3.2.3 Production of chemicals using electrolysis

Question 1

electrolysis

Answer 1

passage of electrical energy from a direct current power supply through a conducting liquid causing redox reactions to occur
- endergonic
- non-spontaneous

Question 2

molten ionic compounds

Answer 2

• ionic compound heated above melting point to become liquid

Question 3

molten advantage/disadvantage

Answer 3

advantage: water is not present
disadvantage: requires more energy to maintain electrolyte in molten state and at required temp

Question 4

aqueous advantage/disadvantage

Answer 4

advantage: heating is not required lowering costs
disadvantage: water can react instead of other ions

Question 5

Downs cell

Answer 5

• NaCl and CaCl2 in 1:2 ratio to lower melting point of NaCl from 801 to600 degrees saving energy costs (since Ca2+ not reduced as it is weaker oxidant) and heat is generated by current flowing through electrolyte
• iron mesh screen separates products and anode and cathode to prevent from forming NaCl
• graphite electrode is inert, iron is cheaper and effectively inert as electrons are continuously supplied (negative electrode cathode reduction) preventing it from oxidising

Question 6

membrane cell

Answer 6

• brine: concentrated NaCl solution (so Cl(g) will be produced at anode not H2O due to similar E0 values)
• semipermeable membrane prevents contact between reactive products and allows positive ions through resulting in pure NaOH output

Question 7

aqueous electrolytes

Answer 7

• if non-standard high concentrations, a weaker reducing/oxidising agent can reduce/oxidise instead of water which may be the stronger reducing/oxidizing agent

Question 8

inert electrodes

Answer 8

• platinum, carbon, graphite
• all metal cathodes since they receive constant supply of electrons and cannot be oxidsed

Question 9

reactive electrodes

Answer 9

• can be impure and then purified via electrolysis with the pure metal deposited at the cathode

Question 10

overpotential

Answer 10

• power supply must be greater than E0
  E0 (reduction/lower)- E0 (oxidation/higher)
  E0<0 (non-spontaneous)

Question 11

electrolytic cell

Answer 11

• power supply pushes electrons towards negative electrode (cathode) so reduction occurs
• electrons pushed out of positive electrode (anode) so oxidation occurs
• usually inert electrodes and reaction occurs on surface of electrodes

Question 12

galvanic vs electrolytic cell

Answer 12

• non-spontaneous as energy input required
• electrical to chemical
• electrodes don’t need to be in separate containers (no salt bridge needed) as reaction is non-spontaneous but products must be separated

Question 13

electroplating

Answer 13

thin surface coating applied over another surface

Question 14

electroplating cell

Answer 14

• object to be plated at cathode (negative terminal) and immersed in electrolyte solution containing ions of metal to be plated
• electrode of metal to be plated at anode (positive terminal)
• ions in electrolyte allow current to pass through cell

Question 15

faraday’s first law of electrolysis

Answer 15

• mass of product and charge are directly proportional to each other
  Q=It

Question 16

faraday’s second law of electrolysis

Answer 16

• to produce one mole of metal another whole number of electrons must to consumed
  Q=n9(e-) x F

Question 17

faraday

Answer 17

96500 coulombs
charge on 1 mole of electrons

Question 18

reasons for electrolysis

Answer 18

• separating molecules in compounds
• electroplating to improve appearance or prevent corrosion
• recharging secondary batteries
• purify

Question 19

limit life of cell

Answer 19

• corrosion of anode
• side reactions like water
• build up of gas around electrode

Question 20

rechargeable batteries requirements

Answer 20

• electrodes not damaged
• products remain in contact with electrodes
• reversible

Question 21

polymer electrolyte membrane electrolyser (proton exchange membrane)

Answer 21

• solid electrolyte
• electrodes are porous to allow gas not liquid to pass through and are catalytic
• membrane contains advanced polymers that allow proton not electron flow to complete the circuit
• can be designed to produce compressed hydrogen gas removing need to separate compressor
• usually assembled in electrolyser stack more cost effective

Question 22

artificial photosynthesis

Answer 22

• solar panel in solution (photoelectrochemical cell) causing water to oxidise into O2 and H+ which migrates to cathode and is reduced to produce hydrogen gas

Question 23

brown hydrogen

Answer 23

from steam reforming fossil fules, releasing CO and CO2

Question 24

grey hydrogen

Answer 24

hydrogen from industrial processes

Question 25

blue hydrogen

Answer 25

derived from fossil fuels with carbon capture

Question 26

green hydrogen

Answer 26

derived using renewable energy

Question 27

electrolyser

Answer 27

system that uses electricity to break water down into hydrogen and oxygen via electrolysis

Question 28

hydrogen advantages

Answer 28

- high energy density - abundant on earth in water and carbon compounds - doesn't produce greenhouse gas emissions when combusted

Question 29

hydrogen disadvantages

Answer 29

- high energy needed for electrolysis - low boiling point so high energy needed to liquify, and high pressures needed to store - explosive requiring safe handling and transport - difficult to transport

Question 30

battery issues

Answer 30

- nonrenewable sources (scarce) - memory effect - toxic or cancerous parts - inhumane practises - difficulty separating materials to recycle - safety, toddlers swalllowing - toxic when disposed and leaks