Transcript Slide 1
ELECTROLYSIS OF AQUEOUS SOLUTIONS 386 When water is present in an electrolysis experiment, it is not just the anion and cation of the solute that can be oxidised or reduced; sometimes the water itself becomes involved. During electrolysis of aqueous solutions the electrode reactions that occur, and so the products formed, depend upon: ■ the nature of the electrolyte ■ the concentration of the ions present ■ the nature of the electrodes. Copper chloride solution the electrolysis of copper chloride solution leads to the deposition of metallic copper at the cathode and the formation of chlorine gas at the anode. There are not many salts that electrolyse in aqueous solution to produce the metal from the cation and the element from the anion as here. Copper bromide and nickel chloride solutions are other examples http://webpages.charter.net/dawill/tmoranwms/Chem_CuCl2_3.jpg http://www.bbc.co.uk/scotland/education/bitesize/standard/img/chemistry/propertiesofsubstances/condu ction/electrolysis_copper.gif Cu2+(aq)+2e– → Cu(s) WARNING. THE PRODUCTS WILL DEPEND ON ELECTROLYTE CONCENTRATION the internet diagram suggests chlorine will form but oxygen gas forms at the anode. In this electrolysis sulfate ions migrate to the anode but they are too stable to be oxidised, so water is oxidised instead: Based on these standard electrode potentials...... If the ion concentration is low the latter will work Sodium sulfate solution With Inert electrodes http://www.crystalgrowing.com/labgrown/cuso4_xx.jpg http://content.answers.com/main/content/wp/en-commons/thumb/0/0b/180pxCopper_sulfate.jpg 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 the anode but because they are too stable to be oxidised, it is again Reaction 8.3 that occurs. Sodium ions migrate to the cathode but they are too stable to be reduced to sodium metal, so instead water is reduced to hydrogen gas: However, if you change the electrodes http://www.ferrarielectroplating.co.uk/index_files/image6231.jpg EXERCISES The electrolysis of molten lead bromide is often used as a demonstration of electrolysis (because lead bromide melts at a much lower temperature than most other simple salts). What would you expect the electrode reactions to be? What would you expect to observe? Why would this experiment have to be carried out in a fume cupboard? Magnesium metal is obtained commercially by electrolysing molten magnesium chloride. Write the electrode reactions for this process and hence the overall reaction http://thispublicaddress.com/tPA4/images/08_05/davy1.j pg Humphry Davy in the early 1800s produced potassium metal for the first time by electrolysing molten potassium hydroxide. At which electrode (cathode or anode) did the potassium form? What formed at the other electrode? Write equations for the electrode reactions and for the overall reaction. Write the electrode reactions you would expect for the electrolysis of a dilute solution of sodium hydroxide. Explain why this can be regarded as the electrolysis of water. Why is it not possible to electrolyse pure water itself? NATURE OF THE ELECTRODES http://www.ferrarielectroplating.co.uk/index_files/image6231.jpg 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 happens because copper is more easily oxidised than water. When we used an inert anode (graphite, platinum, stainless steel), water was more easily oxidised than the electrode material. Copper is still deposited on the cathode as before. This electrolysis is used industrially to purify copper. Blister copper (about 98% pure) from copper smelters is made the anode in an electrolytic cell. As electrolysis proceeds copper goes into solution and deposits onto the cathode. Some impurities go into solution and stay there (do not deposit out) while more inert impurities just fall to the bottom of the cell as the copper is eaten away from around them. In this way 99.9% copper is obtained. Silver nitrate solution With inert electrodes, electrolysis of silver nitrate solution is similar to that of copper sulfate 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 using electrolysis to coat one metal with a thin layer of another metal. To silver-plate a metallic object we make it the cathode of an electrolysis cell, use a solution of silver nitrate and a silver anode. At the anode silver goes into solution and at the cathode silver ions from the solution ‘plate out’ (deposit as a coating on the cathode). These two examples demonstrate that the product of an electrolysis can vary with the nature of the electrodes. http://www.dkimages.com/discover/previews/852/20222826.JPG http://www.jghs.edin.sch.uk/mathscience/chemistrynotes/chemimages/topic11image5.jpg 8.5 CONCENTRATION OF IONS PRESENT Read through these sections....Try to predict outcome before you begin.... Electrolysis of concentrated aqueous solutions of sodium chloride Electrolysis of dilute aqueous solutions of sodium chloride EXERCISES