Question

On heating blue colored powder of copper (II) nitrate in a boiling tube copper oxide (black), oxygen gas and a brown gas X is formed. What could be the pH range of aqueous solution of the gas X?

Answer 1

Hint: The given question is an example of thermal decomposition reaction. When copper nitrate is heated in a test tube, initially the water molecules are evaporated. Further heating, the copper nitrate decomposes to form a reddish brown gas nitrogen dioxide and oxygen. We will elaborate this with chemical reactions and try to understand the process.

Complete answer:
> The thermal decomposition of copper II is an experiment performed in a lab.
> We take a test tube and we add copper crystals to it.
> Then we heat the test tube containing the compound on flame.
> The balanced chemical reaction taking place can be written using the chemical formula as:
$2Cu{(N{O_3})_2}\xrightarrow{{heat}}2CuO + {O_2} + 4N{O_2}$
> The copper (II) nitrate decomposes on heating to form copper oxide, which is black in color.
> Two gases are liberated. One is a colorless gas – oxygen. The other one is a reddish brown colored gas – nitrogen dioxide.
> Hence, the term X asked in the question is nitrogen dioxide gas.
> The brown gas formed dissolves in water to form an acidic solution. The balanced chemical reaction can be written as –
$2N{O_2} + {H_2}O \to HN{O_3} + HN{O_2}$
> We have to remember that the aqueous solution of nitrogen dioxide gas should be less than the pH 7 because the non-metal oxides show acidic property.

Note: We have to remember that the thermal decomposition reaction involves breaking of a certain compound in two or more compounds on heating. This particular thermal decomposition reaction on heating activates at temperature above $170^\circ C$. The brown gas formed, $N{O_2}$ dissolves in water to form an acidic solution. The pH of this solution is below 7.