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Acid Strength of Hydrogen Halides

Halogens combine with hydrogen to form volatile halides of the formula HX. Some characteristics of these hydrides are given below:

(i) Physical Nature. Except HF, all other hydrides viz., HCl, HBr and HI are gases. HF is a· liquid because of intermolecular hydrogen bonding.


- H – F …H – F … H – F …


The bond between hydrogen and halogen is covalent in all the cases.

(ii) Acidic Strength. All .the hydrogen halides act as acids in their aqueous solutions. The acidic strength varies in the order

HF < HCl < HBr < HI

Reason. All the halogen acids ionise to give H+ ion and halide ion, x-.

HX  à H+  +  X-  ; (where X - = F- , Cl- , Br- , I-)

The above order of acidic character can be explained in terms of strength of H-X bonds, which is in the order H-I< H-Br < H-Cl < H-F. Since H-1 bond is weakest, therefore, HI is the strongest acid. On the other hand H-F bond is strongest, hence it is the weakest acid among all the halogen acids.

(iii) Reducing Character. The reducing character of the hydrides increases down the group. It is because the strength of H-X bond decreases from HF to HI. Lower is the strength of bond greater is the ease with which it can be cleaved and hence greater is the reducing character.

Some physical properties of hydrogen halides are summarised in Table 18.11.

 Table 18.11. Properties of Hydrogen Halides