Why do not all reactions reach equilibrium?

Not all reactions reach equilibrium because they may be irreversible, meaning they proceed in one direction until completion.

In more detail, reactions can be classified as either reversible or irreversible. Reversible reactions are those that can proceed in both the forward and reverse directions, and these are the types of reactions that can reach equilibrium. At equilibrium, the rates of the forward and reverse reactions are equal, meaning that the concentrations of the reactants and products remain constant over time.

On the other hand, irreversible reactions are those that proceed in one direction only. Once the reactants have been converted into products, the reaction cannot go in reverse. This is often because the products are removed from the system (for example, in the form of a gas that escapes), or because they react further to form other substances. In such cases, the reaction will continue until one or more of the reactants are completely used up. As a result, irreversible reactions do not reach equilibrium in the traditional sense.

It's also worth noting that the concept of equilibrium is a bit of an idealisation. In reality, many reactions may appear to be at equilibrium, but are actually still proceeding very slowly in one direction or another. This can be due to factors such as temperature, pressure, or the presence of catalysts, which can all influence the rates of the forward and reverse reactions.

Furthermore, the time it takes for a reaction to reach equilibrium can vary widely. Some reactions reach equilibrium almost instantly, while others may take a very long time. If the reaction is very slow, it may appear not to reach equilibrium within the timeframe of an experiment or observation.

In conclusion, while the concept of equilibrium is a fundamental one in the study of chemistry, it's important to remember that not all reactions reach this state. Whether a reaction is reversible or irreversible, and the conditions under which it occurs, can all influence whether or not it will reach equilibrium.