Introduction to ElectrolysisDefinitions and Terminology

Examination Requirementsi) Electrolysis1.39 understand an electric current as a flow of

electrons or ions1.40 understand why covalent compounds do not

conduct electricity1.41 understand why ionic compounds conduct

electricity only when molten or in solution1.42 describe simple experiments to distinguish

between electrolytes and non-electrolytes

Examination Requirements1.43 recall that electrolysis involves the

formation of new substances when ionic compounds conduct electricity

1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead(II) bromide

1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

DefinitionsElectric current - a flow of electrons through a wire, or a flow of electrical charge through a conducting material.Electrode - a device for putting current into or taking current out of a conductor.Electrolysis - a process in which a compound is decomposed by an electric current.

Definitions (continued)Cathode - a negative electrode. An electrode with a surplus of electrons.Anode - a positive electrode. An electrode deficient in electrons.Electrolyte - a substance which, when liquid(molten) or in solution, conducts electricity and is decomposed by the current.

Definitions (continued)Ion - an atom, or group of atoms, with an electric charge. Metal ions are positively charged. Non‑metal ions are negatively charged.Anion - a negative ion which flows towards the anode during electrolysis.Cation - a positive ion which flows towards the cathode during electrolysis.

ElectrolytesTo be an electrolyte, the compound must contain ions to carry the electric current through the compound. The ions must be free to move so that they are attracted to the electrode of opposite charge. The ions are free to move in molten(melted) compounds or when the compound is dissolved in water.Electrolytes are;

a) Ionic compounds (metal/non-metal compounds).b) Acids or alkalis.c) Salts.

Electrolysis of Lead(II) BromideThe electrode connected to the positive side of the battery is the anode.The electrode connected to the negative side of the battery is the cathode.

+ve -ve

graphite anode

graphite cathode

lead(II) bromide

Electrolysis of Lead(II) BromideThe battery is an ‘electric pump’ which provides the energy needed to push the electrons around the external circuit.

Electrolysis of Lead(II) Bromide

Inside the electrolyte, ions are carrying the electric current. Electrons are gained at the cathode and electrons are removed at the anode. Effectively electrons move from the cathode to the anode.

movement of anions

movement of cations

Electrolysis of Lead(II) Bromide

Inside the electrolyte, ions are carrying the electric current. Electrons are gained at the cathode and electrons are removed at the anode. Effectively electrons move from the cathode to the anode.

movement of anions

movement of cations

Movement of IonsDuring electrolysis, ions move through the molten electrolyte.The negative ions, anions, flow towards the anode.The positive ions, cations, flow towards the cathode.The bulb only lights when a current is flowing. If the electrolyte is solid, there are no free ions, so the charge (electric current) cannot be carried through the electrolyte.

Questions 1-31. What could be used instead of the bulb

to show an electric current is flowing through the electrolyte?

2. Why is electrolysis classed as a decomposition reaction?

3. Is electrolysis exothermic or endothermic? Give a reason for your answer.

Ions at the CathodeWhen the positive ions arrive at the cathode, they gain electrons.Atoms are formed on the surface.

M+(l) + e- M(s)

Metals and hydrogen are formed at the cathode.

Ions at the AnodeWhen the negative ions arrive at the anode, they lose electrons.Atoms are formed on the surface.

2X-(l) - 2e- X2(g)

Non-metals are formed at the anode, usually as molecules.

Equations for Lead(II) Bromide

Reaction at cathodePb2+(l) + 2e- Pb(l)

Reaction at anode2Br-(l) - 2e- Br2(l)

Questions 4-54. What are the products for the electrolysis of the

following compounds? For each compound, write the equations for the reactions at the anode and cathode.

a) Sodium chloride,b) Magnesium oxide,c) Calcium fluoride,d) Iron(III) bromide.

5. What would be the products if a mixture of potassium fluoride and lead(II) iodide were electrolysed?

Metal Extraction

Metals low down in the Reactivity Series can be extracted from solutions by electrolysis.This method is more often used for the purification of the metal, rather than the extraction of the metal from its ore.