E and C parameters to many Lewis acids and bases. Each acid is characterized by an EA and a CA. Each base is likewise characterized by its own EB and CB. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is −ΔH = EAEB + CACB + WThe W term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base. The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths. Acid–base equilibrium The reaction of a strong acid with a strong base is essentially a quantitative reaction. For example, HCl(aq) + Na(OH)(aq) → H2O + NaCl(aq) In this reaction both the sodium and chloride ions are spectators as the neutralization reaction, H+ + OH− → H2O does not involve them. With weak bases addition of acid is not quantitative because a solution of a weak base is a buffer solution. A solution of a weak acid is also a buffer solution. When a weak acid reacts with a weak base an equilibrium mixture is produced. For example, adenine, written as AH, can react with a hydrogen phosphate ion, . AH + A− + The equilibrium constant for this reaction can be derived from the acid dissociation constants of adenine and of the dihydrogen phosphate ion. [A−] [H+] = Ka1[AH] [] [H+] = Ka2[] The notation [X] signifies "concentration of X". When these two equations are combined by eliminating the hydrogen ion concentration, an expression for the equilibrium constant, K is obtained. [A−] [] = K[AH] []; K'' = Acid–alkali reaction An acid–alkali reaction is a special case of an acid–base reaction, where the base used is also an alkali. When an acid reacts with an alkali salt (a metal hydroxide), the product is a metal salt and water. Acid–alkali reactions are also neutralization reactions. In general, acid–alkali reactions can be simplified to (aq) + (aq) → by omitting spectator ions. Acids are in general pure substances that contain hydrogen cations () or cause them to be produced in solutions. Hydrochloric acid () and sulfuric acid () are common examples. In water, these break apart into ions: → (aq) + (aq) → (aq) +