PROTON AFFINITY OF ACIDS AND BASES

PROTON AFFINITY OF ACIDS AND BASES The division between acids and bases is not a very sharp one it has been remarked already that the borderline between the two groups is formed by the amphoteric compounds, that have both weak acid and weak basic properties, and thus behave as acids in the presence of strong bases and as bases in the presence of strong acids. To a 

Being the opposite of an acid, a base will be defined as a compound that has a tendency to combine with protons. In this definition the base9 in an alkaline solution is the OH ion. This ion is one of the strongest bases known to exist. When combining with a proton it forms water, that itself is a weak base, since it is able to add one more proton to form an OHJ, hydronium ion, and a weak acid at the same time, since water can dissociate into OH and H+ ions. Water being a base, too, the actual dissociation reaction will be 

In the series NaOH, Mg(OH)2, Al(OH)3, Si(OH)4, PO(OH)3. . ., the basic properties decrease. The OH ion, attached to the Na+ ion in NaOH, is such a strong base that it reacts with the protons of any acid, i.e. any compound that can give off protons. In PO(OH)3 the basicity of the OH ions, under the influence of the P5+, is so far decreased that it will not attract protons from acids of normal acid strength. If, however, unusually strong acids were available, there would be the possibility that PO(OH)3 would still withdraw a proton from this acid and form an ion P(OH) J. 

Such an acid is actually known to exist. Perchloric acid and phosphoric acid form the compound 

Since both ions in this compound are very large, the energy required for dissociation into P(OH) and C104 will be small, and the compound would be expected to be completely ionized when dissolved. However, the solution in water will not be stable, since PO(OH)3 is a very weak base, even weaker than water. In the presence of the latter compound it will react in such a way that one proton is transferred to a water molecule 

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