AQA GCSE C2.7

 When an ionic substance is melted or dissolved in water (to make a solution) the ions become free to move.  Electrolysis can then be carried out by passing an electric current through the molten ionic substances or the solution producing elements.  The substance that is broken down is called the electrolyte.

 In electrolysis the ions move:  Positively charged ions move to the negative electrode (cathode)  Where they gain electrons (reduction)  Negatively charges ions move to the positive electrode (anode)  Where they lose electrons (oxidation)  [OILRIG – Oxidation is Loss (of electrons), Reduction is Gain (of electrons)].

 Aluminium is so reactive it cannot be extracted from its ore using carbon, so it is extracted using electrolysis  Aluminium is extracted from the ore bauxite which is a form of aluminium oxide. molten  It is extracted by electrolysis of molten bauxite. Early attempts at this failed because bauxite is so hard to melt – its melting point is above 2000 o C.  If cryolite is added, the bauxite melts more easily – at about 900 o C. This is an essential step in the extraction process.

A bauxite / cryolite mixture is melted in a steel container containing a carbon lining. Graphite (carbon) anodes Tank lined with carbon cathode Molten electrolyte bauxite + cryolite Steel case Graphite anodes are inserted into the molten electrolyte ready for electrolysis. Electrolytic extraction of Aluminium

 Opposite charges attract.  And so positive aluminium ions move towards the negative electrode (cathode).  At the cathode these ions gain electrons and turn into aluminium atoms.

 Negatively charged oxide ions move to the positive electrode (anode).  Here they lose 2 electrons and so turn into neutral oxygen atoms.  These atoms rapidly join into pairs to form normal oxygen gas.

 Electrolysis is carried out at high temperature.  Under these conditions quite a lot of the oxygen reacts with the carbon (graphite) electrode.  Carbon dioxide is formed and the positive electrode (anode) is rapidly eaten away and frequently has to be replaced.

Extraction of aluminium: overall siphon Graphite / carbon anodes Molten aluminium Molten electrolyte bauxite + cryolite Tank lined with carbon cathode Molten aluminium out Steel case Vented cover

Extraction of aluminium– half equations Positive Electrode (Anode) O 2-  O + 2e - Oxygen is a gas so travels in pairs, therefore: 2O 2- (l)  O 2 (g) + 4e - Negative Electrode (Cathode) Al 3+ (l) + 3e -  Al(l) Overall 2Al 2 O 3 (l)  4Al(l) + 3O 2 (g) The positive electrode (anode) reacts with the oxygen produced to form carbon dioxide C + O 2  CO 2

 Common aluminium ore I axe tub  Added to reduce melting point City role  The electrodes are made out of Right ape  Extracting aluminium is a Cretin duo bauxite cryolite graphite reduction Unscramble the words to end the sentences.

The electrolysis of sodium Chloride solution is not as simple as that of molten Aluminium Oxide. It produces:  Hydrogen  Chlorine  Sodium Hydroxide These products are important reagents for the chemical industry.

Chlorine gas Bleaching agent, e.g. paper industry Sterilisation of water e.g. swimming pools and drinking water Manufacture of HCl Manufacture of Cl-containing organic chemicals Manufacture of PVC and other plastics Pesticides Solvents, e.g. solvent for tippex, & ‘dry cleaning’ dyes

Sodium hydroxide Manufacture of soap Manufacture of Paper Extraction of aluminium Textiles (wool, cotton) Neutralisation of acid effluents

 Salt solution contains NaCl and H 2 O.  Therefore the ions present are:  Na + ions (from the salt)  Cl - ions (from the salt)  H + ions (from the water)  OH - ions (from the water) What happens at each electrode depends on the ability of the ion to gain or lose electrons Na + Cl - H O-O- H+H+ H H O

Negative ions  Chloride ions go to the positive electrode (anode) where they lose an electron.  The neutral chlorine atoms produced join up into pairs and Chlorine gas is formed (Cl 2 ). Cl 2 2Cl e- Chlorine gas is formed

Positive ions  Na + and H + ions are both present.  H + accepts electrons more easily than Na + does. do not  Therefore although Na + ions move to the negative electrode they do not accept electrons.  It is the hydrogen ions (H + ) that gain electrons

 As a result hydrogen gas is formed at the negative electrode. 2H + + 2e-  H 2

 Sodium hydroxide is left in the solution at the end of electrolysis Na+ Cl- H O- H+ Na+ H O- Cl H H SolutionElectrodes