Question

the strength of hydrohalic acids increases down the group, explain

Answer 1

The strength of an acid depends on a number of factors, such as the properties of the solvent, the temperature, and the molecular structure of the acid.  When we compare the strengths of two acids, we can eliminate some variables by considering their properties in the same solvent and at the same temperature and temperature.   That way we can focus on the structure of the acid.  Binary HydridesLets consider a certain acid HA.  The strength of the acid is measured by its tendency to ionize:HA ---> H+  +  A-   Two factors influence the extent to which the acid undergoes ionization.  One is the strength of the H-A bond, the stronger the bond, the more difficult it is for the HA molecule to break up and hence the weaker the acid.  The other factor is the polarity of the H-A bond.  The difference in electronegativities between H and A results in a polar bond.  If the bond is highly polarized, that is if there is a large accumulation of positive and negative charges on the H and A atoms, HA will tend to break up into H+ and A- ions.  So a high degree of polarity characterizes a stronger acid.The halogens form a series of binary acids called the hydrohalic acids.  The strengths of the hydrohalic acids increase in the following order:HF<<HCl<HBr<HIHF has the highest bond dissociation energy of the four hydrogen halides.  Since it takes 568.2 kJ to break the H-F bond, HF is a weak acid.  At the other extreme in the series, HI has the lowest bond energy (298.3 kJ), so HI is the strongest acid of the group.  In this series of acids the polarity of the bond actually decreases from HF to HI.  This property should enhance the acidity of HF relative to the other acids in the series, but its magnitude is not great enough to overcome the trend in bond dissociation energies.In any vertical column (Group) of nonmetallic elements, there is a tendency toward increasing acidity of the hydride with increasing atomic number (as you go down the group).  For example, among the group VIA elements the acid strength increases in the order:H2O< H2S<H2Se<H2TeThis order arises primarily because the bond energies steadily decrease in this series as the central atom grows larger and the overlaps of atomic orbitals grow smaller, just as in the case of the hydrides of the halogens above.

Answer 2

Answer:

Going down a group the acid strength increases because the bond strength decreases as a function of increasing size of the nonmetal, and this has a larger effect than the electronegativity. In fact HF is a weak acid because it is so small that the hydrogen-fluorine bond is so strong that it is hard to break.