Mc Ques Eng Redox Rxns

REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

Public Exam Questions :

1. In which of the following equations does the underlined substance become reduced? A. CuSO4 + Zn → ZnSO4 + Cu B. 2FeCl2 + Cl2 → 2FeCl3 C. Pb(OH)2 + 2HNO3 → Pb(NO3)2 + 2H2O D. MgCO3 + 2HCl → MgCl2 + CO2 + H2O (HKCEE 1995)

2. Refer to the following chemical equation : Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) Which of the following statements is / are correct? (1) Carbon monoxide is an oxidizing agent. (2) The oxidation number of carbon changes from +2 to +4. (3) The oxidation number of iron changes from +2 to 0. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only (HKCEE 1995)

3. Which step in the above flow chart involves the largest change in oxidation number of nitrogen? A. step 1 B. step 2 C. step 3 D. step 4 (HKCEE 1996)

4. X is an element. It can form a cation X2+ which has an electron arrangement 2,8,8. Which of the following statements concerning X are correct? (1) X is a strong oxidizing agent. (2) X is in Period 4 of the Periodic Table. (3) X burns in oxygen with a brick red flame. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only - 1-

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Textbook (Book 2) : p.162 – 187

D. (1), (2) and (3) (HKCEE 1996)

5. Which of the following statements concerning the elements in the third period of the Periodic Table is correct? A. Both sulphur and chlorine can be reduced by aqueous sodium sulphite. B. Magnesium is a stronger reducing agent than sodium. C. Phosphorus and chlorine form a covalent compound. D. Magnesium burns in oxygen to form an acidic oxide. (HKCEE 1997)

6. Which of the following conversions involves the smallest change in oxidation number of the underlined element? A. C(s) → CO2(g) B. NO3-(aq) → NO2(g) C. SO32-(aq) → SO42-(aq) D. MnO4-(aq) → Mn2+(aq) (HKCEE 1997)

7. The following equation represents the reaction of chlorine with dilute sodium hydroxide solution at room temperature : Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H2O(l) Which of the following statements concerning this reaction is correct? A. Chlorine is oxidized and sodium hydroxide is reduced. B. Chlorine is reduced and sodium hydroxide is oxidized. C. Chlorine is simultaneously oxidized and reduced. D. Sodium hydroxide is simultaneously oxidized and reduced. (HKCEE 1997)

8. In which of the following reactions does the underlined substance act as a reducing agent? A. SO2 + 2H2S → 2S + 2H2O B. Pb(NO3)2 + H2SO4 → PbSO4 + 2HNO3 C. 2HCl + MgO → MgCl2 + H2O D. 2KBr + Cl2 → 2KCl + Br2 (HKCEE 1998)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

9. Consider the following chemical equation : Zn + pMnO2 + qNH4+ → Zn2+ + xMn2O3 + yNH3 + zH2O Which of the following combinations is correct? x y z A. 1 2 1 B. 1 3 2 C. 2 3 2 D. 2 2 3 (HKCEE 1998)

10. In which of the following reactions is the underlined reactant reduced? A. Cu2+ + 2OH- → Cu(OH)2 B. SO2 + 2Mg → 2MgO + S C. 2NH3 + 3CuO → 3Cu + N2 + 3H2O D. Zn + 2AgNO3 → Zn(NO3)2 + 2Ag (HKCEE 1999)

11. Which of the following statements concerning bromine and chlorine is INCORRECT? A. They exist as diatomic molecules. B. Their atoms have the same number of outermost shell electrons. C. They form ions with a single negative charge. D. Bromine is a stronger oxidizing agent than chlorines. (HKCEE 1999)

12. 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. nitric acid is more expensive than sulphuric acid. B. dilute nitric acid is an oxidizing agent and would react with the reducing agent. C. nitric acid decomposes more readily than sulphuric acid. D. dilute nitric acid would react with potassium permanganate solution. (HKCEE 1999)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

13. Consider the following chemical equation : 2HNO3 + xFeSO4 + yH2SO4 → zFe2(SO4)3 + 4H2O + 2NO Which of the following combinations is correct? x y Z A. 2 1 1 B. 4 3 2 C. 6 2 3 D. 6 3 3 (HKCEE 1999)

14. Which of the following substance can react with acidified potassium permanganate solution? (1) ethene (2) copper(II) sulphate solution (3) iron(II) sulphate solution A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only (HKCEE 1999)

15. Which of the following reactions will occur when aluminium powder is added to silver nitrate solution? (1) displacement (2) anodization (3) redox A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only (HKCEE 1999)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

16. The equation below represents the reaction of chlorine with hot concentrated potassium hydroxide solution : 3Cl2(g) + 6KOH(aq) → 5KCl(aq) + KClO3(aq) + 3H2O 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. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only (HKCEE 1999)

17. Consider the following chemical equation : 3Zn(s) + 2NO3-(aq) + 8H+(aq) → 3Zn2+(x) + 2NO(y) + 4H2O(z) Which of the following combinations is correct? x y z A. aq g aq B. aq g l C. aq aq l D. l g aq (HKCEE 2000)

18. 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 (HKCEE 2000)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

19. In which of the following substances does nitrogen has the smallest oxidation number? A. NH3 B. NO C. N2O D. N2 (HKCEE 2000)

20. X is an element in Group VI of the Periodic Table. X can form X2- ions. Which of the following statements are correct? (1) The oxidation number of X decreases when X2- ion is formed. (2) Both X atom and X2- ion have the same number of electron shells. (3) Both X atom and X2- ion have the same nuclear charge. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 2000)

21. Consider the following chemical equation : 8H+(aq) + MnO42-(aq) + 5e- → Mn2+(aq) + 4H2O(l) C2O42-(aq) → 2CO2(g) + 2eHow many moles of MnO4-(aq) ions will react completely with one mole of C2O42-(aq) ions? A. 0.4 B. 1.0 C. 2.5 D. 5.0 (HKCEE 2001)

22. 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) (HKCEE 2001)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

23. Consider the following chemical equation : IO3-(aq) + xH2O(l) + ye- → I-(aq) + zOH-(aq) Which of the following combinations is correct? x y Z A. 1 2 2 B. 2 4 4 C. 3 6 6 D. 4 8 8 (HKCEE 2001)

24. The oxidation number of lead in [Pb(OH)4]2- is A. -2 B. +2 C. +4 D. +6 (HKCEE 2001)

25. Which of the following equations 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 (HKCEE 2002)

26. The symbol of vanadium is V. What is the oxidation number of vanadium in NH4VO3? A. -1 B. +3 C. +5 D. +6 (HKCEE 2002)

27. Which of the following methods can be used to distinguish between iron(II) sulphate solution and iron(III) sulphate solution? (1) observing their colours (2) adding acidified potassium permanganate solution (3) adding aqueous ammonia A. (1) and (2) only B. (1) and (3) only - 7-

REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

C. (2) and (3) only D. (1), (2) and (3) (HKCEE 2002)

28. When sulphur dioxide is bubbled into water, a colourless solution is formed. Which of the following statements concerning the solution are correct? (1) The solution conducts electricity better than water. (2) The solution can change iron(III) sulphate solution from yellow to green. (3) The solution can change potassium bromide solution from colourless to brown. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 2002)

29. When a small piece of calcium metal is put into a trough of water, a reaction occurs. Which of the following statements concerning this reaction is correct? A. It is an endothermic reaction. B. It is a redox reaction. C. A slight explosion occurs. D. The calcium metal burns spontaneously in water. (HKCEE 2003)

30. Which of the following combinations is correct? Chemical Hazardous nature A. Sodium Oxidizing B. Mercury Toxic C. Ethyl ethanoate Irritant D. Potassium dichromate Explosive (HKCEE 2003)

31. Which of the following conversions is NOT a reduction? A. Fe2O3 → Fe - 8-

REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

B. Cu(OH)2 → CuO C. CH3CO2H → CH3CH2OH D. H2SO4 → SO2 (HKCEE 2003)

32. Consider the following information about three elements, X, Y and Z. Element X Y Z

Atomic number 12 16 17

Which of the following statements concerning X, Y and Z is correct? A. X reacts with Z to form an ionic compound. B. Y is a stronger oxidizing agent than Z. C. X has a simple molecular structure. D. Y can conduct electricity in the molten state. (HKCEE 2003)

33. Which of the following solutions would NOT undergo a colour change when mixed with sodium sulphite solution? A. iodine solution B. acidified potassium permanganate solution C. chromium(III) sulphate solution D. iron(III) sulphate solution (HKCEE 2003)

34. What is the oxidation number of sulphur in H2S2O7? A. +2 B. +4 C. +6 D. +8 (HKCEE 2003)

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

Suggested Answers :

1.

A

2.

B

3.

B

4.

C

5.

C

6.

B

7.

C

8.

D

9.

A

10 .

B

11.

D

12 .

B

13 .

D

14 .

C

15 .

C

16 .

D

17 .

B

18 .

C

19 .

A

20 .

D

21 .

A

22 .

C

23 .

C

24 .

B

25 .

B

26 .

C

27 .

D

28 .

A

29 .

B

30 .

B

31 .

B

32 .

A

33 .

C

34 .

C

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REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

Explanation of M.C. Questions :

1. A : O.N. of Cu changes from +2 (in CuSO4) to 0 (in Cu) B : O.N. of Fe changes from +2 (in FeCl2) to +3 (in FeCl3) C : no change in O.N. of Pb (+2) D : no. change in O.N. of Mg (+2) 2. O.N. change of Fe : +3 (in Fe2O3) to 0 (in Fe) ⇒ undergoes reduction (oxidizing agent) O.N. change in C : +2 (in CO) to +4 (in CO2) ⇒ undergoes oxidation (reducing agent) (2) is correct statement. Ans. : B 3. O.N. of N in N2 = 0 O.N. of N in NH3 = -3 O.N. of N in NO = +2 O.N. of N in NO2 = +4 O.N. of N in HNO3 = +5 O.N. change in step 1 = 3 O.N. change in step 2 = 5 O.N. change in step 3 = 2 O.N. change in step 4 = 1 Step 2 involves the largest change in O.N. Ans. : B 4. The electronic arrangement of X = 2,8,8,2 - 11-

REDOX REACTIONS

Textbook (Book 2) : p.162 – 187

X is calcium, which is reducing agent. It belongs to Period 4. When it burns in oxygen, a brick red flame is observed. (2) and (3) are correct statements. Ans. : C

5. A : Cl2 is an oxidizing agents which can be reduced by Na2SO3. Cl2(g) + SO32-(aq) + H2O(l) → 2Cl-(aq) + SO42-(aq) + 2H+(aq) S is a reducing agent which can’t react with Na2SO3. B : Na is a stronger reducing agent than Mg because Na is in a higher position in E.C.S. C : P and Cl are non-metals, the compound formed from them is covalent. The formula can be PCl3 or PCl5. D : 2Mg(s) + O2(g) → 2MgO(s) MgO(s) + H2O(l) → Mg(OH)2 Mg(OH)2 is alkaline / basic. This means that MgO is a basic oxide. 6. O.N. of C changes from 0 to +4 O.N. change = 4 O.N. of N changes from +5 to +4 O.N. change = 1 O.N. of S changes from +4 to +6 O.N. change = 2 O.N. of Mn changes from +7 to +5 O.N. change = 2 Ans. : B 7. O.N. of Cl changes from 0 (in Cl2) to –1 (in NaCl) ⇒ undergoes reduction (oxidizing agent) O.N. of Cl changes from 0 (in Cl2) to +1 (in NaOCl) ⇒ undergoes oxidation (reducing agent) - 12-

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Cl2 undergoes reduction and oxidation at the same time. Ans. : C 8. A reducing agent undergoes oxidation, which involves in an increase in O.N. A : O.N. of S changes from +4 (in SO2) to 0 (in S) B : no change in O.N. of Pb (+2) C : no change in O.N. of H and Cl D : O.N. of Br changes from –1 (in KBr) to 0 (in Br2)

9.

The equation is constructed in the following way : at zinc : Zn → Zn2+ + 2e- ------------------------------------ (i) at graphite : 2NH4+ + 2e- → 2NH3 + H2 ------------------ (ii) 2MnO2 + H2 → Mn2O3 + H2O -------------- (iii) (i) + (ii) + (iii), Zn + 2NH4+ + 2MnO2 → Zn2+ + 2NH3 + Mn2O3 + H2O x = 1, y = 2, z = 3 Ans. : A

10. A : no change in O.N. of Cu (+2) B : O.N. of S changes from +4 (in SO2) to 0 (in S) C : O.N. of N changes from –1 (in NH3 to 0 (in N2) D : O.N. of Zn changes from 0 (in Zn) to +2 (in Zn(NO3)2) 11. A : correct. Formulae of bromine and chlorine are Br2 and Cl2 respectively. B : correct. They belong to Group VII which means that they have 7 e-s in their outermost shell. C : correct. They form Br- and Cl- respectively. D : wrong. Cl2 is a stronger oxidizing agent than Br2. Cl2 is in a lower position in E.C.S. than Br2. 12. dilute nitric acid acts as an oxidizing agent which can compete with MnO4-/H+ to react with reducing agent. Ans. : B 14. (1) : it reacts KMnO4/H+(aq) to carry out addition reaction and form ethan-1,2-diol CH2=CH2 → HO-CH2-CH2-OH - 13-

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(2) : CuSO4 can’t react with KMnO4/H+(aq). (3) : FeSO4 is reducing agent with is oxidized by KMnO4/H+(aq). 5Fe2+(aq) + MnO4-(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) (1) and (3) react with KMnO4/H+(aq) Ans. : C

15. The chemical equation involved : Al(s) + 3Ag+(aq) → Al3+(aq) + 3Ag(s) statement (1) : correct. Al is a reactive metal and Ag+ is a less reactive metal ion. Reactive metal can displace less reactive metal ion. It is a displacement reaction. statement (2) : it’s a process which coat a layer of Al2O3 on the surface of aluminium. statement (3) : correct. O.N. of Al changes from 0 to +3 (oxidation) O.N. of Ag changes from +1 to 0 (reduction) (1) and (3) are correct statements. Ans. : C 16. O.N. of Cl changes from 0 (in Cl2) to –1 (in KCl) ------------------- (i) O.N. of Cl changes from 0 (in Cl2) to +5 (in KClO3) ---------------- (ii) There is no change in O.N. of K (+1) statement (1) : wrong. statement (2) : correct. From (i). statement (3) : correct. From (ii). (2) and (3) are correct statements. Ans. : D 18. A : O.N. of Fe changes from +3 (in Fe2(SO4)3) to +2 (in FeSO4) O.N. of S changes from -2 (in H2S) to 0 (in S) B : O.N. of Al changes from 0 (in Al) to +3 (in AlCl3) O.N. of H changes from +1 (in HCl) to 0 (in H2) C : there is no change in O.N. of each of the species D : O.N. of Cl changes from +5 (in KClO3) to -1 (in KCl) O.N. of O changes from -2 (in KClO3) to 0 (in O2) - 14-

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19. A : O.N. of N = -3 B : O.N. of N = +2 C : O.N. of N = +1 D : O.N. of N = 0 20. statement (1) : correct. O.N. of X changes from 0 (in X) to -2 (in X2-) statement (2) : correct. statement (3) : correct. no. of protons does not change when X changes to X2-. (1), (2) and (3) are correct statements. Ans. : D 21. the overall ionic equation : 16H+(aq) + 2MnO4-(aq) + 5C2O42-(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) no. of moles of MnO -4 2 = no. of moles of C 2 O 24- 5 2 no. of moles of MnO4- = 5 = 0.4 mol Ans. : A 22. A : there is no change in O.N. of every species B : there is no change in O.N. of every species C : O.N. of Fe changes from +2 (in FeSO4) to +3 (in Fe2O3) – oxidation O.N. of S changes from +6 (in FeSO4) to +4 (in SO2) – reduction D : there is no change in O.N. of every species 24. let x be O.N. of Pb then x + 4 × (-1) = -2 ∴ x = +2 Ans. : B 25. A : there is no change in O.N. of every species B : O.N. of P changes from +3 (in PCl3) to +5 (in PCl5) – oxidation O.N. of Cl changes from 0 (in Cl2) to -1 (in PCl5) – reduction C : there is no change in O.N. of every species D : there is no change in O.N. of every species 26. let x be O.N. of V then x + 3 × (-2) + 1 = 0 - 15-

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∴ x = +5 Ans. : C

27. statement (1) : correct. FeSO3 is pale green and Fe2(SO3)3 is yellowish brown. statement (2) : correct. KMnO4/H+ can only react with FeSO4. 5Fe2+(aq) + MnO4-(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) statement (3) : correct. Both can react with ammonia to give insoluble hydroxides but they are of different colour. Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) (dirty green)

Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s) (yellowish brown)

(1), (2) and (3) are correct statements. Ans. : D 28. (1) : correct. The solution contains mobile ions but water does not. It thus conducts electricity better. SO2(g) + H2O(l) → H2SO3(aq) H2SO3(aq) → 2H+(aq) + SO32-(aq) (2) : correct. Sulphite solution is reducing agent and Fe2(SO4)3 is oxidizing agent. They can carry out redox reaction. SO32-(aq) + 2Fe3+(aq) + H2O(l) → SO42-(aq) + 2Fe2+(aq) + 2H+(aq) (3) : wrong. KBr(aq) is reducing agent. It can’t react with sulphite solution. (1) and (2) are correct statements. Ans. : A 29. A : wrong. It’s an exothermic reaction. B : correct. Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) O.N. of Ca changes from 0 (in Ca) to +2 (in Ca(OH)2) O.N. of H changes from +1 (in H2O) to 0 (in H2) - 16-

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C : wrong. No explosion observed. D : wrong. It doesn’t burn in water. 30. A : wrong. Na is reducing B : correct. Hg is toxic. C : wrong. D : wrong. K2Cr2O7 is oxidizing 31. A : O.N. of Fe changes from +3 to 0 B : there is no change in O.N. of Cu C : there is a loss of O in the process, thus it’s reduction. D : O.N. of S changes from +6 to +4 32. X (magnesium) is metal but Y (sulphur) and Z (chlorine) are non-metals. A : correct. They give an ionic compound with the formula of XZ2. B : wrong. Z is a stronger oxidizing agent than Y. In E.C.S., Cl2 is in lower position on the L.H.S. than S. C : wrong. X has a giant metallic structure. D : wrong. Y can’t conduct electricity even in molten state because it doesn’t contain mobile ions. 33. A : SO32-(aq) + I2(aq) + H2O(l) → SO42-(aq) + 2I-(aq) + 2H+(aq) the colour of solution changes from reddish brown (I2) to colourless (I-). B : 5SO32-(aq) + 2MnO4-(aq) + 6H+(aq) → 5SO42-(aq) + 2Mn2+(aq) + 3H2O(l) the colour of solution changes from purple (MnO4-) to colourless (Mn2+). C : there is no reaction between Na2SO3 and Cr2(SO4)3. D : SO32-(aq) + 2Fe3+(aq) + H2O(l) → SO42-(aq) + 2Fe2+(aq) + 2H+(aq) the colour of solution changes from yellowish brown (Fe3+) to pale green (Fe2+). 34. let x be O.N. of S then 2x + 2 × 1 + 7 × (-2) = 0 ∴ x = +6 Ans. : C

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