Beatty 17.2 ELECTROCHEM (W2) 2023 Answers

1 Beatty Secondary School Science Department (Chemistry Unit) Chemistry 6092 Name: ___________________________________ ( ) Date: _______________ Class: 4E___ TOPIC: ELECTROCHEMISTRY (WORKSHEET 2) – ELECTROLYSIS OF AQUEOUS & CONCENTRATION SOLUTIONS Learning Objectives: (a) Apply the idea of selective discharge based on (i) cations: linked to the reactivity series (ii) anions: halides, hydroxides and sulfates [e.g. aqueous copper( II) sulfate and dilute sodium chloride solution (as essentially the electrolysis of water)] (iii) concentration effects (as in the electrolysis of concentrated and dilute aqueous sodium chloride) (In all cases above, inert electrodes are used.) (b) Predict the likely products of the electrolysis of an aqueous electrolyte, given relevant information. (c) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information. Multiple-Choice Questions 1 Rubidium is above sodium in the reactivity series. What is formed when concentrated rubidium chloride is electrolysed? cathode (−) anode (+) A chlorine hydrogen B hydrogen rubidium C hydrogen chlorine D rubidium chlorine ( C ) 2 A solid deposit of element R is formed at the cathode(−ve) when an aqueous solution containing ions of R is electrolysed. Which statement about element R must be correct? A R forms negative ions. B R ions gain electrons at the cathode. C R ions lose electrons at the cathode. D R is above hydrogen in the reactivity series. ( B ) 3 Dilute sulfuric acid is electrolysed using inert electrodes. Which equation represents the reaction at the anode (+ve)? A O22− → O2 + 2e− B 2H+ + 2e− → H2 C 4OH− → 2H2O + O2 + 4e− D SO42− → SO2 + O2 + 2e− ( C )

2 4 In which line in the table is all the information correct? reaction at electrode electrode product A 2X− → X2 + 2e− cathode metal B X+ + e− → X anode metal C 2X− → X2 + 2e− anode non-metal D X+ + e− → X cathode non-metal ( C ) 5 Apparatus is set up as shown in the diagram. What occurs at electrode X? A Chloride ions are oxidised. B Chloride ions are reduced. C Nickel ions are oxidised. D Nickel ions are reduced. ( A ) 6 Which statement about the electrolysis of concentrated aqueous sodium chloride using inert electrodes is correct? A Chlorine is released at the cathode. B Oxygen is released at the cathode. C Sodium is released at the cathode. D The pH of the electrolyte increases. ( D ) 7 Four different conditions under which sodium chloride is electrolysed using inert electrodes are listed. 1 concentrated aqueous sodium chloride 2 dilute aqueous sodium chloride 3 molten sodium chloride 4 solid sodium chloride Under which conditions is a green gas formed? A 1 and 2 B 1 and 3 C 3 and 4 D 3 only ( B )

3 8 The diagram shows the electrolysis of dilute sulfuric acid. Which gas collects at Y and at Z? Y Z A hydrogen oxygen B oxygen hydrogen C hydrogen sulfur dioxide D sulfur dioxide hydrogen ( B ) 9 The circuit shown below is set up and an electric current is passed through the four cells. If the increase in mass of Q is greater than the increase of mass of S in the same time, which statement must be true? A The cation of the solution in cell 1 is different from the c ation of the solution in cell 2. B The current flowing in cell 1 is greater than the current flowing in cell 2. C The cation in cell 1 is the same as in cell 2 but the solution in cell 1 is more concentrated than in cell 2. D The loss of mass of electrode P is less than the loss of mass of electrode R.( A ) 10 In an electrolysis experiment, the same amount of charge deposited 16 g of copper and 6 g of titanium. What was the charge on the titanium ion? A 1+ B 2+ C 3+ D 4+ ( D ) dilute sulfuric acid

4 Structured Questions 11 Complete the table below by: (a) writing the formulae of the ions present in the aqueous solutions, (b) identifying the ions that will be discharged at the anode and the cathode during electrolysis, assuming that graphite electrodes are used, (c) writing the ionic equation for the reaction at each electrode. solutions / electrolyte formulae of ions present in the solution ions discharged at the anode and the equation ions discharged at the cathode and the equation dilute nitric acid H+, NO3−, OH− OH− ions 4OH−(aq) → 2H2O(l) + O2(g) + 4e− H+ ions 2H+(aq) + 2e− → H2(g) aqueous zinc sulfate Zn2+, H+, SO42−, OH− OH− ions 4OH−(aq) → 2H2O(l) + O2(g) + 4e− H+ ions 2H+(aq) + 2e− → H2(g) aqueous potassium hydroxide K+, H+, OH− OH− ions 4OH−(aq) → 2H2O(l) + O2(g) + 4e− H+ ions 2H+(aq) + 2e− → H2(g) aqueous silver nitrate Ag+, H+, NO3−, OH− OH− ions 4OH−(aq) → 2H2O(l) + O2(g) + 4e− Ag+ ions Ag+(aq) + e− → Ag(s) concentrated aqueous calcium chloride Ca2+, H+, Cl−, OH− Cl− ions 2Cl−(aq) → Cl2(g) + 2e− H+ ions 2H+(aq) + 2e− → H2(g) concentrated aqueous lithium sulfate Li+, H+, SO42−, OH− OH− ions 4OH−(aq) → 2H2O(l) + O2(g) + 4e− H+ ions 2H+(aq) + 2e− → H2(g) 12 Cell One has platinum electrodes dipping into dilute sulfuric acid. Cell Two has platinum electrodes dipping into copper(II) sulfate solution. (a) Complete the table below. name of products equation for the reaction Cell One Positive electrode oxygen gas, water 4OH− (aq) → 2H2O (l) + O2 (g) + 4e− Negative electrode hydrogen gas 2H+ (aq) + 2e− → H2 (g) Cell Two Positive electrode oxygen gas, water 4OH− (aq) → 2H2O (l) + O2 (g) + 4e− Negative electrode copper metal Cu2+ (aq) + 2e− → Cu (s)

5 (b) Describe what you would observe at each electrode of Cell Two. Effervescence is observed at the positive electrode due to oxygen gas. A layer of reddish-brown metal deposit can be observed at the surface of the negative electrode. (c) Describe changes expected to the solution in Cell Two after some time. Solution in Cell Two turns from blue to colourless / blue solution fades away due to the discharge of copper( II) ions. The solution becomes more acidic / decreases in pH value due to the higher net concentration of hydrogen ions that are not discharged. (d) Give brief reasons for you r answers for the following questions. What change, if any, would you expect to see in the reading of the current meter if, in separate experiments: (i) the solution in Cell Two were replaced by a solution of sugar, No current would be registered. Sug ar solution does not have mobile electrons or mobile ions to function as charge carriers to conduct electricity. (ii) a few drops of b arium hydroxide solution were added to the solution in Cell One? The reading on current will decrease. Barium hydroxi de will react with dilute sulfuric acid to form an insoluble salt, barium sulfate. Thus, the concentration of mobile ions will decrease , resulting in the decrease in electrical conductivity detected by current. 13 An electric current was passed through dilute sulfuric acid using platinum electrodes. (a) Give the formulae of all of the ions present in dilute sulfuric acid. H+ ions, OH− ions and SO42− ions (b) Give the ionic equations for the reactions occurring at the two electrodes in this electrolysis. positive electrode: 4OH− (aq) → 2H2O (l) + O2 (g) + 4e− negative electrode: 2H+ (aq) + 2e− → H2 (g)

6 (c) Hence, explain why the volume of hydrogen formed is twice the volume of oxygen formed. For every 4 moles of electrons involved in the reaction, 2 moles of hydrogen gas and 1 mole of oxygen gas is liberated, hence the volume of hydrogen is twice of oxygen. (d) Oxygen is slightly soluble in water. Explain how the