In the thermal decomposition of potassium chlorate given as 2KClO3 → 2KCl+3O2, law of mass action A) Cannot be applied B) Can be applied C) Can be applied at low temperature D) Can be applied at high temp and pressure

Key Concepts

1. Law of Mass Action (Equilibrium Law)
   For a reaction
   $aA + bB \rightleftharpoons cC + dD$
   the equilibrium constant is
   $K_c = \frac{[C]^c\,[D]^d}{[A]^a\,[B]^b}$
   where square brackets denote molar concentrations (or activities).

2. Heterogeneous vs. Homogeneous Equilibria
   • Homogeneous: All species are in the same phase (e.g., all gases).
   • Heterogeneous: Different phases coexist (solids + gases in this case).

3. Activity of Pure Solids and Liquids
   In heterogeneous equilibria, the activity of a pure solid or liquid is taken as unity (1) because its concentration does not change appreciably.

Applying to $2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)$

• Both $KClO_3$ and $KCl$ are pure solids.
• $O_2$ is a gas.

Therefore, the equilibrium constant expression is

$K_p = P_{\!O_2}^{\;3}$

(or $K_c = [O_2]^3$ if expressed in concentration), since the activities of solids are set to 1.

Hence, the law of mass action is fully applicable; we simply omit the solids in the expression.

Why Each Step Matters

1. Identify Phases: Recognise which reactants/products are solids and which are gases.
2. Set Solid Activities to 1: This simplifies the equilibrium expression.
3. Write the Expression: Only the variable species (here, $O_2$) appear.
4. Draw Conclusion: Because an equilibrium constant can still be written, the law of mass action can be applied.

Imagine a Jar of Marbles

1. Two Types of Marbles Inside: Some marbles never leave the jar (solids: $KClO_3$ and $KCl$). One type keeps running in and out (gas: $O_2$).
2. Rule of Counting: When we write the rule for who is running in and out (the law of mass action), we only count the movers (gases). The sitting marbles (solids) are so many and so packed that their number doesn’t really change, so we pretend their “count” is just 1.
3. So, Can We Use the Rule? Yes! The rule still works; we simply ignore the solids in the math.

Answer: Option B (Can be applied).

Step-by-Step Solution

1. Write the Reaction:
   $2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)$

2. Recognise the Phases:
   $KClO_3$ and $KCl$ are solids, $O_2$ is a gas.

3. Apply the Law of Mass Action:
   For a heterogeneous equilibrium, assign activities of solids as 1.

4. Write the Equilibrium Constant:
   $K_p = P_{\!O_2}^{\;3}$

5. Interpretation:
   Because a valid $K_p$ expression exists, the law of mass action does apply.

6. Select the Correct Option:
   Option B: Can be applied.