ACSBR 16 Electrolysis - Assignment ANS

Anglo-Chinese School (Barker Road) A Methodist Institution Founded in 1886 CHEMISTRY DEPARTMENT OF SCIENCE Name: ANSWERS ( ) Class: SEC 4 ELECTROLYSIS – ASSIGNMENT ALEX LEE/2010 PAGE 1 OF 7 PURE Multiple-Choice Questions [20 Marks] TOTAL SCORE / 30 Write in your selected answer for the multiple-choice questions in the boxes provided. 1 2 3 4 5 6 7 8 9 10 B D B B D C A C C A 11 12 13 14 15 16 17 18 19 20 A B B D A C D D D C 1. The circuit below was set-up, using an unknown substance Q. It was observed that the bulb lit, but there were no decomposition products at the electrodes. What is Q? A aqueous sodium chloride C molten sodium chloride B mercury D sugar solution 2. When the experiment below was set-up, the ammeter showed that no current was flowing. A few drops of dilute sulfuric acid were dissolved in liquid Z and the ammeter then showed that a current was flowing. What could Z be? A aqueous NaOH C ethanol B concentrated NaCl D water

ALEX LEE/2010 PAGE 2 OF 7 3. Two cells, containing substances X a n d Y r e s p e c t i v e l y, were connected in series with a battery and a light bulb, as shown in the diagram below. In which of the following pairs of substances did the bulb light up? in X in Y A aqueous copper(II) sulfate solid lead(II) bromide B aqueous potassium sulfate dilute hydrochloric acid C concentrated hydrochloric acid pure ethanoic acid D concentrated sodium chloride concentrated sugar solution 4. Which one of the following does not yield any gaseous products? electrolyte electrodes A aqueous copper(II) sulfate platinum B aqueous silver nitrate copper C concentrated sodium chloride copper D dilute sulfuric acid platinum 5. A liquid is to be electrolysed, and two test-tubes full of the liquid are placed over carbon electrodes. The diagram below shows the apparatus after some time. Which one of the following could be the liquid undergoing electrolysis? A aqueous CuSO4 C concentrated NaCl B concentrated HCl D dilute H2SO4 6. Electrolysis of molten lead(II) bromide gives lead at the cathode. Why does lead form? A Lead(II) bromide decomposes on heating. B Lead is less reactive than hydrogen. C Positive lead(II) ions are discharged at the cathode. D The anode dissolves to form lead(II) ions. X Y

ALEX LEE/2010 PAGE 3 OF 7 7. Four electrolysis experiments, each using aqueous copper(II) sulfate as the electrolyte, were set-up as shown below. In which of these four experiments did the concentration of the copper(II) sulfate electrolyte remain constant? A C B D 8. In which one of the following instances is there no change in the pH of the solution during electrolysis? A concentrated sodium chloride solution between platinum electrodes B copper(II) chloride solution between platinum electrodes C copper(II) sulfate solution between copper electrodes D dilute sulfuric acid between copper electrodes 9. The electrolysis shown in the diagram is set up. What is observed? at positive electrode at negative electrode A bubbles of colourless gas bubbles of colourless gas B bubbles of colourless gas solution turns brown C solution turns brown bubbles of colourless gas D solution turns brown silvery droplets 10. An aqueous sample of zinc chloride is electrolyzed using iron electrodes. Which of the following statements, pertaining to the above experiment, is false? A Bubbles of colourless gas are formed at the positive electrode. B The pH of the solution gradually increases. C The solution gradually turns green. D Zinc ions are attracted to the negative electrode. copper platinum platinum graphite copper carbon platinum copper carbon electrodes concentrated aqueous rubidium iodide

ALEX LEE/2010 PAGE 4 OF 7 11. What change always takes place when aqueous copper(II) sulfate is electrolysed? A Copper is deposited at the negative electrode. B Oxygen is evolved at the positive electrode. C Sulfate ions move towards the negative electrode. D The colour of the solution fades. 12. In the diagram below each cell contains an aqueous solution of a single salt and all four electrodes are platinum. Electrodes Q and S increase in mass during the electrolysis but no gas is given off at Q or S. If an increase in mass of Q i s g r e a t e r t h a n t h e i n c r e a s e o f ma s s o f S i n t h e s a me t i me , which of the following statements is necessarily true? A The anions of the solutions in Cells A and B are different. B The cations of the solutions in Cells A and B are different. C The current flowing in Cell A is greater than the current flowing in Cell B. D The solution in Cell A is more concentrated than the solution in Cell B. 13. Which one of the following involves the largest number of electrons for complete discharge during electrolysis? A 3 mol of P3- ions C 5 mol of O2- ions B 4 mol of Al3+ ions D 8 mol of Na+ ions 14. In an electrolysis experiment, the same quantity of electricity deposited 16 g of copper and 6 g of titanium. What was the charge on the titanium ion? A 1+ C 3+ B 2+ D 4+ 15. Dilute sulfuric acid was electrolysed using inert electrodes. What volumes (at r.t.p.) of hydrogen and oxygen were formed by the passage of one mole of electrons? volume of H2 / dm3 volume of O2 / dm3 A 12 6 B 12 24 C 24 12 D 24 24 P Q R S Cell A Cell B

ALEX LEE/2010 PAGE 5 OF 7 16. A sample of copper contained, as an impurity, a metal below it in the reactivity series. The diagram shows the apparatus used for refining the sample. The loss in mass of the positive electrode was 50 g and the gain in mass of the negative electrode was 45 g. What was the percentage purity of this sample of copper? A 10.0% C 90.0% B 11.1% D 95.0% 17. The diagram below shows the electrolysis of an unknown electrolyte. Which of the following could be the electrolyte? A aqueous potassium iodide C concentrated copper(II) sulfate B concentrated copper(II) chloride D concentrated hydrochloric acid 18. A metal spoon is to be electroplated with silver using aqueous silver nitrate as the electrolyte. Which of the following should be used as the cathode? A copper block C silver block B platinum block D spoon 19. The heat-reflecting shields of some space rockets are gold-plated, using electrolysis. Which electrodes and electrolyte would be used to gold-plate the heat shield? negative electrode positive electrode electrolyte A carbon heat shield gold compound B gold heat shield copper compound C heat shield carbon copper compound D heat shield gold gold compound impure copper anode pure copper cathode aqueous copper(II) sulfate

ALEX LEE/2010 PAGE 6 OF 7 20. Aqueous sodium sulfate is electrolysed using copper electrodes. The current is constant and the cathode is weighed at regular intervals over a long period of time. Which graph is obtained when the mass of the cathode is plotted against time? A C B D Structured Questions [10 Marks] 21. A strip of moistened filter paper is laid on a microscope slide. A drop of silver nitrate solution is placed near one end of the paper and a drop of potassium iodide solution is placed near the other end. Using inert electrodes, the apparatus is connected to a suitable d.c. supply. After some time, a pale yellow streak appears as shown below. (a) Briefly explain why a pale yellow streak is formed. [2] The positive silver ions migrate towards the cathode, while the negative iodide ions migrate towards the anode. When the two ions collide, an insoluble salt of silver iodide is precipitated, causing a solid yellow streak. (b) Suggest why the streak appears nearer the cathode than the anode. [1] Silver ions have a lower mass than the iodide ions, and hence