The Strength of Acids and Bases

1 Specific and general acid catalysis, p. 74 3.3.2 Specific and general base catalysis, p. 75. 

Here water is acting as a base by accepting a proton, and is thereby converted into its so-called conjugate acid, H30 , while the acid, H2S04, by donating a proton is converted into its conjugate base, HS04e. 

The more generalised picture provided by Lewis, who defined acids as molecules or ions capable of coordinating with unshared electron pairs, and bases as molecules or ions which have such unshared electron pairs available for coordination, has already been referred to (p. 29). Lewis acids include such species as boron trifluoride (1) which reacts with trimethylamine to form a solid salt (m.p. 128°)  

Other common examples are aluminium chloride, tin(rv) chloride, zinc chloride, etc. We shall, at this point, be concerned essentially with proton acids, and the effect of structure on the strength of a number of organic acids and bases will now be considered in turn. Compounds in which it is a C—H bond that is ionised will be considered subsequently (p. 270), however. 

The strength of an acid, HA, in water, i.e. the extent to which it is dissociated, may be determined by considering the equilibrium  

Very werik acids, those with pK greater than 16, will not be detectable as acids at all in water, as the [H3O ] they will produce therein will be less than that produced by the autolysis of water itself  

What is the difference between a strong acid such as hydrochloric acid, sold in hardware stores as muriatic acid and used to clean brick and concrete, and a weak acid such as the acetic acid in vinegar The strength of an add or base is determined by the extent of ionization in aqueous solution. The greater the ionization, the stronger the acid or base. 

Many common acids like sulfuric acid, hydrochloric acid, and nitric acid react completely with water to give hydronium ions and anions. Therefore, like ionic compounds (Section 5.2), they are electrolytes and because they are entirely converted to ions in solution, they are strong electrolytes. The solutions of these strong acids have high concentrations of hydronium ions and are very acidic (Table 9.2). The equation for the reation of a strong acid such as nitric acid with water is written with a single arrow, which indicates that the reaction goes to completion. 

Many acids establish an equilibrium with water at a point where not all the acid molecules have been converted to ions. These are the weak acids— they are only slightly ionized in aqueous solution because an equilibrium is established (Section 8.4). Acetic acid is a typical weak acid  

The GHgGOOH molecules undergo ionization while HjO and GH GOO ions simultaneously recombine to give GH GOOH molecules and water. Since only a few percent of the acetic acid molecules are ionized at any given time, aqueous solutions of acetic acid contain mostly acetic acid molecules with a few hydronium ions and acetate anions, and are only weakly acidic. 

As previously explained, the metal hydroxides are all ionic compounds and, therefore, strong bases because when dissolved in water, they are completely converted to ions (to the extent that they are soluble). Ammonia is a common weak base, a base that establishes an equilibrium with water to produce a solution with relatively few ammonium and hydroxide ions. 

This feature leads to convenient expressions for quantities that measure the strengths of acids and bases and the extent of protonation of bases and deprotonation of acids. 

When we set flH,o = 1 for all the solutions we consider, the resulting equilibrium constant is called the acidity constant, K, of the acid HA  

In elementary applications, the activities are replaced by the numerical values of the molar concentrations, and we write 

Data are widely reported in terms of the negative common (base 10) logarithm of this quantity  

It follows from eqn 4.10a (A G = -RTlnK) that is proportional to Afi for the proton transfer reaction. More explicitly, pK = AfG /(Rrin 10), with 

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In this chapter, we see what acids and bases are and why they vary in strength. We shall use thermodynamics, particularly equilibrium constants, to discuss the strengths of acids and bases quantitatively and thereby develop our insight into their behavior. We then use our knowledge of equilibria involving acids and bases to examine systems in which more than one equilibrium is taking place simultaneously. 

The strength of acids and bases is measured on a pH (potential of hydrogen) scale pH = -logio [H+]... 

It has already been mentioned several times that the strength of acids and bases is expressed by their dissociation constants KA and KB. This is... 

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