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, sp-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.,

SO+ SO2  ↔  SO2+ +  S  

Acid1  Base2    Acid2   Base1

Thus, the protonic definition cannot be used to explain the reactions occurring in non-protonic solvents such as COCl2, S02, N204, etc.
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