ELECTROLYTIC CELLS

Question 1

galvanic vs electrolytic cells

Answer 1

galvanic cells:

• produces electricity
• spontaneous reactions
• convert chemical energy to electrical
• oxidation anode
• reduction cathode
• anode is negative
• cathode is positive

electrolytic cells

• consume electricity
• have non-spontaneous reaction
• convert electrical energy to chemical energy
• oxidation anode
• reduction cathode
• anode is positive
• cathode is negative

Question 2

ELECTROLYSIS

Answer 2

• involves the passage of electrical energy from a power supply such as a battery, through a conducting liquid
  
  • the use of electrical energy drives a non-spontaneous redox reaction

Question 3

electrolysis of molten NaCl

Answer 3

• inert electrodes → platinum or graphite
• the electrolyte (conducting liquid) is molten NaCl → no H2O present
  
  • NaCl melts at 801 degrees Celsius
• negative electrode connected to the negative terminal
• positive electrode connected to the positive terminal
• electrolysis takes place in a single container
  
  • products must be kept apart or they will spontaneously react to reform original reactants

Question 4

competition at electrodes

Answer 4

• in some electrolytic cells there may be several chemicals present that can react
  
  • water is a potential reactant when aqueous electrolytes are used (look for solution word)
  • reactive material used for electrodes may also participate in the reaction

Question 5

standard conditions

Answer 5

• electrochemical series based on standard conditions
• most electrolysis reactions aren’t performed at standard conditions
• reactions are affected by electrolyte concentration, gas pressures. current, voltage and electrode types

Question 6

electrolysis of NaCl solution

Answer 6

at the anode

• electrons are withdrawn from this electrode
• two possible reductants 2Cl- or H2O(l)
• reactions lower in the electrochemical series are stronger reductants and more likely to occur
  
  • reaction would be the oxidation of water

at the cathode

• power supply pushes electrons towards the negative electrode
• 2 possible oxidants H2O(l) and Na+
• reactions higher in the electrochemical series involves stronger oxidants and are more likely to occur
  
  • water will be reduced and H2 formed

Question 7

electrolysis of NaCl solution in real life

Answer 7

• standard conditions
  
  • water react at both anode and cathode
• in practice
  
  • possible for either anode reactions to occurs as they are close to each other on the electrochemical series
  • when [NaCl] increases, Cl2(g) can be produces

Question 8

use of electrolysis in industry

Answer 8

• chemical industries tend to avoid using electrolysis
  
  • high cost of electrical energy and high energy usage
• but process enables some chemical to be produced that could not be readily produced in any other way
• creating sodium metal and chlorine gas
  
  • downs cell - molten NaCl
  • membrane cell - aqueous NaCl

Question 9

why is Fe not oxidised in downs cell

Answer 9

• it is connected to the negative terminal of the power supply
• continuous supply of electrons prevent it from oxidising

Question 10

advantages and disadvantages of molten electrolyte

Answer 10

advantages

• no interference of water in reactions

disadvantage

• process req more energy
• operates at a high temp
• a mixture of NaCl and CaCl2 decreases the melting temperature
  
  • saves energy costs

Question 11

aqueous electrolyte - membrane cell

Answer 11

• preferable
• products of electrolysis
  
  • sodium hydroxide
  • chlorine
  • hydrogen
• highly concentrated NaCl solution used as an electrolyte
• anode and cathode separated by a semipermeable membrane
• prevents contact between products and only allow Na+ thru

Question 12

membrane cell advantage

Answer 12

• NaOH not contaminated w NaCl, Cl2, H2
• enables process to occur between 80-90
• no need to heat electrolyte
• reduced cost of production

Question 13

advantages of use of reactive electrodes

Answer 13

• many commercial electrolytic cells use inert electrodes, however reactive electrodes have benefits
  
  • can be used to purify metals
    
    • impure metal used as anode and pure metal deposited at cathode
    • production of aluminum for alumina

Question 14

electroplating cells

Answer 14

• results in the application of a thin layer of metal over another surface
• when the cell is in operation the power supply acts as an electron pump, pushing electrons onto the negative electrode + removing electrons from positive electrode
  
  • object being plated is at the cathode - negative
  • an electrode of the metal is at the anode - positive

Question 15

faradays first law

Answer 15

• amount of substance deposited is measured directly by taking the mass of metal formed at the cathode
• the amount of any substance deposited, evolved or dissolved at an electrode is directly proportional to the quantity of electrical charge passed through the cell
• 1A indicated that 1C (6.24 x 10^18 electrons) of charge flow every secomd
• the more charge that passes through the cell, the more metals form at the cathode
• charge on metal ION determines how much metal is deposited

Question 16

faraday

Answer 16

• amount of charge on 1 mole of electron
  
  • 1F = 96500 C

Question 17

faraday’s second law

Answer 17

• to produce one mole of a substance, use mole ratio to find the no. moles of electrons that must be consumed
• no. moles of electrons which carry the same no. f faradays correspond to the given charge in coulombs.

Question 18

chemical industry

Answer 18

• to reduce waste and reduce costs, conditions are carefully selected to ensure acceptable yields are obtained
• optimum conditions used to make production of useful chemicals economically viable and environmentally responsible
• reaction conditions are manipulated to
  
  • maximise product yield
  • maximise reaction rate
  • minimise costs
  • maximise overall efficiency

Question 19

conditions to increase reaction rate

Answer 19

• high concentration/pressure
• high temperatures
• high S.A of solids
• use of a catalyst

Question 20

conditions to increase equilibrium yield

Answer 20

• pressure depending on no. particles
• temp depending if exo or endo
• additions of excess reactant
• removal of product as it forms
• consideration incl. product yield, reaction rate, safety hazards, environmental concerns, costs

Question 21

primary vs secondary cells

Answer 21

primary:

• disposable
• non-rechargeable
• go flat when reaction reaches equilibrium
  
  • eg alkaline cells
  • the products slowly migrate away from electrodes or are consumed by side reactions occurring in the cell preventing the cells from being recharged

secondary

• known as rechargeable cells or accumulators
• can be reused many times
• rechargeable
  
  • eg. lithium ion cells

Question 22

secondary cells

Answer 22

• can undergo hundreds of recharges
• chemical reaction gets reversed
• recharged by connecting to an electrical power supply
  
  • input of energy drives a non-spontaneous reaction
  • the positive terminal of the charger is connected to the cell’s positive electrode
  • negative terminal of charger connected to the cell’s negative electrode
• charge of an electrode will
  remain the same during discharge
  and recharge.

Question 23

conditions required for recharging

Answer 23

• the power supply must have a potential difference (voltage) greater than that produced by the cell during discharge
• electrodes must not be damaged and products must remain in contact with the electrodes

Question 24

discharge vs recharge

Answer 24

• when a secondary cell discharges, it acts as a galvanic cell
  
  • chemical to electrical energy
  • spontaneous reaction
  • negative terminal - anode
• when a secondary cell recharges, it acts as an electrolytic cell
  
  • electrical to chemical
  • non-spontaneous
  • positive terminal - anode

Question 25

electrolyser

Answer 25

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

Question 26

PEM electrolyser vs simple electrolyser

Answer 26

• PEM electrolyser does not operate in an aqueous environment like a simple electrolyser
• It uses a conductive polymer electrolyte.
• The overall equation is the same, but the half-equations differ.
• The overall equation for both will be 2H2O l 2H2 + O2. The states will differ.

Question 27

hydrogen gas as a fuel

Answer 27

• is an attractive alternative to carbon based fuels because:
  
  • it is abundant on earth in the form of water and most carbon compounds
  • has a high energy density → 1 g releases 141kJ of energy whereas 1g of methane releases 55.6kJ
  • combusting hydrogen gas only gives water

Question 28

production of hydrogen gas

Answer 28

• most produced from steam reforming of methane
• CO is converted to CO2
• hydrogen produced from fossil fuels is known as brown hydrogen
  
  • as co2 released is harmful for the environment

Question 29

green hydrogen

Answer 29

• H2 produced through the electrolysis of water USING RENEWABLE ENERGY
• overall 2H2O(l) → 2H2(g) + O2(g)
• anode 2H2O(l) → O2(g) + 4H+(aq) +4e-
• cathode 2H+(aq)+2e- → H2(g)

Question 30

industrial production of hydrogen

Answer 30

• doesn’t use simple electrolysis
  
  • renewable energy sources don’t provide stable supply of energy req to run process
  • acidic or alkaline solutions are corrosive
  • hydrogen gas produced is NOT compressed
• industry uses polymer electrolyte membrane (PEM) electrolyser to electrolyse water

Question 31

PEM to electrolyse water

Answer 31

• water is oxidised at the anode
• H+ ions move through conducting polymer to cathode
• H+ ions reduced at cathode to produce COMPRESSED H2 - no need for compressor stage
• electrodes are made from exp metals which allows gases through but not liquids
• metals act as catalysts to improve efficiency of gas production at electrodes
• membrane contains advanced polymers which allow H+ through but not electrons
  ANODE
  2H2O(l) → 4H+(aq) + O2(g) + 4e−
  CATHODE
  4H+(aq) + 4e− → 2H2(g)

Question 32

advantages and disadvantages of PEM

Answer 32

• advantages
  
  • high rate of hydrogen production
• disadvantages
  
  • exp bc of expensive metals used as catalysts

Question 33

hydrogen cars

Answer 33

• comparable to conventional vehicles
• hydrogen gas distribution system needs to be established
• hydrogen tank needs to be fitted such that it won’t explode in the event of a car crash

Question 34

artificial photosynthesis

Answer 34

• photoelectrochemical cell: reactions involving an electrical current generated by the action of light
• sunlight lands on anode
• energy from the sun causes excitation of the metals in the electrode
• H+ migrate from anode to cathode
• H2 produced instead of glucose
• Water oxidation in acid and using catalysts: 2H2O(l) → 4H+ + O2(g) + 4e−
• Proton reduction in the presence of catalysts: 4H+ + 4e− → 2H2(g)
• Overall: 2H2O(l) → 2H2(g) + O2(g)

Question 35

advantages of artificial photosynthesis

Answer 35

• does not create greenhouse gases (liquid water is produced).
• does not require the use of fossil fuels.

Question 36

lead acid batteries

Answer 36

• will need replacing as PbSO4 breaks away from the electrode
• anode reaction during discharge:
  Pb(s)+ SO42-(aq) → PbSO4(s)+ 2e-
• cathode reaction during discharge
  PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- → PbSO4(s) + 2H2O(l)
• OVERALL:
  Pb(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq) → 2PbSO4(s) + 2H2O(l)

Question 37

commercial electrolytic cells

Answer 37

• electrolysis is the only way to produce reactive metals - K, Na, Ca, Li, Al (but not in aq solutions)
• high temp use energy - costs money
• aq electrolysis useful for NaOH and Cl2 production
• products must be kept separate - otherwise will spontaneously react
• electrodes are inert