Electrolysis

Question 1

What is an electrolyte?

Answer 1

An electrolyte is the substance being broken down in electrolysis. It contains free ions to conduct electricity

Question 2

Why are electrodes made of unreactive substances( e.g graphite or platinum)?

Answer 2

So they do not react with the electrolyte or products made in electrolysis

Question 3

What happens during electrolysis?

Answer 3

• Positively charged ions move to the negative. Here they gain electrons (reduction)
• Negatively charged ions move to the positive electrodes. Here they lose electrons(oxidation).

Question 4

Describe electrolysis of molten lead bromide

Answer 4

Needs to be molten because it is an ionic substance which does not conduct electricity when solid

Bromide ions(-)lose electrons and are attracted to positive electrode where it forms bromine atoms 
A pair of  bromine atoms covalently bond to form a bromine molecule and it is released as bromine gas.

Lead ions(+) gain one electron and are attracted to negative electrode to form lead atoms.

Half equations( see notebook or revision guide)

Question 5

What effect does water have on electrolysis(negative electrode)?

Answer 5

In aqueous solutions if 2 elements can be produced at an electrode, the less reactive element will usually be formed.

In a solution we usually have positive metal ions and hydrogen ions (from water) attracted to the negative electrode. Hydrogen will form if it is less reactive than the other metal.

The ion not discharged at negative electrode stays in the solution.

Question 6

What’s the effect of water on the positive electrode?

Answer 6

Hydroxide ions from water are usually discharged at the positive electrode unless the solution contains a halide ion( chloride, iodide, bromide,fluoride)

If hydroxide ions are discharged we see oxygen gas being giving off at the positive electrode.

The ion that is not discharged at the electrode stays in the solution

Question 7

Describe the electrolysis of sodium chloride

Answer 7

At the positive electrode the negative chloride ions are attracted to it, they each lose one electron. The chlorine atoms that form then bond in pairs to form a chlorine molecule and are given off as chlorine gas. Hydroxide ions are also attracted to the positive electrode but chloride ions are discharged because it is a halide ion( so takes precedence)

At the negative electrode, the positive hydrogen ions are attracted to it and gain one electron.Sodium ions are also attracted but since hydrogen is less reactive, hydrogen ions are discharged and sodium ions stay in the solution.

If you were to test the remaining solution with indicator paper, you’ll see it is alkaline as the sodium ions react with hydroxide ions to form sodium hydroxide.

Question 8

What are the uses of the products of the electrolysis of common salt( NaCl)?

Answer 8

Chlorine: bleach( reacted with sodium hydroxide) and plastics

Hydrogen: hydrogenation

Sodium hydroxide: bleach and soap

Question 9

What are the uses of aluminium?

Answer 9

Pens, overhead power cables, aeroplanes

Question 10

What is electrolysis?

Answer 10

The breaking down of a substance using electricity

Question 11

What is the compound electrolysed in the extraction of aluminium?

Answer 11

Al2O3( aluminium oxide)

Question 12

Where do we get the aluminium oxide from?

Answer 12

Bauxite ore which is mined by open cast mining

Question 13

Describe the process of extracting aluminium

Answer 13

Bauxite ore contains rocky impurities. So the first step is to separate aluminium oxide from the impurities( e.g Iron(iii) oxide which colours the waste solution from the separation process brown)

To electrolyse the aluminium oxide we first have to make sure that the ions are free to move to the electrodes( make it molten).

Aluminium oxide has a high melting point though( over 2050 degrees)
However chemists have found a way of saving energy my mixing it with molten cryolite

This brings the temperature down to about 900 degrees which makes it cheaper and easier to use.

Question 14

Why are electrodes often made of carbon?

Answer 14

Good conductor of electricity and graphite is unreactive

Question 15

Describe the electrolysis of aluminium oxide

Answer 15

At the negative electrode:

Aluminium(Al3+) ions gain 3 electrons and are reduced to form Aluminium atoms

At the positive electrode:

Each oxide ion(O2-) loses 2 electrons and the ions are oxidised to form oxygen atoms. These form covalent bonds to form molecules of oxygen gas

Question 16

What is electroplating?

Answer 16

Electroplating is coating an object with a thin layer of metal by electrolysis

Question 17

Why do we electroplate objects?

Answer 17

To protect the metal beneath from corroding
To make the object look more attractive
Increase the hardness of a surface and its resistance to scratching
To save money by using a thin layer of a precious metal instead of the pure expensive metal.

Question 18

What are the disadvantages of electroplating?

Answer 18

It makes things more expensive(better in long term because you don’t have to replace objects as often).

Question 19

What acts as the negative electrode in electroplating?

Answer 19

The metal object to be plated( e. Copper foil)

Question 20

What acts as the positive electrode in electroplating?

Answer 20

The positive electrode is made from the plating metal(e.g nickel)

Question 21

Describe the process of electroplating copper foil with nickel metal

Answer 21

The solution used takes place in a solution containing nickel ions

At the positive electrode( made of plating metal)

-Nickel atoms in the electrode are oxidised. They lose 2 electrons each and form nickel ions.

At the negative electrode( Made of metal to be plated)

-Nickel ions from the solution are reduced. They gain 2 electrons and form nickel atoms which are deposited on the copper electrode.