Electrolysis

ElectrolysisElectrolysis

the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at electrodes and separation of materials.

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Requirements of Requirements of electrolysiselectrolysis

An electrolyte - It is a substance containing free ions which are the carriers of electric current in the electrolyte

A direct current (DC) supply - It provides the electrical energy necessary to create or discharge the ions in the electrolyte. Electrical current is carried by electrons in the external circuit.

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Requirements of Requirements of electrolysiselectrolysis

Two electrodes - Electrode is an electrical conductor which provides the physical interface between the electrical circuit providing the energy and the electrolyte.

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Requirements of Requirements of electrolysiselectrolysis

The following figure shows the electrolyte, battery

( power supply) and electrodes

( cathode and anode ) required for electrolysis.

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ElectrolysisElectrolysis

Molecule of solid copper chloride

CuCl2 (s)

Molecule of solid copper chloride after being dissolved

CuCl2 (aq)chloride

ionCopper

ion

If you melt or dissolve an ionic compound (such If you melt or dissolve an ionic compound (such as NaCl or CuClas NaCl or CuCl22), then the ions become free to ), then the ions become free to move around – and carry electrical current.move around – and carry electrical current.

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ElectrolysisElectrolysis

Electrolysis is Electrolysis is used to used to separate a separate a metal from metal from its its compound.compound.

= chloride ion

= copper ion

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Purifying CopperPurifying Copper

++++

----

Solution containing copper ions

Impure copper

Cu2+

Cu2+

Cu2+

Pure copper

At the anode:

Cu(s) Cu2+(aq) + 2e-

At the cathode:

Cu2+(aq) + 2e- Cu(s)

13/04/23Electrolysis equationsElectrolysis equationsWe need to be able to write “half equations” to show what happens during electrolysis (e.g. for copper chloride):

2 2

2

At the negative electrode the positive ions GAIN electrons to

become neutral copper ATOMS. The half equation is:

Cu2+ + e- Cu

At the positive electrode the negative ions LOSE electrons to

become neutral chlorine MOLECULES. The half equation is:

Cl- - e- Cl2

Oxidation and Reduction in Oxidation and Reduction in ElectrolysisElectrolysis

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At the negative electrode the

positive ions GAIN electrons – this is called Reduction. The half equation

is:

Cu2+ + 2e- Cu

At the positive electrode the

negative ions LOSE electrons –This is called Oxidation.

The half equation is:

2Cl- - 2e- Cl2

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Electrolysis of brineElectrolysis of brine

Positive electrode

Negative electrode

Sodium chloride (brine)

NaCl(aq)

Sodium hydroxide (NaOH(aq)). Used to make soap, paper

and ceramics

Sodium chloride (salt) is made of an alkali metal and a halogen. When it’s dissolved we call the solution “brine”, and we can electrolyse it to produce 3 things…

Chlorine gas (Cl2) – used to kill bacteria and to make acids, bleach and plastics

Hydrogen gas (H2) – used to manufacture ammonia and margarine