chem f.4

In the process of purification of copper using electrolysis, the anode contains mainly copper with some impurities.

At the anode, copper would be oxidized before the less reactive metal (e.g. silver) during the electrolysis. When copper is gradually oxidized (dissolves) to aqueous copper(II) ions, the impure copper anode becomes thinner and thinner. The less reactive metals (e.g. silver), which are originally embedded in the copper anode, do not react but fall into bottom of the solution.

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(1) Write a half equation for the reaction occurs at an electrode (anode or cathode) in a chemical cell or in electrolysis.
For example, the equation for the cathode reaction of the electrolysis of copper(II) sulphate solution using graphite electrodes :
Cu2+(aq) + 2e- → Cu(s)

(2) Write a complete equation for a chemical reaction and overall reaction of electrolysis or chemical cell.
For example, the equation for the reaction of conc. nitric acid and copper :
Cu(s) + 4H+(aq) + 2NO3-(aq) → Cu2+(aq) + 2H2O(l) + 2NO2(g)
You may obtain the complete equation by using two half equations, but never give two half equations as the answer.

因為silver 唔鍾意oxidize
Because silver is located at the lower position in ECS than copper. Silver is a weak reducing agent, i.e. lower tendency to lose electron. Copper is oxidized while silver is not so silver will deposit at the bottom (due to gravity =.=").

唔一定要用half-eqt express, 但到現時為止,你redox reaction的程度只可以寫half equation
會考好少要冇e-果d