Question

In terms of acidic strength, which one of the following is in the correct increasing order?
1. Water < Acetic acid < Hydrochloric acid
2. Water < Hydrochloric acid < Acetic acid
3. Acetic acid < Water < Hydrochloric acid
4. Hydrochloric acid < Water < Acetic acid

Answer 1

Correct Answer - Option 1 : Water < Acetic acid < Hydrochloric acid

Concept:

The acidity of compounds:

• The acidity of compounds is determined by their ability to donate hydrogen ions in solution.
• The greater the ease of donation or liberation of the hydrogen ions, the stronger is the acid.
• The acidic proton of the compound is generally attached to an electronegative atom.
• The strength of the acidity is greatly influenced by the substituents or groups attached.
• The strength of an acid is measured by its pKa value. Lower the pKa, stronger is the acid.

Factors influencing the acid strength-

• Inorganic acids are much stronger than Organic acids.
• The stability of the conjugate base-
  • if the negative charge is resonance stabilized in the conjugate base, then the compound is more acidic compared to the compound whose conjugate base has the charge localized.
• Electronegative substituents or groups like F, Cl, Br, I increase the acidity via inductive electron withdrawal (-I).
• Electron donating groups such as - OR, -Me, etc. decrease the acidity via the +R and +I effect.
• Electron withdrawing groups such as NO2,-CF3, -COOH, -CN increases the acidity via the –R effect.
• Hydrogen attached to sp2 Carbon is more acidic than hydrogen attached to sp3 carbon.
• The acidity order is sp> sp2>sp3.

Explanation:

• Inorganic acids are much more acidic than organic acids.
• HCl is an inorganic acid and is thus stronger than Acetic acid because HCl dissociates completely to give H⁺ions in solution.
• Acetic acid is a weak acid and it dissociates feebly in solution to produce acetate ions CH₃COO^-  and H⁺ ions.
• The conjugate base of acetic acid, acetate ion is resonance stabilized.
• As it is resonance stabilized, the recombination to CH3COO- + H+ → CH₃COOH by taking up the proton is difficult in solution.
• On the other hand, when water H₂O donates a proton, it gives hydroxide ion OH^- and H⁺.
• The solvation of Hydroxyl ions OH^- is much less and thus they are free.
• There is a high possibility of recombination of OH^- + H⁺ to H₂O by taking up the proton. This decreases the concentration of H⁺ ions in the solution.
• Hence, H₂O becomes a weaker acid than acetic acid.
• Thus the correct order of increasing acid strength is Water < Acetic acid < Hydrochloric acid.

• Water can act as an acid as well as a base.
• It can take up a proton to form Hydronium ions H₃O⁺ and behave as a base.
• In water acetic acid is a weak acid.