Experimental Evidence for Dynamic Equilibrium

The concept of dynamic equilibrium – that reactions do not stop at equiibrium – can be confirmed experimentally using isotopic tracers. If the idea that equilibrium is dynamic could not be confirmed by empirical evidence, it would be a very poor theoretical explanation, and would have to be replaced.

These experiments are done as follows:

a reaction is allowed to reach equilibrium, so that its properties are no longer changing.
a small amount of one of the reactants, tagged with an isotope, is added to the equilibrium mixture.
after a time, the tagged element is found in both the reactant and product molecules, even though equilibrium – no observable change – exists in the reacting system at all times.

Using a radioactive tracer

This is only possible if the reaction continues to take place, even after equilibrium has been achieved. Here's why.

Possibility 1: The reaction actually stops when equilibrium is achieved.

This could be represented as:

If the reaction had stopped, then when the tracer, radioactive iodine-131, was added, nothing could happen. There would be a mixture of regular and radioactive iodine-131, but no chemical reaction would take place

so there would be no radioactive iodine present in the form of HI.

Possibility 2: The reaction doesn't stop at equilibrium

When the tracer is added it will react in the forward direction, forming some containing the radioactive iodine-131.

The would all dissappear if the reaction only went forward

but, if the reverse reaction is also taking place, some of the formed will react in the reverse direction to reform the radioactive . The tracer element will now be part of both the reactants and the products.

Possibility 2 is consistent with the experimentally determined evidence – some radioactive iodine in both the reactants and products. This supports the theory that equilibrium is a dynamic process involving equal rates in the forward and reverse directions.