Q14 of 26 Page 1

(a) Indicate the principle behind the method for the refining of zinc.

(b) Account for the following:


(i) It is advantageous to roast sulphide ore to oxide before reduction.


(ii) Zinc oxide can be reduced to metal by heating with carbon but not Cr2O3.


(a) Zinc is refined by electrolysis method. The principle involved in this method is that the impure zinc is made anode while a thin strip of pure zinc is made the cathode, both of them are dipped in a zinc salt solution which acts as electrolyte. When electricity is passed through this circuit, the impure zinc metal at anode gets oxidised and come into the electrolyte


while the zinc ions reduce at cathode to form pure zinc (solid) metal.



(b) (i) It is easy to reduce metal oxides to metals rather than sulphides to metals. Hence sulphide ores are roasted to oxide for easier reduction.


(ii) Carbon has more affinity towards oxygen than Cr2O3 making it a better reducing agent for zinc oxide than Cr2O3.


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For a first order reaction, show that time required for 99% completion is double of the time required for the completion of 90% of reaction.

OR


The rate constant of a first order reaction increases from 4 x 10-2 to 24 x 10-2 , when the temperature changes from 300 K to 350 K. Calculate the energy of activation (Ea). [ Given: log 2 = 0.3010, log 3 = 0.4771, log 4 = 0.6021, log 6 = 0.7782; R = 8.314 J/K/mol]


 15

Account for the following:

(a) Inter-halogen compounds are more reactive than pure halogens.


(b) Nitrogen is less reactive at room temperature.


(c) Reducing character increases from NH3 to BiH3.


 16

(a) For the complex [CoF6]3-, write the hybridization type, magnetic character and spin nature of the complex. (Atomic no. of Co = 27)

(b) Why is the complex [Co(en)3]3+ more stable than [CoF6]3-?