Chemistry: Ionic Compounds: General Concepts of Acids and Bases, Limitations of Arrhenius Concept (For CBSE, ICSE, IAS, NET, NRA 2022)

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General Concepts of Acids and Bases

The concepts of acids and bases are proposed by 3 concepts namely Arrhenius, Bronsted and Lowry & Lewis concepts) .

Concepts of Acid and Bases

Arrhenius Concept

  • It is developed by Svante Arrhenius in 1884 and it most commonly used concept of acids ad bases.
  • It defines acids as a substance that is capable of producing H + ions. It is represented as follows: HA (aq) H + (aq) + A- (aq)

Where HA = acid, A- = Acid molecule without hydrogen ion

The Arrhenius concept of HCl is represented below:

HCl (aq) H + (aq) + Cl- (aq)

The hydrogen ion binds with water molecule and form Hydronium ion (H3O) . H + + H2O H3O +

  • Hydronium ion is also called as ox onium ion or hydroxonium ion.
  • Arrhenius concept is useful to understand acid-base behavior.

Limitations of Arrhenius Concept

  • It is limited to only aqueous solutions and require dissociation of solutions
  • It doesn՚t explain acidic behavior of substance without hydrogen or hydroxide group. E. g. AlCl3

Bronsted and Lowry Concept

  • The main idea of this concept is that acid – base reactions can be interpreted as proton-transfer reactions.
  • According to this concept “an acid is defined as a proton (H + ) donor, and a base is defined as a proton acceptor.”
  • It is clear that hydrogen containing molecules or ions are capable of donating protons in case of acids, whereas, bases are one that accepts protons.
  • E. g. 1: In a reaction between ammonia and hydrogen fluoride, ammonia acts as base while hydrogen fluorides acts as acid.
  • NH3 + HF NH4 + + F-
  • E. g. 2: In the reaction of HCl with NH3 in a solvent like water, the equation is given below:
  • NH3 + H3O + + Cl - H2O + NH4 + + Cl-
  • In this, Hydronium ions give proton to ammonia molecule which results in water and NH4 + . So, Hydronium is proton donor or acid and ammonia is proton acceptor or base. Here, water plays important role in transferring proton from HCl to NH3.
  • Bronsted – Lowry concept, acid and bases can neither be ions or molecular substances.
  • In acid-base equilibrium both forward and reverse reactions involves in proton transfers.
  • E. g. 1: Let us consider the reaction of NH3 with H2O. In the forward reaction, NH3 accepts a proton (as its base) from H2O (is acid) . In the reverse reaction, NH4 + donates a proton (acid) to OH . (base) . NH3 becomes NH4 + ion by gaining a proton, whereas NH4 + ion becomes NH3 molecule by losing a proton. The species NH4 + and NH3 are called conjugate acid-base pair.
  • The acid in conjugate acid-base pair is called as conjugate acid of the base and base is called as conjugate base of acid. i.e.. NH4 + is the conjugate acid of NH3, and NH3 is the conjugate base of NH4
  • The members of each conjugate pair are designated by the same numerical subscript e. g. , pair is marked as 2 while the pair is marked as 1
  • E. g. 2: . When HCI is dissolved in water, the HCI molecules give up H + ions that can be accepted by any of the two bases such as Cl − and H2O. HCl + H2O H3O + + Cl-
  • Here. HCI is completely ionized in dilute aqueous solution. A stronger acid (HCl) has a weaker conjugate base, Cl − and the weaker acid, (HF) has a stronger conjugate base, F − .
  • In a conjugate acid-base pair, the weaker acid stronger is its conjugate base and like versa.
  • Amphiprotic species are species that can act as both acid and base.
  • Bronsted – Lowry concept are more advantageous that Arrhenius concept.

Lewis Concept

  • G. N. Lewis proposed the above concept in the year 1923.
  • According to him, an acid is any atom, molecule or ion that can accept an electron pair from any other atom , molecule or ion.
  • A Lewis base on the other hand is any atom , molecule or ion that can donate a pair of electrons.
  • E. g. 1: Let us take an example AlCl3 + NH3 Cl3Al NH3
  • In the above example AlCl3 is an electron deficient species. It accepts an electron pair from a molecule of NH3 which has a lone pair of electrons on N atom. Thus, AlCl3 is a Lewis acid and NH3 is a Lewis base.

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