Question

Explain catalytic poisoning with example.

Answer 1

Catalysts are important substances for altering or speeding the reaction. The action of these catalysts is hindered by certain substances which act as a poison to them and thus, their action is known as catalytic poisoning.

Complete step-by-step answer: As we know that catalysts are substances which alter the rate of a chemical reaction. They may speed up the reaction or may make the reaction possible under normal and easily achievable conditions. This phenomenon of adding a catalyst and altering the rate of a reaction is known as catalysis. These catalysts remain unchanged chemically and in quantity that means that they are not used up quantitatively in the reaction and at the end of the reaction the same amount of catalyst is obtained as it was added.
Let us consider an example
2{H_2}{O_2} \to 2{H_2}O + {O_2} \\\ \\\
The reaction of decomposition of hydrogen peroxide gives water. When potassium permanganate $(KMn{O_4})$ is added as a catalyst, it speeds up the reaction.
The activity of these catalysts is hindered by certain substances. These substances which slow down or hinder the action of a catalyst are known as poisons and this phenomenon is called catalytic poisoning. These substances act on active sites of the catalysts which affects their activity of enhancing a reaction. Some examples of catalytic poisons are carbon monoxide, phosphates, halides, cyanides and others.
In the Haber’s process which is used to produce ammonia, catalytic action of iron is hindered or poisoned by oxygen containing compounds like carbon monoxide which slows the reaction and synthesis of ammonia is difficult to achieve.
${N_2}(g) + 3{H_2}(g) \xrightarrow {Fe(s)} 2N{H_3}(g) \\\ {{ }} \\\$

Note: The actions of certain catalysts depend on the structure of the catalysts and on the size and structure of the reactants, this is known as shape-selective catalysis. The catalysts which have the same state as that of the reactants of the reaction give homogeneous catalysis and those whose states are different from the reactants of the reaction give heterogeneous catalysis.