Q15 of 26 Page 1

Write the chemical reactions involved in the process of extraction of Gold. Explain the role of dilute NaCN and Zn in this process.

4 Au(s) + 8 CN-(aq)+ 2H2O (aq)+ O2(g) → 4[Au(CN)2]-(aq) + 4OH-


2[Au(CN)2]- (aq)+ Zn (s) → [Zn(CN)4]2- (aq)+ 2Au (s)


Gold is leached with a dilute solution of NaCN in the presence of air (For O2) from which the metal is obtained later by replacement. in the extraction of gold and silver, the metal is leached with NaCN or KCN, which is an oxidation reaction, during this process Ag is formed by the replacement of Zinc.






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13

A first order reaction is 50% completed in 40 minutes at 300 K and in 20 minutes at 320 K. Calculate the activation energy of the reaction.

(Given: log 2 = 0.3010, log 4 = 0.6021, R = 8.314 JK–1 mol–1)

 14

What happens when

(a) a freshly prepared precipitate of Fe(OH)3 is shaken with a small amount of FeCl3 solution?


(b) persistent dialysis of a colloidal solution is carried out?


(c) an emulsion is centrifuged ?

 16

Give reasons:

(a) E° value for Mn3+/Mn2+ couple is much more positive than that for Fe3+/Fe2+


(b) Iron has higher enthalpy of atomization than that of copper.


(c) Sc3+ is colourless in aqueous solution whereas Ti3+ is coloured.

 17

(a) Identify the chiral molecule in the following pair ?


(b) Write the structure of the product when chlorobenzene is treated with methyl chloride in the presence of sodium metal and dry ether.


(c) Write the structure of the alkene formed by dehydrohalogenation of 1-bromo-1-methylcyclohexane with alcoholic KOH.