Surface Chemistry: Comprehensive NEET Chemistry Notes

1. Adsorption

1.1 Definition and Types

• Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.
• • Physical Adsorption (Physisorption): Involves weak van der Waals forces.
  • Chemical Adsorption (Chemisorption): Involves strong chemical bonds.

Formula: $\text{Adsorption Isotherm (Freundlich)}:\frac{x}{m}=k\cdot p^{\frac{1}{n}}$

• Where:
• • $x$ = mass of the gas adsorbed
  • $m$ = mass of the adsorbent
  • $p$ = pressure
  • $k$ and $n$ are constants that depend on the system.

Explanation:

• Freundlich Adsorption Isotherm: It expresses the relationship between the amount of gas adsorbed per unit mass of solid adsorbent and pressure at a constant temperature.

Common Mistake:

• Misinterpreting the constants $k$ and $n$ in the Freundlich equation as universal constants; they are specific to each system.

Example Application:

• Determining the adsorption capacity of activated charcoal for a particular gas in an industrial process.

1.2 Adsorption Isotherms

• Freundlich Isotherm: An empirical relationship that approximates adsorption behavior at moderate pressure ranges.

Formula: $\log \left(\frac{x}{m}\right)=\log k+\frac{1}{n}\log p$

Explanation:

• By plotting $\log \left(\frac{x}{m}\right)$ against $\log p$, one can verify the validity of Freundlich isotherm, where the slope gives $\frac{1}{n}$ and the intercept gives $\log k$.

NEET Problem-Solving Strategy:

• Pay attention to whether the problem requires a linear or non-linear relationship and apply the appropriate form of the Freundlich equation.

NEET Tip:

• Remember that at very high pressures, the Freundlich isotherm fails as it cannot predict saturation.

1.3 Thermodynamics of Adsorption

• Gibbs Free Energy Change ($\Delta G$): For adsorption to be spontaneous, $\Delta G$ must be negative.

Formula: $\Delta G=\Delta H-T\Delta S$

• Where:
• • $\Delta H$ = change in enthalpy (negative for exothermic processes like adsorption)
  • $T$ = temperature
  • $\Delta S$ = change in entropy (often negative in adsorption)

Explanation:

• Adsorption is generally exothermic, hence $\Delta H$ is negative. Despite $\Delta S$ being negative (since the system becomes more ordered), $\Delta G$ can be negative if $\Delta H$ is sufficiently large.

Common Mistake:

• Confusing the signs of $\Delta H$ and $\Delta S$; remember that while $\Delta H$ is negative for adsorption, $\Delta S$ can also be negative, making the process spontaneous depending on the magnitude of $\Delta H$.

2. Catalysis

2.1 Definition and Mechanism

• Catalysis: The process by which the rate of a chemical reaction is increased by the addition of a catalyst.

Formula:

• There is no specific formula, but the catalytic activity can be understood through the Arrhenius equation, where the presence of a catalyst lowers the activation energy $E_a$.

Explanation:

• A catalyst provides an alternative pathway for the reaction with a lower activation energy, hence increasing the reaction rate without being consumed.

Example Application:

• The use of platinum in catalytic converters in vehicles to reduce harmful emissions.

2.2 Enzyme Catalysis

• Enzyme Catalysis: A type of catalysis where enzymes (biological catalysts) speed up biochemical reactions.

Explanation:

• Enzymes work by binding to specific substrates to form an enzyme-substrate complex, lowering the activation energy required for the reaction.

Common Misconception:

• Thinking enzymes are consumed in the reaction; enzymes remain unchanged after the reaction.

Quick Recap:

• Adsorption: $\frac{x}{m}=k\cdot p^{\frac{1}{n}};\Delta G=\Delta H-T\Delta S$.
• Catalysis: Lowers the activation energy, $E_a$, hence increasing the reaction rate.

Concept Connection:

• Adsorption and surface phenomena link closely with the study of equilibrium in chemical reactions (Chemistry) and enzyme activity (Biology).

Practice Questions:

1. Derive the Freundlich adsorption isotherm and explain its limitations.
2. Explain the role of Gibbs free energy in determining the spontaneity of adsorption.
3. Discuss the factors affecting the rate of catalysis and relate it to the activation energy.

This summary captures the key formulae and concepts from the Surface Chemistry chapter of the NCERT textbook, with explanations and applications relevant to NEET UG Chemistry preparation.