summary

In this chapter we have developed several models of acid-base behavior and have applied fundamental equilibrium principles to calculate the pH values for solutions of acids and bases.

The strong electrolytes completely dissociate in aqueous solution, but the apparent dissociation degree is less than 100％. The interionic attractions in an aqueous solution prevent the ions from behaving as totally independent particles.

Arrhenius postulated that acids produce H+ ions in solutions and bases produce  OH- ions. The Brönsted-Lowry model is more general: an acid is a proton donor, and a base is a proton acceptor.

A conjugate base is everything that remains of the acid molecule after the proton is lost. A conjugate acid is formed when a proton is transferred to the base. Two substances related in this way are called a conjugate acid-base pair.

The equilibrium expression for the dissociation of an acid in water is

is called the acid dissociation constant. A small value of  denotes a weak acid, which does not dissociate to any great extent in aqueous solution. The greater the values of , the stronger the acidity. In a similar way it is possible to define the conjugate base dissociation constant , The relation of   and is:. Strong acids form weak conjugate bases, and weak acids form strong conjugate bases.

A polyprotic acid contains more than one ionizable hydrogen atom. Such acids dissociate in steps, with a separate dissociation constant for each step.

The extent of the ionization of a weak acid or weak base can be reduced by adding a strong electrolyte that provides an ion common to the equilibrium. This phenomenon is called the common-ion effect. The addition of a strong electrolyte sightly increases the degree of ionization of a weak acid, this effect is called the salt effect.

The simplest formula of calculation in a weak acid solution is: . And the  calculation is similar to ,.

Amphoteric substances behave either as an acid or as a base. The  calculation is given by the common formula:.