What happens when charcoal burns? The carbon (C) that makes up the charcoal reacts with the oxygen found in air (O2) and give carbon dioxide CO2. This chemical reaction is represented as follows:
C and O are called the reactants and CO2 is the product of the reaction.
Oxygen and hydrogen in presence of a flame explode forming water (H2O):
Gaseous nitrogen (N2) and gaseous hydrogen (H2) react to give ammonia (NH3):
The double arrow indicates that the reaction can take place both ways. Hydrogen and nitrogen will be in equilibrium with ammonia. Which means that their concentrations will not change with time.
This part is somewhat technical but very useful to understand chemical reactions that depend on the pH, which allows one to understand why the action of sulphite depends on the pH.
When there is an equilibrium, concentrations are constant over time. But what will be the equilibrium value of the concentration? Concentrations are written with brackets: the concentration of H+ for example is written [H+].
The molecule of water (H2O) can for instance split into H+ and OH-:
The law of mass action says that the product of the concentrations of the products [H+]*[OH-] divided by the product of the concentrations of the reactants (in this case there is one only) [H2O] is equal to a constant. That is, is constant. This constant is called the equilibrium constant of the reaction and, in general, it is noted K. Water is by far the main component of the solution, so one can say that [H2O] is constant and equal to 1. We can write that [H+][OH-] = K. we also define pK = - log K or, which is equivalent, K = 10-pK.
How to get K? Chemists do this for us. In the literature, the constant of reaction is sometimes given with the reaction. When it is not, there is not much we can do to get it.