5: Electrolysis

Question 1

Define electrolysis

Answer 1

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

Question 2

what happens at anode in electrolysis?

Answer 2

• anode (+ve charge)
• attracts anions (-vely charged ions)
• anions are then oxidised (lose electrons)

Question 3

Oxidation and Reduction

Answer 3

OILRIG
Oxidation - loss of electrons; adding oxygen

Reduction - gain of electrons; taking oxygen away

Question 4

Why molten or in solution? [2]

Answer 4

ions can ONLY move in liquid state

the ELECTROLYTE can conduct electricity

Question 5

what happens at cathode?

Answer 5

• cathode (-ve charge+
• attracts cations (+vely charged ions)
• cations are then reduces (gain electrons)

Question 6

What are electrodes made out of?

Answer 6

Graphite, platinum(expensive)

• high melting point
• conducts electricity
• INERT (unreactive)

Question 7

What is aluminium ore is called?

formula?

Answer 7

bauxite

Al2O3 = aluminium oxide

Question 8

Half equation at cathode of aluminium oxide

reduction or oxidation?

Answer 8

Al 3+ + 3e- ➝ Al (reduction)

Question 9

Half equation at anode of aluminium oxide

reduction or oxidation?

Answer 9

2O2- ➝ O2 (g) + 4e-

oxidation

Question 10

What happens to oxygen? p/s

Answer 10

Problem: Due to high temp, the OXYGEN formed at the anode reacts with the ANODE forming CARBON DIOXIDE
- anode burns away

Question 11

give 2 advantages of fuel cell over a gasoline-fuelled machine (2)

Answer 11

CELL: lightweight, quieter, fewer working parts/less maintenance, more efficient or less energy wasted or more energy produced,

SUSTAINABILITY: conserves a limited resource/petroleum/fossil fuels unlimited supplies of renewable resource(of hydrogen from water)

POLLUTION: No or less greenhouse effect
No or less acid rain
No or less toxic gases
No or less smog

POLLUTANTS: No or less C/soot
No or less CO2
No or less CO
No or less SO2
No or less oxides of nitrogen/NO/NO2/N2O4/NOx
No or less (unburnt) hydrocarbons
No or less low level ozone
H2O is the only product

Question 12

In an electrochemical cell, which electrode made from the more reactive metal, + or - ?
Explain why?

Answer 12

negative electrode

more reactive metals tend to lose their electrons more easily; these electrons make the electrode negative

Question 13

equation for the reaction in a hydrogen fuel cell

Answer 13

O2 + 2H2 → 2H2O

Question 14

hydrogen fuel cell: what happens at the negative electrode?

half equation, r/o, anode/cathode

Answer 14

H2 → 2H+ + 2e–
oxidation
anode

Question 15

hydrogen fuel cell: what happens at the positive electrode?

half equation, r/o, anode/cathode

Answer 15

O2 + 4H+ + 4e– → 2H2O
reduction
cathode

Question 16

Some electrochemical cells can be recharged if an external current is applied. What must happen in the cell for this to happen?

Answer 16

reactions in cell must be reversible

Question 17

Describe the extraction of aluminium from alumina (Al2O3). Include the electrolyte, the electrodes and the reactions at the electrodes (6)

Answer 17

electrolyte alumina / aluminium oxide dissolved in molten cryolite (1)

use cryolite (acts as a solvent) to reduce melting point (1)
electrodes carbon (1)
aluminium formed at cathode / Al 3+ + 3e → Al (1)

oxygen formed at anode/2O2 → O2 + 4e (1)

anode burns/reacts to carbon dioxide/C + O2 → CO2 (1)

Question 18

Chlorine is made by the electrolysis of concentrated aqueous sodium chloride.
Describe this electrolysis.

Write ionic equations for the reactions at the electrodes and name the sodium compound formed. (5)

Answer 18

hydrogen and chlorine / H2 and Cl2 (1)

At the anode(+): 2Cl- → Cl2 + 2e (1) chlorine is formed at anode(+)

At the cathode(-): 2H+ +2e → H2 (1) Hydrogen / H+ is formed at cathode(-) (1)

sodium hydroxide REMAINS IN SOLUTION (1)

Question 19

2 Chemical reactions are always accompanied by an energy change.
(a) Aluminiumisextractedbytheelectrolysisofamoltenmixturewhichcontainsaluminium oxide, Al 2O3. This decomposes to form aluminium at the negative electrode and oxygen at the positive electrode.

i.Write an ionic equation for the reaction at the negative electrode. [2]
ii. Complete the ionic equation for the reaction at the positive electrode.
2O2– → ……. + ……. [2]
iii. Is the reaction exothermic or endothermic? Explain your answer. [1]

Answer 19

i. Al 3+ + 3e → Al
ii. 2O2- → O2 +4e

endothermic AND (electrical) energy supplied// more energy required to break bonds

Question 20

uses of aluminium (2)

Answer 20

food containers / window frames / cooking foil / cars / bikes / drink cans

Question 21

which ions are the easiest to discharge at +ve electrode?

Answer 21

if ions is

• higher in the reactivity series = H+ is discharged
• (sometimes) if concentrated metal = metal discharged

Question 22

which ions are the easiest to discharge at -ve electrode?

Answer 22

halide ions (Cl-, Br-) = halide discharged
-ve ions (NO3 -, SO4 2-, CO3 2-) = OH- discharged = O2 and H2O are formed