Surface ChemistryNEET Chemistry · Class 12 · NCERT Chapter 14

Here are the five differences NEET tests most often. (1) Force of attraction: physisorption uses weak van der Waals forces; chemisorption uses strong covalent or ionic bonds. (2) Enthalpy of adsorption: physisorption has low enthalpy (20-40 kJ/mol); chemisorption has high enthalpy (40-400 kJ/mol). (3) Reversibility: physisorption is reversible (adsorbate can be removed by lowering pressure or raising temperature); chemisorption is irreversible or very difficult to reverse. (4) Layers formed: physisorption forms multi-molecular layers (multilayer); chemisorption forms only a monomolecular layer (monolayer). (5) Effect of temperature: physisorption decreases with increasing temperature (because adsorption is exothermic); chemisorption first increases (to overcome activation energy) then decreases at very high temperature. These five differences appear directly in NEET match-the-following and true/false questions every few years.

The Freundlich adsorption isotherm is an empirical equation that relates the extent of adsorption (x/m) to the pressure (P) of the adsorbate gas at a fixed temperature. The equation is: x/m = k x P^(1/n), where x = mass of adsorbate, m = mass of adsorbent, k and n are constants that depend on the nature of the adsorbate and adsorbent, and n is always greater than 1. The log form is: log(x/m) = log k + (1/n) x log P. This gives a straight line when you plot log(x/m) on the y-axis and log P on the x-axis. Slope = 1/n, intercept = log k. Limitations: (1) It is purely empirical, with no theoretical basis. (2) It works only at intermediate pressures. At very high pressures, x/m approaches a maximum and the equation fails (the graph should plateau, not keep rising). (3) It is not valid for gases where the pressure is near saturation. Despite these limitations, the Freundlich isotherm graph and the log-form straight line are very frequently tested in NEET.

The key difference is particle size. True solutions have particles smaller than 1 nm (individual ions or molecules). You cannot see these particles and light passes through without scattering. Colloidal solutions (also called colloids or sols) have particle sizes between 1 nm and 1000 nm (1 micrometer). These particles are too small to see individually or to filter, but large enough to scatter light (Tyndall effect). Suspensions have particles larger than 1000 nm. These particles settle on standing and can be filtered. Summary: true solution is homogeneous at the molecular level; colloid is apparently homogeneous but heterogeneous at the microscopic level; suspension is heterogeneous and particles settle. The Tyndall effect is the easiest way to distinguish a colloid from a true solution in NEET: shine a beam of light through a colloid and you see the beam; shine it through a true solution and the beam is invisible.

When a beam of light passes through a colloidal solution, the colloidal particles scatter the light in all directions. This makes the beam of light visible when you look at it from the side, even though the solution looks clear from above. This scattering of light by colloidal particles is called the Tyndall effect, named after the scientist John Tyndall who discovered it in 1869. The colloidal particles act as tiny scattering centres because their size (1-1000 nm) is comparable to the wavelength of visible light (400-700 nm). True solutions do NOT show the Tyndall effect because the dissolved particles are too small (sub-nanometre) to scatter visible light significantly. Everyday examples: (1) A beam of sunlight through a dusty room shows a visible shaft of light. (2) Car headlights in fog create a visible cone of light in the fog (water droplets in air = aerosol colloid). (3) The blue colour of the sky is due to scattering of sunlight by colloidal-sized particles in the atmosphere. NEET always tests: "Which of the following shows the Tyndall effect?" Answer: colloid; not true solution.

The Hardy-Schulze rule states: the higher the charge (valence) of the coagulating ion, the greater its power to coagulate a colloid. In other words, you need much less of a high-valence ion than a low-valence ion to coagulate the same colloid. This is because colloidal particles are charged. To coagulate them, you need to neutralise their charge by adding an oppositely charged electrolyte ion. A trivalent ion (like Al3+) is much more effective at neutralising the surface charge than a monovalent ion (like Na+). Coagulating power of cations: Al3+ > Ca2+ > Na+ (for a negatively charged colloid like As2S3 sol). Coagulating power of anions: PO43- > SO42- > Cl- (for a positively charged colloid like Fe(OH)3 sol). Key NEET point: the coagulating ion is always the one opposite in charge to the colloid. Negatively charged As2S3 sol is coagulated by cations (Al3+ is most effective). Positively charged Fe(OH)3 sol is coagulated by anions (PO43- is most effective). The coagulating value (concentration needed) is the inverse of coagulating power: lower coagulating value = more effective.

Lyophilic means "solvent-loving." Lyophobic means "solvent-fearing." Lyophilic colloids: the dispersed phase has a strong affinity for the dispersion medium. Examples: gum, starch, gelatin, proteins, rubber. These colloids are reversible (if you evaporate the solvent and add it back, the colloid re-forms). They are stable and resistant to coagulation because the particles are heavily solvated (surrounded by solvent molecules that form a protective layer). They do NOT need a stabiliser. Their viscosity is much higher than the pure solvent. Lyophobic colloids: the dispersed phase has no affinity for the medium. Examples: gold sol, arsenic sulfide sol, sulfur sol, silver sol. These colloids are irreversible. They are less stable and coagulate readily when electrolyte is added. They require a stabiliser (like a protective colloid or an emulsifier) to remain stable. They do not significantly increase the viscosity of the medium. NEET uses this distinction in coagulation questions: lyophobic colloids coagulate much more easily than lyophilic ones.

Gold number is the minimum amount (in milligrams) of a protective colloid that must be added to 10 mL of a standard gold sol to prevent coagulation when 1 mL of 10% NaCl solution is added. It was introduced by Zsigmondy. The key thing to remember: a lower gold number means a better (more efficient) protective colloid, because you need less of it to protect the gold sol. Examples of gold numbers: gelatin = 0.005-0.01 mg (very good protective colloid), haemoglobin = 0.03-0.07 mg, gum arabic = 0.15-0.25 mg, potato starch = 25 mg (poor protective colloid). Lyophilic colloids like gelatin and starch act as protective colloids for lyophobic colloids. They coat the lyophobic particles and prevent them from being coagulated by electrolytes. This protective action is used in practical applications: gelatin stabilises ice cream, starch stabilises food emulsions. NEET questions test the concept: "Which has the lowest gold number? Answer: gelatin (best protective colloid)."

In homogeneous catalysis, the catalyst and the reactants are in the same phase (all gases, or all liquids). The catalyst participates by forming intermediate compounds with the reactants, which then break down to give products and regenerate the catalyst. Examples: (1) Lead chamber process: 2SO2 + O2 + [NO catalyst] → 2SO3, where all components are gaseous. (2) Friedel-Crafts reaction in organic chemistry uses AlCl3 dissolved in the same solvent as the reactants. In heterogeneous catalysis, the catalyst is in a different phase from the reactants. The catalyst is almost always a solid and the reactants are gases or liquids. The mechanism involves adsorption of reactants on the catalyst surface, weakening of bonds, reaction, and desorption of products. Examples: (1) Haber process: N2(g) + 3H2(g) → 2NH3(g) with solid Fe catalyst and Mo/K2O promoters. (2) Contact process: 2SO2(g) + O2(g) → 2SO3(g) with solid V2O5 catalyst. (3) Hydrogenation of alkenes with Ni or Pt catalyst. Heterogeneous catalysis is more commonly tested in NEET than homogeneous catalysis.