C6 - Electrolysis

Question 1

What happens when an ionic compound is melted or dissolved in water

Answer 1

The ions are free to move about within the liquid or solution

Question 2

Can an ionic compound which is melted or dissolved in water conduct electricty

Answer 2

Yes

Question 3

What are ionic compounds which is melted or dissolved in water called

Answer 3

Electrolytes

Question 4

What happens when you pass an electric current through electrolysis

Answer 4

causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode).

Question 5

What is the negative electrode called (which positively charged ions move to)

Answer 5

The cathode

Question 6

What is the positive electrode called (which negatively charged ions move to)

Answer 6

Anode

Question 7

What do ions produce at the electrodes

Answer 7

Ions are discharged at the electrodes producing ELEMENTS

Question 8

What is the process of passing an electric current through electrolytes and producing elements called

Answer 8

Electrolysis

Question 9

What happens when a simple ionic compound (eg lead bromide) is electrolysed in the molten state using inert electrodes

Answer 9

the metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.

Question 10

What can be extracted from molten compounds using electrolysis

Answer 10

Metals

Question 11

When is electrolysis used for extraction instead on carbon

Answer 11

if the metal is too reactive to be extracted by reduction with carbon (higher than carbon on the reactivity series) or if the metal reacts with carbon.

Question 12

Why is carbon used over electrolysis if possible

Answer 12

Expensive as large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current.

Question 13

How is aluminium manufactured

Answer 13

by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode).

Question 14

What do the ions discharged when an aqueous solution is electrolysed using inert electrodes depend on

Answer 14

the relative reactivity of the elements involved.

Question 15

When is hydrogen produced at the negative electrode (cathode)

Answer 15

if the metal is more reactive than hydrogen

Question 16

When is oxygen produced at the positive electrode (anode)

Answer 16

is produced unless the solution contains halide ions when the halogen is produced.

Question 17

Why does the negative electrode (cathode) sometimes produce hydrogen and why does the positive electrode (anode) sometimes produce oxygen

Answer 17

because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged.

Question 18

What are the reactions at the cathode during electrolysis

Answer 18

Reductions (positively charged ions gain electrons)

Question 19

What are the reactions at the anode during electrolysis?

Answer 19

Oxidations (negatively charged ions lose electrons )

Question 20

Reactions at electrodes can be represented by half equations, for example:

Answer 20

2H+ + 2e- → H2
Or 4OH- → O2 + 2H2O + 4e- or
Or 4OH- – 4e- → O2 + 2H2O

Question 21

Why is aluminium oxide displaced in molten cryolite in the extraction of aluminium

Answer 21

This lowers the melting point to save energy during extraction

Question 22

Calculate the maximum mass of aluminium metal that can be extracted from 25.5 tonnes of aluminium oxide

Answer 22

Al^2O^3 = (2x27)+(3x16)=54+48=102
No of moles = 25500000/102
2Al^2O^3 moles = 250000x2 =500000
500000 x 27 =13500000 = 13.5 tonnes

Question 23

Why does the positive anode need to be continuously replaced

Answer 23

The carbon anodes gradually burn away and need to be replaced regularly

Question 24

What will be the products of this molten ionic compound 
Zinc Chloride(l)

Answer 24

• > Zinc (Zn^2+ move towards cathode, the negative electrode) + chlorine (Cl- move towards anode, the positive electrode)
• > Zn(s/l) + Cl^2(g)

Question 25

What will be the products of the aqueous solution of

Sodium chloride

Answer 25

Hydrogen gas + chlorine gas + sodium hydroxide solution
At anonde (+) - negative chloride ions are attracted to the positive electrode. As they loose electrons they bond togther in pairs (covalent) and are given off as Cl^2 gas. 2Cl-(aq) - 2e- -> Cl^2
At cathode - positive hydrogen ions and positive sodium ions are attracted to the same electrode. The less reactant element gets discharged. In this case hydrogen ions are discharged and sodium ions stay in the solution as aqueous ions H^2O H+(aq) + OH- (aq)