4.7 Acid/base equilibria - Introduction

Ideas about the nature of acids and bases have developed over the centuries. Simple ideas about acids were:

·         Corrosive liquids

·         Have a sour taste

·         Give characteristic colors with indicators

Acids also were known to react with:

·         Bases (and alkali) to give a salt and water – neutralization reaction

·         Carbonates to give salt, carbon dioxide (observed as effervescence) and water

·         Some metals to  give a salt and hydrogen

Ideas have developed about acids and acidity:

 Arrhenius theory

The theory of electrolytic dissociation: it states that when acids, bases or salts dissolve in water they split up partially or completely into their ions.

·         Arrhenius acids give hydrogen ions in aqueous solution

HA _(aq) ⇌   H⁺ _{(aq) +} A^- _(aq)

·         Arrhenius bases give hydroxide ions in aqueous solution

BA _(aq) + H₂O _(aq) ⇌   BH⁺ _{(aq) +} OH^- _(aq)

Limitations of the theory: some acid-base reaction took place in solvents other than water or did not even need water to react. The theory restricts reactions to aqueous solutions only.

 BrØnsted-Lowry theory

The theory proposed that:

·      An acid is a proton (H⁺) donor

·         A base is a proton acceptor

Conjugate acid-base pairs

·         Acid-base Equilibria involve the transfer of protons

·         Each acid has a conjugate base

·         Each base has a conjugate acid

   HA _(aq) + H₂O _(l) ⇌   H₃O⁺ _{(aq) +} A^- _(aq)

The acid HA is transformed into A^- after donating a proton to the base, H₂O.  In the reverse reaction, the A^- ion acts as a base by accepting a proton to form the acid again. Therefore, HA and A^-   are referred to as conjugate acid-base pairs. Similarly, H₂O and H₃O⁺ are conjugate acid-base pairs.