Presentation on theme: "ELECTROLYSIS OF AQUEOUS SOLUTIONS"— Presentation transcript:
1 ELECTROLYSIS OF AQUEOUS SOLUTIONS 386When water is present in an electrolysis experiment, it is not just the anion andcation of the solute that can be oxidised or reduced; sometimes the water itselfbecomes involved.During electrolysis of aqueous solutions the electrode reactions that occur, andso the products formed, depend upon:■ the nature of the electrolyte■ the concentration of the ions present■ the nature of the electrodes.
2 Copper chloride solution the electrolysis of copper chloride solution leads to thedeposition of metallic copper at the cathode and the formation of chlorine gas atthe anode. There are not many salts that electrolyse in aqueous solution to producethe metal from the cation and the element from the anion as here. Copper bromideand nickel chloride solutions are other examples
3 Cu2+(aq)+2e– → Cu(s) the internet diagram suggests chlorine will form WARNING. THE PRODUCTS WILL DEPEND ON ELECTROLYTE CONCENTRATIONthe internet diagram suggests chlorine will formbut oxygen gas forms at the anode. In this electrolysis sulfate ions migrate to theanode but they are too stable to be oxidised, so water is oxidised instead:
4 If the ion concentration is low the latter will work Based on these standard electrode potentials......If the ion concentration is low the latter will work
6 Electrolysis of sodium sulfate solution produces oxygen gas at the anode and hydrogen gas at the cathode. As with copper sulfate, sulfate ions migrate to theanode but because they are too stable to be oxidised, it is again Reaction 8.3that occurs.Sodium ions migrate to the cathode but they are too stable to be reduced tosodium metal, so instead water is reduced to hydrogen gas:
8 EXERCISESThe electrolysis of molten lead bromide is often used as a demonstration ofelectrolysis (because lead bromide melts at a much lower temperature than mostother simple salts). What would you expect the electrode reactions to be? Whatwould you expect to observe? Why would this experiment have to be carried outin a fume cupboard?Magnesium metal is obtained commercially by electrolysing molten magnesiumchloride. Write the electrode reactions for this process and hence the overallreactionHumphry Davy in the early 1800s produced potassium metal for the first time byelectrolysing molten potassium hydroxide. At which electrode (cathode or anode)did the potassium form? What formed at the other electrode? Write equations forthe electrode reactions and for the overall reaction.Write the electrode reactions you would expect for the electrolysis of adilute solution of sodium hydroxide. Explain why this can be regarded asthe electrolysis of water.Why is it not possible to electrolyse pure water itself?
10 However if we use a copper anode, then copper is oxidised instead: That is, the anode is eaten away and goes into solution as copper ions. This happensbecause copper is more easily oxidised than water. When we used an inert anode(graphite, platinum, stainless steel), water was more easily oxidised than theelectrode material. Copper is still deposited on the cathode as before.This electrolysis is used industrially to purify copper. Blister copper (about98% pure) from copper smelters is made the anode in an electrolytic cell. Aselectrolysis proceeds copper goes into solution and deposits onto the cathode.Some impurities go into solution and stay there (do not deposit out) while moreinert impurities just fall to the bottom of the cell as the copper is eaten away fromaround them. In this way 99.9% copper is obtained.
11 Silver nitrate solution With inert electrodes, electrolysis of silver nitrate solution is similar to that of coppersulfate solution: silver deposits on the cathode and oxygen is formed at the anode.However if a silver anode is used, it, silver, not water, is oxidised:Again this is similar to the copper sulfate situation.These two examples are the basis of electroplating, the process of usingelectrolysis to coat one metal with a thin layer of another metal. To silver-plate ametallic object we make it the cathode of an electrolysis cell, use a solution of silvernitrate and a silver anode. At the anode silver goes into solution and at the cathodesilver ions from the solution ‘plate out’ (deposit as a coating on the cathode).These two examples demonstrate that the product of an electrolysis can varywith the nature of the electrodes.
14 8.5 CONCENTRATION OF IONS PRESENT Read through these sections....Try to predict outcome before you begin....Electrolysis of concentrated aqueoussolutions of sodium chlorideElectrolysis of dilute aqueous solutionsof sodium chloride