulphuric acid is a commonly used oxidizing agent. Dilute nitric acid is not used to acidify potassium permanganate solution because A. B. C. D.

nitric acid is more expensive than sulphuric acid. dilute nitric acid is an oxidizing agent and would react with the reducing agent. nitric acid decomposes more readily than sulphuric acid. dilute nitric acid would react with potassium permanganate solution.

[12DSE/MC#13]* 60. The tendency of being reduced of six ionic species increases in the order as shown below: Ba2+(aq) < Na+(aq) < Mg2+(aq) < H+(aq) < Cu2+(aq) < Hg2+(aq) A. B. C. D.

Ba(s) does NOT react with H+(aq) Na(s) has a stronger reducing power than Hg(l) Hg2+(aq) is the weakest oxidizing agent among the six species Displacement reaction occurs when Cu(s) is immersed in MgSO4(aq)

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 13


Topic VII – Redox, Chemical cell & Electrolysis (Part A) [07CE/MC#32] 61. Which of the following statements concerning astatine is correct ? A. B. C. D.

Astatine is soluble in heptane. Astatine is a stronger oxidising agent than chlorine. Astatine is an element in period 5 of the Periodic Table. The formula of the compound formed between astatine and potassium is K2At.

Level 2 Displacement reaction of between halogen and halide

X2 + 2Y-  Y2 + 2X( = feasible;  = not feasible)

+ Cl-

BrI-

[07CE/MC#19] 62. Chlorine is bubbled into an aqueous solution P for some time. The colour of P gradually changes to brown. P is NOT likely to be A. B. C. D.

calcium hydroxide solution. potassium iodide solution. iron(II) chloride solution. zinc bromide solution.

[99CE/MC#10] 64. Which of the following statements concerning bromine and chlorine is INCORRECT? A. They exists as diatomic molecules. B. Their atoms have same number of outermost shell electrons. C. They form ions with a single negative charge. D. Bromine is a stronger oxidizing agent than chlorine [08CE/MC#21] 66. Which of the following substances, when mixed with bromine water, would form a colourless solution? (1) (2) (3)

sodium sulphite sodium chloride sodium iodide

A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

by Ricky Tsui

Cl2

Br2

I2

  

  

  

[00CE/MC#14] 63. Which of the following solutions can react with bromine water to give a colourless solution? A. B. C. D.

sodium chloride solution sodium sulphite solution sodium iodide solution sodium hypochlorite solution

[04CE/MC#18] 65. Which of the following reagents can be used to distinguish sodium bromide solution from sodium chloride solution? A. B. C. D.

bromine water chlorine water 1,1,1-trichloroethane potassium fluoride solution

[07CE/MC#22] 67. Which of the following statements concerning chlorine, bromine and iodine is/are correct? (1) They are all coloured substances. (2) Their reactivity increases with relative atomic mass. (3) They all react with sodium sulphite solution. A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 14


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Conc. / dil. nitric acid as an oxidizing agent 

Both dilute nitric acid (=2M) and concentrated nitric acid (=16M) are not typical acids. They show oxidizing properties apart from acidic properties.

Dilute or concentrated HNO3 reacts with metal oxides, hydroxides, carbonates and hydrogencarbonates in a similar way as very dilute HNO3.

They can oxidize metals to give oxides of nitrogen instead of hydrogen.

Reactions between dilute nitric acid and metals (e.g. copper): Chemical equation: 3Cu(s) + 8HNO3(aq)  3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

Dilute HNO3(aq)

Ionic equation: 3Cu(s) + 8H+(aq) + 2NO3-(aq)  3Cu2+(aq) + 2NO(g) + 4H2O(l) 0

+5

+2

+2

Note: NO2 gas is pungent and toxic, hence the experiment should be carried out in a fume cupboard.

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 15


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Conc. / dil. nitric acid as an oxidizing agent Reaction between concentrated nitric acid and metals (e.g. magnesium): Chemical equation:

Concentrated

Mg(s) + 4HNO3(aq)  Mg(NO3)2(aq) + 2H2O(l) + 2NO2(g)

HNO3(aq)

Ionic equation: Mg(s) + 4H+(aq) + 2NO3-(aq)  Mg2+(aq) + 2H2O(l) + 2NO2(g) 0

+5

+2

+4

Hot concentrated nitric acid can also oxidize some non-metals: C(s) + 4HNO3(aq) 0

+5

 CO2(g) + 4NO2(g) + 2H2O(l) +4

+4

Hot + concentrated

S(s) + 4HNO3(aq)  SO2(g) + 4NO2(g) + 2H2O(l) 0

+5

+4

HNO3(aq)

+4

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 16


Topic VII â&#x20AC;&#x201C; Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Conc. / dil. nitric acid as an oxidizing agent 68. State the expected observation(s) when concentrated nitric acid is added to each of the following substances. Write an ionic equation for each reaction involved. a) Copper(II) carbonate Dilute or concentrated HNO3 reacts with metal oxides, hydroxides, carbonates and b) Copper

hydrogencarbonates in a similar way as very dilute HNO3.

(except metals) c) Sodium sulphite solution

d) Sodium hydroxide solution

e) Potassium iodide solution

[05CE/MC#33] [02CE/MC#19] 69. When a metal X is warmed with an acid Y, they react to 70. Which of the following acids, when form a colourless solution and brown gas. Which of the heated with copper, would produce a gas? following combinations is correct? A. dilute nitric acid X Y B. dilute hydrochloric acid A. zinc concentrated nitric acid C. dilute sulphuric acid B. copper concentrated sulphuric acid D. concentrated ethanoic acid C. zinc concentrated sulphuric acid D. copper concentrated nitric acid

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 17


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

[07CE/MC#46] 71. Which of the following statements concerning the reactions of concentrated nitric acid with copper is/are correct? (1) (2) (3)

A colourless gas is evolved. One mole of NO3‾(aq) ions requires one mole of electrons for reduction. It involves a displacement reaction.

A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

Learn more about conc. HNO3(aq): 

In laboratory, sulphuric acid can be produced by bubbling sulphur dioxide into concentrated nitric acid: SO2(g) + 2HNO3(aq)  H2SO4(aq) + 2NO2(g) +4

+5

+6

+4

Concentrated nitric acid decomposes quickly in the presence of sunlight to give nitrogen dioxide which dissolves in the acid to give a yellow solution:

Hence concentrated nitric acid should be stored in a brown bottle to cut off sunlight. (KMnO4(aq), Cl2(aq) and AgNO3(aq) also decompose quickly in the presence of sunlight and thus they are stored in brown bottles.) 

The corrosive nature of dilute nitric acid and concentrated nitric acid is due to their oxidizing property.

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 18


Topic VII – Redox, Chemical cell & Electrolysis (Part A)

by Ricky Tsui

Answer: 1.

(a)

Cl2(g) + 2e-

(b) (c)

1. 2. 3. 4.

MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l) 

2Cl-(aq)

Cr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+(aq)

5.

write down the product on RHS balance O by adding H2O balance back H by adding H+ balance the charge by adding echeck for balance or not: charge & element

Part B: Balance the following half equations (Hint: e- on RHS)

+ 7H2O(l) Br2(aq) + 2e-

(d)

2Br-(aq)

(a)

H2(g)  2H+(aq) + 2e-

(e)

Ag+(aq) + e-  Ag(s)

(b)

SO32-(aq) + H2O(l)  SO42-(aq) + 2H+(aq) + 2e-

(f)

NO3-(aq) + 2H+(aq) + e- NO2(g) + H2O(l)

(c)

SO2(aq) + 2H2O(l) 

(g)

Fe3+(aq) +e-  Fe2+(aq)

(d)

(h)

I2(aq) + 2e-  2I-(aq)

*(e)

*(i)

O2(g) + 2H2O(l) + 4e-  4OH-(aq)

(f)

2I-(aq)  I2(g) + 2e-

(j)

Cu2+(aq) + 2e-  Cu(s)

(g)

Fe2+(aq)  Fe3+(aq + e-)

(h)

2Br-(aq)  Br2(g) + 2e-

(i)

2Cl-(aq) 

(k)

SO42-(aq) + 4H+(aq) + 2e-  SO2(g) + 2H2O(l) 2H+(aq) + 2e-  H2(g)

(l)

2.

SO42-(aq) + 4H+(aq) + 2e-

Cu(s)  Cu2+(aq) + 2e4OH-(aq)  O2(g) + 2H2O(l) + 4e-

Cl2(g) + 2e-

(a) MnO4-(aq) + 8H+(aq) + 5e-  Mn2+(aq) + 4H2O(l) (b) Cu(s)  Cu2+(aq) + 2e(c) Cr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+(aq) + 7H2O(l) (d) 4OH-(aq)  O2(g) + 2H2O(l) + 4e(e) 2H+(aq) + 2e-  H2(g (f) Fe2+(aq)  Fe3+(aq) + e(g) Ag+(aq) + e-  Ag(s)

3. -3 4. +6 5. C

6. C

7. B

8. A

9. A

10. C 11. D 12. B 13. D

22. C 23. A 24. B 25. D 26. D 27. A 28. A 29. D 30. C 31. C 32. A 33. A 34. C Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 19


Topic VII – Redox, Chemical cell & Electrolysis (Part A) Oxidizing agent(s)

Reducing agent(s)

45.

SO2(g)

Mg(s)

46.

I2(aq)

S2O32-(aq)

47.

S(s)

C(s)

48.

CuO(s)

NH3(g)

49.

Stage 1: SiO2(s) Stage 2: Cl2(g) Stage 3: SiCl4(l)

Stage 1: C(s) Stage 2: Si(s) Stage 3: H2(g)

50.

Cl2(g)

Cl2(g)

by Ricky Tsui

68. a) Copper(II) carbonate dissolves to give a blue solution. Gas bubbles are evolved.

CuCO3(s) + 2H+(aq)  Cu2+(aq) + CO2(g) + H2O(l) b) Copper dissolves to give a blue solution. Brown gas is evolved. Cu(s) + 4H+(aq) + 2NO3-(aq)  Cu2+(aq) + 2H2O(l) + 2NO2(g) c) Brown gas is evolved. SO32-(aq) + 2NO3-(aq) + 2H+(aq)  SO42-(aq) + 2NO2(g) + H2O(l) d) No observable change. / Heat is evolved.

H+(aq) + OH-(aq)  H2O(l) e) Brown gas is evolved. The solution turns from colourless to brown.

2I-(aq) + 2NO3-(aq) + 4H+(aq)  I2(aq) + 2NO2(g) + 2H2O(l)

Ch 31 Redox Reactions / Ch 32 Redox Reaction in a chemical cell (with inert electrodes)

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Page 20

Topic 7 redox reactions part a redox  
Topic 7 redox reactions part a redox  
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