The purpose of electrolysis is to split up ionic compounds using electricity to produce useful products.

Electrolysis is used a lot in industry and is a multi-billion pound industry. It is used to produce all of the Copper, Aluminium, Chlorine and Caustic Soda (sodium hydroxide) that we use today as well as to produce gold and silver plated jewellery and chrome-plated taps and bicycle parts

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AQA OCR EdexcelEdexcel

From 2011

They’re too small to see but the cathode has billions of electrons on its surface

The Basics:

1) Positive ions are attracted to the negative electrode and GAIN electrons.

2) Negative ions are attracted to the POSITIVE electrode and LOSE electrons

More detailed:

1) Positive ions (cations) are attracted to the negative electrode (the cathode) and GAIN electrons. These ions are REDUCED and the element is formed. For example, the positive ions above will each gain 2 electrons to become the atom of the element

2) Negative ions (anions) are attracted to the POSITIVE electrode (the anode) and LOSE electrons, they are OXIDISED. The anions in the example above will each lose 1 electron to become atoms of the element.

Click here to find out how ionic compounds dissolve

To find out how these ions got into the solution, click here

The Basics:1) When the current is switched on the Copper

ions are attracted to the negative electrode and gain electrons. You’d see a light brown coating form on the electrode as the copper is deposited.

2) The Chloride ions are attracted to the POSITIVE electrode and LOSE electrons. You’d observe bubbles of Chlorine gas being produced.

More detailed:

At the cathode (negative electrode): Each Copper ion gains 2 electrons to form Copper metal

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

At the anode (positive electrode): Each Chloride ion loses one electron to form Chlorine atoms. These combine to form molecules of Chlorine gas.

2Cl- (aq) - 2e- Cl2 (g)

Cl-1

Cl-1

Cl-1

Cl-1

Cl-1

Cl-1 Cu+2

Cu+2

Cu+2

The Basics:1) When the current is switched on the

Potassium ions are attracted to the negative electrode and gain electrons. You’d expect to see Potassium metal coating the electrode wouldn’t you? Instead you’d see Bubbles

2) The Bromide ions are attracted to the POSITIVE electrode and LOSE electrons. You’d observe water around the electrode turning a red / brown colour as BROMINE is formed

More detailed:

At the cathode (negative electrode): Each Potassium ion gains 1 electron to form Potassium metal. The Potassium immediate reacts with the water to produce Potassium Hydroxide and Hydrogen gas

K+ (aq) + 1e- K (s)

K (s) + H2O (l) KOH (aq) + H2 (g)

At the anode (positive electrode): Each Bromide ion loses one electron to form Bromine atoms. These combine to form Bromine, which is a liquid and dissolves in water.

2Br- (aq) - 2e- Br2 (g)

Br -1

Br -1

Br-1

Br -1

Br -1

Br-1

K +

K+

K +

K+

K+

These are called “electrode equations” and show how electrons are lost or gained at the electrode.

The Basics:1) Sodium Chloride from seawater is

electrolysed to make a number of important products. The company then sells these to make a profit.

2) Because the raw material is so cheap, the company only has to pay for the electricity to electrolyse the salt solution (called brine).

3) The products are Sodium Hydroxide (detergents), Chlorine (bleach and water treatment) and Hydrogen (fuel)More detailed:

At the cathode (negative electrode): Each Sodium ion gains 1 electron to form Sodium metal. The Sodium immediate reacts with the water to produce Sodium Hydroxide and Hydrogen gas

Na+ (aq) + 1e- Na (s)

Na (s) + H2O (l) NaOH (aq) + H2 (g)

At the anode (positive electrode): Each Chloride ion loses one electron to form Chlorine atoms. These combine to form Chlorine, which is a green coloured gas.

2Cl- (aq) - 2e- Cl2 (g)

Cl -1

Cl -1

Cl-1

Cl -1

Cl -1

Cl-1

Na +

Na+

Na +

Na+

Na+

These are called “electrode equations” and show how electrons are lost or gained at the electrode.

O HNa+

O HNa+

O HNa+

O HNa+

Cl2

H2

Cl2

Cl2

Cl2

H2

H2

H2

The Basics:

1) Impure Copper starts off at the anode. When the current starts Copper ions from the solution are attracted to the cathode and form copper metal.

2) Copper atoms LEAVE the anode and go into the solution, replacing the ones that have already gone to the cathode.

3) As this process continues the cathode becomes heavier since it is gaining Copper metal and the anode becomes lighter as it loses Copper atoms and impurities.

4) These impurities fall to the bottom of the vessel.

More detailed:

At the cathode (negative electrode): Each Copper ion gains 2 electrons to form Copper metal.

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

At the anode (positive electrode): Each Bromide ion loses one electron to form Bromine atoms. These combine to form Bromine, which is a liquid and dissolves in water.

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

Remember: Whatever is attracted to the cathode GAINS electrons. We say that it is REDUCED

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+Cu2+

SO4-2

SO4-2

SO4-2

SO4-2

SO4-2

SO4-2

Cu

Cu

Cu Cu2+

Cu2+

Cu2+

Remember: Electrons are always lost at the anode. We call this Oxidation. To find out more about oxidation and reduction, click here.

Note: Nothing happens to the Sulphate (SO4 -2 ) ions in this process

The Basics:

1) Some substances react with water or are completely insoluble. To electrolyse these you have to melt them in order to free the ions so they can move.

2) Molten electrolytes are usually much easier to figure out; positive ion goes to negative electrode / negative ion goes to the positive electrode.

For example: Lead Bromide (PbBr2)

At the cathode (negative electrode): Each Lead ion gains 2 electrons to form Lead metal

Pb+2 (l) + 2e- Pb (s)

At the anode (positive electrode): Each Bromide ion loses one electron to form Bromine atoms. These combine to form Bromine which, at that temperature, is a brown gas.

2Br- - (l) - 2e- Br2 (g)

For example: Potassium Oxide(K2O)

At the cathode (negative electrode): Each Potassium ion gains 1 electron to form Potassium metal

K+ (l) + 1e- K (s)

At the anode (positive electrode): Each Oxide ion loses 2 electrons to form Oxygen atoms. These combine to form Oxygen gas.

O -2(l) - 2e- O2 (g)

Click the button to watch a video of molten Lead Bromide being electrolysed

The Basics:

1) The bauxite ore is purified and called alumina. It’s chemical name is Aluminium Oxide (Al2O3) It has to be melted so that the ions are free to move. A compound called cryolite is used to lower the melting point of Alumina.

2) The negatively charged Oxide ions move to the positive electrode, lose electrons and form Oxygen gas.

3) The positively charged Aluminium ions move to the negative electrode, gain electrons and form Aluminium metal.

Click below to watch a video of Aluminium being extracted industrially

More detail

At the cathode (negative electrode): Each Aluminium ion gains 3 electrons * to form Aluminium metal.

Al+3 (l) + 3e- Al (s)

At the anode (positive electrode): Each Oxide ion loses 2 electrons * and Oxygen gas is released

2O-2 (l) - 4e- O2 (g)

This would probably be good enough to get all of the marks but, to be completely accurate we have to balance the overall equation – this means balancing the electrons, which, as you can see, aren’t balanced above:

Reduction: 4 Al+3 + 12e- 4AlOxidation: 6 O-2 - 12 e- 3O2

Overall: 2 Al2O3 4 Al + 3 O2

To learn moreabout charges on ions, click here.

Test YourselfHave a go at these questions then click the button at the bottom of the page for the answers.

Click here for the answers

Even though these questions aren’t specifically about electrolysis, the examiner is testing whether you appreciate the industrial, economic and environmental impact of Chemistry.Click below to find out more about this part of the course

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AnswersCheck your answers to the questions – click the button to go to the relevant slide in the turorial.

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Problems? Click this button to revise this topic

Chloride ions are negatively charged and are electrostatically attracted to the positive electrode

Hydrogen

Even though these questions aren’t specifically about electrolysis, the examiner is testing whether you appreciate the industrial, economic and environmental impact of Chemistry.Click below to find out more about this part of the course

Chlorine is a poisonous gas. An accident releasing Chlorine would damage plants and animals

Closing the factory would mean people losing their jobs, which would be bad for the community.

Scientists working for the chemical company might be biased.

Dissolving Ionic Compounds

As you probably remember, ionic compounds are almost always solids at room temperature. The ions are held together by strong electrostatic forces of attraction ( +ive ions attract –ive ions). They have high melting points and DO NOT conduct electricity in the solid state

Na+

Cl-

Cl-

Cl-

Na+

Na+

Na+Na+Na+

Cl-

Cl-

Cl-

The water particles break the bonds holding the ions together allowing the ions to move freely and randomly.In this dissolved state, or when molten, ionic compounds will conduct electricity because THE IONS ARE FREE TO MOVE when a voltage is applied

Click here to see why some ionic compounds dissolve and others don’t

Na+

Dissolving Ionic Compounds

As you probably remember, ionic compounds are almost always solids at room temperature. The ions are held together by strong electrostatic forces of attraction ( +ive ions attract –ive ions). They have high melting points and DO NOT conduct electricity in the solid state. The ionic bonds require a lot of energy to break them.

Cl-

Na+

Cl-

If you look closely you will see that the d- Oxygen atoms are clustering around the Na+ ions and the d+ Hydrogen atoms pack closely around the Cl- ions. Many many tiny little bonds are formed which make the Sodium and Chloride ions stable in the water. This is why Sodium Chloride dissolves in water. Note: The Sodium ions and the Chloride ions float around randomly in solution until a voltage is applied

Click here to see why some ionic compounds dissolve and others don’t

O

H

H d

+

d+

d-

O

HH d

+ d

+

d-

Water is called a “polar molecule”. This means that the Oxygen atom has a slightly negative charge (d- means slightly negative) and each Hydrogen atom is slightly positive (d+ means slightly positive)

Na+ O H

H d

+ d+

d-

O

HH

d

+

d+

d

-

O

H

H

d+

d+

d-

O H

H

d+

d+

d

-

O H

H d +

d +

d -

O H

H d+

d+

d- O

HH

d + d +

d -

O H

H d+

d+

d-

O

H

H

d+

d

+

d

-

O

H

H d +

d +

d -