Adsorption

• The difference between physisorption and chemisorption across five key parameters
• How to apply the Freundlich isotherm equation and interpret its log-log graph
• What the Langmuir isotherm assumes and when saturation occurs
• Which factors control the extent of adsorption (surface area, temperature, pressure, nature of materials)
• Real-world applications: gas masks, froth flotation, chromatography, sugar decolourisation, heterogeneous catalysis

What is Adsorption?

Adsorption is the phenomenon by which molecules of a gas or liquid (the adsorbate) accumulate on the surface of a solid or liquid (the adsorbent). It is a surface phenomenon — only the surface atoms/molecules of the adsorbent are involved.

• Adsorbent — the substance on whose surface accumulation occurs (e.g., activated charcoal, silica gel, alumina)
• Adsorbate — the substance that gets adsorbed (e.g., gases like NH₃, SO₂, CO₂; dyes in solution)

Do not confuse with absorption, where the adsorbate penetrates uniformly into the bulk. Adsorption is a surface effect; absorption is a bulk effect. The term sorption covers both.

Cause of adsorption: Surface atoms of an adsorbent have unsatisfied (residual) valencies because they lack neighbours on one side. These residual forces attract and retain adsorbate molecules — releasing energy as heat of adsorption (ΔH_ads).

Adsorption is always exothermic (ΔH < 0) and therefore ΔS < 0 (decrease in freedom of adsorbate). For the process to be spontaneous, ΔG = ΔH − TΔS < 0, which is satisfied at lower temperatures.

Physisorption vs Chemisorption

Mnemonic — "Physical is Fast and Feeble; Chemical is Committed and Strong":

• Physisorption: Fast (no Ea needed), Feeble (low ΔH), Fleeting (reversible)
• Chemisorption: Specific, Strong (high ΔH), Stable (irreversible)

JEE Trap: At low temperatures physisorption dominates; as temperature rises it may convert to chemisorption. So the adsorption vs temperature curve shows a characteristic shape — initial decrease (physisorption fading) followed by a rise (chemisorption activating) then a plateau.

Freundlich Adsorption Isotherm

An isotherm describes the relationship between extent of adsorption and pressure (for gases) or concentration (for solutions) at constant temperature.

The Freundlich equation is empirical:

$\frac{x}{m} = k \cdot p^{1/n} \quad (n > 1)$

• x = mass of adsorbate adsorbed
• m = mass of adsorbent
• p = equilibrium pressure of adsorbate
• k, n = empirical constants depending on adsorbent-adsorbate pair and temperature

Taking logarithm:

$\log\frac{x}{m} = \log k + \frac{1}{n} \log p$

This is a straight-line equation: plot of log(x/m) vs log p gives a straight line with:

• Slope = 1/n (between 0 and 1)
• Intercept = log k

Limits of the equation:

• At very low pressure: x/m ∝ p (linear region, 1/n ≈ 1)
• At very high pressure: x/m = constant (saturation, 1/n ≈ 0)
• Intermediate region: x/m ∝ p^(1/n)

Limitation: The Freundlich isotherm is empirical — it does not predict saturation correctly at high pressures.

Langmuir Adsorption Isotherm

Langmuir (1916) proposed a theoretical isotherm based on these assumptions:

1. Adsorption is monolayer (only one layer of adsorbate on the surface)
2. All adsorption sites are equivalent (same energy)
3. There is no interaction between adsorbed molecules
4. Adsorption reaches a dynamic equilibrium between adsorption and desorption

$\frac{x}{m} = \frac{ab \cdot p}{1 + b \cdot p}$

where a = maximum adsorption (monolayer capacity) and b = Langmuir constant.

At high pressure: x/m → a (saturation, monolayer complete) At low pressure: x/m ≈ ab·p (linear, Freundlich-like)

Factors Affecting Adsorption

Applications of Adsorption

Adsorption from Solution Phase

Freundlich isotherm applies to adsorption from solutions too:

$\frac{x}{m} = k \cdot C^{1/n}$

where C = equilibrium concentration of adsorbate in solution.

The log form: log(x/m) = log k + (1/n) log C gives a straight line with slope 1/n.

Mechanism of Heterogeneous Catalysis — The Adsorption Role

Steps in heterogeneous catalysis (e.g., SO₂ → SO₃ over V₂O₅):

1. Adsorption of reactants on catalyst surface (chemisorption — forms surface intermediates)
2. Reaction occurs at the surface between adsorbed species
3. Desorption of products from the surface
4. Products diffuse away, surface sites regenerated

The key: catalyst provides an alternate reaction pathway with lower activation energy — enabled by the ability of the surface to chemisorb reactants.

Intermediate Compound Theory vs Adsorption Theory

Shape of Adsorption Isotherms — JEE Graph Points

For Freundlich isotherm (log-log plot):

• Straight line with positive slope (= 1/n, where 0 < 1/n < 1)
• Y-intercept = log k (positive)
• Slope < 1 confirms n > 1, which means adsorption increases less than proportionally with pressure

For x/m vs p (Freundlich, linear plot):

• Curved upward initially, then flattening — but does NOT reach a clear plateau (unlike Langmuir)

For x/m vs p (Langmuir):

• Hyperbolic curve, clearly asymptotes to x/m = a at high p

Example 1: Freundlich Isotherm — Finding k and n from Graph

Problem: A log(x/m) vs log p graph for adsorption of NH₃ on charcoal gives
a straight line with slope 0.5 and y-intercept 1.2. Find the values of n and k,
and write the Freundlich equation.

Step 1: From the Freundlich equation:
        log(x/m) = log k + (1/n) log p
        
Step 2: Identify from graph:
        Slope = 1/n = 0.5  →  n = 1/0.5 = 2
        Y-intercept = log k = 1.2  →  k = 10^1.2 ≈ 15.85

Step 3: Write the equation:
        x/m = k·p^(1/n) = 15.85 × p^0.5 = 15.85 √p

Answer: n = 2, k ≈ 15.85, equation: x/m = 15.85 p^(1/2)

Verification check: Since 1/n = 0.5, which is between 0 and 1, the result is
physically valid — adsorption increases with pressure but less than linearly.

Example 2: Physisorption vs Chemisorption — MCQ Reasoning

Problem: Which of the following statements is INCORRECT about chemisorption?
(A) It is irreversible
(B) It requires activation energy
(C) It forms multilayers
(D) It is highly specific

Step 1: Recall chemisorption features:
        - Involves chemical bond formation between adsorbate and surface
        - Bond formation requires activation energy ✓
        - Irreversible (strong bonds) ✓
        - Highly specific (reaction with particular sites) ✓
        
Step 2: Evaluate option (C):
        Chemisorption forms a MONOLAYER only — because once a chemical bond is
        formed on a site, that site is occupied. Further layers would only be
        physisorption layers (different nature). Multi-layer formation is a
        feature of PHYSISORPTION.

Answer: (C) — Chemisorption forms monolayers only, NOT multilayers.
        Multilayer adsorption is a characteristic of physisorption.

1. What is the difference between an adsorbent and an adsorbate? Give one example of each.
2. A Freundlich plot of log(x/m) vs log p gives slope = 0.4 and intercept = 0.7. Write the Freundlich equation.
3. Why does physisorption decrease with increase in temperature while chemisorption may initially increase?
4. List three applications of adsorption that are relevant to everyday life, and name the adsorbent used in each.
5. (Stretch) The Langmuir isotherm is based on four assumptions. Identify which assumption fails in real systems (hint: think about surface heterogeneity and multilayer formation), and explain how this limitation is addressed by the BET isotherm (Brunauer–Emmett–Teller theory).

• NCERT Class 12 Chemistry, Chapter 5 — Surface Chemistry, Sections 5.1–5.2
• Adsorption appears in 1–2 JEE MCQs per year, often testing Freundlich equation (slope/intercept), physisorption vs chemisorption comparison, and applications
• Common MCQ formats: identify incorrect statement, calculate x/m from graph, match application with adsorbent

Study strategy: Draw the log(x/m) vs log p graph from scratch twice. Memorise the 5-row physisorption vs chemisorption comparison table. For applications, link each to its adsorbent — the exam often gives the application and asks for the adsorbent.

Practice MCQs on adsorption isotherms and physisorption vs chemisorption in the Surface Chemistry — Adsorption topic bank. Start with Easy (conceptual), then attempt Medium (graph-based numericals).

Confused about why the Freundlich log-log graph gives a straight line? Ask the Drishti AI tutor to walk through the logarithmic transformation step by step, or to explain the shape of the Langmuir isotherm curve.

Answer all 5 Quick Check questions and mark them in your Drishti progress tracker. Target 4/5 before moving to Colloids.

• Read: Surface Chemistry — Colloids — Tyndall effect, Brownian motion, Hardy-Schulze rule
• Then: Surface Chemistry — Catalysis — enzyme catalysis, zeolites, activation energy diagrams
• Practice: Mixed Surface Chemistry MCQs (Medium difficulty)