Question

Is there a general effect on the rate of reaction of a dynamic equilibrium when an inert gas is introduced at a constant volume? I know that the position of equilibrium won't change, but much like a catalyst which increases the rate of both forward and reverse reactions, I'm wondering if the introduction of an inert gas will have a similar effect.

My preliminary thoughts were that the inert gas would 'block' successful collisions by the reactants and products, thereby decreasing both forward and reverse rates. However, these 'blocked' collisions could simply be the result of more collisions taking place since there are more molecules in the same volume so the rate doesn't actually change.

My other thought was that certain inert gases might increase the rate of reaction by acting as a heterogenous catalyst – a surface for the reaction to take place.

Clearly, my thoughts haven't led to a definite answer so I was wondering if anyone could clear up exactly what happens to the rate of reaction when an inert gas is introduced. Thanks!

Answer 1

Adding a catalyst to an equilibrium system does not affect the position of equilibrium. Changes in volume can shift the equilibrium in a direction that accommodates the volume change. Systems at equilibrium can be disturbed by changes in temperature, concentration, volume, and pressure.

Step-by-step explanation:

Adding a catalyst to this, or any other equilibrium system, will not affect the position of an equilibrium. A catalyst speeds up both the forward and the reverse reactions, so there is no uneven change in reaction rates. Generally, a catalyst will help a reaction to reach the point of equilibrium sooner, but it will not affect the equilibrium otherwise.

These results illustrate the relationship between the stoichiometry of a gas-phase equilibrium and the effect of a volume-induced pressure (concentration) change. If the total molar amounts of reactants and products are equal, as in the first example, a change in volume does not shift the equilibrium. If the molar amounts of reactants and products are different, a change in volume will shift the equilibrium in a direction that better "accommodates" the volume change.

In the second example, two moles of reactant (NO₂) yield three moles of product (2NO+ O₂), and so decreasing the system volume causes the equilibrium to shift left since the reverse reaction produces less gas (2 mol) than the forward reaction (3 mol). Conversely, increasing the volume of this equilibrium system would result in a shift towards products.

Summary:
Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure; volume and pressure changes will disturb equilibrium if the number of moles of gas is different on the reactant and product sides of the reaction.

The system's response to these disturbances is described by Le Chatelier's principle: The system will respond in a way that counteracts the disturbance. Not all changes to the system result in a disturbance of the equilibrium. Adding a catalyst affects the rates of the reactions but does not alter the equilibrium, and changing pressure or volume will not significantly disturb systems with no gases or with equal numbers of moles of gas on the reactant and product side.