Chapter 20B: Electrolytic Cells Flashcards
Define electrolytic cells.
It is a cell in which the redox reaction is non spontaneous and electrical energy is used to cause it to occur.
Define electrolysis.
It is a process that involves the lysing/splitting of a substance, often to its component elements, by supplying electrical energy. The electrical energy from an external source causes a non-spontaneous redox reaction to occur.
What is selective discharge?
It is discharging the ion/substance at each electrode that requires the least energy.
What are the 3 factors affecting selective discharge?
1) position of ion/substance in the electrochemical/redox series
2) concentration of ion (which affects electrode potential)
3) nature of electrode
How does the position of an ion in the electrochemical series (its Eº value) affect discharge? (2)
1) The more positive the Eº value, the higher its tendency for reduction.
2) the less positive the Eº value, the higher its tendency for oxidation.
How does ion concentration affect discharge and why? Describe one exception.
Ions which are present in high concentrations may be discharged preferentially even though its relative Eº value suggests otherwise. This is because it would cause the position of equilibrium to shift and change its Eº value, making it preferentially discharged.
(Exception: highly reactive metals are never reduced in an aqueous electrolyte during electrolyte, since the difference between their Eº values and water is too large)
How does the nature of electrodes affect discharge?
Electrodes that are not inert should be considered for discharge, but it ultimately depends on its Eº values.
What are some rules/exceptions when predicting products of electrolysis of aqueous electrolytes? (2C, 2A, 2O)
Cathode
1) cations of reactive metals are never reduced and H2O (l) is always reduced preferentially due to the large difference between their Eº values.
2) H2(g) can be produced from the reduction of water/H+ when an acid is electrolysed.
Anode
1) NO3- and SO4 2- are not oxidised even at high concentrations.
2) O2 (g) can be produced from oxidation of water/OH- if an alkali is electrolysed.
Others
1) consider electrodes that can undergo oxidation themselves if they are used as anodes
2) if O2 is produced at a graphite anode, CO(g) and CO2(g) may be formed, especially at high temperatures.
What is the Faraday’s Law of Electrolysis?
The number of moles of a substance that undergoes oxidation/reduction at each electrode during electrolysis is directly proportional to the quantity of charge that passes through the cell.
What happens during the electrolytic purification of copper?
1) the impure copper is the anode while the cathode is a bar of pure copper. The electrodes are placed in a solution of copper (II) sulfate. Electricity is then passed through the cell.
2) at the anode: metal impurities with Eº values less positive than that of Cu will be oxidised (as they are more readily oxidised than copper) and will enter the electrolyte as metal ions.
3) at the anode: metal impurities with Eº values more position than that of Cu will not be oxidised, and drop off as anode sludge.
4) at the cathode: Cu2+ ions are reduced to form Cu solid on the pure copper.
5) the metal impurities dissolved at the anode will not be reduced since their Eº values are less positive than that of Cu (less readily reduced than Cu2+ ions), and will remain in the electrolyte.
6) anode decreases in mass while cathode increases in mass.
What does it mean to anodise aluminum?
Anodising of aluminum is a surface treatment to increase the thickness of the corrosion resistant Al2O3 layer on the surface of an aluminum object by electrolysis.
What happens during the anodising of aluminum? (5)
1) the Al object to be anodised is made the anode, while the cathode is Pt/graphite. The electrolyte is dilute H2SO4.
2) at the anode, Al(s) is oxidised to form Al2O3 (s).
3) at the cathode, H+ is reduced to form H2 gas.
4) the Al2O3 layer formed would be porous, where the pores are sealed by dipping the Al object in hot water.
5) the thickness of the deposit of aluminum oxide increases with electrolysis time.
What is the purpose of anodising of Al?
The oxide coating would be porous enough to accept and hold dyes that would not otherwise stick to aluminum. Since the thickness of the deposit of Al2O3 increases with electrolysis time, the amount of dye deposited and the intensity of colour developed also increase with electrolysis time. Anodised objects can therefore be decorative.
