NJC 2026 Electrochemistry Part 2 (Student)

Electrolytic Cell (Part 2) Content • Electrolysis (i) factors affecting the amount of substance liberated during electrolysis (ii) the Faraday constant; the Avogadro constant; their relationship (iii) industrial uses of electrolysis Learning Outcomes Candidates should be able to: (l) state the relationship, F = Le, between the Faraday constant, the Avogadro constant and the charge on the electron (m) predict the identity of the substance liberated during electrolysis from the state of electrolyte (molten or aqueous), position in the redox series (electrode potential) and concentration (n) calculate: (i) the quantity of charge passed during electrolysis (ii) the mass and/or volume of substance liberated during electrolysis (o) explain, in terms of the electrode reactions, the industrial processes of: (i) the anodising of aluminium (ii) the electrolytic purification of copper [technical details are not required] References • Advanced Chemistry, by Michael Clugston and Rosalind Flemming. • Chemistry for Advanced Level, by Peter Cann and Peter Hughes • Chemistry, by Raymond Chang Copyright © National Junior College All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any other information storage and retrieval system, without prior permission in writing from the copyright owner.

Comparison of Electrochemical (Part 1) and Electrolytic cell (Part 2) ELECTROCHEMICAL CELL ELECTROLYTIC CELL Energy change chemical energy → electrical energy electrical energy → chemical energy Cell Reaction Spontaneous Chemical reaction takes place to generate electric current within celll (battery). Non-spontaneous External electric current is applied to drive the reaction to occur. Cathode (reduction) Positive (+) electrode Negative (–) electrode Anode (oxidation) Negative (–) electrode Positive (+) electrode Note: 1. In both electrochemical and electrolytic cells, oxidation always takes place at the anode while reduction always takes place at the cathode. 2. Polarity of electrode always follow the polarity of the battery. [ElectroCHEMICAL cell] The two half-cells form the battery. Anode is the negative terminal of the battery that releases electrons while cathode is the positive terminal of the battery that accepts electrons. [ElectroLYTIC cell] The polarities of the electrode are reversed, as the electrodes follows the polarity of the battery terminal it is connected to. • anode (+ve) is connected to the +ve terminal of battery and attracts anions (−ve) • cathode (−ve) is connected to the −ve terminal of battery and attracts cations (+ve)

Pause and ponder Which half-reaction occurs in which electrode? Anode, Oxidation; Reduction, Cathode 1 The Electrolytic Cell Electrolysis is the process in which electrical energy is used to cause a NON-spontaneous redox reaction to occur. Success Criteria: • I can draw the set-up of an electrolytic cell to illustrate what constitutes the cell. • I can identify the reduction or oxidation taking place at each electrode based on the polarity of the electrodes in the electrolytic cell. • I can explain the differences between electrochemical and electrolytic cells in terms of energy change, spontaneity of the cell reaction and sign of the cathode and anode. The set-up An electrolytic cell is a container that consists of a pair of electrodes dipped in an electrolyte and connected to an external source of electromotive force (the battery). I. The battery drives electrons out of one electrode (anode) and into the other electrode (cathode), providing the driving force for the non -spontaneous chemical reaction to occur. II. Electrodes (Polarity of electrode follows the polarity of the battery terminal it is connected to) chemical process polarity Cathode reduction (gain of electrons) Negative (gain e− from -ve terminal of battery) Anode oxidation (loss of electrons) Positive (Release e− to +ve terminal of battery) III. An electrolyte is a liquid or an aqueous solution, that contains mobile ions and conducts electricity. The electrolyte could either be: • a molten liquid of ionic salt (e.g. molten PbBr2), or • an aqueous solution containing ions (e.g. aqueous CuSO 4 or dilute H2SO4). electrodes electron flow Useful aid: (An Ox, Red Cat) electron flow

1.1 Electrolysis of Molten Salts Success Criteria • I can predict the products formed at each electrode (cathode and anode) for the electrolysis of molten salts by identifying the ions present in the electrolyte. Some ionic salts, when molten, can be decomposed into their constituent elements by using electrical energy. Worked Example 1 Consider the electrolysis of molten lead( II) bromide using inert platinum electrodes. a) What are the ions present in the electrolyte? Pb2+ and Br− b) What happens to these ions when the setup is connected to a battery? Pb2+ ions (cation) move towards the cathode (-ve) Br− ions (anion) move towards the anode (+ve) c) What chemical processes happen at the following electrodes? Write equations for the reactions that occur. (i) Cathode (attracts cations) Chemical process: reduction Equation: [R] Pb2+ + 2e− ⎯→ Pb (ii) Anode (attracts anions) Chemical process: oxidation Equation: [O] 2Br− ⎯→ Br2 + 2e− d) Deduce what would be observed at each of the electrodes after some time. Cathode: small beads of molten lead formed Anode: brown fumes of bromine evolved Note: During electrolysis, cations receive electrons from the cathode while anions give up electrons to the anode. This process of gaining or losing electrons at the electrodes is termed as discharge. Useful aid: Oxidation Is Loss of e− Reduction Is Gain of e−

1.2 Electrolysis of Aqueous Solutions Success Criteria • I can predict the products formed in the electrolysis of aqueous solutions by identifying the ions/species with more positive E values at the cathode (–) electrode. • I can predict the products formed in the electrolysis of aqueous solutions by identifying the ions/species with less positive (more negative) E values at the anode (+) electrode. In the electrolysis of aqueous solutions, the ions/species which need to be considered for discharge are: • water • cations and anions (possibly mixtures) Only one ion/ species is discharged at an electrode at any one time. This is called selective discharge of species. The factors that affect the selective discharge of ions/species at each electrode are: • position of the ions in the electrochemical series (their relative E values) • concentration of the ions • nature of the electrode 1.2.1 Relative electrode potential chemical process species discharged electrode potential, E Cathode (−ve) reduction species that is more easily reduced (Compare cations and H2O) More positive E Anode (+ve) oxidation species that is more easily oxidised (Compare anions and H2O) Less positive E Reduction takes place for the species with more positive E values (E measures the tendency for reduction to take place). For electrolysis of an aqueous solution, at the cathode, metal cations such as Cu2+ and Ag+ would be reduced since their E values are more positive than E (H2O/H2) and they are more readily reduced than H2O. Cu2+ + 2e− ⇌ Cu E = +0.34 V Ag+ + e− ⇌ Ag E = +0.80 V 2H2O + 2e− ⇌ H2 + 2OH– E = −0.83 V

Note: NO3− ions are not oxidised (N of NO3− is in its maximum possible oxidation state of +5 and thus cannot be oxidised) Note: NO3− cannot be oxidised Worked