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SECTION 3
CHEMICAL CELLS AND ELECTROLYSIS
1986 12. (NH4)2Cr2O7 heat → N2 + 4H2O + Cr2O3 When ammonium dichromate is decomposed according to the above equation, which of the following correctly describes the changes in oxidation numbers of nitrogen and chromium? O.N. of nitrogen O.N. of chromium A increases increases B increases decreases C decreases increases D decreases decreases 13. In the following reaction: 2 I- + H2O2 + 2 H+ → I2 + 2 H2O A I- is reduced and H+ is oxidized. B I- is reduced and H2O2 is oxidized. C I- is oxidized and H2O2 is reduced. D I- is oxidized and H+ is reduced. 14. If 0.1 M aqueous solutions of the following compounds were electrolyzed using graphite electrodes, which one would give hydrogen at the cathode and oxygen at the anode? A copper(II) sulphate B sodium hydroxide C potassium iodide D silver nitrate 17. In which of the following reactions is sulphur dioxide acting as an oxidizing agent? A 3 SO2 + Cr2O72- + 2 H+ → 3 SO42- + 2 Cr3+ + H2O B SO2 + 2 H2S → 3 S + 2 H2O
C D
SO2 + PbO2 → PbSO4 2 SO2 + O2 → 2 SO3
20. In which of the following compounds does nitrogen have the lowest oxidation number? A HNO2 B KNO3 C NO D NH3 39. When aqueous copper(II) sulphate is electrolyzed using copper electrodes, the equation(s) for the reaction(s) which occur(s) at the anode is/are: (1) Cu(s) → Cu2+(aq) + 2 e(2) Cu2+(aq) + 2 e- → Cu(s) (3) SO42-(aq) → SO2 + O2 + 2 eA (1) only B (3) only C (1) & (2) only D (2) & (3) only 1987 10. During electrolysis, electricity passes through the electrolyte as a result of the movement of A cations only. B anions only. C cations and anions. D mobile electrons. 11.
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C D
Referring to the above diagram, the largest current flowing from iron to metal X in the external circuit can be obtained when X is A copper. B magnesium. C silver. D zinc. 14.
The colour of the solution around X and Y fades. The colour of the solution becomes blue around X and red around Y.
42. Which of the following are redox reactions? (1) Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s) (2) Al(OH)3(s) + NaOH(aq) → NaAlO2(aq) + 2 H2O(l) (3) MnO2(s) + 4 HCl(aq) → MnCl2(aq) + Cl2(g) + 2 H2O(l) A (1) & (2) only B (1) & (3) only C (2) & (3) only D (1), (2) & (3) 1988 11. In which of the following will a redox reaction occur? A adding sodium to ethanol B bubbling sulphur dioxide into sodium hydroxide solution C heating copper(II) hydroxide D adding zinc oxide to concentrated nitric acid 14. Consider the reaction: MnO4-(aq) + 8 H+(aq) + 5 Fe2+(aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l) Which of the following statements concerning the above reaction is true? A 1 mole of electrons is transferred per mole of MnO4-(aq) ions reacted. B 1 mole of the reductant is required to react with 0.2 mole of the oxidant. C H+(aq) ions are reduced to H2O(l). D The oxidation number of manganese increases from -1 to +2.
Referring to the above diagram, which of the following changes would occur when a current is passed through the solution for some time? A Sodium is formed at X and sulphur dioxide is liberated at Y. B Oxygen is liberated at X and hydrogen is liberated at Y.
37. Which of the following elements show more than one oxidation state in their compounds? (1) nitrogen (2) chlorine (3) aluminium
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A B C D
(1) & (2) only (1) & (3) only (2) & (3) only (1), (2) & (3)
1989 6.
When 250 cm3 of 2.00 M hydrochloric acid are added to 13.0 g of zinc, the volume of gas produced at room temperature and pressure is A 2.40 dm3. B 4.80 dm3. C 6.00 dm3. D 12.0 dm3. (Relative atomic masses: H=1.00, Cl=35.5, Zn=65.0; Molar volume of a gas at room temperature and pressure = 24.0 dm3)
10. A solution of silver nitrate was electrolyzed under a steady current, using silver electrodes. Which of the following graphs correctly represents the change in mass of the anode with time?
12. When a solution of magnesium sulphate of known concentration is electrolyzed using platinum electrodes, which of the following is a correct description of what happens? No. of moles of Mg2+(aq) No. of moles of SO42-(aq) pH value of solution
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A B C D
decreases remains unchanged decreases remains unchanged
decreases decreases remains unchanged remains unchanged
increases decreases remains unchanged remains unchanged
13. The anode of a dry cell is usually made of A zinc. B iron. C tin. D lead. 14. In which of the following reactions is the underlined substance reduced? A Cu(OH)2 (s) + 4 NH3(aq) → [Cu(NH3)4](OH)2(aq) B 2 H+(aq) + CO32-(aq) → CO2(g) + H2O(l) C Cl2(g) + 2 Fe2+(aq) → 2 Fe3+(aq) + 2 Cl-(aq) D 2 Ag+(aq) + Zn(s) → 2 Ag(s) + Zn2+(aq) 15. Chlorine gas A decolorizes bromine water. B liberates iodine from potassium iodide solution. C turns acidified potassium dichromate solution from orange to green. D turns iron(III) chloride solution from yellow to green. 16. Chlorine reacts with cold dilute aqueous sodium hydroxide according to the equation: Cl2(g) + 2 NaOH(aq) → NaClO(aq) + NaCl(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 both oxidized and reduced. D Sodium hydroxide is both oxidized and reduced.
20. When zinc reacts with 1 M nitric acid, hydrogen is not evolved. This is because A zinc reduces the nitric acid to nitrogen. B the concentration of hydrogen ions in dilute nitric acid is very low. C nitric acid is an oxidizing agent. D zinc forms a protective oxide layer. 38. K2Cr2O7(aq) + 3 K2SO3(aq) + 4 H2SO4(aq) → Cr2(SO4)3(aq) + 4 K2SO4(aq) + 4 H2O(l) In the above equation, which of the following elements undergo(es) a change in oxidation number? (1) Cr (2) S (3) O A (1) only B (1) & (2) only C (2) & (3) only D (1), (2) & (3) 1990 19. In which of the following pairs of substances is the oxidation number of the sulphur atom and the nitrogen atom the same? A H2SO4 and HNO3 B SO2 and HNO2 C SCl2 and NO D NaHS and NH3 20. Which of the following gases would support the combustion of a piece of burning magnesium ribbon? (1) carbon dioxide (2) sulphur dioxide (3) chlorine A (1) & (2) only
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B C D
(1) & (3) only (2) & (3) only (1), (2) & (3)
21. Which of the following pairs of substances would react to produce hydrogen? (1) iron and steam (2) sodium and ethanol (3) magnesium and concentrated sulphuric acid A (1) & (2) only B (1) & (3) only C (2) & (3) only D (1), (2) & (3)
A B C D
Zn and Pb Mg and Ag Pb and Cu Fe and Mg
1991 Direction: Questions 6 and 7 refer to the following electrolysis experiment.
23. A student tries to plate a copper ring with nickel using the set-up below:
Which of the following combinations is correct? Anode Cathode Electrolyte A copper ring nickel rod Ni2+(aq) B nickel rod copper ring Ni2+(aq) C copper ring nickel rod Cu2+(aq) D nickel rod copper ring Cu2+(aq) 24. Which of the following pairs of metals would be expected to give the largest voltage when they are used as electrodes in a simple chemical cell using potassium nitrate solution as the electrolyte?
6.
The gases collected at electrodes X and Y respectively are in the volume ratio of A 1 : 1. B 1 : 2. C 1 : 4. D 2 : 1.
7.
Which of the following statements concerning the experiment is/are correct? (1) The pH value of the potassium sulphate solution remains unchanged at the end of the experiment. (2) The concentration of the potassium sulphate solution remains unchanged at the end of the experiment. (3) The products of electrolysis at electrodes X and Y would remain unchanged if 2 M sulphuric acid were used instead of 2 M potassium sulphate solution.
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A B C D
(1) I-(aq) acts as a reducing agent. (2) Chlorine is a stronger oxidizing agent than iodine. (3) The reaction is a displacement reaction. A (1) & (2) only B (1) & (3) only C (2) & (3) only D (1), (2) & (3)
(1) only (2) only (1) & (3) only (2) & (3) only
9.
1992 8.
Which of the following combinations would produce the largest current flowing from metal X to metal Y in the external circuit? Metal X Metal Y A Fe Cu B Mg Ag C Ag Zn D Cu Pb 37. Which of the following oxidation numbers can nitrogen display in its compounds? (1) -3 (2) +2 (3) +3 (4) +4 A (1) & (4) only B (2) & (3) only C (1), (2) & (4) only D (1), (2), (3) & (4) 43. Cl2(aq) + 2 I-(aq) → I2(aq) + 2 Cl-(aq) Which of the following statements concerning the above reaction are correct?
The manufacture of sulphuric acid can be represented by the following flow diagram: Which stage involves the greatest change in the oxidation number of sulphur? A Stage I B Stage II C Stage III D Stage IV
Direction: Questions 9 an 10 refer to the following diagrams:
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A B C D
H2(g) CO(g) NH3(g) C(s)
dil. H2SO4 dil. HNO3 dil. HCl conc. HCl
12. Using 1 mole of reactant, which of the following chemical changes involves the highest number of electrons? A CrO42-(aq)→ Cr3+(aq) B Cr2O72-(aq) → Cr3+(aq) C MnO4-(aq) → Mn2+(aq) D MnO42-(aq) → MnO2(s)
9.
Which of the following represents the correct order of activity of the metals P, Q, R and S? A R, P, Q, S B Q, P, R, S C P, Q, S, R D P, R, S, Q
10. Which of the two metals used as electrodes would give the largest cell voltage? A Q and R B Q and S C S and R D S and P 11. Consider the following flow diagram:
Which of the following combinations is correct? Reactant X Reactant Y
13. When 2 moles of oxygen gas are collected at the anode during the electrolysis of dilute sulphuric acid, the number of moles of electrons released at the anode is A 2. B 4. C 6. D 8. 37. Which of the following aqueous solutions, when electrolyzed using carbon electrodes, will liberate only gaseous products at both electrodes? (1) KOH(aq) (2) AgNO3(aq) (3) MgCl2(aq) A (1) & (2) only B (1) & (3) only C (2) & (3) only D (1), (2) & (3) 38. Which of the following statements concerning the reaction between dry chlorine and hot iron wire is/are correct? (1) Iron(II) chloride is formed. (2) A solid product is obtained after cooling to room temperature.
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(3) Chlorine is reduced. A (1) only B (3) only C (1) & (2) only D (2) & (3) only
C D
The mass of the copper electrode decreases. The mass of the electrode made of metal X decreases.
19.
1993 15. The oxidation number of copper remains unchanged when A magnesium ribbon is added to copper(II) sulphate solution. B sodium carbonate solution is added to copper(II) nitrate solution. C carbon is heated with copper(II) oxide. D copper foil is burnt in chlorine. 16. In which of the following equations does the underlined substance undergo reduction? A 2 H2O + 2 K → 2 KOH + H2 B Fe2(SO4)3 + 2 KI → 2 FeSO4 + K2SO4 + I2 C 2 H2S + SO2 → 3 S + 2 H2O D NaClO + SO2 + H2O → NaCl + H2SO4 17.
With reference to the above diagram, which of the following statements is correct? A The electrode made of metal X is the positive pole. B Copper is at a higher position in the electrochemical series that metal X.
In the above experiment, which of the following major products will be liberated at the electrodes? cathode anode A magnesium oxygen B magnesium bromine C hydrogen bromine D hydrogen oxygen 1994 12. In which of the following experiments will a redox reaction occur? A adding copper turnings to iron(II) nitrate solution B adding bromine water to potassium chloride solution C adding iron fillings to silver nitrate solution D adding sodium chloride solution to silver nitrate solution 13. When a substance X is electrolyzed using platinum electrodes, a gas is collected at each electrode. X may be A silver nitrate solution. B potassium chloride solution. C molten sodium chloride. D molten copper(II) chloride.
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17. In which of the following reactions does sulphur dioxide act as an oxidizing agent? A SO2 + 2 Mg → 2 MgO + S B SO2 + 2 KOH → K2SO3 + H2O C SO2 + Cl2 + 2 H2O → H2SO4 + 2 HCl D 5 SO2 + 2 KMnO4 + 2 H2O → K2SO4 + 2 MnSO4 + 2 H2SO4