Electrolysis for AQA Chem

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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

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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Electrolysis of Copper Sulfate

During the electrolysis:

Copper atoms in the anode lose two electrons to become copper ions

These ions are attracted to the cathode, where they gain two electrons and are deposited as pure copper

Anode loses mass, cathode gains mass

Impurities from the ions collect as a sludge beneath the anode

Half equations:

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

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

Electroplating

A technique using electrolysis to coat one metal with a thin layer of the other

Usually a more expensive metal, such as gold or silver, is plated in a thin layer onto a cheaper metal such as steel

Advantages of electroplating objects include:

Improves appearance

Increases value

Cheaper than using large amount of expensive metal

Can improve resistance to corrosion and resistance to abrasion and wear

Can reduce electrical resistance

When using copper electrodes, include variable resistor in experimental set up

Wash electrodes in distilled water and then dip them in propanone before use

Record the masses of the dry electrodes before the start of the experiment

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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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Production of Chlorine Gas by Electrolysis

When the sodium chloride is dissolved in water, the ions forming the ionic solid separate out. This means that there are actually 4 ions present in the solution: H+, OH-, Na+ and Cl-.

This electrolysis can be demonstrated using a U-Tube, although this is not the industrial set up

Anode:

The indicator turns red initially, and is then bleached to colourless.

This indicates the presence of chlorine.

The negative ions are attracted to the positive electrode.

The chloride ions are discharged (giving chlorine) in preference to the hydroxide ions.

These are left behind in solution.

Cathode:

The product at the negative electrode is hydrogen.

At the negative electrode the indicator turns purple.

At the negative electrode, the hydrogen ions are discharged (producing hydrogen gas) in preference to the sodium ions.

These are also left behind in solution.

Thus sodium hydroxide solution remains. This is the cause of the purple colour of the indicator at the negative electrode.

Some of the water will ionise, that is, turn to hydrogen (H+) and hydroxide (OH-) ions.

The remainder of the solution stays green.

In time, the green colour of the indicator in the middle would change too, as the ions diffuse through the resulting solution.

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Industrial Production of Chlorine Gas

Chlorine is industrially produced by the electrolysis of brine (a concentrated solution of NaCl)

This is called chloroalkali process

The non-permeable ion exchange membrane allows sodium ions to pass into the second chamber where sodium hydroxide forms

Chlorine gas is collected from the anode and hydrogen gas from the cathode.

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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.