The electrolysis of aqueous solutions of ionic compounds Specification references C4.3.1 The process of electrolysis C4.3.4 Electrolysis of aqueous solutions C4.3.5 Representation of reactions at s as half equations WS1.2 Aims In this activity you will observe the electrolysis of aqueous solutions of ionic compounds. After describing your observations you will answer questions about what happens during the electrolysis. Learning outcomes After completing this activity, you should be able to: describe the experimental set-up for electrolysis state that oxygen can be produced at the anode and hydrogen at the cathode when some solutions are electrolysed write a word equation to describe electrolysis of a solution describe electrolysis of solutions in terms of movement of ions write a balanced symbol equation including state symbols for the overall electrolysis of a solution predict the products at each for the electrolysis of aqueous ionic compounds explain how hydrogen ions and hydroxide ions can be present in solutions, including a balanced symbol equation with state symbols, for the reversible reaction in which water ionises describe electrolysis with half equations at the s explain the classification of reactions at the s as oxidation or reduction. This resource sheet may have been changed from the original. 1
Safety chemical splash proof eye protection 1 mol/dm 3 copper(ii) chloride LOW HAZARD 1 mol/dm 3 potassium bromide LOW HAZARD 1 mol/dm 3 sodium sulfate LOW HAZARD The practical should be carried out in a well-ventilated laboratory as significant amounts of toxic chlorine and bromine can be produced, as well as highly flammable hydrogen. Take care if you are asthmatic. Equipment 3 beakers (100 cm 3 ) retort stand and clamp to hold s DC power supply (6 V) leads and crocodile clips low voltage lamp 1 mol/dm 3 copper(ii) chloride 1 mol/dm 3 potassium bromide 1 mol/dm 3 sodium sulfate 2 carbon s (about 5 mm diameter) blue litmus paper Setting the scene In aqueous solutions of ionic compounds, four different ions are present: the metal ion and the non-metal ion from the ionic compound, plus hydrogen ions and hydroxide ions from the water. The elements produced at the s when an aqueous solution is electrolysed using inert s depend on the relative reactivities of the elements involved. This activity involves the electrolysis of aqueous solutions of potassium bromide, sodium sulfate, and copper(ii) chloride. Method 1 Pour about 50 cm 3 of potassium bromide solution into a beaker. 2 Put in the carbon s and set up the circuit as shown below. 3 Turn on the current. 4 After a few minutes, record what you see at the two s in the results table below. This resource sheet may have been changed from the original. 2
5 Repeat using a new beaker containing sodium sulfate solution and then repeat with copper(ii) chloride solution. Task 1 Use the information from Setting the scene (above) to work out which ions are present in each solution. Add this into the relevant column in the results table below. Then colour in red the ions that are attracted to the negative, and colour in green the ions that are attracted to the positive. 2 Use your observations to work out which element is actually produced at each. Add this into the relevant column in the results table below. Additional information If a gas is produced at the negative, it is hydrogen. If a gas is produced at the positive that does not bleach blue litmus paper, it is oxygen. Solution Ions present Observations at negative Product at negative Observations at positive Product at positive potassium bromide sodium sulfate copper(ii) chloride Results table 1 This resource sheet may have been changed from the original. 3
Questions 1 Which of the solutions contain metals that are more reactive than hydrogen? 2 Which solutions contain halide ions? 3 Complete the following sentences: a In the aqueous solution water molecules break down producing ions and ions. b Write an equation, with state symbols, to show how water breaks down into H + ions and OH ions. c At the negative (cathode), hydrogen is produced if the metal is reactive than hydrogen. d At the positive (anode), is produced unless the solution contains halide ions. In this case the is produced. (2 marks) 4 State how you would test for the presence of the following gases: a hydrogen b oxygen c chlorine. 5 Brine is a concentrated solution of sodium chloride. When it is electrolysed, three products are formed: chlorine gas is produced at the anode (positive ) hydrogen gas is produced at the cathode (negative ) the remaining solution is sodium hydroxide. This resource sheet may have been changed from the original. 4
a Explain why hydrogen is produced at the cathode. b i Explain why sodium hydroxide is left in the solution. (2 marks) ii Predict what you would see if you tested the solution with universal indicator. Explain your answer. (2 marks) c Write an overall word equation for the electrolysis of brine. d Explain why this is an important industrial process. 6 Predict the products when the following aqueous solutions are electrolysed. Use the reactivity series to remind you which metals are more reactive than hydrogen. Aqueous solution Product at negative Product at positive potassium chloride copper(ii) nitrate magnesium chloride silver nitrate 7 Complete the following sentences: a During electrolysis, at the cathode (negative ), positively charged ions (4 marks) electrons and so the reactions are. (2 marks) b At the anode (positive ), negatively charged ions electrons and so the reactions are. (2 marks) This resource sheet may have been changed from the original. 5
8 Complete the following half equations to show what happens when a hydrogen is produced at the cathode: 2H + H 2 b oxygen is produced at the anode 4OH O 2 2H 2 O 9 Complete the table by writing half equations for the reactions at the s for the three solutions that you observed in the practical activity. Solution potassium bromide sodium sulfate copper(ii) chloride Half equation at negative Half equation at positive 10 Electrolysis is a redox reaction. Discuss this statement by explaining the changes that take place at the anode and cathode during the electrolysis of aqueous sodium chloride. Illustrate your answer by writing half equations for the reactions that occur at each. (6 marks) (8 marks) This resource sheet may have been changed from the original. 6