Arrhenius concept of acids and bases

Arrhenius was first to define acids and bases in aqueous solution. An acid is a substance that dissociate in aqueous solution to give hydrogen ions. All mineral acids like HCl, HNO₃ and H₂SO₄ containing hydrogen when dissolved in water dissociates completely into H+ ions as:

HCl → H++ Cl⁻

HNO₃ → H+ + NO3

H₂SO₄ → 2 H+ + SO₄²-

Arrhenius name these acids which dissociates completely in water as strong acids.

Other acids like acetic acid CH₃COOH, carbonic acid H2CO3 and phosphoric acid H3PO4 when dissolved in water does not dissociate completely into ions. As a result an equilibrium is setup between ions and and undissociated acid molecules as:

CH₃COOH ⇔  H+ + CH₃COO-

H2CO3 ⇔ 2 H+ + CO2−

H3PO4 ⇔ 3 H+ + PO4 3

Arrhenius named these as weak acids.

According to Arrhenius a base is a substance which contains hydroxyl group and which when dissolved in water gives hydroxyl ion OH- as

NaOH →Na+ + OH⁻

KOH →  K+ + OH⁻

Arrhenius named these bases which dissociate completely as strong bases.

The substances which do not completely dissociate into hydroxide ions named as weak bases. For example- NH4OH, Mg(OH)2 , Al(OH)3 do not dissociate completely to give OH⁻ ions.

NH4OH    ⇔  NH₄⁺    +  OH⁻

Mg(OH)2  ⇔  Mg2+   +  OH⁻

Al(OH)3    ⇔  Al3+     +  OH⁻

Arrhenius named these as weak bases.

H+ obtained from acid and OH⁻ from bases combine together to form unionized water. Arrhenius described this combination of H+ and OH⁻ ions as neutralization reaction.

Click here for Limitations of Arrhenius Theory.

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