Notes on Electrolysis

Notes on Electrolysis.

Colour of  solution before electrolysis Colour of  solution after  electrolysis Anode material Reaction at Anode /Observations

Equation at Anode Cathode material Cathode observation

Cathode reaction

Electrolysis of aqueous Copper (II) sulphate blue

Electrolysis of aqueous Copper (II) sulphate blue

blue

colourless

Copper Anode Anode ionization occurs. Copper anode dissolves and decreases in mass

Carbon anode Hydroxide ions and sulphate ions migrate . However only hydroxide ion is Preferentially discharged. Bubbles of oxygen is seen

Cu Cu2+ +2e (oxidation)

4OH-

Electr Electrolp olpati ating ng : Spoon, Spoon, object object be plated plated

Carbon Carbon materi material al

1. Brow Brown n sol solid id depos deposit ited ed . 2. Incr Increas easee in mas masss of cat cathod hode. e. (the increase in mass is equal to the loss in mass at cathode) Copper ions discharged were from the anode which dissolved

Cathode Cu2+ +2e  Cu equation ( from the anode) Overall Remains unchanged .There is no loss of  concentration of  ions from the solution electrolyte

Observati on which is seen in Both Observati on which is not seen in Both



2H2O + O2+ 4e(oxidation)

Bubbles .

Hydrogen ions and Cu 2+ from electrolyte migrate here but only Cu 2+ is discharged( Lower in the ECS series and less stable) Cu2+ +2e  Cu ( from electrolyte) There is loss of Cu2+ hence decrease of concentration

Bro Brown sol solid id in Cat Cathode hode

Bro Brown sol solid id in Cath Cathod ode e

1. No change in colour of electrolyte 2. Decrease in Anode

1. There is colour change 2. Bubbles at anode.

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/ Differenc es

1. A to F are six six aqueous soluti solutions. ons. Each soluti solution on was tested tested with with litmus litmus paper and and then connected as shown in the electrical circuit . The electrodes used were carbon electrodes. The following table shows the results. Solution Litmus test Current A B C D E F

Anode product

No effect 0. 0 No n e Turns red 1 .6 Oxygen No effect 1 .1 Chlorine Turns red 1 .4 Chlorine No effect 1.0 Oxygen Turns blue 1 .5 Oxygen a. Ident Identif ify y the the six six solu soluti tion onss  b. Give an an ionic ionic equation equation for for the reaction reaction in solution solution B at at the I

anode

II.

Cathode Product None Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen

Cathode.

c. If a few drops of universal indicator were added to solution C before electrolysis  began, the indicator appeared green. As electrolysis progresses , the indicator  gradually turned blue. Explain the colour change. 2.

The diagram below shows the electrolysis of an aqueous solution containing an electrolyte P using graphite as electrodes. It is known that P contains nitrate ions. a. Why must must we we use d.c. d.c. elec electri tricit city y for elec electro trolys lysis? is?  b. During During the the electr electroly olysis sis , a gas Q colle collecte cted d above elec electro trode de R. (i) What is gas Q?

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(ii) (ii)

Shows Shows by means means of an an ionic ionic equatio equation n , how how gas gas Q is formed formed at the the elec electro trode de R. (iii (iii)) Why Why is oxi oxida dati tion on said said to to have have take taken n place place at elec electr trode ode R? R? c. At electro electrode de S, no gas was given given off off but a pink deposit deposit formed formed over over the electrode electrode S. d. Describe Describe two two other other changes changes that that you would observe observe after after sometim sometimee . Explain Explain your answers.

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