Question

A brown substance  $ 'X' $  on heating in air forms a substance  $ 'Y' $ . When hydrogen gas is placed over heated  $ 'Y' $ it again changes back into  $ 'X' $ .

Name the substances  $ 'X' $ and  $ 'Y' $ .

Name the type of chemical reaction occurring during both the changes.

Write the chemical reactions.

Answer 1

Hint: Here we will proceed by using the approach of oxidation and reduction chemical reactions which is known as redox reaction.

Complete answer:

The brown substance  $ 'X' $ is copper that is  $ Cu $  when heated copper oxide is formed.

Hence compound  $ 'Y' $  is copper oxide.

This brown substance is copper as it is brown in colour and on passing oxygen in presence of heat, it forms copper oxide which on reacting with hydrogen again forms copper.

The reaction is as follows. It is a combination reaction.

 $ 2Cu + {O_2} \to 2CuO $ 

If hydrogen gas is passed over heated copper  $ \left( {2CuO} \right) $ , the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

The chemical reaction is as follows

 $ CuO + {H_2} \to Cu + {H_2}O $ 

Reactions taking place here are oxidation and reduction. Also known as redox reaction.

 $ 2Cu + O2 \to 2CuO $  (Oxidation)

 $ 2CuO + H2 \to Cu + H20 $  when heated (Reduction)

As during this reaction, the copper oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidised while the other gets reduced.

Note: In this particular question, one must know that redox reaction is a type of chemical reaction that involves a transfer of electrons between two species. Oxidation of molecules by removal of hydrogen or combination with oxygen normally liberates large quantities of energy. By using this approach one can easily solve questions related to redox reactions.