TJC 2025 IP4 Electrochemistry Notes (Filled Blanks)

2024 IP4 Chemistry Topic 11: Electrochemistry Notes Learning objectives At the end of this notes, you should be able to: (a) Describe electrolysis as the conduction of electricity through an ionic compound (an electrolyte), when molten or dissolved in water, leading to chemical changes (including decomposition) at the electrodes (b) Describe electrolysis as evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution (c) Describe, in terms of the mobility of ions present and the electrode products, the electrolysis of molten sodium chloride, using inert electrodes (d) Predict the likely products of the electrolysis of a molten binary ionic compound using inert electrodes (e) 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.) (f) Predict the likely products of the electrolysis of an aqueous electrolyte, given relevant information (g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information (h) Describe the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper (no technical details are required) (i) Describe the electroplating of metals, e.g. copper plating, and state one use of electroplating (j) Describe the production of electrical energy from simple cells (from two electrodes in an electrolyte) linked to the reactivity series and redox reactions (in terms of electron transfer) (k) Describe hydrogen, derived from water or hydrocarbons, as a potential fuel, reacting with oxygen to generate electricity directly in a fuel cell (details of the construction and operation of a fuel cell are not required)

2 Topic Outline 1 Introduction to Electrolysis 1.1 Definition 1.2 Components of an electrolytic cell 1.2.1 Electrolyte 1.2.2 Electrodes 1.2.3 Set-up of an Electrolytic cell 2 Electrolysis of Molten Ionic Compounds 2.1 Electrolysis of molten sodium chloride 3 Electrolysis of Aqueous Ionic Compounds 3.1 Electrolysis of water 3.2 Electrolysis of dilute sodium chloride 3.3 Electrolysis of concentrated sodium chloride (brine) 3.4 Identification test for gases 4 Ease of Discharge 4.1 Ease of discharge of cations 4.2 Ease of discharge of anions 5 Nature of the Electrodes 5.1 Electrolysis of copper(II) sulfate solution using inert electrodes 5.2 Electrolysis of copper(II) sulfate solution during copper electrodes 6 Applications of Electrolysis 6.1 Electroplating 6.2 Purification of copper 7 Galvanic Cell 7.1 Definition of a galvanic cell 7.2 Set-up of a galvanic cell 7.3 Reactivity Series and Electromotive Force (e.m.f.)

3 8 Fuel Cells References: Chemistry Matters Tan Yin Toon & Chen Ling Kwong (pg 261-282) Chemistry Matters Workbook Tan Yin Toon & Chen Ling Kwong (pg 94-102)

4 1 Introduction to Electrolysis 1.1 Definition • Electrolysis is the process of using electricity to break down or decompose a compound. • Electrolysis takes place in an electrolytic cell, which consists of a battery, electrodes and electrolyte. 1.2 Components of an electrolytic cell 1.2.1 Electrolyte • Electrolytes are molten (liquid) compounds or aqueous solutions which can conduct electricity. • In molten or aqueous state, there are free mobile ions that can act as charge carriers to conduct electricity. Molten ionic compound: NaCl(l) → Na+(l) + Cl–(l) Aqueous solution: HCl(aq) → H+(aq) + Cl–(aq) 1.2.2 Electrodes • Electrodes are metal plates or carbon (graphite) rods which can conduct electricity. • Inert electrodes are often used as they do not take part in any chemica l reaction during electrolysis. They provide the surface for electron transfer to occur during electroly tic reactions. • Carbon and platinum (Pt) electrodes are considered to be inert electrodes. 1.2.3 Set-up of an Electrolytic cell • Anode is the electrode where oxidation occurs. Anode is the positive electrode connected to the positive terminal of the battery. • Cathode is the electrode where reduction occurs. Cathode is the negative electrode connected to the negative terminal of the battery. • The process of gaining or losing electrons at the electrodes is called discharge.

5 Set-up of Electrolytic Cell – Learn how to draw and label (a) Anode • Positive electrode (attached to positive terminal of battery) • Anions migrate to anode • oxidAtion of Anions occurs Oxidation half-equation: 2Br–(l) → Br2(g) + 2e– ----- (1) (b) Cathode • Negative electrode (attached to negative terminal of battery) • Cations migrate to cathode • reduCtion of Cations occurs Reduction half-equation: Pb2+(l) + 2e– → Pb(l) ----- (2) (c) Overall equation The overall equation is (1) + (2): PbBr2(l) → Pb(l) + Br2(g) Note that electrons are only present in the half equations. The number of electrons lost at the anode is equal to the number of electrons gained at the cathode. Electrons do not appear in the overall equation. The flow of ions towards the electrodes constitutes the flow of electric current through the electrolyte. Battery Flow of electrons Cations Anions Anode Cathode + – + – Electrolyte e.g. PbBr2(l)

6 2 Electrolysis of Molten Ionic Compounds • When solid ionic compounds are heated strongly, they melt to give the molten form. Molten ionic compounds contain mobile ions to act as charge carriers and can hence act as electrolytes. • Binary ionic compounds contain only two elements to form the metal cation and non-metal anion. An example of binary compounds is sodium chloride. • The electrolysis of a molten binary ionic compound is straightforward since the electrolyte contains only type of one cation and one anion. 2.1 Electrolysis of molten sodium chloride using inert electrode Ions present in electrolyte: Na+(l) and Cl–(l) At the anode: o Cl–(l) migrates to the anode o Cl–(l) is oxidised to form Cl2(g) o Oxidation half-equation: 2Cl–(l) → Cl2(g) + 2e– o Observation: greenish-yellow fumes of chlorine are produced at the anode. At the cathode: o Na+(l) migrates to the cathode o Na+(l) is reduced to form Na(l) o Reduction half-equation: Na+(l) + e– → Na(l) o Observation: Grey sodium metal is produced at the cathode Overall equation: 2NaCl(l) → 2Na(l) + Cl2(g) Watch molten sodium chloride conduct electricity! Click here or scan the QR code or key in the link: https://www.youtube.com/watch?v= d3SDxNhPhtE

7 3 Electrolysis of Aqueous Ionic Compounds 3.1 Electrolysis of water • An aqueous solution of an ionic compound contains cations, anions and water molecules, H2O. • The electrolysis is not as straightforward since water may be involved in the process. • The equations for oxidation and reduction of water are shown below: Oxidation of water at the anode: 2H2O(l) → O2(g) + 4H+(aq) + 4e– o H2O is oxidised as the oxidation state of O increases from –2 in H2O to 0 in O2. o Since H+ ions are produced, the solution around the anode becomes more acidic. Reduction of water at the cathode: 2H2O(l) + 2e– → H2(g) + 2OH–(aq) o H2O is reduced as the oxidation state of H decreases from +1 in H2O to 0 in H2. o Since OH– are produced, the solution around the cathode b