Electrolysis Flashcards
Define electrolysis
The breaking down of a compound using an electric current. Only works for ionic compound when they are molten/dissolved in water because then they can conduct electricity
State the general principle of electrolysis
Metals or hydrogen are formed at the cathode, and
that non-metals are formed at anode
Explain the discharge series
- A particular form of the metal reactivity series
- The lower down the series, the more likely the ion will be discharged
- For positive ions: Na+, Mg2+, Al3+, H+, Cu2+
- For negative ions: SO42-, NO3-, OH-, Cl-, Br-, I-
In the electrolyte,
it is the ions that move not the electrons
Describe the electrode products in the
electrolysis of molten lead(II) bromide
- At the cathode (-) molten lead is formed:
Pb2+(l) + 2e- → Pb(l) - At the anode, gaseous bromine is evolved:
2Br-(l) → Br2(g) + 2e- - Overall: PbBr2(I) -> Pb(I) + Br2(g)
Describe the electrode products in the
electrolysis of concentrated hydrochloric acid
- At the cathode, hydrogen is formed:
2H+ + 2e- → H2+ - At the anode, chlorine gas is formed:
2Cl- → Cl2 + 2e-
Describe the electrode products in the
electrolysis of concentrated aqueous sodium chloride
- At the anode, chlorine gas is formed:
2Cl- → Cl2 + 2e- - At the cathode, hydrogen gas is formed:
2H+ + 2e- → H2+ - The ions remaining in solution are Na+ and OH- => aqueous sodium hydroxide is formed
Predict the products of electrolysis of a specified halide in dilute aqueous solution
When a dilute aqueous solution of acids are electrolysed, oxygen is formed at the anode, and hydrogen is formed at the cathode
- Cathode: 2H+ + 2e- → H2+
- Anode: the OH- ions in the water lose electrons
4OH- → O2 + 2H2O + 4e-
Reduction and oxidation is electrolysis
During electrolysis, ions gain electrons at the cathode and lose electrons at the anode
Purifying copper using copper electrodes
- Copper sulfate solution
- It is purified by using an impure copper at anode and pure copper at cathode
- At the anode, because the anode isn’t inert, it loses electrons and copper ions go into solution => anode gets smaller: Cu(s) → Cu2+(aq) + 2e-
- At the cathode, copper ions are more likely to be discharged (lower in reactivity series): Cu2+ + 2e- → Cu(s)
- Electrolyte remains deep blue because copper ions removed at cathode are replaced by copper ions formed at anode
Purifying copper using inert electrodes (graphite or platinum)
- Copper sulfate solution
- At the anode, because it is inert, cannot lose electrons. Hydroxide ions rather than sulfate ions are discharged: 4OH-(aq) → O2(g) + 2H2O(l) + 4e-
- At the cathode, copper ions are more likely to discharged than hydrogen: Cu2+ + 2e- → Cu(s)
- Electrolyte gradually loses its blue because copper ions in solution are turning into copper atoms and are not replaced by copper ions
Define electroplating
- Used to put a thin layer of one metal on top of another
- Object to be electroplated is connected to the cathode
- Plating metal connects to anode
- Electrolyte is a solution of ionic compound of the plating metal
- When an article is electroplated, the plating metal gains electrons at the cathode and become metal atoms
- When an object is electroplated, the metal ions formed at the anode are transferred to the cathode where they are deposited as metal
Electroplating nickel spoon
- Anode: silver
- Solution: Silver nitrate (AgNO3)
- At the anode: silver atoms lose electrons, become silver ions that enter the solution: Ag(s) → Ag+ (aq) + e-
- At the cathode: silver ions gain electrons and to become silver atoms which form a thin layer on the nickel spoon: Ag+(aq) + e- → Ag(s)
Uses of electroplating
- Protection of metals from corrosion. e.g. steel cans are electroplated with tin to prevent rusting
- Improving the appearance of metal. e.g. chromium plating gives a shiny surface to objects
Extracting aluminium from aluminium oxide
- Aluminium oxide dissolves in large amounts of molten cryolite (sodium aluminium fluoride) to reduce the melting point => less energy needed => cheaper
- At the cathode (carbon lining), aluminium ions gain electrons to become aluminium atom: Al3+ +3e- → Al
- At the anode, oxide ions lose electrons and oxidised to oxygen: 2O2- → O2 + 4e-
- Oxygen reacts with the hot carbon anodes to form carbon dioxide => carbon anodes need to be replaced from time to time
- Overall equation: 2Al2O3 → 4Al + 3O2
