MCQ Detailed View

Bond energy refers to the amount of energy required to break one mole of a bond in a gaseous molecule. It is an important measure of bond strength and chemical stability. For hydrogen fluoride (HF), the bond energy of the H–F bond is 562 kJ/mol, which is relatively high compared to many other single covalent bonds.

The high bond energy of the H–F bond is due to several important factors:

1. 
   

   Electronegativity difference: Fluorine is the most electronegative element in the periodic table, with an electronegativity value of 3.98 (Pauling scale). Hydrogen has a lower electronegativity (2.20). The large difference between them results in a strong polar covalent bond. This polarity increases the electrostatic attraction between the hydrogen nucleus and fluorine’s valence electrons, strengthening the bond.

   
   


2. 
   

   Bond length: The H–F bond is very short, approximately 0.92 Å. A shorter bond length means that the bonding electrons are closer to both nuclei, which strengthens the bond.

   
   


3. 
   

   Covalent character with partial ionic nature: The H–F bond is covalent, but due to the high polarity, it also exhibits partial ionic character. This dual nature enhances the bond’s stability and energy requirement for dissociation.

   
   

Because of this strong H–F bond, hydrogen fluoride is relatively stable but also highly reactive due to its polarity. In aqueous solution, HF behaves as a weak acid compared to HCl, HBr, or HI, even though its bond is stronger. The weak acidity in water arises because the H–F bond is so strong that it resists dissociation into H⁺ and F⁻ ions easily.

In chemical thermodynamics and kinetics, knowing the bond energy of HF is useful for calculating enthalpy changes in reactions involving fluorine compounds. It is also critical in understanding the trends of hydrogen halides (H–X). The bond energy decreases as we go down the group from HF to HI:

• 
  

  H–F: 562 kJ/mol

  
  


• 
  

  H–Cl: 431 kJ/mol

  
  


• 
  

  H–Br: 366 kJ/mol

  
  


• 
  

  H–I: 299 kJ/mol

  
  

This trend shows that as the halogen atom becomes larger, the bond length increases and the bond strength decreases.

Key Point:
The H–F bond energy of 562 kJ/mol highlights the exceptional stability and strength of this bond, making it one of the strongest single covalent bonds known in inorganic chemistry.