Question

What is the difference between a strong acid and a weak acid and a strong base and a weak base?

Answer 1

The strenght of an acid or base is related to its reactivity and stability, and depends on how ionized they are when dissolved in water.

According to the Bronsted-Lowry theory of acids and bases, when an acid is dissolved in water, the proton is transferred to a water molecule to produce a hydroxonium cation and an anion which depends on the acid we used initially.

This is the general equation:

Reactions like the one above are reversible. Nevertheless, in some cases, the acid donates its proton so easily that the reaction becomes one-way; therefore, a strong acid is the one who completely (100% or close) ionizes. For instance, when hydrogen chloride dissolves in water, it becomes hydrochloric acid and the totally of its molecules are ionized as follows:

So it is virtually impossible that you can find molecules of HCl after dissolution. All of it will be hydroxonium cation and chloride anion.

And a weak acid is not totally, but partially ionized in water. The reaction of dissolution is now reversible. For instance, ethanoic acid has about 1% of its molecules in an ionized state while the rest of them remains as an acid.

So, the main difference between a strong and a weak acid is its ability to fully ionize in water (or not!). It is the same principle with bases. The more ionizable the base is, the more strenght it has!