Adsorption: Physisorption vs Chemisorption
When a gas or liquid substance comes into contact with the surface of a solid or liquid, it accumulates or concentrates at the surface rather than penetrating into the bulk of the material. This phenomenon is known as adsorption. It is vital to distinguish adsorption from absorption, where the substance is uniformly distributed throughout the bulk of the absorbing medium. If both occur simultaneously, the combined process is termed sorption.
The substance that gets concentrated at the surface is called the adsorbate, while the material on whose surface the adsorption takes place is known as the adsorbent. Adsorption is fundamentally a surface-driven phenomenon, meaning that higher surface area per unit mass drastically increases the extent of adsorption. Examples include activated charcoal trapping toxic gases or silica gel trapping moisture in packaging.
Thermodynamically, adsorption is always an exothermic process. Since molecules lose translational degrees of freedom when trapped on a surface, the entropy change ($\Delta S$) is negative. For adsorption to be spontaneous, the Gibbs free energy change ($\Delta G = \Delta H – T\DeltaS$) must be negative, which confirms that $\Delta H$ must be sufficiently negative to overcome the negative $T\Delta S$ term.
Adsorption is broadly classified into two categories based on the nature of forces operating between the adsorbate and the adsorbent:
- Physisorption (Physical Adsorption): Caused by weak intermolecular van der Waals forces. It is reversible, non-specific, and decreases with an increase in temperature. It has a low enthalpy of adsorption (20–40 kJ/mol) and can form multi-molecular layers under high pressure.
- Chemisorption (Chemical Adsorption): Caused by forces akin to chemical bond formation. It involves high activation energy, is highly specific, irreversible, and increases initially with temperature due to activation requirements. It has a high enthalpy of adsorption (80–240 kJ/mol) and forms strictly mono-molecular layers.
Adsorption Isotherms: Freundlich and Langmuir
An adsorption isotherm is a graphical representation showing the variation in the amount of gas adsorbed by an adsorbent with pressure (or concentration in solution) at a constant temperature. Understanding these isotherms helps in predicting how surface phenomena behave under varying thermodynamic conditions in industrial applications like gas purification and heterogeneous catalysis.
The Freundlich Adsorption Isotherm, proposed in 1909, gives an empirical relationship between the quantity of gas adsorbed per gram of adsorbent ($x/m$) and equilibrium pressure ($P$). Mathematically, it is expressed as:
$\frac{x}{m} = k \cdot P^{1/n}$ (where $n > 1$)
Taking the logarithm on both sides linearizes the equation:
$\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n} \log P$
A plot of $\log(x/m)$ versus $\log P$ yields a straight line with a slope equal to $1/n$ and an intercept equal to $\log k$. Although highly successful for intermediate pressure ranges, the Freundlich isotherm fails at very high pressures where adsorption reaches saturation.
To overcome this limitation, Irving Langmuir proposed a theoretical model in 1916 based on kinetic principles. The Langmuir Adsorption Isotherm assumes that adsorption sites are equivalent, each site holds only one adsorbed molecule, and there is no lateral interaction between adjacent adsorbed molecules. The Langmuir equation is given by:
$\frac{x}{m} = \frac{aP}{1 + bP}$
At very low pressures, the term $bP$ in the denominator is negligible, reducing the Langmuir equation to a linear dependence on pressure ($\frac{x}{m} = aP$). At very high pressures, $bP$ becomes much greater than 1, making adsorption independent of pressure ($\frac{x}{m} = \frac{a}{b}$), which accounts for surface saturation.
Catalysis and Catalytic Mechanisms
A catalyst is a substance that alters the rate of a chemical reaction without itself undergoing any permanent chemical change. Catalysts work by providing an alternative reaction pathway with a lower activation energy ($E_a$). Catalysis is broadly divided into two major classifications: homogeneous and heterogeneous.
In homogeneous catalysis, the reactants and the catalyst exist in the same phase, typically liquid or gas. A classic example is the lead chamber process for manufacturing sulfuric acid, where nitric oxide acts as a gaseous catalyst for sulfur dioxide oxidation. In heterogeneous catalysis, the catalyst exists in a different phase than the reactants, most commonly a solid catalyst reacting with gaseous or liquid reactants.
Heterogeneous catalysis typically occurs via the adsorption theory, involving five sequential steps: diffusion of reactants to the surface, adsorption of reactants onto the surface, chemical reaction on the surface to form intermediates, desorption of products from the surface, and diffusion of products away from the catalytic surface.
Key features of catalysts include activity (the ability to accelerate a reaction manifold times) and selectivity (the ability of a catalyst to direct a reaction to yield a specific product, such as using different metal catalysts to convert carbon monoxide and hydrogen into methane, formaldehyde, or methanol).
Enzyme catalysis represents biochemical catalysis mediated by complex protein molecules. Enzymes are extraordinarily efficient and highly specific catalysts. Their action is often explained by the lock-and-key mechanism or induced-fit model, where the substrate fits precisely into active sites on the enzyme surface.
Colloids: Preparation, Properties, and Purification
A colloid is a heterogeneous system in which one substance is dispersed as very fine particles (the dispersed phase) in another substance called the dispersion medium. The size of colloidal particles typically ranges between 1 nm and 1000 nm. Colloids are classified based on the physical state of the dispersed phase and dispersion medium (e.g., sols, gels, emulsions, aerosols) and the affinity between phases (lyophilic and lyophobic).
Lyophilic colloids (solvent-loving) have a strong affinity between the dispersed phase and dispersion medium, are reversible, and are quite stable (e.g., starch, gum, gelatin). Lyophobic colloids (solvent-hating) have little to no affinity, are irreversible, and require stabilizing agents to prevent precipitation (e.g., metal sols, metal sulfides).
Methods for preparing colloids fall into two main categories:
- Dispersion Methods: Breaking down bulk materials into colloidal sizes. Examples include Bredig’s Arc Method (used for preparing gold, silver, or platinum sols) and peptization (converting a freshly precipitated ionic precipitate into a colloidal sol by adding a suitable electrolyte).
- Condensation Methods: Aggregating smaller molecular or ionic species into colloidal aggregates through chemical reactions such as double decomposition, oxidation, reduction, or hydrolysis.
Colloidal solutions exhibit unique properties, including the Tyndall effect (scattering of a light beam by colloidal particles making the path visible), Brownian movement (continuous zig-zag random motion of particles due to uneven bombardment by solvent molecules), and electrophoresis (movement of charged colloidal particles under an applied electric field).
To ensure stability or study properties, purification of colloids is necessary to remove excess electrolytes. Common purification techniques include dialysis, electrodialysis, and ultrafiltration.
Emulsions and Surface Active Agents
Emulsions are colloidal systems in which both the dispersed phase and the dispersion medium are liquids. Since pure liquids do not form stable mixtures permanently, emulsions are categorized into two main types:
- Oil-in-Water (O/W) Emulsions: Oil acts as the dispersed phase while water acts as the dispersion medium. Examples include milk and vanishing cream.
- Water-in-Oil (W/O) Emulsions: Water acts as the dispersed phase while oil acts as the dispersion medium. Examples include butter, cold cream, and cod liver oil.
To stabilize an emulsion and prevent the separation of liquid layers, a third component called an emulsifying agent (or emulsifier) is added. Emulsifiers form an interfacial film between the suspended droplets and the continuous medium. Common emulsifiers include soaps, detergents, proteins, and long-chain alcohols.
The stability of hydrophobic colloids and emulsions is dictated by the presence of electrical charges on their particles. According to the Hardy-Schulze rule, the flocculation power of an effective ion depends directly on its valency. Higher valency ions precipitate oppositely charged colloids much more effectively (e.g., $\text{Al}^{3+} > \text{Ba}^{2+} > \text{Na}^+$ for negatively charged sols).
Important Facts / Formulas
| Property | Physisorption | Chemisorption |
|---|---|---|
| Nature of Forces | Weak van der Waals forces | Strong chemical bond forces |
| Heat of Adsorption | Low (20–40 kJ/mol) | High (80–240 kJ/mol) |
| Reversibility | Reversible | Irreversible |
| Effect of Temperature | Decreases with increase in temperature | Increases initially, then decreases |
| Activation Energy | Very low or negligible | High activation energy required |
Previous Year Question Hints
- Freundlich Isotherm Slope: Expect questions calculating the value of $1/n$ from a $\log(x/m)$ vs $\log P$ plot. Remember that $1/n$ usually lies between 0 and 1. At high pressures, $1/n \approx 0$, making adsorption independent of pressure.
- Hardy-Schulze Rule Applications: You may be asked to identify which electrolyte coagulates a given negatively charged arsenic sulfide sol most rapidly. Choose the electrolyte containing the cation with the highest positive valency (e.g., $\text{AlCl}_3$ over $\text{NaCl}$).
- Enzyme Kinetics: Questions often test the temperature dependence of enzyme catalysis, noting that efficiency peaks at an optimum temperature and drops sharply due to denaturation at higher temperatures.
Quick Revision Summary
- Adsorption is a surface phenomenon where adsorbate molecules concentrate on an adsorbent surface; it is always exothermic ($\Delta H < 0$) and spontaneous ($\Delta G < 0$).
- Physisorption is reversible with low activation energy, whereas chemisorption is irreversible and requires activation energy.
- Freundlich isotherm equation: $\frac{x}{m} = k P^{1/n}$; Langmuir isotherm equation: $\frac{x}{m} = \frac{aP}{1 + bP}$.
- Heterogeneous catalysis operates primarily through adsorption of reactants, surface reaction, and desorption of products.
- Colloidal particles range from 1 nm to 1000 nm and exhibit properties like the Tyndall effect, Brownian motion, and electrophoresis.
- Lyophobic sols are inherently unstable and require stabilizing electrolytes or protective colloids, whereas lyophilic sols are self-stabilizing.
- Emulsions are liquid-liquid colloidal systems stabilized by emulsifying agents such as soaps and detergents.
- The Hardy-Schulze rule states that the coagulating power of an ion varies directly with its valency.
