Chemical Equilibrium

Chemical Equilibrium
In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the system. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are generally not zero, but equal. Thus, there are no net changes in the concentrations of the reactant(s) and product(s). Such a state is known as dynamic equilibrium.
In discussing stoichiometry, we often assume that all reactions go to completion and that the amount of product(s) obtained depends on the amount and stoichiometric relationship of the limiting reactant to product(s). Some reactions go to completion. The following class reactions are examples of these types of reactions:
1. Acid-base reactions;
2. Precipitation reactions;
3. Oxidation-reduction reactions (electron transfer reactions).
4. Reactions in which nonelectrolytes are formed.
However, many reactions, especially those involving gases, generally do not go to completion. In fact, reverse reactions occur before the limiting reactants get completely used up. All gaseous reactions in a closed system undergo reversible processes and the following are some examples:
N2(g) + 3H2(g) ? 2 NH3(g);
In aqueous solutions, the dissociations of weak acids and bases and the dissolution of slightly soluble salts are reversible processes.
NH3(aq) + H2O(l) ? NH4+(aq) + OH- (aq);