Chapter 8: Production of Chemicals by Electrolysis

Question 1

Electrolysis

Answer 1

• The passage of electrical energy from a power supply (e.g. a battery) through a conducting liquid
• Allow non-spontaneous reactions to occur

Question 2

Galvanic vs electrolytic cells

Answer 2

• Galvanic cells
  
  • Spontaneous reactions
  • Produce electricity
  • Chemical → electrical energy
  • Oxidation @ anode, reduction @ cathode
  • Anode = negative, cathode = positive
  • Negative gradient
• Electrolytic cells
  
  • Non-spontaneous reactions
  • Consume electricity
  • Electrical → chemical energy
  • Oxidation @ anode, reduction @ cathode
  • Anode = positive, cathode = negative
  • Positive gradient

Question 3

Competition at electrodes

Answer 3

• Water is a potential reactant when aqueous electrolytes are used
  
  • TIP: look for the word ‘solution’
• Reactive electrodes may also participate in the reaction

Question 4

Features of commercial electrolytic cell

Answer 4

• Separation and continuous removal of products
  
  • Prevents products from mixing & spontaneously reacting
  • E.g. a semipermeable membrane separates reactive products
• Inert or reactive electrode materials
  
  • Inert = not consumed in reaction, reactive = consumed
  • Choice depends on cost and the cell conditions
  • E.g. carbon anode and iron cathode in a Downs cell are inert, relatively cheap and have high melting points
• Molten or aqueous electrolyte
  
  • Depends on whether the presence of water will interfere with the electrolytic production of the desired products
• Chemical additives to electrolyte
  
  • Lower the melting point of a molten electrolyte
  • E.g. adding CaCl₂ to molten NaCl lowers its melting point

Question 5

Whilst commerical electrolytic cells typically use inert electrodes, what benefits can reactive electrodes have?

Answer 5

• Can be used to purify metals (impure metal used as the anode and pure metal deposited at cathode)
• Production of aluminium from alumina

Question 6

Molten ionic compounds

Answer 6

• When an ionic compound is heated to above its melting point and melts to become a liquid

Question 7

Downs cell design

Answer 7

• Molten electrolyte – water is not present to interfere with the desired reactions
• Iron mesh – used to separate the products at the anode and cathode

Question 8

Advantages and disadvantages of using a molten electrolyte

Answer 8

• Advantage
  
  • No interference of water in reactions
• Disadvantages
  
  • Process requires much more energy
  • Operates at higher temperatures

Question 9

Membrane cell design

Answer 9

• Aqueous electrolyte – preferable when possible and it is more cost-effective
  
  • Highly concentrated NaCl solution (brine) is used
• Semipermeable membrane – separates reactive products
  
  • Made from a polymer that only allows positive ions to pass through

Question 10

Electroplating

Answer 10

• A commercial application of electrolysis
• Results in a thin layer of metal over another surface
• E.g. tin cans are mostly steel with only a thin layer of tin over the steel surface (prevents corrosion)

Question 11

Faraday’s laws

Answer 11

• First law of electrolysis
  
  • The amount of substance deposited at an electrode is directly proportional to the electrical charge applied to the cell (m ∝ Q)
  • The higher the charge, the more metal that forms
• Second law of electrolysis
  
  • To produce 1 mole of a metal, 1,2, 3 or another whole number of moles of electrons must be consumed

Question 12

Factors that determine the amounts of products that form in electrolytic cells

Answer 12

• Charge on the ion in the electrode reaction
• Current flowing through the cell
• Length of time that the current flows

NOTE: Faraday’s laws describe the relationship between these factors.