Question

What happens when hydrogen gas is added to copper oxide?

Answer 1

On addition of hydrogen to copper oxide, the reduction of copper oxide to copper metal takes place. Copper oxide is black in color which reduces to brown color copper metal. On the other hand oxidation of hydrogen gas takes place.

Complete Step By Step Solution
Copper oxide is an inorganic black color solid compound. It is used for production of many wood preservatives. The addition of hydrogen gas changes its black color to brown as the oxide gets reduced to copper metal and the hydrogen gets oxidized to form water.
$\text{CuO}+{{\text{H}}_{2}}\to \text{Cu}+{{\text{H}}_{2}}\text{O}$
The end products formed are copper and water. Reduction is referred to as loss of oxygen, copper oxide loses its oxygen i.e. gets reduced and forms copper. On the other hand oxidation is referred to as gain of oxygen so hydrogen gas gains oxygen i.e. oxidizes and forms water. Hydrogen removes oxygen from copper oxide thus it acts as a reducing agent. The copper formed in the product does not react with water as copper metal is present below hydrogen in the reactivity series.
$\text{CuO}\xrightarrow{\text{Reduction}}\text{Cu}$
${{\text{H}}_{\text{2}}}\xrightarrow{\text{Oxidation}}{{\text{H}}_{2}}\text{O}$ $$
$\underset{\text{oxidising Agent}}{\mathop{\text{CuO}}}\,+\underset{\text{Reducing Agent}}{\mathop{{{\text{H}}_{2}}}}\,\to \text{Cu}+{{\text{H}}_{2}}\text{O}$
Copper oxide is a semiconducting compound with molar mass of $79\cdot 545\text{ g}$ per mole. Copper oxide can be produced by heating copper hydroxide.

Note
The black color copper oxide is known as cupric oxide or Copper (II) oxide. There also exists Copper (I) oxide which has a reddish appearance. It is known as cuprous oxide with the formula $\text{C}{{\text{u}}_{2}}\text{O}$ . Cuprous oxide is more stable than cupric oxide. Both of these are p-type semiconductors.