IONIZATION OF ACIDS

We know that acids are proton donors in aqueous solutions- that is, they provide hydrogen cations when dissolved in an aqueous solution- (since hydrogen ions have no neutrons and a singly positive charge due to the loss of an electron, they are protons). The factor that makes these compounds acidic is the dissociation (or breaking up) of the acids to provide these Hydrogen ions in solution. This can be referred to as the ionization of acids, which is simply the dissociation of an acid into ions. This can be seen in the equation below when hydrogen chloride (a gas) is added to water.

\( HCl_{(g)} + H_2 O_{(l)} \rightarrow H^+{(aq)} + Cl^-{(aq)} \)

Note: Hydrogen Chloride is anhydrous as a pure gas, and can be termed ‘Hydrochloric Acid’ only when dissolved in a solution where it dissociates into Hydrogen and Chloride ions.

But where did the water molecule go, you may ask. Well, in actuality, after the acid is dissolved, the Hydrogen ion(s) provided by the acid is accepted by the oxygen atom in a water molecule, as the oxygen atom has a partially negative charge, and as a result, bonds with the hydrogen ion to form a hydronium ion (or hydroxonium ion- both refer to the same compound).

\( Hydronium: H_3O^+ \)

Following this, the actual reactions that occur can be written as:

\( HCl_{(g)} + H_2O_{(l)} \rightarrow H^+{(aq)} + Cl^-{(aq)} \)

\( H^+{(aq)} + H_2O{(l)} \rightarrow H_3O_{(aq)}^+ \)

However, the latter reaction is understood, as all acids undergo the same reaction after dissociation, and so this is not usually written.

ACID ANHYDRIDES

An acid anhydride is a compound that reacts with water to form an acidic solution, the compound usually being a nonmetal oxide. Some examples of acid anhydrides are:

Seen Below: Nitrogen dioxide, Carbon dioxide, Sulfur trioxide, Dinitrogen Pentoxide, Sulfur dioxide

\( NO_2, CO_2, SO_2, SO_3, N_2O_5 \)

Chemical reactions between acid anhydrides and water:

Nitrogen dioxide and water react, forming nitrous acid and nitric acid:

\( 2NO_{2(g)} + H_2O_{(l)} \rightarrow HNO_{2(aq)} + HNO_{3(aq)} \)

Carbon dioxide and water react, forming carbonic acid:

\( CO_{2(g)} + H_2O_{(l)} \rightarrow H_2CO_{3(aq)} \)

Sulfur dioxide and water react, forming sulfurous acid:

\( SO_{2(g)} + H_2O_{(l)} \rightarrow H_2SO_{3(aq)} \)

Sulfur trioxide and water react, forming sulfuric acid:

\( SO_{3(g)} + H_2O_{(l)} \rightarrow H_2SO_{4(aq)} \)

Dinitrogen pentoxide and water react, forming nitric acid:

\( N_2O_{5(s)} + H_2O_{(l)} \rightarrow 2HNO_{3(aq)} \)