Danyal Education. Electrolysis TOPIC. E rtnuhiple Choice Questions

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1 TOPC Electrolysis E rtnuhiple Choice Questions ) Dilute zinc sulfate solution was electrolysed using carbon electrodes. What were the volumes of hydrogen and oxygen formed by the passage of one mole of electrons? Volume of, / dml Volume of H, dmj A. 6 t2 B. t2 6 C. t2 24 D Element X is extracted by electrolysis of a molten compound of elements X and Y. The overall ionic equation is given as: 2X3* + 6Y -+ 2X+3Y, Which of the following could be the molten compound used? A. iron () chloride B. magnesium bromide C. aluminium chloride D. calcium oxide 3. When an aqueous solution containing ions of X is electrolysed, a solid deposit of element X is formed at the negative electrode. Which statement about element X must be correct? A. An ion of X is more stable than a hydrogen ion. B ons of X lose electrons at the cathode. C. ons of X gain electrons at the cathode. D. X is above hydrogen in the reactivity series. 4. Which of the following requires the least number of electrons for discharge to take place? A. two moles of aluminium ions B. five moles of hydroxide ions C. three moles of oxide ions D. six moles of hydrogen ions Electrolysis 9

2 5. Which substances produce two gases of 2 : volume ratio when electrolysed using graphite electrodes? A. aqueous copper () sulfate and dilute sulfuric acid B. aqueous copper () sulfate and concentrated aqueous sodium nitrate C. dilute nitric acid and concentrated aqueous sodium nitrate D. concentrated aqueous sodium chloride and concentrated aqueous sodium sulfate 6. A similar electric current was passed through the contents of two crucibles. Crucible A contained molten magnesium bromide while container B contained molten scandium bromide. 4.8 g of magnesium was formed in crucible A white 6. g of scandium was formed in crucible B. What is the charge of a scandium ion? A. 1+ B. 2+ C. 3+ D Which statement explains why electrical energy is produced by a simple cell? A. ons are formed from atoms gaining or losing electrons. B. Both the electrodes dissolve. C. Electrons are transferred from a more reactive element to a less reactive element. D. Battery cells provide electrons from the negative electrode to the positive electrode 8. A piece of blue litmus paper was soaked in concentrated sodium chloride solution and then placed on top of a glass slide. The paper was then connected to a direct current source as shown in the diagram below. glass slide blue litmus paper Which of the following shows the observations made near the negative and positive electrodes after the current had flowed for some time? Positive electrode A. red bleached B. bleached red C. bleached blue D. blue bleached 9. During electrolysis,.15 moles of chromium is deposited on the cathode when.9 moles of electrons is passed through a molten electrolyte containing chromium. Which of the following substances could be the electrolyte? A. CrClo B. CrBr, C. Crr(SOo). D. Cr(NO.,)u Negative electrode 92 Topic 14

3 1. What is the volume of chlorine gas that is produced when 1. kg of molten sodium chloride is electrolysed at room temperature and pressure? A. 194 dm3 B. 25 dm3 C. 386 dm3 D. 415 dm3 11. A current is passed through two electrolytic cells which are connected in series. The first cell contains XC/r(aq) while the second cell contains YC/(aq). The mass of metal X and metal y deposited at the electrode is in the ratio : 3. What is the ratio of the relative atomic mass of X to the relative atomic mass of Y? A. l:6 B. 6:1 C. 2:3 D. 3:2 12. Electrolysis of a concentrated solution of X using graphite electrodes produced a brown colouration around one electrode and a deposition of metal on the other. what could X be? A. zinc bromide B. sodium bromide C. copper () bromide D. silver iodide 13. An electrolytic cell is set up using platinum electrodes W, X, y andz. molten lead () bromide w x Y Z At which electrode(s) is/are halogens produced? A. W only B. WandY C. X only D. XandZ concentrated hydrochloric acid 14' A simple cell is set up with magnesium and copper as the electrodes and dilute sulfuric acid as the electrolyte. Which of the following will not occur? A. The bulb will light up. B. A gas will be given off at the copper electrode. C. A gas will be given off at the magnesium electrode. D. The electrons will flow from the magnesium electrode to the copper electrode. Etectrotysis 93

4 15. Given the following ionic equation, wal(s) + xcu2*(aq) -+ya/3*(aq) + zcu(s) What is the total charge on each side of the balanced equation? A. 2+ B. 3+ C. 5+ D When the metal X is added to a solution of copper () sulfate, heat is evolved and copper is displaced from the solution. What does this indicate? The reaction is exothermic. Metal X is more reactive than copper. m n a simple cell with metal X and a copper electrode, the copper electrode is the positive electrode. A. and only B. and only C. and only D. All of the above 17, The following solutions were electrolysed using platinum electrodes concentrated hydrochloric acid aqueous sodium chloride m dilute sulfuric acid Which solutions produced hydrogen? A. and only B. and only C. and only D. All of the above 18. A sample of copper contained a metal impurity which is below copper in the reactivity series. The cell shown below was set up to refine the copper sample. impure copper anode aqueous copper () sulfate At the end of the experiment, it was noted that the loss in mass of the anode was 5. g and the gain in mass of the cathode was 45. g. What was the percentage purity of the sample of copper? A. 1. Vo B. ll.t 7 C. 9O.Vo D. 95.Vo pure copper cathode Education 94 Topic 14 Danyal

5 19. Two cells were set up as shown below, A B C D CuSOo(aq) HrSOo(aq) Which of the following combinations of metal would allow the deposition of copper on one of the electrodes? Metal A Metal B A. copper copper iron lron B. lron iron copper copper C copper copper copper 1ron D. lron copper lron copper 2. Five metal strips V, W, X, Y and Z were tested for their reactivity by using a set-up as shown below. Readings from the voltmeter were recorded in a table. metal under test Unknown metal lead Voltage reading mv V r.3 w x -.6 Y {.78 Z.74 Which of the following shows the correct arrangement of the five unknown metal strips in order of decreasing reactivity? A. Y,Z,W,X,Y B. X,Y,W,Z,V C. Y,Z,W,Y,X D. W,X,Y,V,Z Metal C Metal D Electrotysis 95

6 21. Two experiments were conducted on the electrolysis of aqueous copper () sulfate. n experiment, copper electrodes were used but in experiment, platinum electrodes were used instead' Which graph shows the electrical conductivity of the electrolytes in the first five minutes? A. C. electrical conductivity electrical conductivity tlme electrical conductivity electrical conductivity time 22. Crude copper extracted from its ore is called 'blister'copper and contains about 98 Vo pwe copper. Before it can be used in electrical wiring, it must be further purified. Very pure copper (99.9 Vo purity) is obtained from crude copper by electrolysis. Several statements about copper purification by electrolysis are given below. Aqueous copper () nitrate can be used as the electrolyte solution. 11 Crude copper is placed at the cathode while pure copper is placed at the anode. m The concentration of the electrolyte solution remains constant during electrolysis. Which of the above statements are correct? A. and only B. and only C. and only D. All of the above B D. time time 96 Topic 14

7 E Stn ctrrred Questions t. n an electrolysis experiment, a circuit was connected as shown in the diagram below and the switch was left switched on to allow current to pass through the circuit for a period of time. ) electrolyte A copper wlre electrolvte B At the end of the experiment, it was found that 32. g of copper and 12. g of titanium were deposited at electrodes 2 and 4 respectively. Calculate the charge of a titanium ion. Electrical cells are devices which convert chemical energy to electrical energy to be used for practical purposes. (a) (i) Tony plans to set up an electrical cell using zinc and silver as the electrodes. Draw a labelled diagram showing what the cell should look like and include the following in the diagram: o voltmeter. An appropriate electrolyte. The direction of the flow of electrons. The positive and negative terminal on the electrodes (ii) Write the ionic equations to represent the reactions at both the positive and the negative electrodes. (iii) What will be observed at both the electrodes? (iv) Write an equation with state symbols for the overall reaction in the cell. (b) What will be observed with the voltmeter when the zinc electrode is replaced by magnesium? Give a reason for your answer. A few drops of Universal ndicator solution are added to a beaker of concentrated lithium chloride solution and electrolysed using carbon electrodes. Determine if each of the following statements is true or false. Circle T or F accordingly and state your reason. (a) A silvery shiny solid is formed at the negative electrode. T/F (b) A colourless gas is evolved at the anode. TF (c) Effervescence is observed at both electrodes. TF (d) The solution around the cathode tums red. TF (e) The ph of the electrolyte solution remains at7 atthe end of the experiment. T/r' Electrotysis 97

8 4. An electrolysis experiment is conducted based on the electrolytic cell below, water containing a small amount of sodium chloride and phenolphthalein platinum electrodes (a) (i) What is observed at the cathode? (ii) Write the ionic equation for the reaction occurring at the cathode. (b) (i) What is observed at the anode? (ii) Write the ionic equation for the reaction occurring at the anode. (c) During the course of the electrolytic process, it is observed that the solution near one of the electrodes tums pink. Which electrode is it? Account for this observation. (d) Do you expect the electrolyte to be more acidic or alkaline at the end of the experiment? Give a reason for your answer. (e) Write the overall equation for this electrolytic cell, with state symbols. 5. Two electrolytic cells are connected in series silver + copper silver silver nitrate solution + copper copper () sulfate solution Cell contains silver electrodes placed inside aqueous silver nitrate solution while cell 2 contains copper electrodes placed inside aqueous copper () sulfate solution. A current is passed through the circuit. (a) (i) denrify the positive and negative electrode by indicating (+) and (-) in the diagram for cell l. Education (ii) State the ions present in the electrolyte of cell 1. (iii) Write the ionic equations for the reactions at the electrodes of cell 1. Hence, describe what is observed at the two electrodes. (b) What will happen to the concentration and colour of the electrolyte solution in cell2? (c) what is rhe mass of copper deposited it.27 g of silver is deposited? Fl 2 98 Topic 14 Danyal

9 6. Brine is known as concentrated sodium chloride. When a sample of brine is electrolysed using graphite, two different gases are produced at the electrodes. The resulting solution X produces a blue precipitate which does not dissolve in excess X when copper () sulfate solution is added. When solution X is added to solution Y, a white precipitate which is insoluble in excess X is formed. brine blue precipitate insoluble in excess solution X solution X + white precipitate insoluble in excess solution X two gases Upon electrolysing concentrated solution Y using platinum electrodes, a reddish-brown solution is formed around the anode and a colourless and odourless gas is formed at the cathode. When a sample of the reddish-brown solution is extracted and added into a beaker of aqueous potassium salt solution Z, the colour of the solution changes to brown and a black solid is formed upon standing. (a) (i) State the types of ions present in brine. (ii) Write the ionic equation for the reactions occurring at both electrodes when brine is electrolysed. (iii) Write the formula of the main ions present in solution X. (iv) What will be the colour of litmus paper after it is used to test for solution X? (v) Explain how solution X is obtained from the electrolysis ofbrine. (vi) What is observed at the positive and negative electrodes when solution X is electrolysed? (vii) Write down the ionic equation involving the formation of blue precipitate which is insoluble in excess of solution X. (b) (D Name the salt present in solution Y. (ii) Write down the ionic equation involving the formation of the reddish-brown solution. Which cathode is involved? add copper () sulfate solution electrolysis add solution Y (iii) What is the identity of the colourless gas when solution Y is electrolysed? (c) (i) What is the identity of potassium saltz? Write a balanced chemical equation and explain the chemistry involved. (ii) Explain, with reason, whether the above reaction is a redox reaction. Electrolysis 99

10 7. The diagram shows a cell for purifying copper. r + 8. pure copper cathode impure copper anode electrolyte (aqueous copper () sulfate) (a) Describe what would be observed during electrolysis and write the equations for the reactions at the electrodes. (b) The electrodes and the electrolyte conduct electricity. (i) Explain how the structure of metals allows copper electrodes to conduct electricity. (ii) Explain why solid copper () sulfate does not conduct electricity but an aqueous solution of copper () sulfate does conduct electricity. (c) Describe how the apparatus shown in the diagram could be modified in order to electroplate an iron object, such as a knife, with nickel. Aconstant current was passed through aqueous copper () sulfate using inert electrodes as shown in the diagram below. (a) Name a suitable material for the inert electrodes. + variable reslstor aqueous copper () sulfate (b) What was the purpose of the variable resistor? (c) The cathode was removed at intervals, carefully washed, dried and weighed. The results are shown in the table below. Complete the table by calculating the total increase in mass after each 1 minute interval..ni*^ / *:- Mass of cathode / g l.l Total increase in mass / g r Topic 14

11 (d) Plot total increase in mass / g against time / min on the grid below and draw the line which best fits these results. increase in mass / g r..+'i''+-*-+.-ii--i..h 1 ir il i:t1 t::) '111 ; iil"r i r.r.i..l time / min (e) Name the substance that was deposited on the cathode. (f) What was the time taken for 2. g to be deposited? (g) Suggest why the last two mass readings were the same. (h) What was the appearance of the electrolyte (i) at the start of the experiment, and (ii) at the end of the experiment? (i) (i) The inert electrodes were replaced by copper electrodes and the experiment was repeated. l;:..:l The same current was passed for the same length of time. Draw a line on your graph, labelled T, to show the results you would expect for this second experiment. (ii) What was the colour of the electrolyte at the end of the experiment? l'f!ili- -J ;J.-l--i--!-r--:.-i-i.r..i.-l-:: ' l-i-i -]+-+i+! i{{-fj-+.-ti +-r +.{ i..l.i.i+-i+iij +i :i..i-i_1'j-_t..i::l:l-.1. i + -;-.i iil i +- l:'1 _i.i.i c ':., :*li tirl.ii-.j-j Electrotysis O