Q.Colourless $\text{H}_2(g)$ and violet $\text{I}_2(g)$ are sealed together in a closed vessel at constant temperature. The vessel is observed to reach a constant pink-violet colour that never changes further, yet radioactively-tagged iodine atoms introduced into the vessel are found, after equilibrium, distributed throughout both $\text{I}_2$ and $\text{HI}$ molecules. Explain what this shows about the true nature of chemical equilibrium.

Chemical equilibrium is the state a reversible reaction reaches, in a closed vessel, once the

concentrations of every reactant and every product stop changing with time — but this apparent

stillness is misleading, because at the molecular level the reaction has not actually stopped at all.

Consider a reaction $A \rightleftharpoons B$ starting from pure $A$. At first the forward reaction

proceeds at its maximum rate, since $[A]$ is highest and no $B$ yet exists to drive any reverse

reaction. As $A$ is consumed the forward rate falls, while the accumulating $B$ makes the reverse rate

rise from zero; eventually the two rates become exactly equal, and from that moment the net

concentrations of $A$ and $B$ stop changing. Equilibrium has been reached — but molecules of $A$ are

still continuously being converted to $B$, and molecules of $B$ are still continuously being converted

back to $A$, at equal rates. It is this never-ceasing two-way traffic, precisely balanced, that earns

the name "dynamic" equilibrium, as opposed to a reaction that has genuinely and permanently stopped.

This dynamic character is not merely inferred from the mathematics — it has been demonstrated directly

using isotopic labelling: introducing a small quantity of radioactively-tagged atoms into a mixture

already at equilibrium shows the label soon distributed across both reactant and product molecules,

even though the bulk, measured concentrations of every species never change. Only continuous forward

and reverse reaction can explain that redistribution.

Chemical equilibrium can be reached from either direction — starting from pure reactants, or from pure

products (in stoichiometrically equivalent amounts) — and, at a given temperature, the same

equilibrium state results either way. This reversibility and dynamic equality of rates is what

distinguishes true chemical equilibrium from a reaction that has simply run out of reactant, and it is

the foundation on which the quantitative equilibrium constant is built.