Electrolysis

Question 1

when do ionic compounds conduct electricity?

Answer 1

• ionic compounds are not electrical conductors when they are solid.
• if you melt them, or dissolve them in water, they are able to conduct electricity
• as they have ions that can move and carry charge

Question 2

what is electrolysis?

Answer 2

• the breaking down of a substance using ELECTRICITY.

- an electric current is passed through an ELECTROLYTE (a molten/dissolved ionic compound), causing it to decompose.

Question 3

what does electrolysis involve?

Answer 3

OXIDATION (loss of electrons) and REDUCTION (gain of electrons)

Question 4

where do positive ions go?

Answer 4

• the positive ions (CATIONS) in the electrolyte
• move towards the cathode (negative electrode)
• and are REDUCED (gain electrons)

Question 5

where do negative ions go?

Answer 5

• the negative ions (ANIONS) in the electrolyte
• move towards the anode (positive electrode)
• and are OXIDISED (lose electrons)

Question 6

what happens when ions gain or lose electrons?

Answer 6

as ions gain or lose electrons they form the uncharged substances and are discharged from the electrolyte.

Question 7

what are the cations?

Answer 7

the POSITIVE IONS

Question 8

what are the anions?

Answer 8

the NEGATIVE IONS

Question 9

what do ionic half equations show?

Answer 9

• show how electrons are transferred during reactions.

- really useful for showing what happens ar each electrode during electrolysis.

Question 10

how do you write an ionic half equation?

Answer 10

1. put one of the things being oxidised or reduced on one side and the thing it gets oxidised or reduced to on the other.
2. balance up the numbers of atoms just like in a normal equation
3. then add electrons (e-) on to one side to balance up the charges.

Question 11

what are molten ionic compounds usually broken up into?

Answer 11

their elements , e.g. molten lead bromide to lead and bromine

Question 12

EXAMPLE OF ELECTROLYSIS

Answer 12

MOTLEN LEAD BROMIDE (PbBr2).

• the positive Pb2+ ions are attracted to the negative cathode.
• at the cathode a lead ion accepts two electrons and is reduced to a lead atom.
• the negative Br- ions are attracted to the positive anode.
• at the anode two bromide ions lose one electron each and are oxidised to a bromine molecule.

Question 13

what are electrodes made of?

Answer 13

an INERT material so they don’t take part in the reaction.

Question 14

how do you set up an electrochemical cell?

Answer 14

• an electrochemical cell is a circuit, made up of the anode, cathode, electrolyte, a power source and wires that connect the two electrodes.
  1. get two inert electrodes e.g. graphite or platinum
  2. clean the surfaces of the electrodes using emery paper
  3. from this point on, don’t touch the surfaces of the electrodes as you could transfer grease back onto the strips.
  4. place both electrodes into a beaker filled with your electrolyte
  5. connect the electrodes to a power supply using crocodile clips and wires. when you turn the power supply on, a current will flow through the cell.

Question 15

what does electrolysis of aqueous solutions involve?

Answer 15

• the ions from water
• in aqueous solutions, as well as the ions from the ionic compound, there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water

Question 16

what happens at the cathode in electrolysis of aqueous solutions?

Answer 16

• at the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced IF the metal is more reactive than hydrogen (e.g. sodium)
• if the metal is less reactive than hydrogen (e.g. copper or silver), then a solid layer of pure metal will be produced instead.

Question 17

what happens at the anode in electrolysis of aqueous solutions?

Answer 17

• at the anode, if OH- and halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed.
• if no halide ions are present then oxygen will be formed.

Question 18

what happens in the electrolysis of sodium chloride?

Answer 18

• a solution of sodium chloride (NaCl) contains four different ions: Na+, Cl-, OH- and H+.
• sodium metal is MORE REACTIVE than hydrogen. so at the CATHODE, hydrogen gas is produced.
• chloride ions are present in the solution. so at the ANODE, chlorine gas is produced.

Question 19

what will the product at the cathode be when aqueous copper (II) sulphate is electrolysed?

Answer 19

COPPER

Question 20

what will the product at the cathode be when aqueous dilute sulphuric acid is electrolysed?

Answer 20

HYDROGEN

Question 21

what will the product at the anode be when aqueous copper (II) sulphate is electrolysed?

Answer 21

oxygen

Question 22

what will the product at the anode be when aqueous dilute sulphuric acid is electrolysed?

Answer 22

oxygen