C3.4 - Electrolysis

Question 1

What is electrolysis?

Answer 1

Electrolysis is a process in which an electric current is passed through a compound, causing a chemical change.

Question 2

What three components do you need for electrolysis to work?

Answer 2

• An electrolyte.
• Two electrodes.
• An electrical supply such as a power pack or
  battery.

Question 3

What is an electrolyte?

Answer 3

An electrolyte is a compound in its liquid state or in solution which contains mobile ions and conducts electricity.

Question 4

What are electrodes?

Answer 4

• Electrical conductors used in electrolysis.There are two electrodes used in electrolysis:
• The negative electrode - The cathode
• The positive electrode - The anode.

Question 5

What happens during electrolysis?

Answer 5

• Positive ions (cations) gain electrons at the
  cathode and become atoms - They become
  discharged.
• Negative ions (anions) lose electrons at the anode
  and become atoms - They become discharged.
• If the atoms formed are non-metal atoms, covalent
  bonds may form between them, making
  molecules.

Question 6

What is a binary ionic compound?

Answer 6

A binary ionic compound contains just two elements.

Question 7

Explain how diffusion and convection occurs in electrolysis.

Answer 7

• Ions move to an oppositely charged electrode
  during electrolysis.
• The concentration of ions close to each electrode
  goes down as ions gain or lose electrons to
  become atoms.
• Other ions in the electrolyte can move to replace
  them by diffusion and convection.
• Cannot happen which ionic compounds in the
  solid state and therefore cannot conduct
  electricity.

Question 8

What are inert electrodes and what are they usually made from?

Answer 8

• Inert electrodes are not changed during
  electrolysis.
• Usually made from unreactive metals such as
  copper or platinum, or from graphite.
• Inert electrodes can be used for the electrolysis
  of many ionic compounds in their liquid state or
  in aqueous solution.

Question 9

What happens during the electrolysis of water?

Answer 9

Water is naturally partially ionised - It contains small concentrations of hydrogen ions and hydroxide ions.

At the cathode hydrogen ions are discharged - Reduced as they gain electrons - Producing hydrogen in the gas state.

At the anode hydroxide ions are discharged - Oxidised as they lose electrons - Forms water and oxygen.

Question 10

What are the ionic equations for the reactions with hydrogen ions at the cathode and hydroxide ions at the anode?

Answer 10

• 4H+ (aq) + 4e- -> 2H2 (g)

- 4OH- (aq) -> 2H2O (l) + O2 (g) + 4e-

Question 11

How do you know what gets discharged as the cathode and anode during the electrolysis of an solution?

Answer 11

• Hydrogen is produced at the cathode, but if ions
  from a less reactive metal than hydrogen are
  present, they are discharged more easily and
  therefore that metal is produced instead.
• Oxygen is produced at the anode, but if ions from
  an element in Group 7 are present at a high
  enough concentration, the Group 7 element is
  produced instead.

Question 12

What is electroplating?

Answer 12

• Coating an object with a metal by electrolysis.

- Type of electrolysis that uses non-inert electrodes.

Question 13

What are non-inert electrodes?

Answer 13

Electrodes that are changed during electrolysis.

Question 14

Describe the things in electroplating?

Answer 14

• The cathode is the object you want to coat.
• The anode is a piece of metal you want to coat
  the object with.
• The electrolyte is a solution containing ions of
  the coating metal.

Question 15

How does electroplating work?

Answer 15

• Metal ions from the electrolyte are discharged on
  the surface of the object (the cathode).
• These ions are replaced by metal ions leaving the
  surface of the anode.
• Overall, metal leaves the anode and is deposited
  on the object.
• The process continues until the anode is used up.

Question 16

Describe the apparatus for the purification of copper.

Answer 16

• Cathode of pure copper.
• Anode of impure copper.
• Electrolyte of copper (II) sulphate solution.
• Power supply connected to the electrodes.

Question 17

How does the purification of copper work?

Answer 17

• Cathode gains copper atoms and increases in
  mass.
• Copper is deposited on the cathode.
• Anode loses copper atoms and decreases in
  mass.
• Impurities from the anode fall off and collect
  underneath it.