What is Chemical Equilibrium?

What is chemical equilibrium?

To define chemical equilibrium we need to ask some important questions about reactions:

1. Does a reaction always run its full course so that all the reactants are used up?

   • When all the reactants in a reaction are used up the reaction is said to have gone to completion. However, in some reactions not all the reactants are used. \(\color{red}{\textbf{Not all reactions go to completion}}\).

2. Does a reaction always proceed in the same direction or can it be reversed? In other words, does a reaction always proceed reactants \(\to\) products, or is it possible that a reaction will reverse and go products \(\to\) reactants?

   • Reactions that go to completion are irreversible. However, in some reactions the reactants form products (in a \(\color{red}{\textbf{forward reaction}}\)), and the products can change back into reactants (in a \(\color{red}{\textbf{reverse reaction}}\)).

3. Can a reaction reach a point where reactants are still present, but there does not seem to be any further change taking place in the reaction?

   • In all reactions, as the amount of reactant in a reaction decreases the product is formed more slowly. In a reversible reaction, as the amount of product increases the reactant is formed more quickly. Eventually the rate of the forward reaction (reactants \(\to\) products) equals the rate of the reverse reaction (products \(\to\) reactants).

     At this point there \(\color{red}{\textbf{are still reactants present}}\) but there does not appear to be any further change taking place. The reaction is said to be in \(\color{blue}{\textbf{chemical}}\) \(\color{blue}{\textbf{equilibrium}}\).

Chemical equilibrium is the state of a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction. While a reaction is in equilibrium the concentration of the reactants and products are constant.

There are many examples of chemical equilibrium all around you. One example is a bottle of fizzy cooldrink. In the bottle there is carbon dioxide (\(\text{CO}_{2}\)) dissolved in the liquid. There is also \(\text{CO}_{2}\) gas in the space between the liquid and the cap. There is a constant movement of \(\text{CO}_{2}\) from the liquid to the gas phase, and from the gas phase into the liquid. However, if you look at the bottle there does not appear to be any change. The system is in equilibrium.

\(\text{CO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{l})\) \(\rightleftharpoons\) \(\text{H}_{2}\text{CO}_{3}(\text{aq})\)

Without chemical equilibrium life as we know it would not be possible. Another example of equilibrium in our everyday lives goes on within our very bodies. Haemoglobin is a macromolecule that transports oxygen around our bodies. Without it we would not survive. The haemoglobin has to be able to take up oxgen, but also to release it and this is done through changes in the chemical equilibrium of this reaction in different places in our bodies.

haemoglobin(aq) + \(4\text{O}_{2}(\text{g})\) \(\rightleftharpoons\) haemoglobin(\(\text{O}_{2})_{4}\)(aq)

Fact:

Haemoglobin, inside red blood cells in the lungs, bonds with oxygen. This oxyhaemoglobin moves with the red blood cells through the blood stream to cells throughout the body.

Optional Video: Demonstration of Simulated Chemical Equilibrium

This video is a good demonstration of how a reaction reaches equilibrium with time.