topic 3 - electrolysis

Question 1

electrolysis

Answer 1

the decomposition of a substance/compound using electricity

an electric current is passed through an electrolyte causing it to decompose

Question 2

electrolyte

Answer 2

a molten or dissolved ionic compound where electrolysis occurs

Question 3

electrochemical cell

Answer 3

a circuit made up of the anode, cathode, electrolyte, a power supply and the wires connecting the two electrodes

Question 4

how to set up an electrochemical cell when the electrolyte is in solution (5)

Answer 4

1) take two inert electrodes, such as those made of graphite or platinum
2) clean the surfaces of the electrodes with emery or sand paper
3) place both electrodes in a beaker with the electrolyte
4) connect the electrodes to the power supply with crocodile clips and wires
5) once turned on, a current will flow through the electrolyte

Question 5

what happens to the voltage of the cell as the experiment continues

Answer 5

as the reactants are used up, the voltage of the cell decreases

Question 6

how to set up an electrochemical cell when the electrolyte is a molten ionic substance

Answer 6

1) ionic substance is placed in a crucible
2) heat substance over a Bunsen burner until molten (in a fume cupboard to avoid releasing toxic fumes)
3) once molten, two clean and inert electrodes should be dipped into the electrolyte
4) connect the electrodes to the power supply with crocodile clips and wires
5) once turned on, a current will flow through the electrolyte

Question 7

what happens at the cathode

Answer 7

positively charged metal ions (cations) move towards the cathode where they are reduced (gain electrons) to metal atoms

Question 8

what happens at the anode

Answer 8

negatively charged non metal ions (anions) move towards the anode where they are oxidised (lose electrons) to form non-metal atoms

Question 9

what happens at each electrode in the electrolysis of : MOLTEN LEAD BROMIDE (with half equations)

Answer 9

at the cathode:
lead is reduced, forming a silver coloured liquid (molten lead)
Pb^2+ + 2e^- -> Pb

at the anode:
bromine is oxidised, forming a grown gas vapour (bromine gas)
2Br^- -> Br2 + 2e^-

Question 10

what are the rules for what happens at the cathode when there is an aqueous solution

Answer 10

if the metal is more reactive than hydrogen, hydrogen will be produced
if metal is less reactive, pure metal will be produced

Question 11

what are the rules for what happens at the anode when there is an aqueous solution

Answer 11

if halide ions are present (eg. fluoride, chloride, bromide, iodide, astatide), then halogen molecules will be produced
if no halide ions are present, oxygen will be produced (from hydroxide)

Question 12

what happens at each electrode in the electrolysis of : SODIUM CHLORIDE SOLUTION (with half equations)

Answer 12

at the cathode:
hydrogen is more reactive than sodium so hydrogen gas is produced
2H^+ + 2e^- -> H2

at the anode:
chloride ions (halide ions) are present so chlorine gas is produced
2Cl^- -> Cl2 + 2e^-

Question 13

what is formed at the cathode and anode when there is a solution of copper chloride

Answer 13

cathode:
copper (Cu^2+ + 2e^- -> Cu)

anode:
chlorine (2Cl^- -> Cl2 + 2e^-)

Question 14

what is formed at the cathode and anode when there is a solution of sodium sulfate

Answer 14

cathode:
hydrogen (2H^+ + 2e^- -> H2)

anode:
oxygen (4OH^- -> O2 + 2H2O + 4e^-)

Question 15

what is formed at the cathode and anode when there is water acidified by sulfuric acid

Answer 15

cathode:
hydrogen (2H^+ + 2e^- -> H2)

anode:
oxygen (4OH^- -> O2 + 2H2O + 4e^-)

Question 16

what happens in the electrolysis of copper sulfate when you use INERT electrodes

Answer 16

ions inside were:
Cu^2+ 
SO4^2-
H^+
OH^-

cathode: copper is less reactive than hydrogen so metal is produced (Cu^2+ + 2e^- -> Cu) (copper forms on the cathode)
anode: no halide ions are present so oxygen and water are produced at the anode (4OH^- -> O2 + 2H2O + 4e^-)

Question 17

what happens when you perform electrolysis of copper sulfate with copper electrodes

Answer 17

the mass of the anode decreases and the mass of the cathode increases

Question 18

how do you record the change in mass of copper electrodes and how can you increase this change

Answer 18

measure the mass of the electrodes before the reaction

afterwards, dry the electrodes and measure the mass

Question 19

how can you increase the change in mass of copper electrodes

Answer 19

if you increase the current, the rate of electrolysis will increase so there will be a greater difference in mass

Question 20

how to purify copper

Answer 20

copper is extracted using carbon and then purified using electrolysis
the impure copper anode is oxidised and dissolves to form copper ions which are reduced at the cathode to form a layer of pure copper
any impurities are left as a sludge at the bottom of the electrochemical cell