Assertion (A): In the dissociation of PCl5 at constant pressure and temperature addition of helium at equilibr

NCERT Exemplar · Q50

Q. Assertion (A): In the dissociation of PCl5 at constant pressure and temperature addition of helium at equilibrium increases the dissociation of PCl5. Reason (R): Helium removes Cl2 from the field of action.

(i) Both A and R are true and R is correct explanation of A.

(ii) Both A and R are true but R is not correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

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Concept understanding — Phase Equilibrium

Phase Equilibrium: From Intuition to Precision

Imagine a glass of ice water on a warm day. The ice cubes are melting, but the water around them stays cold. If you wait long enough, the ice stops melting — not because the room got colder, but because the system has reached a balance. The ice and water coexist without any net change. That balance is phase equilibrium.

The Intuition: A Tug-of-War Between Two Tendencies

Every substance wants to exist in the phase (solid, liquid, or gas) that has the lowest energy under the given conditions. But there's a catch: nature also loves disorder (entropy). A solid is low-energy but highly ordered; a gas is high-energy but very disordered. At a specific temperature and pressure, these two opposing drives — minimising energy and maximising disorder — exactly cancel each other out. The result? Two phases can coexist peacefully, with molecules constantly jumping between them but no net change in the amount of each phase.

Think of it as a tug-of-war. On one side, molecules in the solid phase are tightly bound and low in energy. On the other side, molecules in the liquid phase have more freedom and higher entropy. At the melting point, the rope doesn't move — the rates of melting and freezing are equal.

The Precise Statement

Phase equilibrium is the condition in which two or more phases of a substance coexist in thermodynamic equilibrium, with no net transfer of mass between phases. For a pure substance, this occurs at a unique temperature for a given pressure (or vice versa), defined by the equality of the chemical potential (or Gibbs free energy per mole) in all phases.

In simpler terms: at equilibrium, the driving force for a molecule to move from phase A to phase B is exactly the same as the driving force to move from B to A. The system is dynamic — molecules are still crossing the boundary — but macroscopically static.

The Key Condition: Equal Chemical Potential

The rigorous condition for phase equilibrium in a pure substance is:

\mu_{\text{solid}} = \mu_{\text{liquid}} = \mu_{\text{gas}}

where $\mu$ (mu) is the chemical potential — the Gibbs free energy per mole. When chemical potentials are equal, no phase has a "preference" to grow at the expense of another.

Important

For a pure substance, phase equilibrium occurs only along specific lines on a phase diagram (the melting curve, boiling curve, and sublimation curve). At the triple point, all three phases coexist.

A Concrete Example: Water at 0°C

At 1 atm pressure and 0°C, ice and liquid water are in equilibrium. Here's what's happening at the molecular level:

Water molecules at the surface of the ice are vibrating. Some gain enough energy to break free and enter the liquid.
Simultaneously, water molecules in the liquid near the ice surface lose energy and attach to the ice crystal.
The rates of these two processes are equal. So the amount of ice and the amount of liquid water remain constant — even though individual molecules are constantly switching sides. …

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The addition of an inert gas at constant pressure shifts equilibrium toward more dissociation (Le Chatelier), but the given reason (that helium removes Cl₂) is chemically wrong. So Assertion is true, Reason is false — option (iii).

The question tests a classic nuance in chemical equilibrium: the effect of adding an inert gas like helium. Many students memorise that "adding inert gas at constant volume does nothing" and "adding inert gas at constant pressure shifts equilibrium toward more moles" — but they often confuse why. The reason given here is a tempting but incorrect explanation.

Let’s break it down.

What happens when PCl₅ dissociates? The reaction is:

\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g) + \text{Cl}_2(g)

One mole of gas gives two moles of gas — so the forward direction increases the total number of moles.

The effect of adding helium at constant pressure

At constant pressure, adding an inert gas increases the total volume of the system (since $PV = nRT$ , if $P$ and $T$ are fixed, $V \propto n_{\text{total}}$ ).

The partial pressures of the reacting gases decrease because they are now spread over a larger volume.

Le Chatelier’s principle says: the system will shift in the direction that increases the number of moles of gas, to counteract the dilution.

Since the forward reaction produces more moles, equilibrium shifts to the right — more PCl₅ dissociates.

So Assertion (A) is true.
Why the Reason (R) is wrong

The reason claims: "Helium removes Cl₂ from the field of action."

This is chemically false. Helium is inert — it does not react with Cl₂, nor does it selectively absorb or remove it. The only effect of helium is to change the partial pressures of all gases equally by increasing the total volume. There is no specific removal of Cl₂.

So Reason (R) is false.

Watch out

A common mistake is to think that adding an inert gas "dilutes" only the product side — but it dilutes all gases equally. The shift occurs only because the forward reaction produces more gas molecules, not because any particular species is removed.

Evaluating the options
- (i) Both true, R explains A — No, R is false.
- (ii) Both true, R does not explain A — No, R is false. …

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Q.Assertion (A): In the dissociation of PCl5 at constant pressure and temperature addition of helium at equilibrium increases the dissociation of PCl5. Reason (R): Helium removes Cl2 from the field of action.

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Q.Assertion (A): In the dissociation of PCl5 at constant pressure and temperature addition of helium at equilibrium increases the dissociation of PCl5.
Reason (R): Helium removes Cl2 from the field of action.

Unlock everything free for 14 days