Surface Chemistry
Where chemistry happens at the boundary — adsorption, catalysis on a surface, and the in-between world of colloids
- The difference between adsorption, absorption and sorption, and what drives it.
- Physisorption vs chemisorption, and the factors that govern adsorption.
- The Freundlich adsorption isotherm and how to read its log–log plot.
- Homogeneous and heterogeneous catalysis, promoters, poisons, and enzymes.
- Colloids — their classification, lyophilic/lyophobic sols, and micelles.
- Colloid properties: Tyndall effect, Brownian motion, coagulation, and the Hardy–Schulze rule.
Adsorption — the Basic Idea
Adsorption is the accumulation of a substance at a surface rather than throughout the bulk. The substance that gathers is the adsorbate; the surface that holds it is the adsorbent. It differs from absorption, where a substance spreads uniformly through the whole bulk; when both happen together we call it sorption. Adsorption is driven by unbalanced surface forces and is always exothermic.
| Term | Meaning | Example |
|---|---|---|
| Adsorption | at the surface only | \(\ce{O2}\) on charcoal |
| Absorption | uniform through the bulk | water in a sponge |
| Sorption | both at once | dyes on cotton |
Physisorption vs Chemisorption
Adsorption comes in two strengths. Physisorption uses weak van der Waals forces; chemisorption forms actual chemical bonds. The two differ in almost every property.
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces | van der Waals (weak) | chemical bonds (strong) |
| Enthalpy | low (≈ 20–40 kJ mol⁻¹) | high (≈ 80–240 kJ mol⁻¹) |
| Specificity | none | highly specific |
| Layers | multilayer | monolayer |
| Temperature | falls as \(T\) rises | rises then falls with \(T\) |
| Reversibility | reversible | often irreversible |
Factors Affecting Adsorption
The extent of adsorption of a gas on a solid depends on the adsorbent, the gas, and the conditions. Finely divided solids and porous materials (charcoal, silica gel) adsorb most, because adsorption is a surface phenomenon.
| Factor | Effect |
|---|---|
| Surface area | more area ⇒ more adsorption |
| Nature of gas | easily liquefiable gases (high critical \(T\): \(\ce{SO2},\ \ce{NH3}\)) adsorb more |
| Pressure | adsorption rises with pressure (at constant \(T\)) |
| Temperature | physisorption falls; chemisorption rises then falls |
The Freundlich Adsorption Isotherm
An adsorption isotherm plots the amount adsorbed against pressure at constant temperature. The empirical Freundlich isotherm captures the curve: adsorption rises with pressure, then levels off as the surface saturates.
\(x/m\) is mass adsorbed per gram of adsorbent. A plot of \(\log(x/m)\) vs \(\log p\) is a straight line of slope \(1/n\) and intercept \(\log k\). At low \(p\), \(x/m\propto p\); at high \(p\), \(x/m\) becomes independent of \(p\).
Catalysis: Homogeneous & Heterogeneous
A catalyst speeds a reaction by offering a lower-energy path and is recovered unchanged. In homogeneous catalysis the catalyst is in the same phase as the reactants; in heterogeneous catalysis it is in a different phase — usually a solid catalyst with gaseous or liquid reactants.
| Type | Example | Catalyst |
|---|---|---|
| Homogeneous | ester hydrolysis | \(\ce{H+}\) (aqueous) |
| Homogeneous | lead-chamber process | \(\ce{NO}\) (gas) |
| Heterogeneous | Haber process (\(\ce{NH3}\)) | \(\ce{Fe}\) (solid) |
| Heterogeneous | contact process (\(\ce{SO3}\)) | \(\ce{V2O5}\) (solid) |
| Heterogeneous | oil hydrogenation | \(\ce{Ni}\) (solid) |
How Surface (Heterogeneous) Catalysis Works
The adsorption theory explains heterogeneous catalysis as a sequence on the catalyst surface: reactant molecules diffuse to the surface and are adsorbed on active sites, which weakens their bonds; they react; the products desorb and diffuse away, freeing the site. Adsorption lowers the activation energy and concentrates the reactants — both speed the reaction.
Enzyme Catalysis
Enzymes are biological catalysts — complex protein molecules that run the reactions of life with astonishing speed and specificity. Each enzyme has an active site shaped to fit one substrate, the famous lock-and-key picture, and works best at a particular temperature and pH.
| Enzyme | Reaction catalysed |
|---|---|
| Invertase | sucrose → glucose + fructose |
| Zymase | glucose → ethanol + \(\ce{CO2}\) |
| Urease | urea → ammonia + \(\ce{CO2}\) |
| Pepsin | proteins → peptides |
Colloids & Their Classification
A colloid sits between a true solution and a suspension, with particle sizes of about \(1\text{–}1000\ \text{nm}\). It has a dispersed phase spread through a dispersion medium. Colloids are classified three ways: by physical state, by affinity for the medium, and by particle type.
| Dispersed phase | Medium | Name | Example |
|---|---|---|---|
| Solid | Liquid | sol | paint, ink |
| Liquid | Liquid | emulsion | milk |
| Liquid | Solid | gel | jelly, cheese |
| Gas | Liquid | foam | whipped cream |
| Solid | Gas | aerosol | smoke |
By affinity, lyophilic ("solvent-loving") sols are stable and reversible (starch, gelatin), while lyophobic ("solvent-hating") sols are unstable and need a stabiliser (metal sols). A special class, associated colloids or micelles, form when molecules like soap aggregate above the critical micelle concentration — the basis of detergent cleaning action.
Properties of Colloids
Colloidal particles are large enough to scatter light and carry charge, yet small enough to stay suspended — giving a distinctive set of properties used to detect and control them.
| Property | What it is |
|---|---|
| Tyndall effect | scattering of light into a visible cone |
| Brownian motion | random zig-zag motion that resists settling |
| Electrophoresis | charged particles migrate in an electric field |
| Coagulation | charge neutralised ⇒ particles clump and settle |
To coagulate a negative sol, the effectiveness of cations is \(\ce{Al^3+} > \ce{Ba^2+} > \ce{Na+}\); for a positive sol, anions follow \(\ce{PO4^3-} > \ce{SO4^2-} > \ce{Cl-}\). The ion of opposite charge to the sol does the work.
Emulsions, Purification & Uses
An emulsion is a colloid of one liquid dispersed in another: oil-in-water (milk) or water-in-oil (butter). An emulsifier (soap, protein) stabilises it. Colloids are purified by dialysis — passing the impure sol through a semipermeable membrane so small ions diffuse out while colloidal particles remain.
Putting It to Work
Problem. Classify each: water taken up by a sponge; \(\ce{O2}\) gathering on charcoal; a dye taken up by cotton.
Solution. Decide whether it is surface-only, bulk, or both:
Problem. A \(\log(x/m)\) vs \(\log p\) plot has slope \(0.5\) and intercept \(\log k=0.301\). Find \(x/m\) at \(p=4\).
Solution. So \(1/n=0.5\) and \(k=2\); apply \(x/m=k\,p^{1/n}\):
Problem. An adsorption has enthalpy \(\approx150\ \text{kJ mol}^{-1}\), is monolayer and highly specific. Which type is it?
Solution. High enthalpy, monolayer, specific all point one way:
Problem. In the Haber process, \(\ce{N2 + 3H2 ->[Fe] 2NH3}\). Classify the catalysis and name the promoter.
Solution. Solid \(\ce{Fe}\) with gaseous reactants is a different phase:
Problem. Order the coagulating power of \(\ce{Na+},\ \ce{Ba^2+},\ \ce{Al^3+}\) for a negative arsenious sulphide sol.
Solution. A negative sol is coagulated by cations; power rises with charge:
Problem. A beam of light is shone through a starch dispersion and through a salt solution. What is seen, and what does it prove?
Solution. Particle size decides whether light scatters:
Chapter Summary
Surface accumulation, always exothermic; absorption is bulk, sorption is both.
Physisorption (weak, multilayer, reversible) vs chemisorption (strong, monolayer, specific).
\(x/m=k\,p^{1/n}\); log–log plot is linear with slope \(1/n\).
Homo- vs heterogeneous; surface catalysis = adsorb, react, desorb; promoters and poisons.
1–1000 nm; lyophilic vs lyophobic; micelles above the CMC.
Tyndall, Brownian, electrophoresis, coagulation; Hardy–Schulze: power rises with charge.
Problems
For each item, first decide which phenomenon it tests — adsorption, catalysis, or colloids — then apply the relevant rule. Difficulty rises down the list.
- Define adsorbate and adsorbent, and explain why adsorption is exothermic.
- Give three differences between physisorption and chemisorption.
- Why does powdered charcoal adsorb more gas than a single lump of the same mass?
- Arrange \(\ce{H2},\ \ce{N2},\ \ce{SO2}\) in order of increasing adsorption on charcoal, and justify by critical temperature.
- For a Freundlich isotherm, a \(\log(x/m)\) vs \(\log p\) plot gives slope \(0.4\) and intercept \(0.6\). Write \(x/m\) as a function of \(p\).
- Classify as homogeneous or heterogeneous: ester hydrolysis by \(\ce{H+}\); \(\ce{SO2}\) oxidation over \(\ce{V2O5}\); hydrogenation of oil over \(\ce{Ni}\).
- Explain, using the adsorption theory, how a solid catalyst speeds a gas-phase reaction.
- What is a promoter and a poison? Give one example of each.
- List two reasons enzymes are described as the most efficient and specific catalysts known.
- Distinguish lyophilic and lyophobic sols in terms of stability and reversibility.
- Using the Hardy–Schulze rule, predict which of \(\ce{NaCl},\ \ce{BaCl2},\ \ce{AlCl3}\) most effectively coagulates a negative sol, and why.
- Explain how a soap molecule forms a micelle and removes grease from cloth.