5 Electricity and Chemistry

Question 1

Define electrolysis

Answer 1

The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity

Question 2

What are electrolytes

Answer 2

Compounds that can conduct electricity

has to be molten or dissolved in water (ions must be free to move around)

Question 3

What is the anode

Answer 3

positive terminal of the power supply

Question 4

What is the cathode

Answer 4

negative terminal of the power supply

Question 5

Explain the electrolysis of molten lead(II) bromide (5)

Answer 5

Lead(II) bromide (PbBr2) is ionically bonded and contains Pb2+ ions and Br- ions.

When the solid is melted and voltage is applied, the ions are free to move

The positive lead ions move to the cathode and the bromide ions move to the anode

The products are lead and bromine

silvery deposits of lead form near the bottom of the dish and brown bromine vapour is found need the anode

Question 6

What happens when an electric current passes through an electrolyte

Answer 6

new substance is formed

Question 7

Why doesnt the electrodes undergo any chemical change

Answer 7

electrodes are usually made of carbon, which is inert

Question 8

Explain electrolysis of concentrated sodium chloride solution

Answer 8

When concentrated sodium chloride solution is electrolysed, hydrogen ions (from the water solvent) form hydrogen molecules at the cathode and chloride ions form chlorine molecules at the anode

Question 9

Explain electrolysis of copper(II) sulfate solution using carbon electrodes

Answer 9

Products: copper and oxygen

copper forms as a red-brown coating on the cathode (Cu2+ and H+ move to cathod and only Cu2+ ions are discharged)

bubbles of colourless gas, oxygen seen next to the anode
(SO4 2- and OH- move to the anode, but only OH- ions are discharged)

Question 10

Explain electrolysis of dilute sulfuric acid (5)

Answer 10

hydrogen at cathode

oxygen at anode

Colourless gases at both electrodes
Cathode gas makes a lighted splint go ‘pop’
Anode gas relights a glowing splint

Question 11

How to predict products of electrolysis of simple MOLTEN ionic compounds

Answer 11

metal form at cathode
non-metal form at anode

EG: electrolysis of aluminium oxide

aluminium —> cathode
oxygen —> anode

Question 12

How to predict products of electrolysis for AQUEOUS solutions

Answer 12

cathode: metal below hydrogen in reactivity series —> deposited on the cathod

metal above hydrogen —> hydrogen gas deposited on cathode

Anode: depends on concentration of solution

Question 13

What is electroplating

Answer 13

When a piece of one metal is coated in a thin layer of another metal

Question 14

Why are metal objects electroplated (2)

Answer 14

prevent corrosion

improve appearance

Question 15

Basic principles of electroplating

Answer 15

The object to be electrolplated is the cathode

The anode is composed of the metal to be plated onto the object

The electrolyte solution is a salt containing ions of the metal to be plated onto the objcet

Question 16

Explain electrolysis of copper(II) sulfate solution using copper solution (2)

Answer 16

Copper deposited as a red coating at the cathode(Cu2+ ions gain two electrons and are discharged)

The anode slowly dissolves and loses mass (copper atom loses two electrons and Cu2+ ions are formed)

Question 17

How to refine copper (3)

Answer 17

At the anode (+), copper atoms lose electrons and go into solution as copper(II) ions:
Cu(s) → Cu 2+ (aq) + 2e –

The Cu 2+ ions are attracted to the cathode (–) where they gain electrons, forming copper atoms:
Cu 2+ (aq) + 2e – → Cu(s)
A layer of pure copper builds up on the cathode.

Precious metals such as platinum, silver and gold, that might have been present in the impure copper anode, drop to the bottom. This mixture is known as anode slime

Question 18

How is aluminium extracted

Answer 18

Impurities are removed from bauxite to produce pure aluminium oxide.
The aluminium oxide is dissolved in molten cryolite (Na 3 AlF 6 ), another ore of aluminium. This is because the melting point of Al 2 O 3 is very high, at around 2000 °C. A mixture of Al 2 O 3 dissolved in molten cryolite has a lower melting point, which saves heat energy, and also has a higher conductivity.
The molten Al 2 O 3 undergoes electrolysis in a tank where both the anode and the cathode are made of carbon:

Al 3+ ions move to the lining of the tank (the cathode (–)) where they gain three electrons to form aluminium atoms: ​​
Al 3+ (l) + 3e – ​→ Al(l) reduction
The liquid aluminium is run off the bottom of the tank​.

O 2– ions move to the carbon anodes (+) where they lose 4 electrons to form oxygen:
2O 2– (l) → O 2 (g) + 4e – oxidation

The oxygen forming at the anodes reacts with the carbon anode, forming carbon dioxide, CO 2 :
C(s) + O 2 (g) → CO 2 (g)
This means that the anodes wear away and have to be replaced regularly.

Question 19

Properties of aluminium(3)

Answer 19

impermeable

resistant to corrosion

high strength to weight ratio

Question 20

Uses of aluminium (3)

Answer 20

packaging drinks can etc

transport - aeroplanes

building material

Question 21

Manufacturing sodium hydroxide,, hydrogen and chlorine