C2 - Electrolysis

Question 1

Define electrolysis.

Answer 1

Passing an electric current through a liquid to separate it.

Question 2

Define electrolyte.

Answer 2

A substance which has been broken down. It must be a liquid which has been obtained by either dissolving or melting because the ions become free to move.

Question 3

Describe the electrolysis of a melted substance such as lead bromide.

Answer 3

In a melted substance there is just the ions from the melted substance so in the case of lead bromide there will be Pb²⁺ ions and Br^- ions.

The Br^- ions go to the positive anode.

The Pb+ ions move to the negative cathode.

These happen because opposite charges attract each other.

Current is a flow of electrons. It will add electrons to the Pb²⁺ ions at the cathode turning them into atoms and metal will coat on the cathode. At the anode, electrons are taken from the Br^- ions turning them into Br atoms.

2Br^--2e = Br₂ and Pb²⁺+ 2e -> Pb

So, using OILRIG

Oxidation

Is

Loss

Reduction

Is

Gain

The bromide ions have been oxidised. The lead ions have been reduced.

Question 4

Explain electrolysis for a dissolved substance such as sodium chloride (brinewater).

Answer 4

A dissolved substance has both the ions from the substance as well as the ions from water.

NaCl H₂O

So we get these ions: Na²⁺, Cl^-, H⁺, OH^-

Anode: Cl^- OH^- ANODE RULE - GROUP 7 GIVEN OFF

Cathode: Na²⁺, H⁺ CATHODE RULE - LEAST REACTIVE GIVEN OFF

Cathode, the least reactive element is given off - Hydrogen.

Anode, the group 7 element is given off - Chlorine.

Cathode: 2Cl^-- 2e -> Cl₂

Anode: 2H⁺ + 2e -> H₂

Using OILRIG we can say the chlorine has been oxidised and the hydrogen has been reduced.

Question 5

How can we test for and use the products of electrolysis of brine?

Answer 5

NaOH will remain in the solution and can be tested with universal indicator to be around pH 10.

Chlorine will bleach damp litmus paper.

Hydrogen gas can be collected and tested for with a lighted splint. If it does a squeaky pop it is hydrogen.

Chlorine can be used to kill bacteria in swimming pools.

NaOH can be used to make soap.

Hydrogen can be used to make margarine or ammonia.

Question 6

What can we use electroplating for?

Answer 6

Plating cheap jewellery.

Question 7

How do we do electrolysis in aliminium oxide?

Answer 7

We add cryolite. This lowers the melting point of the aliminium oxide as it is very high owing to it’s ionic bonds.

Carbon rods are made of graphite because they conduct electricity. The carbon in the electrode reacts with the oxygen to form carbon dioxide which escapes into the air meaning the anode has to be regularly replaced as it wears away.

Question 8

What happens if there is no halogen at the anode?

Answer 8

4OH^-- 4e -> 2H₂O