The process of accumulation of molecular species at the surface of solid or liquid is called adsorption. It may be physisorption or chemisorption. Low temperature is favourable for physisorption. Amount of gas adsorbed by a solid depends upon nature of gas. Easily liquefiable gases are readily adsorbed. Catalyst increases the rate of reaction. Catalysis may be homogeneous or heterogeneous. Enzymes are biological catalysts. They work at optimum pH and temperature. Solutions are of three types i.e. true solutions, colloids and suspensions. Depending on the interaction between dispersed phase and dispersion medium colloids may be Lyophilic and Lyophobic. Depending on type of particles there may be multimolecular, macro-molecular and associated colloids. Sol, gel, emulsions are also various types of colloids.
An example of homogeneous catalysis is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(2SO_2 (g) + O_2(g) \overset{NO (g)}{\longrightarrow}2SO_3 (g)\)
In homogeneous catalysis the reactants and the catalyst are in the same phase (all liquid or all gas). In heterogeneous catalysis the catalyst is in a different phase from the reactants, usually a solid catalyst with gaseous reactants.
In the lead chamber process, sulphur dioxide is oxidised to sulphur trioxide by dioxygen with nitric oxide as the catalyst:
\( \displaystyle \ce{2SO2(g) + O2(g)} \xrightarrow{\ce{NO(g)}} \ce{2SO3(g)} \)
The reactants \( \ce{SO2} \) and \( \ce{O2} \) and the catalyst NO are all gases, so this is homogeneous catalysis.
Why the other options are incorrect
Option (1) \(2SO_2 (g) \overset{Pt (s)}{\longrightarrow} 2SO_3 (g)\) — Incorrect. The platinum catalyst is a solid and the reactants are gases (contact process type), so the catalysis is heterogeneous.
Option (2) \(N_2 (g) + 3H_2 (g) \overset{Fe (s)}{\longrightarrow}2NH_3 (g)\) — Incorrect. Haber’s process uses solid iron with gaseous \( \ce{N2} \) and \( \ce{H2} \); heterogeneous.
Option (3) \(4NH_3(g) + 5O_2(g) \overset{Pt (s)}{\longrightarrow} 4NO(g) + 6H_2O(g)\) — Incorrect. Ostwald’s process uses solid platinum gauze with gaseous reactants; heterogeneous.
Note: It seems that there is a mistake in the original NTA paper, which prints 2SO2(g) → 2SO3(g) in Option (1) when it should read 2SO2(g) + O2(g) → 2SO3(g).
NCERT (Pre-rationalised edition, 2022-23):
Homogeneous Catalysis
