Question

There is also formation of insoluble $CuS$ (black) when copper reacts with conc. $H_{2}S{O}_4$. What is the change in oxidation number of sulphur?

Answer 1

In $CuS$, Sulphur oxidation state is $+2+x=0;x=-2$

In $H_{2}S{O}_4$, Sulphur oxidation state is

$2(+1)+x+4(-2)=0$

$2+x-8=0$

$x=6$

Change in oxidation number of sulphur is $6-(-2)=8$