Surface Chemistry — Adsorption, Catalysis, Colloids, Emulsions

Chemistry at the Surface

Surface chemistry studies phenomena occurring at interfaces — between solid-gas, solid-liquid, or liquid-liquid phases. Adsorption, catalysis, and colloids are the three pillars. This chapter is theory-heavy and scoring for boards (4-6 marks) since questions test definitions and classifications.

NEET asks 1-2 questions, usually on adsorption types, colloid properties, or catalyst behaviour. The chapter has no numericals (except Freundlich isotherm plotting) — it is purely conceptual.

Key Concepts

Adsorption vs Absorption

Adsorption — accumulation of a substance on the surface of another. The substance adsorbed is the adsorbate; the surface is the adsorbent. Example: gas molecules sticking to activated charcoal.

Absorption — penetration into the bulk of another substance. Example: water soaking into a sponge.

Sorption — when both occur simultaneously. Example: dye on a fibre.

Types of adsorption

Feature	Physisorption	Chemisorption
Forces	Weak van der Waals	Strong chemical bonds
Enthalpy	Low (20-40 kJ/mol)	High (80-240 kJ/mol)
Reversibility	Reversible	Usually irreversible
Specificity	Non-specific	Highly specific
Temperature effect	Decreases with T increase	Increases initially, then decreases
Layers	Multilayer possible	Only monolayer
Activation energy	Low/none	May need activation energy

Key relationship: Physisorption can transform into chemisorption at higher temperatures. At low T, gas is physically adsorbed; as T increases, activation energy is available for chemical bond formation. This explains why chemisorption initially increases with temperature.

\frac{x}{m} = kp^{1/n} \quad (1 \leq n \leq 10)

Log form: $\log\frac{x}{m} = \log k + \frac{1}{n}\log p$

Plot $\log(x/m)$ vs $\log p$ — a straight line with slope $1/n$ and intercept $\log k$ .

At very low pressure: $x/m \propto p$ (linear). At very high pressure: $x/m$ approaches a constant (surface saturated). The Freundlich equation works at intermediate pressures.

Factors affecting adsorption

Nature of adsorbate — easily liquefied gases (high critical temperature) are adsorbed more. SO $_2$ > CO $_2$ > CH $_4$ > H $_2$ on charcoal.
Nature of adsorbent — larger surface area = more adsorption. Finely divided metals, activated charcoal, silica gel are excellent adsorbents.
Temperature — physisorption decreases with increasing T. Chemisorption increases initially then decreases.
Pressure — adsorption increases with pressure until the surface is saturated.

Catalysis

A catalyst alters the rate of a reaction without being consumed. It provides an alternative pathway with lower activation energy ( $E_a$ ).

Feature	Homogeneous	Heterogeneous
Phase	Same as reactants	Different from reactants
Example	H $^+$ in ester hydrolysis	Fe in Haber process, V $_2$ O $_5$ in Contact process
Mechanism	Intermediate compound formation	Surface adsorption → reaction → desorption
Selectivity	Moderate	Can be high (shape-selective)

Key facts about catalysts:

Do NOT change the equilibrium position — they speed up both forward and reverse equally
Lower the activation energy for both directions
Are regenerated at the end of the reaction
Small amounts can catalyse large amounts of reactant

Heterogeneous catalysis — mechanism

Five steps:

Diffusion of reactants to the catalyst surface
Adsorption of reactants on the surface (forms bonds with surface atoms)
Reaction on the surface — bonds rearrange
Desorption of products from the surface
Diffusion of products away from the surface

The catalyst surface has active sites — specific locations where adsorption and reaction occur. These are often edges, corners or defects in the crystal structure.

Promoters increase catalyst activity without being catalysts themselves (e.g., Mo promotes Fe in the Haber process). Poisons decrease catalyst activity by blocking active sites (e.g., CO poisons the catalyst in catalytic converters).

Shape-selective catalysis

The product depends on the pore size of the catalyst. Zeolites (aluminosilicates with porous structure) are the classic example.

Example: ZSM-5 zeolite converts methanol to gasoline-range hydrocarbons — only molecules that fit the pore can form. This selectivity is used in petrochemical refining.

Enzyme catalysis

Enzymes are biological catalysts — proteins with a specific 3D active site that fits only a particular substrate (lock-and-key model or induced fit model).

Properties:

Extremely specific (one enzyme, one substrate)
Work at mild conditions (body temperature, neutral pH)
Very efficient (turnover number can exceed 10 $^6$ per second)
Can be inhibited by competitive inhibitors (bind active site) or non-competitive inhibitors (bind elsewhere, change shape)

Example: Invertase converts sucrose to glucose + fructose.

Colloids

Particle size: 1-1000 nm — between true solutions (<1 nm) and suspensions (>1000 nm).

Property	Description	Application
Tyndall effect	Scattering of light by colloidal particles (Rayleigh scattering)	Distinguishing colloid from true solution; headlights in fog
Brownian motion	Random zigzag movement due to bombardment by solvent molecules	Prevents settling, maintains stability
Electrophoresis	Movement of charged colloidal particles under electric field	Separating proteins, identifying particles
Coagulation/flocculation	Precipitation by adding electrolyte (neutralises surface charge)	Water treatment, rubber manufacture
Dialysis	Removal of dissolved ions from colloid using a membrane	Purifying colloidal solutions, kidney dialysis

Types of colloids

Lyophilic (solvent-loving) — strong interaction between dispersed phase and medium. Self-stabilising, reversible, difficult to coagulate. Examples: starch, gelatin, gum arabic in water.

Lyophobic (solvent-hating) — weak interaction. Need stabilisers, irreversible, easily coagulated. Examples: metal sols (gold, silver), As $_2$ S $_3$ sol.

Multimolecular — aggregates of many small molecules (e.g., sulphur sol — S $_8$ molecules cluster). Macromolecular — single large molecules (e.g., proteins, polymers in solution). Associated/Micellar — surfactant molecules aggregate above the critical micelle concentration (CMC) to form micelles (e.g., soap in water).

Hardy-Schulze rule

The coagulating power of an ion increases with its valence. For a negatively charged sol, cations coagulate it: Al $^{3+}$ > Ba $^{2+}$ > Na $^+$ . For a positively charged sol, anions coagulate it: [Fe(CN) $_6$ ] $^{4-}$ > PO $_4^{3-}$ > SO $_4^{2-}$ > Cl $^-$ .

Higher the valence of the coagulating ion (opposite charge to the colloid), greater is its coagulating power and lower is the flocculation value (amount needed).

Al $^{3+}$ coagulates As $_2$ S $_3$ sol (negative) much more effectively than Na $^+$ .

Emulsions

A colloid of two immiscible liquids. Two types:

Oil-in-water (O/W) — oil droplets dispersed in water. Examples: milk, vanishing cream. Stabilised by hydrophilic emulsifiers (e.g., soap).
Water-in-oil (W/O) — water droplets dispersed in oil. Examples: butter, cold cream. Stabilised by hydrophobic emulsifiers (e.g., lanolin).

Emulsifiers (surfactants) stabilise emulsions by forming a film at the oil-water interface, preventing coalescence.

Protective colloids

A lyophilic colloid added to a lyophobic sol to prevent its coagulation. The lyophilic particles coat the lyophobic ones, providing a stable charged layer. Example: gelatin added to gold sol — gelatin coats the gold particles and stabilises them. The protective power is measured by the gold number — the milligrams of protective colloid needed to just prevent the coagulation of 10 mL of gold sol by 1 mL of 10% NaCl. Lower gold number = better protection.

Solved Examples

Example 1 (Easy — CBSE)

Distinguish between physisorption and chemisorption (any 3 points).

Property	Physisorption	Chemisorption
Forces	van der Waals	Chemical bonds
Reversibility	Reversible	Usually irreversible
Activation energy	Low/none	May be significant
Layers	Multilayer	Monolayer only

Example 2 (Medium — CBSE)

Colloidal particles carry surface charge. An ion of higher valence provides more charge per ion, neutralising the colloid’s charge more effectively. Al $^{3+}$ provides three units of positive charge per ion, while Na $^+$ provides only one. Fewer Al $^{3+}$ ions are needed to neutralise the same amount of negative charge, so Al $^{3+}$ has higher coagulating power.

Example 3 (Application)

Activated charcoal has an enormous surface area (~1000 m $^2$ /g due to its porous structure). When contaminated air passes through, toxic gases (Cl $_2$ , SO $_2$ , phosgene) are adsorbed on the charcoal surface (physisorption and chemisorption). Clean air passes through. The mask works until the charcoal surface is saturated, after which it must be replaced.

Example 4 (NEET-style)

For a positively charged Fe(OH) $_3$ sol, arrange the following in order of increasing coagulating power: NaCl, Na $_2$ SO $_4$ , Na $_3$ PO $_4$ .

The positively charged sol is coagulated by anions. Higher anion valence = greater coagulating power. Cl $^-$ (1-) < SO $_4^{2-}$ (2-) < PO $_4^{3-}$ (3-).

Order: NaCl < Na $_2$ SO $_4$ < Na $_3$ PO $_4$ .

Common Mistakes to Avoid

Confusing adsorption and absorption. Adsorption is a surface phenomenon (gas on charcoal surface). Absorption is a bulk phenomenon (water soaking into a sponge). If the question says “surface,” think adsorption.

Forgetting that catalysts do not change equilibrium. A catalyst speeds up both forward and reverse reactions equally. It changes the rate of reaching equilibrium, not the equilibrium position itself. The equilibrium constant $K$ is unchanged.

Wrong classification of colloids. Lyophilic (starch, gelatin) are self-stabilising and difficult to coagulate. Lyophobic (metal sols, As $_2$ S $_3$ ) are unstable and need protective agents. Do not mix them up.

Thinking the Tyndall effect occurs in true solutions. True solutions have particles <1 nm — too small to scatter light. The Tyndall effect is specific to colloids (1-1000 nm particles).

Confusing gold number values. A lower gold number means better protective power (less protective colloid needed). Gelatin has a very low gold number (0.005-0.01) and is an excellent protective colloid.

Exam Weightage and Strategy

Surface Chemistry carries 4-6 marks in CBSE Class 12 boards and 1-2 NEET questions per year. The questions are definitional and classification-based. Learn the difference tables (physisorption vs chemisorption, lyophilic vs lyophobic, O/W vs W/O emulsion) and the Hardy-Schulze rule. These cover 90% of PYQs.

Three tables to memorise: (1) physisorption vs chemisorption (5 differences), (2) homogeneous vs heterogeneous catalysis (4 differences), (3) lyophilic vs lyophobic colloids (4 differences). Plus the Hardy-Schulze rule with one example. That covers the entire chapter for exams.

Practice Questions

Q1. What is the Tyndall effect? Give one application.

Scattering of light by colloidal particles, making the beam visible when passing through the colloid. Application: headlights become visible in fog (fog is an aerosol — a colloid of water droplets in air). Also used in the lab to distinguish colloids from true solutions.

Q2. Arrange NaCl, BaCl $_2$ , AlCl $_3$ in order of increasing coagulating power for a negative sol.

NaCl < BaCl $_2$ < AlCl $_3$ (Hardy-Schulze rule: higher valence of the cation = greater coagulating power for a negative sol. Na $^+$ (1+) < Ba $^{2+}$ (2+) < Al $^{3+}$ (3+)).

Q3. What is an emulsion? Give examples of both types.

An emulsion is a colloid of two immiscible liquids stabilised by an emulsifier. Oil-in-water (O/W): milk (fat droplets in water), vanishing cream. Water-in-oil (W/O): butter (water droplets in fat), cold cream. The type depends on which phase is dispersed and which is the medium.

Q4. What is shape-selective catalysis? Give an example.

Catalysis where the product depends on the pore size and shape of the catalyst. Only reactant molecules that fit the pore can enter, react, and exit. Example: ZSM-5 zeolite converts methanol to gasoline-range hydrocarbons — only molecules small enough to fit through the zeolite channels are produced.

Q5. Why does adsorption decrease with temperature for physisorption but initially increase for chemisorption?

Physisorption is exothermic and involves weak van der Waals forces. By Le Chatelier’s principle, increasing temperature shifts equilibrium toward desorption. Chemisorption requires activation energy — at low temperatures, molecules do not have enough energy to form chemical bonds with the surface. As temperature increases, more molecules cross the activation energy barrier, so chemisorption increases. At very high temperatures, chemisorption also decreases because desorption becomes dominant.

FAQs

What is an enzyme?

A biological catalyst — a protein that speeds up biochemical reactions with high specificity. Each enzyme works on a specific substrate (lock-and-key model). Enzymes work at mild conditions (body temperature, neutral pH) and have turnover numbers far exceeding inorganic catalysts.

Why does adsorption decrease with temperature increase?

For physisorption, the process is exothermic. By Le Chatelier’s principle, increasing temperature shifts equilibrium towards desorption. However, chemisorption initially increases (needs activation energy to form chemical bonds) before decreasing at very high temperatures.

What is a protective colloid?

A lyophilic colloid that is added to a lyophobic colloid to prevent its coagulation. The lyophilic particles coat the lyophobic ones, providing steric and charge stabilisation. Example: gelatin added to gold sol prevents it from coagulating when electrolyte is added.