Question

Name the gas evolved in each of the following cases:
(i) Copper carbonate is heated strongly.
(ii) Action of dilute hydrochloric acid on sodium sulphite.
(iii) Nitrogen combines with hydrogen.
(iv) Action of dilute sulphuric acid on sodium carbonate.
(v) Addition of sodium to cold water.

Answer 1

Hint:
(a) This reaction involves the decomposition of copper carbonate.
(b) Apart from the gas evolved, this reaction also results in the formation of common salt and water.
(c) The gas evolved is one of the most important compounds used in the fertiliser industry.
(d) The same gas as (i) is evolved in this reaction as well.
(e) Apart from the evolved gas, this reaction results in the formation of one of the strongest bases found in nature.

Step-by-Step Solution:
Let us go through each of these reactions one at a time.
(i) This reaction is the thermal decomposition of Copper Carbonate, which breaks down when heated into two or more products from the initial reactant. The products evolved in this particular reaction are Copper Oxide and Carbon Dioxide.

$CuC{{O}_{3}}~\to \text{ }CuO\text{ }+\text{ }C{{O}_{2}}\uparrow$

Therefore, the gas evolved here is Carbon Dioxide.

(ii) When hydrochloric acid reacts with sodium sulphite, it forms sodium chloride and sulphur dioxide which gives smell of burning sulphur.$N{{a}_{2}}S{{O}_{3}}~+\text{ }2HCl\text{ }\to \text{ }2NaCl\text{ }+\text{ }{{H}_{2}}O\text{ }+\text{ }S{{O}_{2}}\uparrow$

Therefore, the gas evolved as a result of this reaction is Sulphur Dioxide.

(iii) Nitrogen and hydrogen gases combine under specific conditions to form ammonia. This is an example of a combination reaction. It is also known as a synthesis reaction.

${{N}_{2}}+3{{H}_{2}}\xrightarrow{773K}2N{{H}_{3}}\uparrow$

Therefore, the gas evolved here is Ammonia.

(iv) When dilute sulphuric acid is added to sodium carbonate, the corresponding salt, sodium sulphate and water are formed and carbon dioxide gas is evolved. ${{H}_{2}}S{{O}_{4}}\text{ }+\text{ }N{{a}_{2}}C{{O}_{3}}\text{ }\to \text{ }N{{a}_{2}}S{{O}_{4}}\text{ }+\text{ }{{H}_{2}}O\text{ }+\text{ }C{{O}_{2}}\uparrow$
(v) When sodium is dropped in cold water, it reacts explosively and burns with a golden yellow flame. The reaction is exothermic and vigorous and forms sodium hydroxide with the liberation of hydrogen gas.

$2\text{ }Na\text{ }+\text{ }2{{H}_{2}}O\text{ }\to \text{ }2NaOH\text{ }+\text{ }{{H}_{2}}~\uparrow ~$

Note: For reactions which result in multiple products, be very careful of their natures so as to ensure you do not make a mistake when naming the gas evolved. For instance, in (i) you must remember that Copper Oxide is in fact a precipitate.