Electrolysis for AQA Comb

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Basic Principles of Electrolysis

Electrolysis: Chemical decomposition produced by passing an electric current through a liquid or solution containing ions.

Electrolyte: An ionic substance with freely moving ions. Electrolytes conduct electricity.

Cation: A positively charged ion

Anion: A negatively charged ion

Electrolysis uses electricity to break down an electrolyte.

Two electrodes (usually a carbon or a metal) are connected to a d.c. current and placed in the electrolyte.

The positive electrode is called the anode

The negative electrode is called the cathode

The ions migrate towards the electrode with the opposite charge

Cations collect at the cathode

Anions collect at the anion

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Oxidation and Reduction (HT Only)

Reduction: Loss of oxygen or gain of electrons

Oxidation: Gain of oxygen or loss of electrons

Use the mnemonic OILRIG to help you to remember this

Oxygen Is Loss, Reduction Is Gain

Anions lose electrons at the anode

Cations gain electrons at the cathode

This results in charged ions becoming atoms or molecules

Example: In the electrolysis of molten zinc chloride (\(ZnCl_2\)), \(Zn^{2+}\) ions are attracted to the cathode, where they gain two electrons and become zinc atoms. At the same time, chloride ions are attracted to the anode where they lose electrons to form chlorine molecules.

These changes can be represented by half equations, which show what takes place at each electrode:

Cathode reaction \(Zn^{2+} + 2e^- \rightarrow Zn\) Reduction

Anode reaction: \(2Cl^- \rightarrow Cl_2 + 2e^-\) Oxidation

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Manufacture of Aluminium

Very reactive metallic elements are extracted from ores by electrolysis

Because they are higher up in the reactivity series than carbon, they cannot be displaced from their oxides by carbon

The usual source of aluminium is the ore, bauxite.

This is treated chemically to leave a white aluminium oxide powder, \(Al_2O_3\), commonly known as alumina

Alumina has a very high melting point .

Therefore, in order for it to be electrolysed, it has to be dissolved in molten cryolite

Acts as a solvent for the alumina

Cryolite also has a high melting point

Cathode:

Attracts Al ions where they gain electrons to form aluminium metal

\(Al^{3+}+3e^- \rightarrow Al\)

Anode:

Attracts oxide ions

Lose electrons and form oxygen gas

Oxygen gas reacts with carbon anodes, so they have to be replaced from time to time

\(2O^- \rightarrow O_2 + 4e^-\)

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Electrolysis of an Ionic Compound

Lead bromide has a high melting point and so must be heated in a crucible so that it will melt

Cathode:

Zinc ions are changed into zinc atoms as they gain electrons

\(Zn^{2+}(l) +2e^- \rightarrow Zn (s) \)

Anode:

Bromide ions are changed into bromine gas as they give up electrons to the anode

\(2Br^-(l) \rightarrow Br_2(g) +2e^-\)

Note: The above details are given for lead chloride, but a similar situation will occur if any other ionic compound is used. The anions will attracted to the anode where they will be oxidised. THe cations will be attracted to the cathode where they will be reduced

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Electrolysis of Copper(II) chloride

When salts dissolve in water, positive and negative ions that can move through the water molecules are released

Electrolysis can be used, in some cases, to separate these two types of ions

Cathode:

Copper ions are attracted to the cathode, where they gain electrons to form copper atoms

The salmon pink discolouration seen at the cathode is a thin layer of copper metal

\(Cu^{2+}(aq) +2e^- \rightarrow Cu (s) \)

Anode:

Chloride ions are attracted to the anode, where they lose electrons and combine to form molecules of chlorine gas

Bubbles can be see at the anode - this is the chlorine gas

\(2Cl^-(aq) \rightarrow Cl_2(g) +2e^-\)

Note:

This is not the case for solutions of ionic salts containing more reactive metals such as sodium

Water also decomposes to form hydrogen and hydroxide ions

Hydrogen ions gain electrons more easily than sodium and so will be preferentially reduced

Hydrogen gas will be seen at the cathode

The reactive metal remains in solution

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Electrolysis of water

The electrolysis of water with sodium sulfate or sulfuric acid breaks it down into hydrogen and oxygen gases. This is because hydroxide is discharged more readily than sulfate ions and hydrogen ions are discharged more readily than sodium ions.

Since the formula of water is \(H_2O\), twice as much hydrogen as oxygen gas will be produced.

The apparatus used to carry out this experiment is known as a Hofman Voltameter

The Hofman Voltameter has platinum electrodes.

(REMEMBER! A voltmeter is used to measure potential difference or voltage, in this case we are using a voltAmeter!)

Anode:

Oxidation reaction

\(4OH^-(aq) \rightarrow 2H_2O(l) + O_2(g) + 4e^-\)

Cathode

Reduction reaction

\(4H^+(aq) + 4e^- \rightarrow 2H_2(g)\)

The electrolysis of water with sodium chloride will produce hydrogen and chlorine.