Question

$A$ and $B$ are two ores of copper.
$A$ undergoes calcination to form a solid, $C{O}_{2\left(g\right)}$ and $O_{2\left(g\right)}$
$B$ undergoes roasting to form a solid and a gas $C$ which turns acidified $K_{2}C{r}_2{O}_7$ to green solution.
$B$ also reacts with dil $HCl$ to form a solid and gas $D$ which turns lead acetate solution black. $D$ also reacts with $C$ to form colloidal sulphur in the presence of moisture.

The gas ($C$) acts as a/an:

• A. Oxidising agent
• B. Reducing agent
• C. Both (a) & (b)
• D. Fluxing agent

Answer 1

The correct option is C Both (a) & (b)

A = $Cu\left(C{O}_3\right)$ and B = $CuS$

Reactions involved are:

Calcination of A:
$\underset{\left(A\right)}{Cu\left(C{O}_3\right)}\to \underset{\left(solid\right)}{CuO}+C{O}_2+O_2$

Roasting of B:
$\underset{\left(B\right)}{CuS}\to \underset{\left(solid\right)}{CuO}+\underset{\left(C\right)}{S{O}_2}$

$\underset{\left(C\right)}{S{O}_2}+K_{2}C{r}_2{O}_7\to K_2(SO{)}_4+\underset{\left(green\right)}{C{r}^{3+}}$

Gas '$C$' is $S{O}_2$ i.e. Sulfur is in +4 oxidation state. It can go to its minimum oxidation state ($-2$) and to maximum oxidation state ($+6$), hence can act as both oxidising and reducing agent.