Class 12 Chemistry P Block Elements Trends in chemical reactions

Trends in chemical reactions

  1. Reaction with hydrogen: They form their respective halogen acids of formula HX that is :

     HF  (Hydrogen Fluroide)    HCl (Hydrogen Chloride)  HBr(Hydrogen Bromide)

     HI (Hydrogen Iodide)


The properties of halogen acids are:

(a) All have linear structure with bond angle 180 degree.

(b) Out of all only HF is liquid otherwise all are gases.

HF is liquid due to hydrogen bonding they occur as associated molecules.

(c) Stability: The stability of hydrides depends upon the comparability of size. The order of stability of hydrides is:

  HF > HCl  >  HBr > HI 

  Due to increase in size, bond length increase due to which bond dissociation energy decrease therefore stability decrease.

(d) Reducing nature: The order of reducing character of hydrides is:

  HF < HCl  <  HBr < HI                           

All are Arrhenius acids. The HI has weakest bond therefore release of H is much easier.

(e) Boiling point: The boiling point depends upon Vander wall force and this Vander wall force increases with increase in size.

             HF > HCl  <  HBr < HI                       

         Due to hydrogen bonding HF has highest boiling point.

(f)Polarity: All halogen acids are polar in nature. Greater the size of anion more is the polarizibility more is the covalent character. Out of all, HF is highly polar.


  1. Reaction with oxygen: They react with oxygen to form their respective oxides.

Due to less electro negativity difference the oxides are un-stable.


The reason behind this trend is due to some thermodynamic reasons.


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