Acids, bases and salts

When acids and bases are mixed in the right proportion they react with each other to give salts and water

NaOH + HCl → NaCl + H₂O

Arrhenius Concept of Acids and Bases:

According to Arrhenius theory, acids are substances that dissociates in water to give hydrogen ions H ⁺ (aq) and bases are substances that produce hydroxyl ions OH^– (aq).

HNO₃ (aq) → H⁺ (aq) + NO^-₃ (aq)

A bare proton, H+ is very reactive and cannot exist freely in aqueous solutions. Thus, it bonds to the oxygen atom of a solvent water molecule to give trigonal pyramidal hydronium ion, H₃O^+.

Bases are defined as a compound or element that releases hydroxide (OH^-) ions into the solution.

LiOH (s) → Li⁺ (aq) + OH^- (aq)

In this reaction lithium hydroxide (LiOH) dissociates into lithium (Li⁺) and hydroxide (OH^-) ions when dissolved in water.

Arrhenius concept of acid and base, however, suffers from the limitation of being applicable only to aqueous solutions and also, does not account for the basicity of substances like, ammonia which do not possess a hydroxyl group.

The Brönsted-Lowry Acids and Bases:

According to Brönsted-Lowry theory, acid is a substance that is capable of donating a hydrogen ion H⁺ and bases are substances capable of accepting a hydrogen ion, H⁺. In short, acids are proton donors and bases are proton acceptors.

The acid-base pair that differs only by one proton is called a conjugate acid-base pair. Therefore, OH^– is called the conjugate base of an acid H₂O and H₂CO₃ is called conjugate acid of the base HCO₃^-.

If Brönsted acid is a strong acid then its conjugate base is a weak base and viceversa.

Water acts both as an acid and a base. In case of reaction with acids, water acts as a base while in case of bases, it acts as an acid by donating a proton.

Lewis Acids and Bases:

An acid as a species which accepts electron pair and base which donates an electron pair.

However, in Lewis concept many acids do not have proton. A typical example is reaction of electron deficient species BF₃ with NH₃.

BF3 does not have a proton but still acts as an acid and reacts with NH₃ by accepting its lone pair of electrons. The reaction can be represented by,

BF₃ + :NH₃ → BF₃:NH₃

Electron deficient species like AlCl₃, Co³⁺, Mg²⁺, etc. can act as Lewis acids while species like H₂O, NH₃, OH^– etc. which can donate a pair of electrons, can act as Lewis bases.