Periodic variations of acidic and basic properties:

(a)  Hydracids of the elements of the same periods:

Consider the hydracids of the elements of II period, Viz., CH₄, NH₃, H₂0 and HF. These hydrides become increasingly acidic as we move from CH₄ to HF. CH₄ has negligible acidic properties while HF is a fairly stronger acid. The increase in acidicproperties is due to the fact that the stability of their conjugate bases increases in the order

          CH^-₃< NH^-₂  < OH^- < F^-

The increase in acidic properties is supported by the succes­sive increase in the dissociation constant.

CH₄(=10^-58)<NH₃ (=10^-35)<H₂0(=10^-14)<HF(=10^-4)

(b)  Hydracids of the elements of same group:  

(i)   Hydrides of V group elements (NH₃, PH₃, AsH₃, SbH₃, BiH₃) show basic character which decreases due to increase in size and decrease in electronegativity from N to Bi. There is a decrease in electron density in, sp³ -hybrid orbital and thus electron donor capacity decreases.

(ii)  Hydracids of VI group elements (H₂0, H₂S, H₂Se, H₂Te) act as weak acids. The strength increases in the order H₂0 < H₂S < H₂Se < H₂Te.

The increasing acidic properties reflects decreasing trend in the electron donor capacity of OH^-, HS^-, HSe^- or HTe^- ions.

(iii) Hydracids of VII group elements (HF, HCI, HBr, HI) show acidic propertieswhich increase from HF to HI. This is explained by the fact that bond energies decrease. (H-F = 135 kcal/mol, HCI = 103, HBr = 88 and HI = 71 kcal/mol).

(c)  Oxyacids:

(i)     The acidic properties of oxyacids of the same element which is in different oxidation states increases with increase in oxidation number.

       + 1            +3          +5            +7

     HCIO  <  HC1O₂   <   HC1O₃   <  HCIO₄

       +4            +6         +3            +5

     H₂SO₃  <   H₂SO₄;    HNO₂     <  HNO₃

But this rule fails in oxyacids of phosphorus.

H₃PO₂  >  H₃PO₃  >  H₃PO₄

(ii)  The acidic properties of the oxyacids of different elements which are in the same oxidation state decreases as the atomic number increases. This is due to increase in size and decrease in electronegativity.

HC1O₄ > HBrO₄ > HIO₄

  H₂SO₃  > H₂SeO₃

Limitations:   

There are a number of acid-base reactions in which no proton transfer takes place, e.g.,

SO₂ + SO₂  ↔  SO²⁺ +  S  

Acid₁  Base₂    Acid₂   Base₁