Chemical equations can be written unbalanced, that is, with an unequal number of atoms in the reactants versus the products, or balanced, with an equal number of atoms on each side of the equation. A balanced chemical equation adheres to the law of conservation of matter. This law states the number of atoms before and after a chemical reaction are equal to ensure atoms are not created or destroyed. Therefore, a balanced equation better represents the way the reaction takes place in the real world. In the example of the combustion of methane, the unbalanced chemical equation reads CH4 + O2 → CO2 + H2O. In this equation, there are four hydrogen atoms in the reactants, but only two hydrogen atoms in the products. Adding a coefficient of two to H2O corrects the number of hydrogen atoms, but means that the number of oxygen atoms is no longer balanced. Thus, a coefficient of two must be added to oxygen in the reactants as well. The equation is now fully balanced.
Chemical equilibrium is a state of balance in which forward and reverse chemical reactions are happening at the same rate. Chemical equilibrium plays a role in the reversibility of a chemical reaction. The concentration ratio of reactants and products is also stabilized during equilibrium.The arrow's direction in a chemical equation indicates how a chemical reaction proceeds. Chemical reactions are irreversible when the arrow only points toward the products. This indicates that the reaction will proceed until all reactants are used. For example, when sodium hydroxide (NaOH) is combined with hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2O) are produced. Sodium chloride and water do not react to produce sodium hydroxide and hydrochloric acid.