Why do acids and bases dissociate

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Strength of acids and bases

Acids and bases do not dissociate completely in water, but are also present as undissociated molecules. The degree of dissociation depends on the strength of the acid. Let us consider the dissociation equilibrium of an acid (the same applies to bases):

For strong acids, the equilibrium is far to the right, i.e. the acid is almost completely present as and. She is completely dissociated. A weak acid, on the other hand, is only slightly dissociated and is mostly present as HA.

The law of mass action also applies to the dissociation reactions:

Since the concentration of the water hardly changes in a dilute solution, it is included in the constant.

Definition of the acid constant:

The same procedure is used for the bases:

= Base constant

The values ​​of and measure whether an acid or a base is strong or weak. The negative decadic logarithm is usually given in tables:

Negative decadic logarithm of the acid and base constants:
= - log and = - log

A strong acid (equilibrium on the right) has a large acid constant and thus a small value and vice versa. and a conjugate acid-base pair are related via the ionic product of water. If you write down the reactions and multiply the acid by the base constant you get. Written logarithmically:

Relationship between the negative decadic logarithms:
+ = 14