Question

# The following is an extract from 'Metals in the service of man, Alexander and street / Pelican 1976' : Alumina (aluminium oxide) has a very high melting point of over 2000°C, so that it cannot readily be liquefied. However,conversion of alumina to aluminium and oxygen, by electrolysis, can occur when it is dissolved in some other substance. (a) Which solution is used to react with bauxite as a first step in obtaining pure aluminium oxide? (b) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction. (c) Name the element which serves both as the anode and the cathode in the extraction of aluminium. (d) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium. (e) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis. (f) Give the equation for the reaction that occurs at the anode when aluminium is purified by electrolysis.

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Solution

## (a) Hot solution of sodium hydroxide is used to react with bauxite as a first step in obtaining pure aluminium oxide. (b) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. The equation for this reaction is: $2\mathrm{Al}\left(\mathrm{OH}{\right)}_{3}\stackrel{heated}{\to }{\mathrm{Al}}_{2}{\mathrm{O}}_{3}+3{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{u}\mathrm{m}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{A}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{u}\mathrm{m}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}$ (c) Carbon is the element that serves both as the anode and the cathode in the extraction of aluminium. (d) During the extraction of aluminium, aluminium metal is deposited at the cathode. Aluminium ions accept electrons and get reduced to aluminium metal at the cathode. The equation for the reaction is: ​${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$ (e) During the extraction of aluminium by electrolysis, aluminium metal is deposited at cathode. Aluminium ions accept electrons and get reduced to aluminium metal at cathode. The equation for the reaction is: ${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$ (f) During the extraction of aluminium by electrolysis, oxygen gas is liberated at the anode. Oxygen ions lose electrons to form oxygen gas at the anode. The equation for the reaction is: ​ ​ ${\mathrm{O}}^{2-}\stackrel{}{\to }\mathrm{O}+2{\mathrm{e}}^{-}\phantom{\rule{0ex}{0ex}}\mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_{2}$

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