Hello Dear,

For example: 2HX(g)->H2(g)+X2(g,l,s)

By plunging red-hot wire into a test tube of the gas:

HI: easily decomposed

HBr: may or may not decompose depending on the exact temperature of the wire

HCl and HF: not decomposed

As the halogen atom gets bigger, the bonding pair gets more and more distant from the nucleus. The attraction is less, and the bond becomes weaker exactly what is shown in the example. This is important in the thermal stability of the hydrogen halides-how easily they are broken up into hydrogen and the halogen on heating. Hydrogen fluoride and hydrogen chloride are very stable to heat.

Hope this helps you.

All the Best!!!

The correct order of the thermal stability of hydrogen halides (H – X) is

HF > HCl > HBr > HI

EXPLANATION- As the size of the halogen atom increases from F to I, H – X bond length in HX molecules also increases from H – F to H – I (H – F < H – Cl < H – Br < H – I).  The increase in H – X bond length decreases the strength of H – X bond from H – F to H – I (H – F > H – Cl > H – Br > H – I). The decrease in the strength of H – X bond is evident from the fact that H – X bond dissociation energies decrease from H – F to H – I. Due to successive decrease in the strength of H – X bond from H – F to H – I, thermal stability of HX molecules also decreases from HF to HI (HF > HCl > HBr > HI).