Unit 4 - Electrochemistry Flashcards
Electrolysis
The breakdown of an ionic compound, molten or in aqueous solution by the passage of electricity in an electrolytic cell
Cathode
Negative terminal of the battery
Anode
Positive terminal of the battery
Main components of an electrolytic cell
- Battery
- Two electrodes - one connected to the cathode as the negative electrode and another connected to the anode as the positive electrode
- Electrolyte - aqueous or molten substance that conducts electricity
Electrolyte
Substance being broken down - usually an ionic compound that is molten or aqueous
Electrode materials
Carbon/platinum/graphite to be inert and good conductors of electricity
Stages of electrolysis
1.Electrolyte is made molten or dissolved in water to become aqueous
2.Electric current in passed through the cell
3.Cations move towards the cathode
4.Anions move towards the anode
5.Cations come from metal ions, gains an electron
6.Anions come from non-metal ions, loses an electron
Process of electrodes
1.Electrons flow from negative terminal of the battery to charge up the cathode
2. Attracts cations and makes them move toward the cathode to gain electrons (reduction)
3.Anions move toward anode and lose electrons (oxidation)
4.Electrons flow from the anode to the cathode
Ionisation
Process of making ions by either gaining or losing electrons
Dissociation
Compounds splitting to form ions - usually acids
Reactivity series and products
-Cations below hydrogen on reactivity series will be reduced over the H+ ions and metal will form
- Cations above hydrogen on reactivity series will cause H+ ions to be reduced, hydrogen gas will form and the metal ion with stay in solution
Reactivity series for non-metals
The electrochemical series
Anode products
- Halide ions will form at the anode e.g. Cl- or Br-
- Other negative ions e.g. SO4- will form oxygen from dissociation of OH- ions since sulfate is more reactive than hydroxide ions
Molten lead (II) bromide half-equation of cathode reduction and anode oxidation
- Cathode (-)= Pb^2+(1) + 2e^- –> Pb(s)
- Anode (+)= 2Br^-(aq) –> Br2(g) + 2e^-
Concentrated aqueous sodium chloride half-equation of cathode reduction and anode oxidation
- Cathode (-)= 2H^+(aq) + 2e^- –> H2(g)
- Anode (+)= 2Cl^-(aq) –> Cl2(g) + 2e^-
Dilute sulfuric acid half-equation of cathode reduction and anode oxidation
- Cathode (-)= 2H^+(aq) + 2e^- –> H2(g)
- Anode (+)= 40H^-(aq) –> 2H2O(1) + O2(g) + 4e^-
Cathode reduction equation
Electrons are on the left hand side
Anode oxidation equation
Electrons are on the right-hand side
Main applications of electrolysis
- Electroplating
- Purification of copper/refining copper
- Electrolysis of concentrated sodium chloride/brine
Electroplating
Coating one metal with a thin layer of another metal either to improve appearance or prevent corrosion
Electroplating in electrolysis
The object to be electroplated is made at the cathode and the anode is made from the metal that will be the coating layer
Example equations for aqueous silver nitrate electroplating
- Cathode (-)= Ag^+(aq) + e^- –> Ag(s)
- Anode (+)= Ag(s) –> Ag^+(aq) + e^-
Copper refining and purification
Electrolysis is used to get 99.9% copper to use in electrical wiring
Steps of copper refining and purification
1.At the anode copper atoms lose electrons and go as copper (II) ions in the solution: Cu(s) –> Cu^2+(aq) + 2e^-
2.Cu^2+ ions are attracted to the cathode to gain electrons and form copper atoms: Cu^2+(aq) + 2e^- –> Cu(s) - layer of pure copper build up on cathode
3.Platinum, silver and gold might have been present in the impure copper anode and drop to the bottom to make a mixture - slime
4.Anode loses mass and cathode gains mass
Electrolysis of concentrated sodium chloride
Aqueous concentrated sodium chloride is produced by solution mining of underground rock salt deposits and then electrolyzed
Electrolysis of concentrated sodium chloride equations
- Cathode = 2H^+ +2e^- –> H2
- Anode = 2Cl^-(aq) –> Cl2(g) + 2e^-
Hydrogen used as a fuel
Hydrogen + Oxygen in a hydrogen-oxygen fuel cell produces electricity and water
Energy production of hydrogen as fuel
Exothermic reaction creates 143kJ/g
Fuel cell
Electrochemical cell converts chemical energy from fuel to electricity
Disadvantages of hydrogen-oxygen fuel cells
- Lack of facilities and methods for widespread use
- Compressed gases are hazardous
- Reactants are gases so must be compressed for storage
- Hydrogen sourced from methane - non-renewable energy source
- Hydrogen sourced from water - expensive to produce
Advantages of hydrogen-oxygen fuel cells
- Plentiful cheap supply of reactants
- Highly efficient process (>60% compared to -33% for petrol)
- Renewable energy source
- Produces zero carbon emissions
- Similar energy content
