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MHT-CET Chemistry · Surface Chemistry

Adsorption: Surface vs Bulk, Physisorption vs Chemisorption, and the Freundlich Isotherm

Adsorption is the gathering of a substance on the surface of another; it is exothermic, falls as temperature rises, and grows with pressure and with how easily the gas liquefies.

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Why this matters

12 PYQs, none HARD. Four separate adsorption from absorption, sorption and desorption; six ask which gas adsorbs most, whether a case is physisorption, or how temperature acts; two are the Freundlich equation. Three cards.

Concept 1 of 3: Adsorption, Absorption, Sorption and Desorption

Adsorption stays on the surface; absorption soaks right through the bulk. When both happen at once — the dye sticks to the chalk's surface while the water soaks in — it is sorption. Desorption is adsorption run backwards, freeing the surface for more.

Definition

  • Adsorption: a surface phenomenon; the concentration is higher at the surface than in the bulk; exothermic; usually reversible.
  • Absorption: a bulk phenomenon; uniform concentration throughout; independent of surface area.
  • Sorption: adsorption and absorption together — chalk dipped in ink.
  • Desorption: removing the adsorbed substance, freeing the sites for more adsorbate.

Worked example

Which is an example of sorption: charcoal in methylene blue, chalk dipped in ink, H₂ over platinum, O₂ over nickel?
Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q80Moderate

Example 1 · Surface Chemistry · Adsorption, Physisorption, Chemisorption and Isotherms

Identify example of sorption from following.

Calling a dye on charcoal sorption

Charcoal decolourising methylene blue is the textbook ADSORPTION example. Sorption needs something to be absorbed into the bulk as well.
Drill 3 more on adsorption, absorption, sorption and desorption

Concept 2 of 3: Physisorption vs Chemisorption, and What Controls the Extent

Physisorption is weak van der Waals attraction — non-specific, so charcoal takes up any gas, and more of a gas that liquefies easily. Chemisorption forms real chemical bonds with particular metals, like H₂ on nickel or N₂ on iron. Both release heat, so cooling increases adsorption.

Definition

  • Physisorption: van der Waals forces, non-specific, low enthalpy, reversible, multilayer — all gases on charcoal.
  • Chemisorption: chemical bonds, specific, high enthalpy, often irreversible, monolayer — O₂ on W, H₂ on Ni, N₂ on Fe.
  • Nature of gas: higher critical temperature (easier to liquefy) → more adsorbed: SO₂ ≈ Cl₂ > NH₃ > … > O₂ > N₂ > H₂.
  • Temperature: adsorption is exothermic, so it decreases as temperature rises — the lowest-temperature isobar sits highest.
  • Pressure and surface area: more of each → more adsorption.

Extent of physisorption

xm↑ as Tc↑, p↑, T↓\frac{x}{m} \uparrow \ \text{as}\ T_c \uparrow,\ p \uparrow,\ T \downarrow

Worked example

Which gas is adsorbed least on a solid under the same conditions: Cl₂, NH₃, SO₂, H₂?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 26 April Shift I · Q66Moderate

Example 2 · Surface Chemistry · Adsorption, Physisorption, Chemisorption and Isotherms

Which among the following gases is least adsorbed on solid at similar conditions of temperature and pressure?

Reading 'inversely proportional' as critical temperature

Critical temperature raises adsorption; the TEMPERATURE of the process lowers it. The question lists both on purpose.
Drill 5 more on physisorption vs chemisorption, and what controls the extent

Concept 3 of 3: The Freundlich Adsorption Isotherm

At a fixed temperature, the amount adsorbed per gram of adsorbent rises with pressure — but less than proportionally, as a fractional power 1/n with n > 1. Taking logs turns it into a straight line whose slope is 1/n and intercept log k.

Definition

  • Freundlich equation: x/m = k p^(1/n), with n > 1 (for a solution, p is replaced by the concentration C).
  • Log form: log(x/m) = log k + (1/n) log p — a straight line.
  • Slope = 1/n; intercept = log k.

Freundlich isotherm

xm=k p1/n⟹log⁡xm=log⁡k+1nlog⁡p\frac{x}{m} = k\,p^{1/n}\quad\Longrightarrow\quad \log\frac{x}{m} = \log k + \frac{1}{n}\log p

Worked example

In a Freundlich plot of log(x/m) against log C, what are the slope and the intercept?
Practice this conceptself-check · 1 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 16th May Shift 1 · Q97Easy

Example 3 · Surface Chemistry · Adsorption, Physisorption, Chemisorption and Isotherms

What is the value of slope in Freundlich adsorption isotherm log⁡(x/m)\log(x/m) against log⁡C\log C?

Inverting x/m or the exponent

The options swap x/m for m/x and 1/n for n. It is x/m on the left, and the power is the FRACTION 1/n.
Drill 1 more on the freundlich adsorption isotherm

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Watch out for (3)

Test yourself on Surface Chemistry

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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