Unit 4 - Electrochemistry

Question 1

Electrolysis

Answer 1

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

Question 2

Cathode

Answer 2

Negative terminal of the battery

Question 3

Anode

Answer 3

Positive terminal of the battery

Question 4

Main components of an electrolytic cell

Answer 4

• Battery
• Two electrodes - one connected to the cathode as the negative electrode and another connected to the anode as the positive electrode
• Electrolyte - aqueous or molten substance that conducts electricity

Question 5

Electrolyte

Answer 5

Substance being broken down - usually an ionic compound that is molten or aqueous

Question 6

Electrode materials

Answer 6

Carbon/platinum/graphite to be inert and good conductors of electricity

Question 7

Stages of electrolysis

Answer 7

1.Electrolyte is made molten or dissolved in water to become aqueous
2.Electric current in passed through the cell
3.Cations move towards the cathode
4.Anions move towards the anode
5.Cations come from metal ions, gains an electron
6.Anions come from non-metal ions, loses an electron

Question 8

Process of electrodes

Answer 8

1.Electrons flow from negative terminal of the battery to charge up the cathode
2. Attracts cations and makes them move toward the cathode to gain electrons (reduction)
3.Anions move toward anode and lose electrons (oxidation)
4.Electrons flow from the anode to the cathode

Question 9

Ionisation

Answer 9

Process of making ions by either gaining or losing electrons

Question 10

Dissociation

Answer 10

Compounds splitting to form ions - usually acids

Question 11

Reactivity series and products

Answer 11

-Cations below hydrogen on reactivity series will be reduced over the H+ ions and metal will form
- Cations above hydrogen on reactivity series will cause H+ ions to be reduced, hydrogen gas will form and the metal ion with stay in solution

Question 12

Reactivity series for non-metals

Answer 12

The electrochemical series

Question 13

Anode products

Answer 13

• Halide ions will form at the anode e.g. Cl- or Br-
• Other negative ions e.g. SO4- will form oxygen from dissociation of OH- ions since sulfate is more reactive than hydroxide ions

Question 14

Molten lead (II) bromide half-equation of cathode reduction and anode oxidation

Answer 14

• Cathode (-)= Pb^2+(1) + 2e^- –> Pb(s)
• Anode (+)= 2Br^-(aq) –> Br2(g) + 2e^-

Question 15

Concentrated aqueous sodium chloride half-equation of cathode reduction and anode oxidation

Answer 15

• Cathode (-)= 2H^+(aq) + 2e^- –> H2(g)
• Anode (+)= 2Cl^-(aq) –> Cl2(g) + 2e^-

Question 16

Dilute sulfuric acid half-equation of cathode reduction and anode oxidation

Answer 16

• Cathode (-)= 2H^+(aq) + 2e^- –> H2(g)
• Anode (+)= 40H^-(aq) –> 2H2O(1) + O2(g) + 4e^-

Question 17

Cathode reduction equation

Answer 17

Electrons are on the right hand side

Question 18

Anode oxidation equation

Answer 18

Electrons are on the right-hand side

Question 19

Main applications of electrolysis

Answer 19

• Electroplating
• Purification of copper/refining copper
• Electrolysis of concentrated sodium chloride/brine

Question 20

Electroplating

Answer 20

Coating one metal with a thin layer of another metal either to improve appearance or prevent corrosion

Question 21

Electroplating in electrolysis

Answer 21

The object to be electroplated is made at the cathode and the anode is made from the metal that will be the coating layer

Question 22

Example equations for aqueous silver nitrate electroplating

Answer 22

• Cathode (-)= Ag^+(aq) + e^- –> Ag(s)
• Anode (+)= Ag(s) –> Ag^+(aq) + e^-

Question 23

Copper refining and purification

Answer 23

Electrolysis is used to get 99.9% copper to use in electrical wiring

Question 24

Steps of copper refining and purification

Answer 24

1.At the anode copper atoms lose electrons and go as copper (II) ions in the solution: Cu(s) –> Cu^2+(aq) + 2e^-
2.Cu^2+ ions are attracted to the cathode to gain electrons and form copper atoms: Cu^2+(aq) + 2e^- –> Cu(s) - layer of pure copper build up on cathode
3.Platinum, silver and gold might have been present in the impure copper anode and drop to the bottom to make a mixture - slime
4.Anode loses mass and cathode gains mass

Question 25

Electrolysis of concentrated sodium chloride

Answer 25

Aqueous concentrated sodium chloride is produced by solution mining of underground rock salt deposits and then electrolyzed

Question 26

Electrolysis of concentrated sodium chloride equations

Answer 26

• Cathode = 2H^+ +2e^- –> H2
• Anode = 2Cl^-(aq) –> Cl2(g) + 2e^-

Question 27

Hydrogen used as a fuel

Answer 27

Hydrogen + Oxygen in a hydrogen-oxygen fuel cell produces electricity and water

Question 28

Energy production of hydrogen as fuel

Answer 28

Exothermic reaction creates 143kJ/g

Question 29

Fuel cell

Answer 29

Electrochemical cell converts chemical energy from fuel to electricity

Question 30

Disadvantages of hydrogen-oxygen fuel cells

Answer 30

• Lack of facilities and methods for widespread use
• Compressed gases are hazardous
• Reactants are gases so must be compressed for storage
• Hydrogen sourced from methane - non-renewable energy source
• Hydrogen sourced from water - expensive to produce

Question 30

Advantages of hydrogen-oxygen fuel cells

Answer 30

• Plentiful cheap supply of reactants
• Highly efficient process (>60% compared to -33% for petrol)
• Renewable energy source
• Produces zero carbon emissions
• Similar energy content