Chemistry » Acid-Base and Redox Reactions » Acids And Bases Continued

Strong and Weak Acids and Bases

Strong and weak acids and bases

The term dissociate should be used for ionic solids that break into their component parts when dissolved in water. Ionise should be used when a reaction with water takes place and ions, that were not present in the original compound, are formed.

For example, \(\text{NaOH}\) is an ionic solid composed of \(\text{Na}^{+}\) ions and \(\text{OH}^{-}\) ions. Upon dissolution it dissociates into its component parts.

On the other hand, \(\text{HCl}\) is a covalent molecule. When dissolved in water it ionises to form \(\text{H}^{+}\) and \(\text{Cl}^{-}\) ions.

In general, strong bases dissociate, bases such as ammonia and organic amines ionise. All acids ionise.

Strong acids and bases

A strong acid or base is one that will almost completely dissociate or ionise to form ions in solution. That is, a large percentage of the moles of a strong acid or base will form ions when added to water.

Definition: Strong acid and strong base

A strong acid or base is one that almost completely dissociates to form ions in solution.

\(\text{HCl}\) is a strong acid. For example if \(\text{100 000}\) molecules of \(\text{HCl}\) are added to water and \(\text{90 000}\) ionise to form \(\text{H}^{+}\) and \(\text{Cl}^{-}\) ions, then there is a large amount of ionisation. This is what makes \(\text{HCl}\) a strong acid.

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The unequal double arrows in the reaction equation indicate that the equilibrium position favours the formation of ions.

There are three strong acids that we commonly find: \(\text{HCl}\) (hydrochloric acid), \(\text{HNO}_{3}\) (nitric acid) and \(\text{H}_{2}\text{SO}_{4}\) (sulfuric acid). Two strong bases that are commonly found are: \(\text{NaOH}\) (sodium hydroxide) and \(\text{KOH}\) (potassium hydroxide).

Weak acids and bases

A weak acid or base is one where only a small percentage of molecules will dissociate to form ions in solution.

Definition: Weak acid and weak base

A weak acid or base is one where only a small percentage of molecules dissociate to form ions in solution.

\(\text{HF}\) is a weak acid. For example if \(\text{100 000}\) molecules of HF are added to water and only \(\text{100}\) ionise to form \(\text{H}^{+}\) and \(\text{F}^{-}\) ions, then there is only a small amount of ionisation. This is what makes \(\text{HF}\) a weak acid.

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The unequal double arrows in the reaction equation indicate that the equilibrium position does not favour the formation of ions.

An example of a weak base is \(\text{Mg}(\text{OH})_{2}\) which will only dissociate partially into \(\text{Mg}^{2+}\) and \(\text{OH}^{-}\) ions.

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