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Thermal stability order of HX is- HF>HCl>HBr>HI  [But the order should be HF>HI>HBr>HCl, due vanderwall forces in HI, HBr(due to large size)]. Can you explin me please why this has happened?

Thermal stability is directly proportional to bond dissociation energy.Bond dissociation enthalpy is defined as the energy required to break a bond between two atoms. 

As we move from fluorine to iodine, atomic sizes increases, therefore, H-X bond length increases. This causes the H-X bond dissociation energy to decrease and hence H-X bond breaks more easily. Thus, the thermal stability is decreased in the order: HF>HCl>HBr>HI.

 

We can explain it in another manner also:

As the size of the halogen gets bigger from F to I, the valence orbital used for bonding is larger and more diffuse. As a result, the overlap of the orbital with the 1s orbital of the hydrogen atom becomes less effective and this accounts for the weaker bond strength that is reflected in the bond energies. So HI is weakest of all.

 

 

 

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  •  thermal stability depends on bond strength and bond dissociation enthalpy....flourine being highly electronegative forms very strong bond with hydrides therefore the bond dissociation enthalpy is more and hence thermal stability is more and it decreases down the group
  • bond dissociation enthalpy is defined as the energy given to break a bond between two atoms ...hence you can see that  thermal stability is directly propotional to bond dissociation enthalpy
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