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CHEMISTRY U3/4 > ELECTROLYTIC CELLS > Flashcards

ELECTROLYTIC CELLS Flashcards

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1
Q

galvanic vs electrolytic cells

A

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
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2
Q

ELECTROLYSIS

A
  • 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
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3
Q

electrolysis of molten NaCl

A
  • 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
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4
Q

competition at electrodes

A
  • 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
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5
Q

standard conditions - factors affecting reactions

A
  • 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
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6
Q

electrolysis of NaCl solution

A

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  • electrons are withdrawn from this electrode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
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7
Q

electrolysis of NaCl solution in real life

A
  • 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
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8
Q

use of electrolysis in industry

A
  • 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
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9
Q

why is Fe not oxidised in downs cell

A
  • it is connected to the negative terminal of the power supply
  • continuous supply of electrons prevent it from oxidising
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10
Q

advantages and disadvantages of molten electrolyte

A

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
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11
Q

aqueous electrolyte - membrane cell

A
  • 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
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12
Q

membrane cell advantage

A
  • NaOH not contaminated w NaCl, Cl2, H2
  • enables process to occur between 80-90
  • no need to heat electrolyte
  • reduced cost of production
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13
Q

advantages of use of reactive electrodes

A
  • 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
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14
Q

electroplating cells

A
  • 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
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15
Q

faradays first law

A
  • 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
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16
Q

faraday

A
  • amount of charge on 1 mole of electron
    • 1F = 96500 C
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17
Q

faraday’s second law

A
  • 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.
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18
Q

conditions used in chemical industry

A
  • 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
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19
Q

conditions to increase reaction rate

A
  • high concentration/pressure
  • high temperatures
  • high S.A of solids
  • use of a catalyst
20
Q

conditions to increase equilibrium yield

A
  • 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
21
Q

primary vs secondary cells

A

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
22
Q

secondary cells

A
  • 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.
23
Q

conditions required for recharging

A
  • 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
24
Q

discharge vs recharge

A
  • 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
25
Q

electrolyser

A

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

26
Q

PEM electrolyser vs simple electrolyser

A
  • PEM electrolyser does not operate in an aqueous environment like a simple electrolyser
  • It uses a conductive polymer electrolyte.
  • increasing the PEM thickness would decrease efficiency of fuel cell -> due to added resistance to proton flow
  • 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.
27
Q

hydrogen gas as a fuel

A
  • 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
28
Q

production of hydrogen gas

A
  • 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
29
Q

green hydrogen

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

SUSTAINABILITY CHALLENGES WITH USING WATER AS RENWABLE FEEDSTOCK:
* CHALLENGE TO MAINTAIN SUFFICIENT WATER SUPPLY DURING DROUGHT
* LIMITED SUPPLY OF WATER FOR DIRNKING
* LIMITED SUPPLY OF WATER FOR SANITATION, WASHING AND BATHING

30
Q

industrial production of hydrogen

A
  • 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
31
Q

PEM to electrolyse water

A
  • 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)
32
Q

advantages and disadvantages of PEM

A
  • advantages
    • high rate of hydrogen production
  • disadvantages
    • exp bc of expensive metals used as catalysts
33
Q

hydrogen cars

A
  • 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
34
Q

artificial photosynthesis

A
  • 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)
35
Q

advantages of artificial photosynthesis

A
  • does not create greenhouse gases (liquid water is produced).
  • does not require the use of fossil fuels.
36
Q

lead acid batteries

A
  • 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)
37
Q

commercial electrolytic cells

A
  • 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
38
Q

green hydrogen advantage and disadvantage

A

advantage
* the only product of combustion is water

disadvantage
* limited infrastructure for production, storage and distribution
* electrolysis process is energy intensive

39
Q

why is it necessary to employ hydrogen compressor

A
  • crucial for storing hydrogen gas efficiently due to its very low
    density at standard atmospheric pressure
  • Compressing hydrogen reduces its volume, making it feasible to store larger quantities in smaller, more manageable volumes
40
Q

features of electrolytic cells

A
  • distance between electrodes affect current and thus rate of reaction
  • increase in current can increase reaction rate
  • powersource must have greater EMF than spontaneous galvanic cell would produce
  • surgace area of electrode is proportional to rate
  • conc of electrolyte remain constant
41
Q

role of membrane in electrolytic cell

A
  • allows H+ ions / protons to pass through in order to complete the circuit.
  • The membrane prevented a spontaneous reaction between the products.
42
Q

how does the operation of a secondary cell change when it is nearing the end of its cell life

A
  • The cell will not be able to be fully recharged.
  • The battery does not last as long between recharges.
43
Q

pruposes of the membrane in the electrolyser

A
  • separation of products that would spontaneously react if they came in contact
  • completion of the internal circuit
  • prevention of the direct flow of electrons / direct reaction of reactants
44
Q

why don’t cells last forever?

A

● Loss of active materials (reactants and products)

● Formation of other chemicals in side reactions that
prevent the cell from working properly

● Impurities in cell materials, including electrodes
➔ These can react with active materials
● Decrease in contact of electrolyte with electrodes
➔ Either through leakage of electrolyte or its transformation
into a non-conductive material

● Corrosion or failure of internal components

45
Q

self-discharge

A

● Even when a battery/cell is not in use (anode and cathode not connected), side reactions and deterioration still occur
● These reactions can be slowed by storage at cold temperatures

CHEMISTRY U3/4 (11 decks)

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