Question

If \[Cu\] reacts with conc. \[{H_2}S{O_4}\] , then what reactions take place:
A. \[Cu\] is oxidised to \[C{u^{2 + }}\]
B. \[S{O^{2 - }}_4\] is reduced to \[S{O_2}\]
C. Both of the above reactions take place
D. None of the above reactions take place

Answer 1

Hint: The first question occurs here is whether copper reacts with sulphuric acid or not. Copper is a metal which has a high value of reduction potential and negative value of oxidation potential. In electrochemical series copper is placed below hydrogen so it does not liberate hydrogen gas when it reacts with it. So copper does not react with dilute sulphuric acid but reacts with concentrated sulphuric acid. When copper is heated with concentrated sulphuric acid. Concentrated sulphuric acid is an oxidising agent .

Complete answer:
When copper reacts with concentrated sulphuric acid sulphur dioxide is produced by the reduction of \[{H_2}S{O_4}\]. The copper (II) ions are formed in the aqueous solution. The redox reaction if \[Cu\] reacts with conc. \[{H_2}S{O_4}\] is given below, $Cu + 2{H_2}S{O_4} \to CuS{O_4} + S{O_2} + 2{H_2}O$; When we add concentration solution of ammonia, the copper (II) ion in solution reacts to form copper-ammonia complex which is deep in colour. Thus the reactions given in option A and B both occurs simultaneously that is \[Cu\] is oxidised to \[C{u^{2 + }}\] and \[S{O^{2 - }}_4\] is reduced to \[S{O_2}\] hence here option C is correct answer to this problem that is Both of the above reactions take place.

Note: We know that copper does not react with dilute sulphuric acid because the reduction potential of copper is higher than hydrogen. Hence copper is not able to displace hydrogen from non-oxidising acids like dilute sulphuric acid and nitric acid but concentrated sulphuric acid is a good oxidising agent, so it gives redox reaction with copper metal which is given above where \[Cu\] is oxidised to \[C{u^{2 + }}\] and \[S{O^{2 - }}_4\] is reduced to \[S{O_2}\].