The Acidity of Hydrogen Halides
This section examines the acidity of hydrogen halides – hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), and hydrogen iodide (HI). These are all binary compounds of hydrogen with halogens.
The Acidity of the Hydrogen Halides
Hydrogen Chloride as an Acid
All hydrogen halides are gases at room temperature and are covalent molecules. However, when they dissolve in water, they ionize to produce hydronium ions (H3O+) and halide ions (X–), which makes them acidic:
Equation: H2O + HX → H3O+ + X–
HF ionizes only slightly while HCl, HBr, and HI ionize completely. Therefore, HF is the weakest acid, and the strength of these acids increases from HF to HI, i.e., HF (weakest acid) < HCl < HBr < HI (strongest acid). This trend can be attributed to the bond dissociation energies:
- HF: High bond dissociation energy, making it hard to ionize.
- HCl, HBr, HI: Successively lower bond dissociation energies, making them easier to ionize in water.
As a result, HF is the weakest acid because it ionizes the least, while HI is the strongest because it ionizes completely.
Explanation of Acidity Order
The acidity strength can also be explained by considering the stability of the conjugate base (X–). As you move down the group, the size of the halide ion increases, leading to better stabilization of the negative charge due to its larger volume.
- HF: F– is small and holds the negative charge tightly, making it less stable and HF a weaker acid.
- HCl, HBr, HI: Larger ions (Cl–, Br–, I–) better stabilize the negative charge, making the corresponding acids stronger.
