NEET Surface Chemistry Previous Year Questions with Solutions

When the catalyst and the reactants are in different phases, the catalytic process is termed heterogeneous catalysis. The reaction typically proceeds via the adsorption theory at the solid catalyst’s active surface sites:

In the industrial synthesis of ammonia by Haber’s process, dinitrogen gas and dihydrogen gas react over solid finely divided iron containing molybdenum or potassium oxide promoter:

The reactants (N₂ and H₂) are in the gaseous phase, whereas the catalyst (Fe) is in the solid phase. Because the phases are different, this is a classic canonical example of heterogeneous catalysis.

In all other options, reactants and catalysts share the identical phase:

Pumice stone is a highly vesicular, porous volcanic rock formed when gas bubbles remain trapped in rapidly cooling molten lava:

In pumice stone, the trapped gas bubbles constitute the dispersed phase, while the solid rock matrix constitutes the dispersion medium:

Referring to the standard eight colloidal classes based on physical states (Gas in Gas forms a true mixture):

When a converging beam of light is passed through a colloidal solution in a dark room, the path of the beam becomes illuminated by a blueish light when viewed at right angles. This phenomenon is known as the Tyndall effect, caused by the scattering of light by colloidal particles.

According to NCERT Surface Chemistry, the Tyndall effect is observed only when two conditions are satisfied: 1. The diameter of the dispersed particles is not much smaller than the wavelength of the light used ($1\text{ nm} - 1000\text{ nm}$). 2. The refractive indices of the dispersed phase and the dispersion medium differ greatly.

- NaCl, Glucose, and Urea solutions: Solute molecules/ions dissolve completely at the molecular level with particle diameters $< 1\text{ nm}$, forming true homogeneous solutions. True solutions DO NOT scatter visible light. - Starch solution: Starch is a high-molecular-weight polysaccharide that forms a macromolecular colloidal sol in water. The large macromolecular aggregates (size $1-100\text{ nm}$) scatter light effectively, producing a visible Tyndall cone.

Colloidal particles acquire positive or negative charge by preferential adsorption of ions from solution, forming a firmly attached "fixed layer" on the particle surface.

The fixed layer attracts counter-ions from the dispersion medium, forming a second, mobile "diffused layer". The arrangement of double layers of opposite charges around the colloidal particle is called the Helmholtz electrical double layer.

The potential difference between the fixed layer and the diffused layer of opposite charges is termed the electrokinetic potential or zeta potential (denoted by $$\zeta$$).

If the zeta potential is sufficiently large (typically exceeding $$\pm 30\text{ mV}$$), the electrostatic repulsion between approaching colloidal particles is strong enough to overcome attractive van der Waals forces, preventing agglomeration/coagulation. Thus, measurement of zeta potential is universally employed to determine and predict the stability of colloidal dispersions.