i) Physical state
Hydrogen fluoride is a low boiling liquid (boiling point 292K) while HCl, HBr and HI are gases. The anomalous property of HF is due to presence of hydrogen bonding in the molecules. Due to hydrogen bonding in HF molecules it exists as associated molecule (HF)n.
(ii) Thermal stability
The thermal stability of the hydrides decreases from HF to HI. HF is most stable whereas HI is least stable. For e.g., HF and HCl are stable up to 1500K while HBr dissociates to the extent of 10% and HI is dissociated to the extent of 20% at 700 K.
The decrease in stability of the hydrides is due to decrease in bond strength, which decreases when we go down the group.(iii) Reducing character
The decreasing thermal stability of hydrogen halides from HF to HI indicates that the reducing character increases down the group as HF < HCI < HBr < HI.
Thus, HF is not a reducing agent at all. HCl is a weak reducing agent, HBr is a stronger reducing agent while HI is the strongest reducing agent among all the hydrides.(iv) Acidic strength
In gaseous state, hydrogen halides are covalent. But in aqueous solutions, they ionize and behave as acids. The acidic strength of these acids decreases in the order: HI > HBr > HCI > HF
Thus, HF is the weakest acid and HI is the strongest acid among these halogen hydrides.Explanation:
The above order of acidic strength is reverse of that expected on the basis of electronegativity. Fluorine is the most electronegative halogen; therefore, the electronegativity difference will be maximum in HF and should decrease gradually as we move towards iodine through chlorine and bromine. Thus, HF should be more ionic in nature and consequently it should be strongest acid. Although many factors contribute towards the relative acidic strengths, the major factor is the bond dissociation energy. The bond dissociation energy decreases from HF to HI so that HF has maximum bond dissociation energy and HI has the lowest value.
