Explain why transition metals (i) have high melting points, (ii) have variable oxidation states, (iii) exhibit paramagnetism.

 Explain why transition metals (i) have high melting points, (ii) have variable oxidation states, (iii) exhibit paramagnetism. (b)(i) Name the impurities present in bausite, (ii) State how the impurities in bauxite are removed, (iii) Explain why aluminium oxide is said to be amphoteric. (c)(i) Describe the electrolysis of copper (II) tetraoxosulphate (VI) solution, using copper electrodes, (ii) Will the color of the copper (II) tetraoxosulphate (VI) solution change at the end of the electrolysis described in (c)(i) above? Give reasons for your answer. 

Solution:

The transition metals are hard and dense with high melting points because in addition to the 4s electrons, the 3d electrons in the atoms of these metals are also available for bonding.

(ii) Transition metals have variable oxidation states because the 3d electrons are available for bond formation. Example; Manganese  have variable oxidation states (a) MnO=+2 (b) MnO2=+4 (c) MnO42-=+6 (d) MnO4-=+7 

(iii) Magnetism is associated with the spinning of inner electrons about their axes. Atoms or ions that have one or more unpaired electrons and are attracted into a magnetic field are said to exhibit paramagnetism. Those that have one paired electrons and are repelled by a magnetic field exhibit diamagnetism.  

(b)(i) Impurities present in bauxite are iron (III) oxides and trioxosilicates (IV), (ii) The impurities in bauxite are removed by heating the bauxite with concentrated sodium hydroxide solution under pressure to form soluble sodium aluminate (III), NaAl(OH)4 as purification stage. 

Al2O3(s) + 2NaOH(aq) + 3H2O(l) ---> 2NaAl(OH)4

(iii) Aluminium oxide Al2O3 is amphoteric  because it reacts with dilute acids and strong alkali to form salts, that is it behaves both as basic oxides and as acidic oxides. 

(c)(i) At the cathode, Cu2+ are discharged preferentially. The Cu2+ takes two electrons each from the cathode to be deposited as metallic copper on the cathode. 

Cu2++2e- --> Cu(s). At the anode, no ions are discharged. The conversion of copper atoms to ions is favoured because it requires less energy. One atom of copper anode ionizes by dissolving from the anode into the electrolyte to form Cu2+ ions.

Cu2+  --2e- --> Cu2+(aq) --2e- --> Cu(s).

(ii) There is no colour change in the electrolyte because the copper ions discharged at the cathode are being replaced by the dissolving copper anode i.e. transfer of copper ions from the anode to the cathode.

QUESTION SOURCE: WAEC 1988 THEORY QUESTION 12(a)(i)(ii)(iii)(b)(i)(ii)(iii)(c)(i)(ii)