The extent of a certain adsorption first increases and then decreases with rising temperature. The process is:

Impossible — adsorption can only decrease with temperature

The slow start followed by rapid completion in the titration of oxalic acid with KMnO₄ is due to:

Autocatalysis by the Mn²⁺ produced

The Tyndall effect is observed in a colloid because the particles:

Are comparable in size to the wavelength of light and scatter it

Delta formation where a river meets the sea is an example of:

Coagulation of a colloid by electrolytes

Molybdenum added to the iron catalyst in the Haber process acts as a:

Second catalyst that replaces iron

Surface Chemistry — Adsorption, Catalysis and Colloids

Surface chemistry is the smallest scoring unit in physical chemistry and also the easiest to secure, because it is almost entirely conceptual — there is one graph, two isotherms and a great deal of vocabulary that has to be used precisely. The whole subject exists because an atom at a surface has unsatisfied bonds on one side. That single fact explains adsorption, explains why catalysts are made porous or finely divided, and explains why colloidal particles carry charge and refuse to settle. Read the chapter looking for that thread and the terminology stops feeling arbitrary.

Adsorption and the Two Isotherms

The distinctions that questions turn on
Term	Meaning	Example
Adsorption	Concentration at the surface only	Charcoal removing colour from sugar solution
Absorption	Uniform distribution throughout the bulk	Water taken up by anhydrous CaCl₂
Sorption	Both happening together	Dyes on cotton fibre
Adsorbate	The species that gets stuck	The gas, in gas-on-charcoal
Adsorbent	The surface doing the sticking	The charcoal

Adsorption is always exothermic, and there is a clean thermodynamic reason: a freely moving gas molecule loses translational freedom when it lands on a surface, so ΔS is negative, and since the process happens spontaneously with ΔG negative, ΔH must be negative and large enough to overcome the entropy penalty. This immediately predicts the temperature behaviour — by Le Chatelier, heating reduces the extent of adsorption, which is why activated charcoal filters work better cold and why an adsorbed gas can be driven off by warming.

Physisorption versus chemisorption
Feature	Physisorption	Chemisorption
Force involved	van der Waals	Genuine chemical bond
Enthalpy	20–40 kJ/mol	80–240 kJ/mol
Reversibility	Readily reversible	Usually irreversible
Specificity	Non-specific — any gas on any solid	Highly specific
Layers formed	Multilayer possible	Monolayer only
Effect of rising temperature	Falls steadily	First rises, then falls

🎯 Why chemisorption rises before it falls with temperature

This is the one genuinely counter-intuitive result in the chapter, and it is a favourite. Chemisorption forms a real bond, so it needs activation energy — like any chemical reaction. At low temperature the molecules simply lack the energy to react with the surface, so warming helps and adsorption increases. Once enough molecules can clear the barrier, the exothermic equilibrium takes over and further heating drives them off again, so the curve turns down. Physisorption needs no activation energy at all, so it has no rising branch and falls monotonically from the start. When a graph in a question rises then falls, it is telling you the process is chemisorption.

The Freundlich isotherm is x/m = k·p^(1/n) with 1/n between 0 and 1, so a plot of log(x/m) against log p is a straight line of slope 1/n. Its behaviour matches experiment in three regimes: at low pressure x/m ∝ p (1/n → 1), at high pressure x/m becomes independent of p (1/n → 0), and at intermediate pressure the fractional power applies. The Langmuir isotherm derives the same shape from a monolayer assumption and explains the plateau physically — the surface simply runs out of sites.

Catalysis and Its Vocabulary

A catalyst provides an alternative reaction path of lower activation energy, unchanged in amount at the end. Homogeneous catalysis has catalyst and reactants in the same phase, as in the lead-chamber process where NO(g) catalyses SO₂(g) oxidation. Heterogeneous catalysis has them in different phases, as in Haber's iron catalysing gases — and this is where surface chemistry matters, because the reaction happens on the surface and so finely divided or porous catalysts work best.

Terms that must be used precisely
Term	Meaning	Example
Promoter	Increases the activity of a catalyst	Mo with Fe in the Haber process
Poison	Destroys activity, often by chemisorbing irreversibly	CO on the Fe catalyst; S on Pt
Autocatalysis	A product catalyses the reaction, so the rate rises with time	Mn²⁺ in KMnO₄ titrations
Shape-selective catalysis	Pore size decides which molecules can react	ZSM-5 zeolite converting alcohols to petrol
Enzyme catalysis	Highly specific, works near 298–310 K and a narrow pH	Maltase converting maltose to glucose

⚠️ Autocatalysis explains the slow start of a permanganate titration

When you first add KMnO₄ to warm acidified oxalic acid, the purple colour lingers — then suddenly the reaction becomes fast and the colour vanishes on contact. Nothing was wrong with the first drops: the product Mn²⁺ is itself the catalyst, so at the start there is none and the reaction crawls, but each drop makes more catalyst and the rate accelerates. This is the reverse of the normal picture where rate falls as reactants deplete, and a rate-versus-time graph that rises before falling is the signature of autocatalysis rather than an experimental error.

Colloids — Classification and Behaviour

A colloid has dispersed particles between 1 and 1000 nm — larger than a true solution, smaller than a suspension. That size range is the whole reason for its behaviour: the particles are big enough to scatter light but small enough that Brownian motion keeps them from settling. Lyophilic colloids such as starch or gelatin are reversible and stabilised by strong solvent interaction; lyophobic colloids such as metal sols are irreversible and stabilised only by charge, which is why they are easily coagulated.

The properties the paper asks about by name
Property	What is observed	Cause
Tyndall effect	A visible beam through the sol	Scattering by particles comparable to λ of light
Brownian motion	Ceaseless zig-zag movement	Uneven bombardment by solvent molecules
Electrophoresis	Particles migrate in an electric field	Every colloidal particle carries a charge
Coagulation	The sol settles out as a precipitate	Added electrolyte neutralises the charge
Dialysis	Electrolyte removed through a membrane	Ions pass, colloidal particles cannot

🎯 Why a delta forms where a river meets the sea

River water carries a colloidal sol of clay whose particles all carry the same charge and therefore repel one another, which is what keeps them suspended over hundreds of kilometres. Sea water is a concentrated electrolyte; its ions neutralise that charge, the repulsion disappears, the particles clump and settle, and a delta grows. The same chemistry explains three other everyday facts: alum stops a shaving cut bleeding by coagulating blood, which is a negatively charged sol; smoke is precipitated in a Cottrell precipitator by charged plates; and rubber is coagulated from latex by adding acid. Hardy–Schulze puts a number on it — the higher the charge on the coagulating ion, the more effective it is, so Al³⁺ beats Ba²⁺ which beats Na⁺ for a negative sol.

An emulsion is a colloid of one liquid in another, needing an emulsifier to persist: soap stabilises oil-in-water, which is how washing works, while milk is fat-in-water stabilised by casein and butter is the reverse, water-in-oil. A micelle forms above a critical concentration and temperature when soap molecules aggregate with hydrocarbon tails inward and ionic heads outward, trapping grease in the core — the reason soap cleans at all, and the reason it fails in hard water, where Ca²⁺ and Mg²⁺ precipitate the soap as an insoluble scum before micelles can form.

Key takeaways

Adsorption is a surface phenomenon, absorption is bulk, sorption is both happening at once.
Adsorption is always exothermic, because the adsorbate loses freedom so ΔS is negative.
Physisorption is weak, reversible, non-specific and multilayer; chemisorption is strong, specific and monolayer.
Chemisorption first rises then falls with temperature, because it needs activation energy; physisorption only falls.
Freundlich: x/m = kp^(1/n), so log(x/m) against log p is linear with slope 1/n.
A promoter boosts a catalyst, a poison destroys it, and in autocatalysis a product is the catalyst.
Colloidal particles are 1–1000 nm: big enough to scatter light, small enough to resist settling.
Hardy–Schulze: the higher the charge on the coagulating ion, the more effective — Al³⁺ > Ba²⁺ > Na⁺.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Adsorption of a gas on a solid is always accompanied by:
1. An increase in entropy
2. A decrease in enthalpy
3. An increase in enthalpy
4. No change in free energy
Show answer
Answer: B — A decrease in enthalpy
A gas molecule landing on a surface loses translational freedom, so ΔS is negative; for the process to be spontaneous with ΔG negative, ΔH must be negative — adsorption is always exothermic. This is also why heating reduces the extent of physisorption, and why adsorbed gases can be driven off by warming the adsorbent.
The extent of a certain adsorption first increases and then decreases with rising temperature. The process is:
1. Physisorption
2. Chemisorption
3. Absorption
4. Impossible — adsorption can only decrease with temperature
Show answer
Answer: B — Chemisorption
Chemisorption forms a real chemical bond and therefore requires activation energy, so at low temperature warming supplies the energy needed and adsorption increases; once the barrier is cleared, the exothermic equilibrium takes over and further heating desorbs the gas. Physisorption needs no activation energy and so falls monotonically from the start, which is exactly the contrast the graph is testing.
In the Freundlich isotherm x/m = kp^(1/n), the plot of log(x/m) against log p gives a straight line of slope:
1. k
2. n
3. 1/n
4. log k
Show answer
Answer: C — 1/n
Taking logarithms gives log(x/m) = log k + (1/n) log p, so the slope is 1/n and the intercept log k. Since 1/n lies between 0 and 1, the line is always less steep than 45°; the limiting cases 1/n = 1 and 1/n = 0 correspond to the low-pressure proportional region and the high-pressure saturated plateau respectively.
The slow start followed by rapid completion in the titration of oxalic acid with KMnO₄ is due to:
1. A promoter present in the acid
2. Autocatalysis by the Mn²⁺ produced
3. Catalyst poisoning
4. Shape-selective catalysis
Show answer
Answer: B — Autocatalysis by the Mn²⁺ produced
Mn²⁺ is a product of the reduction of MnO₄⁻ and it catalyses the very reaction that makes it, so at the outset there is no catalyst and the reaction crawls, then accelerates as product accumulates. A promoter would have to be added deliberately alongside a catalyst, and poisoning would slow the reaction rather than speed it up — both point the wrong way for the observation described.
Which ion would be most effective in coagulating a negatively charged As₂S₃ sol?
1. Na⁺
2. Ba²⁺
3. Al³⁺
4. Cl⁻
Show answer
Answer: C — Al³⁺
By the Hardy–Schulze rule the coagulating power of an ion rises steeply with its charge, so for a negative sol the trivalent Al³⁺ is far more effective than Ba²⁺ or Na⁺. Cl⁻ is negative like the sol itself and would simply be repelled, so it cannot coagulate it at all — the sign of the ion must be opposite to that of the particles.
The Tyndall effect is observed in a colloid because the particles:
1. Absorb all the incident light
2. Are comparable in size to the wavelength of light and scatter it
3. Are charged
4. Undergo Brownian motion
Show answer
Answer: B — Are comparable in size to the wavelength of light and scatter it
Scattering requires particles of a size comparable to the wavelength of visible light, which is precisely the 1–1000 nm colloidal range — a true solution has particles far too small to scatter, so no beam is visible. Charge explains electrophoresis and coagulation, and Brownian motion explains why the sol does not settle, but neither produces the visible beam.
Delta formation where a river meets the sea is an example of:
1. Dialysis
2. Coagulation of a colloid by electrolytes
3. Emulsification
4. Peptisation
Show answer
Answer: B — Coagulation of a colloid by electrolytes
River water carries a clay sol whose like-charged particles repel and stay suspended; the concentrated electrolytes in sea water neutralise that charge, so the particles clump and settle out as a delta. Dialysis is the opposite process of removing electrolyte to preserve a sol, and peptisation is the conversion of a precipitate back into a colloid — both move in the wrong direction here.
Molybdenum added to the iron catalyst in the Haber process acts as a:
1. Catalyst poison
2. Promoter
3. Emulsifier
4. Second catalyst that replaces iron
Show answer
Answer: B — Promoter
A promoter is a substance with no catalytic activity of its own that increases the activity of the actual catalyst, which is exactly molybdenum's role alongside iron. A poison would destroy activity — CO does that to this very catalyst — and molybdenum does not substitute for iron, since removing the iron would stop the reaction regardless of how much Mo were present.