Strength of acids

When an excess of a solution of any strong base, such as NaOH, is added to solid aluminum hydroxide, the Al(OH)3 acts as an acid and dissolves. The equation for the reaction is 

Other amphoteric metal hydroxides undergo similar reactions. 

The ease of ionization of binary protic acids depends on both (1) the ease of breaking H — X bonds and (2) the stability of the resulting ions in solution. Let us consider the relative strengths of the Group 7A hydrohalic acids. Hydrogen fluoride ionizes only slightly in dilute aqueous solutions. 

and HI, however, ionize completely or nearly completely in dilute aqueous solutions because the H — X bonds are much weaker. 

A weak acid may be very reactive. For example, HF dissolves sand and glass. The equation for its reaction with sand is 

As fluorine is the most electronegative element, it could be expected that the introduction of a fluorine atom or polyfluoroalkyl group into the carbon chain of an organic acid, such 

The strong inductive effect of fluoroalkyl groups has a corresponding additive acidifying effect on alcohols (Table 4.2). For instance, perfluoro-t-butanol is of the same order of acidity as acetic acid. The hydrates of fluoroketones are also remarkably acidic [10]. 

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As we shall see later, the limitations imposed by most solvents may prevent us from being able to utilise the very strong basic characteristics of some anions. However, at this point it is more useful to consider other factors affecting the strengths of acids. 

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

Table 11-1V. relative strengths of acids in aqueous solution... 

We can use this more general view to discuss the strengths of acids. In our generalized acid-base reaction (52), the proton transfer implies the chemical bond in HB, must be broken and the chemical bond in HB2 must be formed. If the HB, bond is easily broken, then HB, will be a strong acid. Then equilibrium will tend to favor a proton transfer from HB, to some other base, B2. If, on the other hand, the HB, bond is extremely stable, then this substance will be a weak acid. Equilibrium will tend to favor a proton transfer from some other acid, HB2, to base B, forming the stable HB, bond. 

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. 

Predict the relative strengths of acids from molecular structures (Example 10.6). 

Sections 10.9 and 10.10 discuss the relationship between molecular structure and the strengths of acids. The same ideas can be applied to bases, (a) Explain the relative strengths of the bases OH, NH,, and CH. (see Table 10.3). 

This is always the case for any two acids, and by measuring the positions of the equilibrium the relative strengths of acids and bases can be determined. Of course, if the two acids involved are close to each other in strength, a measurable reaction will occur from both sides, though the position of equilibrium will still be over to the... 

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

Figure 12. Effect of calcium on jelly strength of acid-demethylated pectinates...

Solutions of stabilised chlorine dioxide have been claimed to provide a simpler system for delivering chlorine dioxide. However the effectiveness of such solutions depends on the amount of chlorine dioxide released, which depends on the strength of acid used and length of time for reaction(8). 

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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