Question

Which of the following is an irreversible reaction?
A. $PC{l_5} \to PC{l_3} + C{l_2}$
B. $2S{O_2} + {O_2} \to 2S{O_3}$
C. ${N_2} + 3{H_2} \to 2N{H_3}$
D. $2KCl{O_3} \to 2KCl + {O_2}$

Answer 1

A reaction is said to be irreversible if the either of the products formed escape out as gas or gets separated as liquid.

Complete step by step answer:
Potassium chloride and chlorine react to form potassium pentachloride. Also, Sulphur trioxide decomposes to form Sulphur dioxide along with the release of oxygen gas. Finally, ammonia also decomposes to give nitrogen and hydrogen gas as byproducts. Hence, the only reaction which cannot be reversed here, is the decomposition reaction of potassium chlorate.
The correct option is D)$2KCl{O_3} \to 2KCl + {O_2}$. This is because the oxygen so released in this
reaction as a byproduct does not react with potassium chloride and hence it does not lead to the formation of potassium chlorate. Generally, this happens because the oxygen formed as a byproduct is in a gaseous state and hence it escapes from the surroundings. Whereas, the other reactions can be easily reversed.
Thus Option (D) IS CORRECT .

Additional Information:
The decomposition of potassium chlorate to form potassium chloride and oxygen gas is a thermal decomposition reaction as heat is required. When sufficient quantity of heat is supplied potassium chlorate breaks down to form a solid residue of potassium chloride which is an ionic compound. Along with this oxygen gas is also released.

Note: It should be noted that not all decomposition reactions are irreversible. Only those reactions in which all the reactants are consumed to form products are considered irreversible.