Electrolytes

Electrolytes are compounds which when molten or dissolved in water conduct electric current and are

decomposed in the process .

Non-electrolyte

A non-electrolyte is a liquid which does not allow the passage of electricity.

Molten Solution

This is composed of lead(II) ions, Pb2 + , and bromide ions, Br-. Its chemical formula is therefore

PbBr2.

A suitable apparatus which could be used to carry out this electrolysis is shown in Figure above.

The bulb helps to show when electricity is flowing in the circuit, and until the lead(II) bromide is

completely molten, the bulb does not light up . This confirms that electrolytes have to be molten

for the ions to start to move to the electrodes and thereby conduct electricity.

At the Cathode At the Anode

Observation

When electricity is flowing, a silvery

deposit of lead metal forms on the

cathode. In fact, as it is molten, it is more

likely to drip off in a molten blob.

Observation

When electricity is flowing, brown

fumes of bromine gas are seen at

the anode.

Half equation 

Pb2+ + 2e ---> Pb

Half equation

2Br- ---> Br2 + e

Explanation

The lead(II) ions, as they are positive,

move to the negative cathode, where each

ion gains two electrons to form a lead

atom.

Any reaction at a cathode involved is

again in electrons. This is called

reduction or more exactly, cathodic

reduction .

Explanation

The bromide ions, as they are

negative, move to the positive

anode, where each loses an

electron to form a bromine atom.

Then two of these newly formed

atoms combine to form bromine

gas.

Any reaction at an anode involves

a loss of electrons.

In summary, the lead(II) bromide is split into its component elements :

PbBr2 ---> Pb + Br2

Electrolysis of Aqueous Sulphuric Acid

As sulphuric acid is aqueous, it is composed not only of hydrogen ions (H+ ) and sulphate ions

(SO42-), but also of hydroxide ions (OH-) from the water.

H2SO4 + H2O --> 2H+ + SO42- + H+ + OH-

The apparatus used to carry out this electrolysis and collect the gases given off is shown in Figure

9 .8 .

When we have more than one type of ion moving to an electrode, selective discharge (or

preferential discharge) takes place.

This means that the ion which can lose or gain electrons with the greatest ease is discharged,

and the other ions, which are harder to discharge, remain in solution .

With the electrolyte aqueous sulphuric acid, migration of ions to the electrodes also occurs.

At the Cathode At the Anode

Here we have only one ion, the

hydrogen, H+ (aq), and each ion gains

an electron to become a hydrogen

atom.

Two of these newly formed atoms

then combine to form a hydrogen gas

molecule .

Here we have a choice of either

sulphate, SO42-(aq), or hydroxide

OH- (aq) ions.

Hydroxide is easier to discharge, so

oxygen gas is given off at the anode.

Equation:

2H+ + 2e ---> H2

Equation:

OH- + 4e ---> O2 + H2O

Notes

With electrolysis of aqueous solutions of dilute acids or alkalis, the volume of

hydrogen given off at the cathode is roughly twice that of the oxygen gas at the

anode.

Accordingly, the elements of water are lost and as the electrolysis continues, the

concentration of the acid or alkali increases .

Essentially, the electrolysis of aqueous sulphuric acid is the electrolysis of water,

with hydrogen and oxygen gas being given off in a ratio of 2 : 1 .

Extraction of Metal

The extraction of metals from their ores, in particular aluminium and sodium, is important

industrial uses of electrolysis.

The diagram below shows the methods of extraction for different metals.

We can see that those metals which are less reactive than carbon in reactivity series are

extracted from their ore by displacement reaction using carbon. This will be discussed in detail in

chapter 3, form 5, Oxidation and Reduction.

Copper and mercury can be extracted from their ore by burning directly in air.

Silver (Ag) and gold (Au) need no extraction because they exist as element in nature.

Those metals which are more reactive than carbon are extracted by electrolysis.

Extraction of Aluminium

Aluminium is the most abundant metal found in the earth's crust. It makes up about 8% by weight

of the Earth’s solid surface.

It is also a very useful metal due to its low density and ability to resist corrosion.

The main source of aluminium is bauxite ore (Aluminium Oxide).

In industry, aluminium is extracted by electrolysis from bauxite ore.

Adding Cryolite

In electrolysis, molten aluminium oxide must be used to extract aluminium. Aluminium oxide

decompose to form aluminium and oxide ions when melted.

Al2O3 ---> 2Al3+ + 3O2-

However, the melting point of aluminium oxide is very high (over 2 000°C), so another

aluminium compound called cryolite (Na3AIF6) is added to lower down the melting point

(about 980oC).

The diagram above shows how aluminium is extracted from molten aluminium oxide by

electrolysis.

Graphite is used as the anode and cathode.

During electrolysis, the aluminium ions are attracted towards the graphite cathode.

The ions is discharged and become molten aluminium metal.

The partial equation of this reaction is as follow:

Al3+ + 3e ---> Al

At the anode, oxygen gas which also has commercial value is collected. The partial equation

of this reaction is as follow:

2O2- ---> O2 + 4e

At the temperature of 980 °C, the oxygen burns the carbon anode. Therefore the anode has

to be replaced periodically.

Also, this cell uses large quantities of electricity, and therefore needs cheap sources of power.

Extraction of sodium chloride

In industry, sodium is extracted from molten sodium chloride. Molten sodium chloride is put

into the apparatus as showing in the diagram above.

When sodium chloride is melted, the sodium and chloride ions disassociate to become freely

move ions, as shown in the chemical equation below.

NaCl ---> Na+ + Cl-

In this electrolytic cell, graphite was used as anode while iron is used as cathode.

The negative chloride ions are attracted to the anode and then discharged to form chlorine

gas.

2Cl- ---> Cl2 + 2e

Since chlorine gas is also significant in industry, it is collected and stored.

In cathode, the sodium ions are discharged to form sodium atom.

Na+ + e ---> Na

Due to high temperature, the sodium metal formed is in molten form.

Metal sodium have lower density. Therefore it moves upward and been collected.

Purification Of Copper

In the refining or purification of copper, the impure copper is made the anode and a thin, pure

copper plate is used as a cathode.

The electrolyte is usually acidified copper(II) sulphate solution.

When electricity flows, the copper dissolves from the impure anode and goes into solution as

copper ions.

Impurities in the copper do not dissolve, and instead fall off the anode as anode sludge. At the

cathode, the copper ions are deposited as pure copper metal.

Reaction in anode (impure copper)

In anode, the copper atoms from the electrode are ionised to form copper(II) ions.

Cu ---> Cu2+ + 2e

Reaction in cathode (pure copper)

Cu2+Cu ---> Cu + 2e

Electroplating

Electroplating: Coating with a Thin Protective Layer of Metal

A very common use of electrolysis is to form a thin protective coating of a metal on the

surface of another which is likely to corrode.

The diagram above illustrate the electroplating of a key with copper.

In this process, we need to make the cathode the object for plating (the key.

The anode is then made of the metal we wish to plate with (copper), and the electrolyte needs

to be a solution of a salt of this metal (copper(II) sulphate).

Anode

In anode, the copper atoms from the electrode are ionised to form copper(II) ions.

Cu ---> Cu2+ + 2e

Cathode

In cathode, the copper ions are discharged to form copper atom and then deposit on the

surface of the key

Cu2+ ---> Cu + 2e

Cells and Batteries

A device which converts chemical energy into electrical energy is called a cell or battery. Battery

is a collection of cells.

A cell consists of a pair of dissimilar metals in an electrolyte.

Figure above shows an example of a simple voltaic cell consist of a magnesium electrode and a

copper electrode immerse in magnesium sulphate solution.

When chemical reaction happens, the more reactive metal, magnesium, dissolves in the

magnesium sulphate solution and become magnesium ions, thereby producing electrons, as

shown in the half equation below:

Mg ---> Mg2+ + 2e

As electrons are produced, the magnesium acts as the negative electrode.

These electrons then travel to the copper electrode.

The hydrogen ions around the copper electrode receive the electrons and are discharged to

produce bubbles of hydrogen gas:

2H+ + 2e ---> H2

As electrons are taken in, the copper is the positive electrode.

This production and movement of electrons is electricity, so electrical energy has been

generated and the galvanometer is deflected. *Overall, the chemical reaction can be

represented by the ionic equation:

Mg + 2H+ ---> Mg2+ + H2

In voltaic cell, the negative electrode is the anode whereas the positive electrode

is the cathode, which is the opposite of the electrolytic cell.