Chemical reactions are always written with the reactants on the left and the products on the right. Many chemical reactions can and do proceed simultaneously in both a forward and a reverse direction, often at different rates or to different extents. When a chemical reaction is proceeding in both the forward and reverse directions at equal rates, a condition of chemical equilibrium is said to exist. Although an equilibrium chemical reaction is actually proceeding simultaneously in both directions, the species on the left are still referred to as the reactants and the species on the right are still referred to as the products. If the molecular formula of a compound is unknown, or if it has none, the compound is represented by its correct empirical formula.
Chemists always prefer to write a balanced chemical reaction whenever it is possible to do so.
Although a balanced chemical reaction is most commonly wrotten in the simplest possible form, there is no requirement that this be done. The second form shown in the above example is unusual, but still correct and balanced. The fifth form shown in the example above is both correct and common. The first form in the above example is the most common, and the simplest, form of this balanced chemical reaction. The chemical nature of the species is always given at least in terms of its molecular formula.