Understanding equilibrium – the basics

A good example of a reversible reaction is that of blue hydrated copper sulfate that can be heated to form white anhydrous copper sulfate. The heating process drives off the water. This reaction can easily be reversed by adding water to the white anhydrous copper sulfate. The first reaction is endothermic as it requires heat energy to drive off the water. The reverse reaction is exothermic and heat will be evolved when the anhydrous copper sulfate reacts with the water.

In a reversible reaction all of the reactants will be converted to products and vice versa. An equilibrium reaction it’s very similar to this idea as reactants can be converted to products and vice versa but importantly, the reaction never goes to completion. There will always be reactants and products in the reaction mixture.

A classic example of a reaction at equilibrium is the Haber process. Nitrogen and Hydrogen I reacted and the pressure in the presence of an iron catalyst to form ammonia. If this reaction were to be carried out in a sealed container that initially contained just hydrogen and nitrogen we would observe, over time, the percentage of nitrogen and hydrogen decreasing and the percentage of ammonia increasing up to a point where the relative amount of each would not change over time. At this point we would say that equilibrium had been reached.

When equilibrium is reached it is important to appreciate that the reaction has not finished. What is actually occuring is that the rate of the reaction moving from products to reactants is occuring at the same rate as the reaction moving from reactants to products. As the reaction is still occurring the equilibrium is said to be dynamic.

It is also important to realize that at equilibrium there are not equal concentrations of reactants and products. The concentration of reactants and products will remain the same but will not be equal.

It is possible to increase or decrease the concentration of reactants and products using Le Chatelier’s principle. This principle will involve manipulating the temperature, pressure, concentration of reactants and products or by using a catalyst, but this is more advanced work!

Le Chatelier

By http://en.wikipedia.org/wiki/Image:Lechatelier.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=428795