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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

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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 spcarbon.
  • 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. 

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