Surface Chemistry: One Mark Questions, Book Back Answers and Notes

About Surface Chemistry

Medium ~120 min study

Charcoal removes colour from sugar solution. A little finely divided nickel lets vegetable oil take up hydrogen. Light from a torch shows a clear beam through fog. All three depend on what happens at a surface, where molecules of one phase meet another.

The chapter moves in three steps. It starts with adsorption, the collection of molecules on a surface, and separates weak physical adsorption from chemical adsorption. It then uses adsorption to explain how many catalysts work. Finally it turns to colloids, where particles are so small that their total surface area is enormous, and explains why they stay dispersed and how they can be made to settle.

Most questions here are short and factual. You may be asked to compare physisorption with chemisorption, identify the dispersed phase and dispersion medium in examples like milk, fog or butter, explain the Tyndall effect and Brownian movement, and apply the Hardy-Schulze rule to decide which electrolyte coagulates a sol best. Learn the examples from the textbook tables, since questions often name them directly.

What you'll learn

• Distinguish adsorption from absorption with examples.
• Compare physisorption and chemisorption on nature of forces, enthalpy, reversibility and effect of temperature.
• Apply the Freundlich isotherm to gas adsorption on a solid.
• Explain heterogeneous catalysis using the adsorption theory.
• Classify colloids by the physical states of dispersed phase and dispersion medium.
• Describe the optical, kinetic and electrical properties of colloids.
• Explain how emulsifying agents stabilise emulsions.

Before you start

• States of matter and the idea of a phase boundary.
• Solutions and concentration terms.
• Intermolecular forces, especially van der Waals forces.

Surface Chemistry explained

Adsorption, Catalysis and Colloids

Adsorption and Its Types

Adsorption is the accumulation of a substance on the surface of another, not throughout its bulk. The substance that collects is the adsorbate; the surface is the adsorbent. In physisorption, molecules are held by weak van der Waals forces; it is reversible, not specific, has a low enthalpy of about 20–40 kJ mol^-1, and decreases as temperature rises. In chemisorption, chemical bonds form; it is specific, usually irreversible, has a high enthalpy of about 40–400 kJ mol^-1, and needs activation energy. Adsorption increases with the surface area of the adsorbent and with how easily the gas liquefies. At constant temperature, the Freundlich isotherm relates the amount adsorbed per gram, x/m, to pressure: x/m = kP^1/n, so a plot of log(x/m) against log P is a straight line.

Catalysis

A catalyst changes the rate of a reaction and is recovered unchanged at the end. In homogeneous catalysis the catalyst and reactants are in the same phase, like NO in the lead chamber process. In heterogeneous catalysis they are in different phases, like iron in Haber's process for ammonia. The intermediate compound formation theory explains homogeneous catalysis. The adsorption theory explains heterogeneous catalysis: reactants are adsorbed on active sites, react there, and the products desorb. Enzymes such as invertase and zymase are biological catalysts that are highly specific and work best near body temperature and a particular pH. Their action is described by the lock-and-key model.

Colloids and Their Classification

A colloid is a heterogeneous system in which particles of about 1–1000 nm, the dispersed phase, are spread through a dispersion medium. Colloids are classified by the physical state of the two phases (smoke is solid in gas, milk is liquid in liquid, gels are liquid in solid) and by how the phase interacts with the medium. Lyophilic sols like starch and gelatin form readily and are stable. Lyophobic sols like gold and arsenious sulphide need special methods and are easily coagulated. Preparation methods include Bredig's arc method and peptisation for dispersion, and oxidation, reduction and hydrolysis for condensation. Sols are purified by dialysis, electrodialysis and ultrafiltration.

Properties of Colloids

Colloidal particles scatter light, so the path of a beam becomes visible: the Tyndall effect. Seen under an ultramicroscope, they move in a continuous zig-zag path, called Brownian movement, caused by uneven hits from molecules of the medium. The particles carry a charge, positive for Fe(OH)₃ sol and negative for As₂S₃ sol. In electrophoresis the particles move towards the oppositely charged electrode; in electro-osmosis the medium moves instead. Adding an electrolyte neutralises the charge and makes a lyophobic sol coagulate. By the Hardy-Schulze rule, the higher the charge of the oppositely charged ion, the stronger its coagulating power, so Al³⁺ coagulates As₂S₃ sol far better than Na⁺.

Emulsions and Applications

An emulsion is a colloid of one liquid in another immiscible liquid. Milk is oil in water; butter and cold cream are water in oil. An emulsifying agent such as soap or protein stabilises it by forming a film around the droplets. Surface chemistry explains the cleansing action of soap, tanning of leather, purification of water by alum, and removal of smoke particles in the Cottrell precipitator, where charged carbon particles are attracted to electrodes.

Common mistakes to avoid

• Confusing adsorption with absorption. Silica gel adsorbs water on its surface; anhydrous CaCl2 absorbs it throughout.
• Calling chemisorption reversible. Because chemical bonds form, it is usually irreversible, while physisorption is reversible.
• Treating all catalysis the same. Homogeneous catalysis is explained by intermediate compound formation, heterogeneous catalysis by adsorption.
• Assuming that removing the charge always coagulates a sol. It does for lyophobic sols, but lyophilic sols are also stabilised by a layer of solvent, so they need a large amount of electrolyte or removal of that layer.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

How does temperature affect physisorption and chemisorption?

Physisorption is exothermic and depends on weak forces, so it decreases steadily as temperature rises. Chemisorption needs activation energy, so it first increases with temperature. At still higher temperatures it also decreases, since adsorption is exothermic overall.

What is the difference between a sol and an emulsion?

A sol has solid particles dispersed in a liquid, such as gold sol or starch in water. An emulsion has one liquid dispersed in another liquid it does not mix with, such as oil droplets in water in milk. Emulsions usually need an emulsifying agent to stay stable.

Why is the Tyndall effect not seen in true solutions?

Particles in a true solution are smaller than about 1 nm, too small to scatter visible light noticeably, so the beam passes through unseen. Colloidal particles are roughly 1–1000 nm across, large enough to scatter light sideways and show the path of the beam.

How does Brownian movement help keep a colloid stable?

Molecules of the dispersion medium strike each colloidal particle unequally from different sides, so the particle keeps moving in a zig-zag path. This constant stirring opposes the pull of gravity, so the particles do not settle.

How does a catalyst increase the rate of a reaction?

It provides a different reaction path with lower activation energy. At the same temperature, more molecules now have enough energy to cross the lower barrier, so the reaction goes faster. The catalyst does not change the equilibrium position.

How does soap clean greasy cloth?

A soap molecule has a long hydrocarbon tail that dissolves in grease and a polar carboxylate head that stays in water. In water, soap molecules gather around a grease droplet with tails pointing in, forming a micelle. The micelles stay suspended in water and are rinsed away with the dirt.