topic 3 - electrolysis Flashcards
electrolysis
the decomposition of a substance/compound using electricity
an electric current is passed through an electrolyte causing it to decompose
electrolyte
a molten or dissolved ionic compound where electrolysis occurs
electrochemical cell
a circuit made up of the anode, cathode, electrolyte, a power supply and the wires connecting the two electrodes
how to set up an electrochemical cell when the electrolyte is in solution (5)
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
what happens to the voltage of the cell as the experiment continues
as the reactants are used up, the voltage of the cell decreases
how to set up an electrochemical cell when the electrolyte is a molten ionic substance
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
what happens at the cathode
positively charged metal ions (cations) move towards the cathode where they are reduced (gain electrons) to metal atoms
what happens at the anode
negatively charged non metal ions (anions) move towards the anode where they are oxidised (lose electrons) to form non-metal atoms
what happens at each electrode in the electrolysis of : MOLTEN LEAD BROMIDE (with half equations)
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^-
what are the rules for what happens at the cathode when there is an aqueous solution
if the metal is more reactive than hydrogen, hydrogen will be produced
if metal is less reactive, pure metal will be produced
what are the rules for what happens at the anode when there is an aqueous solution
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)
what happens at each electrode in the electrolysis of : SODIUM CHLORIDE SOLUTION (with half equations)
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^-
what is formed at the cathode and anode when there is a solution of copper chloride
cathode:
copper (Cu^2+ + 2e^- -> Cu)
anode:
chlorine (2Cl^- -> Cl2 + 2e^-)
what is formed at the cathode and anode when there is a solution of sodium sulfate
cathode:
hydrogen (2H^+ + 2e^- -> H2)
anode:
oxygen (4OH^- -> O2 + 2H2O + 4e^-)
what is formed at the cathode and anode when there is water acidified by sulfuric acid
cathode:
hydrogen (2H^+ + 2e^- -> H2)
anode:
oxygen (4OH^- -> O2 + 2H2O + 4e^-)
what happens in the electrolysis of copper sulfate when you use INERT electrodes
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^-)
what happens when you perform electrolysis of copper sulfate with copper electrodes
the mass of the anode decreases and the mass of the cathode increases
how do you record the change in mass of copper electrodes and how can you increase this change
measure the mass of the electrodes before the reaction
afterwards, dry the electrodes and measure the mass
how can you increase the change in mass of copper electrodes
if you increase the current, the rate of electrolysis will increase so there will be a greater difference in mass
how to purify copper
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
