Water as a Bronsted acid or base

A Bronsted acid can act as a proton donor, and a Bronsted base can function as a proton acceptor. 

Equilibrium 6.1 illustrates that water can function as both a Bronsted acid and a Bronsted base. In the presence of other Bronsted acids or bases, the role of water depends on the relative strengths of the various species in solution. When HCl is bubbled into water, the gas dissolves and equilibrium 6.3 is established. 

In dealing with acid-base equilibria in aqueous solution, three equilibrium constants are of special significance  

Essential equations relating to acid-base equilibria are listed below. Expressions involving concentrations are approximations, since we should strictly be using activities (see main text). Moreover, for a weak acid, HA, we assume that the concentration in aqueous solution of the dissociated acid at equilibrium is negligible with respect to the concentration of acid present initially similarly for a weak base. 

Review example 1 Calculate the pH of aqueous 0.020 m acetic acid (Kg = 1.7 x 10 ) 

In an aqueous solution of NH3, water behaves as a Bronsted acid, donating H+ (equation 7.4). In equation 7.4, [NH4]+ is the conjugate acid of NH3, while H2O is the 

Equation 7.5 gives the value of K for equihbrium 7.4 and shows that NH3 acts as a weak base in aqueous solution. This is explored further in worked example 7.2. 

Worked example 7.2 Manipulating equilibrium constant data 

These exercises all refer to the equilihiia in the worked example. 

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