what is the principal difference between a strong and weak acid.?

All acids can ionized in water to give hydrogen ions (or hydronium ions). The principal difference between a strong and weak acid is that they have different degrees of ionization to form hydrogen ions (or hydronium ions).

A strong acid can completely ionizes in water to give hydrogen ions (or hydronium ions).
A weak acid can only incompletely ionizes in water to give hydrogen ions (or hydronium ions).

Firstly you should know that all acids that are weak in water can be strong in other solvents. For example, HCN is weak in water but strong in NH3. So its properties in these solvents are absolutely different. But now i`m telling about water solutions.

Strong acids can dissociate fully, while weak acids don`t dissociate at all or do it partially. There`s a difference between them in degree of dissociation. This degree is ration of dissociated molecules to the total number of molecules. Strong acids have degree of dissociation that tends to one (and it is usually about 30%). Examples: HNO3, HCl, H2SO4 etc.

For weak acids this magnitude tends to zero (so there are not more than 3% of dissociated molecules). E.g.: H2S, CH3COOH, H2CO3 etc.

Generally all popular acids and their type you can see in handbook tables. But it`s necessary to know dissociation constant to define the strength of acid by calculating. E.g., for HCl (HCl = H(+) + Cl(-) in solution) dissociation constant is

K(d) = [H(+)][Cl(-)]/[HCl]
When K(d) is big, there are lots of ions in solution. So acid is strong. When K(d) is small, acid is weak.


If it is interesting for you, also we often compare the strength of acids by a reaction: an acid that is stronger can push out a weaker one from the solution of its salt.

E.g. 2HCl + Na2CO3 = 2NaCl + H2CO3 (that is CO2 + H2O). So this reaction goes to the end because there`s a gas product. You can get it from H2CO3 – this acid is so weak that it degrades to carbon dioxide CO2 and H2O immediately. HCl is a strong acid so it can push out H2CO3 from its salt Na2CO3. H2CO3 can`t do the same. It`s different for them too.

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