3.1 What is Equilibrium?

Defining equilibrium

While studying chemical reactions, we need to know when they start and end. To do this, we define the state of equilibrium when reactions are not proceeding. In thermodynamics, systems are at perfect rest, with no gradients or inhomogeneities. Under equilibrium conditions, systems have two attributes:

1. None of the properties change with time - regardless of the time observed

2. The system will return to equilibrium if its attributes are disturbed.

A state in which there are no observable changes as a function of time. This state is achieved when (i) the forward and reverse reaction rates are equal and (ii) concentrations of reactants and products remain constant.

For example, when we place a sample of dinitrogen tetroxide (\(\ce{N2O4}\), a colorless gas) in a glass tube, it forms nitrogen dioxide (\(\ce{NO2}\), a brown gas) as shown in the reaction below.

\[\ce{N2O4(g) <=> 2 NO2(g)} \]

The color becomes darker as \(\ce{N2O4}\) is converted to \(\ce{NO2}\). When the system reaches equilibrium, both \(\ce{N2O4}\) and \(\ce{NO2}\) are present (Fig. 34).

Fig. 34 A mixture of \(\ce{NO2}\) and \(\ce{N2O4}\) moves toward equilibrium. Colorless \(\ce{N2O4}\) reacts to form brown \(\ce{NO2}\). As the reaction proceeds toward equilibrium, the color of the mixture darkens due to the increasing concentration of \(\ce{NO2}\). Image source: 13.1 Chemical Equilibria - Chemistry: Atoms First | OpenStax

The formation of \(\ce{NO2}\) from \(\ce{N2O4}\) is a reversible reaction, which is identified by the equilibrium arrow (\(\ce{<=>}\)). Theoretically, all reactions are reversible, but many proceed in one direction until the reactants are exhausted and will reverse only under certain conditions. Such reactions are often depicted with a \(\ce{->}\) from reactants to products. Reactions, such as the formation of \(\ce{NO2}\) from \(\ce{N2O4}\), are reversible under more easily obtainable conditions and, therefore, are named as such. In a reversible reaction, the reactants can combine to form products, and the products can react to form the reactants. Thus, not only can \(\ce{N2O4}\) decompose to form \(\ce{NO2}\), but the \(\ce{NO2}\) produced can react to form \(\ce{N2O4}\). As soon as the forward reaction produces any \(\ce{NO2}\), the reverse reaction begins, and \(\ce{NO2}\) starts to react to form \(\ce{N2O4}\). At equilibrium, the concentrations of \(\ce{N2O4}\) and \(\ce{NO2}\) no longer change because the rate of formation of \(\ce{NO2}\) is exactly equal to the rate of consumption of \(\ce{NO2}\). The formation rate of \(\ce{N2O4}\) equals the consumption rate of \(\ce{N2O4}\). Equilibrium is a dynamic process.

Types of Equilibrium

There are two types of equilibrium: (i) physical equilibrium involves a change in the states of matter but not a change in the chemical composition. (ii) chemical equilibrium involves a change in chemical composition.

Example: Types of equilibrium

\[\ce{H2O(l) <=> H2O(g)} \]

This is an example of a reaction depicting physical equilibrium, where the liquid form of water is transformed into a gaseous form.

\[\ce{N2O4(g) <=> 2 NO2(g)} \]

This is an example of a reaction depicting a system in chemical equilibrium, where \(\ce{N2O4(g)}\) is being transformed into \(\ce{NO2(g)}\).