(i) What is the role of activated charcoal in a gas mask? (ii) A colloidal sol is prepared by the method shown

Question

Q. (i) What is the role of activated charcoal in a gas mask?

(ii) A colloidal sol is prepared by the method shown in the figure:

FeCl_3

solution is added dropwise into

NaOH

solution taken in a test tube. What is the charge on the hydrated ferric oxide colloidal particles formed in the test tube? How is the sol represented?

(iii) How does chemisorption vary with temperature?

CBSECBSE Class XII Board 2019Subjective· 3mImportance★★★★★est

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Physisorption and Chemisorption

Physisorption and Chemisorption: The Two Faces of Adsorption

Imagine a wall. If you throw a tennis ball at it, the ball might stick for a moment because of static cling or a bit of moisture — but a gentle tap knocks it off. That's one kind of sticking. Now imagine you throw a blob of superglue at the same wall. It bonds, changes the wall's surface, and you'd need to scrape it off. That's a different kind of sticking entirely.

Adsorption — the accumulation of a substance (the adsorbate) on the surface of another (the adsorbent) — happens in exactly these two ways. The first is physisorption (physical adsorption). The second is chemisorption (chemical adsorption).

Physisorption: The Gentle Touch

Physisorption is driven by van der Waals forces — the same weak, universal attractions that make gases deviate from ideal behaviour. These forces are present between any two molecules that are close enough, so physisorption is non-specific: any gas will physisorb on any solid surface, given the right conditions.

Because the forces are weak, the process is reversible. The adsorbate can be removed easily by lowering the pressure or raising the temperature — no chemical change occurs. The heat released (enthalpy of adsorption) is small, typically in the range of 20–40 kJ/mol.

Note

Physisorption is like condensation: it increases as temperature decreases and as pressure increases. At low temperatures, gas molecules have less kinetic energy and are more easily trapped by the surface.

Key characteristics at a glance:

Property	Physisorption
Force	van der Waals (weak)
Specificity	Non-specific — occurs on any surface
Reversibility	Reversible
Enthalpy change	Low (20–40 kJ/mol)
Activation energy	None — occurs instantly
Layers formed	Multilayers possible

Chemisorption: The Chemical Grip

Chemisorption involves the formation of a chemical bond (covalent or ionic) between the adsorbate and the adsorbent surface. This is a true chemical reaction — electrons are shared or transferred, and the adsorbate molecule often gets dissociated into atoms or fragments.

Because a chemical bond is formed, chemisorption is highly specific. Hydrogen will chemisorb on nickel but not on gold; oxygen will chemisorb on platinum but not on glass. The process is often irreversible — you cannot simply pump away the adsorbed gas; you may need to heat the surface to extreme temperatures or chemically react it off.

The enthalpy of chemisorption is large, typically 80–400 kJ/mol — comparable to the energy of a covalent bond.

Watch out

A common mistake: students think chemisorption is always slow. In fact, it can be fast if no activation energy is needed (e.g., oxygen on clean tungsten). But many chemisorption processes do have an activation barrier, making them slow at low temperatures — just like any chemical reaction.

Key characteristics at a glance:

Property	Chemisorption
Force	Chemical bond (covalent/ionic)
Specificity	Highly specific
Reversibility	Often irreversible
Enthalpy change	High (80–400 kJ/mol)
Activation energy	Often present — increases with temperature
Layers formed	Monolayer only (bonds saturate)

The Critical Difference: Activation Energy

This is the single most important distinction for exams.

Physisorption has zero activation energy — it happens the instant a molecule touches the surface. Chemisorption often requires an activation energy $E_a$ , just like any chemical reaction. This means:

At very low temperatures, chemisorption may be impossible because molecules lack the energy to overcome the barrier. …

Start your 14-day free trial to unlock the full solution →

Activated charcoal adsorbs toxic gases in a gas mask. The hydrated ferric oxide sol formed by adding FeCl₃ to NaOH is negatively charged and represented as $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} / \text{OH}^-$ . Chemisorption first increases with temperature (activation energy), then decreases at very high temperatures due to desorption.

(i) Role of activated charcoal in a gas mask

Activated charcoal is a highly porous form of carbon with an enormous surface area (up to 1000 m²/g). In a gas mask, it acts as an adsorbent — toxic gases and vapours in the inhaled air get physically adsorbed onto its surface. The pores trap molecules like chlorine, ammonia, or nerve agents, preventing them from reaching the lungs. This is a classic example of physical adsorption (physisorption), which is reversible and non-specific, but effective because of the large surface area.

Watch out

Activated charcoal does not chemically destroy the gases — it only holds them on its surface. Once the pores are saturated, the mask becomes ineffective.

(ii) Charge on hydrated ferric oxide sol from FeCl₃ + NaOH

Concept first: The charge on a colloidal particle depends on which ion from the dispersion medium gets preferentially adsorbed on its surface. In this preparation, we add FeCl₃ dropwise into excess NaOH. The reaction is:

\text{FeCl}_3 + 3\text{NaOH} \rightarrow \text{Fe(OH)}_3 + 3\text{NaCl}

The Fe(OH)₃ formed is insoluble and aggregates into colloidal-sized particles. But the key is the medium: since NaOH is in excess, the solution contains plenty of OH⁻ ions. These OH⁻ ions get adsorbed onto the surface of the Fe(OH)₃ particles, giving them a negative charge.

Why not positive? If you instead added NaOH to excess FeCl₃ (the reverse order), the excess Fe³⁺ ions would adsorb, giving a positive sol. Here, the order is reversed — excess OH⁻ dominates.
Representation of the sol: The hydrated ferric oxide is written as $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$ (since Fe(OH)₃ readily loses water to form the oxide hydrate). The adsorbed ion is written after a slash. So the sol is represented as:

$\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} / \text{OH}^-$ …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set D1 markMCQ
$Q. Which of the following adsorptions forms multimolecular layer?$
(a) Physical adsorption
(b) Chemical adsorption
(c) Both (A) and (B)
(d) None of these
›Reveal solutionSolution
Physical adsorption (physisorption) forms multimolecular layers; chemisorption forms only a monolayer.

Adsorption is classified into two types by the nature of the attractive force:

Physical adsorption (physisorption): the adsorbate is held by weak van der Waals forces. These forces are non-specific and long-range, so once the first layer of molecules is adsorbed, further layers can pile up on top. Hence physisorption gives a multimolecular (multilayer) film, especially at low temperature and high pressure. …
Start your 14-day free trial to see the full solution →
CBSE 2021Set A1 markMCQ
$Q. How many layers are involved in chemical adsorption?$
(a) One
(b) Two
(c) Many
(d) Zero
›Reveal solutionSolution

Chemical adsorption (chemisorption) is unilayer — only one layer forms.

In chemisorption the adsorbate is held to the surface by strong chemical (covalent) bonds. Because chemical bonding can only occur directly with the surface atoms, chemisorption stops after a single layer (monolayer) is formed — it cannot build up further layers.

…

Start your 14-day free trial to see the full solution →
CBSE 2021Set A1 markMCQ
$Q. In physical adsorption the approximate heat energy evolved in kJ/mol is$
(a) 20-40
(b) 40-60
(c) 60-80
(d) 40-400
›Reveal solutionSolution
Physisorption is due to weak van der Waals forces, releasing a small heat of about 20-40 kJ/mol.

In physical adsorption (physisorption), the adsorbate molecules are held to the adsorbent surface by weak van der Waals forces. Because these forces are weak, only a small amount of energy is released.

Physical adsorption: enthalpy of adsorption ≈ 20-40 kJ/mol. …
Start your 14-day free trial to see the full solution →
CBSE 2020Set ANNUAL1 mark
$Q. Why does physisorption decrease with rise of temperature ?$
›Reveal solutionSolution

Since physisorption releases heat and relies only on weak attractions, warming the system pushes the equilibrium back towards the gas phase.

Physisorption (physical adsorption) is an exothermic process held together only by weak van der Waals forces between the adsorbate and the adsorbent surface. By Le Chatelier's principle, since heat is a "product" of an exothermic process, raising the temperature shifts the adsorption–desorption equilibrium towards desorption (away from the adsorbed state). Additionally, at higher temperature, adsorbed molecules gain more kinetic energy, making it easier for them to overcome the we …

Start your 14-day free trial to see the full solution →

Q.(i) What is the role of activated charcoal in a gas mask?

(i) Role of activated charcoal in a gas mask

(ii) Charge on hydrated ferric oxide sol from FeCl₃ + NaOH

🎓Unlock everything free for 14 days

Unlock everything free for 14 days