Question:

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

Updated On: Jul 29, 2022
  • $HI > HCl < HF > HBr$
  • $HCl < HF > HBr < HI$
  • $HF > HCl > HBr > HI$
  • $HI > HBr > HCl > HF$
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The Correct Option is C

Solution and Explanation

In a group, $\Delta G^{\circ}_{f}$(HX) changes from - ve to + ve downwards. $HF \left(g\right) \Delta G =- 273 .20 \,kJ \,mol^{-1}$ $HI\left(g\right) \Delta G = + 1.72 \,kJ\, mol^{-1}$ Thus HF is thermally stable and HI not. Thus $HF > HCI > HBr > HI$
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Concepts Used:

Group 17 Elements

Halogens are the group 17 elements of the periodic table. The term ‘halogen’ means ‘salt-producing’, hence the name halogens as they possess the tendency to form salts after reacting to metals. It generally has five elements:

  • Fluorine (F)
  • Chlorine (Cl)
  • Bromine (Br)
  • Iodine (I)
  • Astatine (At)

These are all naturally occurring halogens but Tennessine (Ts) is an artificially created halogen.

Halogens:

Halogens are highly reactive elements and are highly electronegative. They have a high tendency to react with metals to form salts. They are also known as Group 17 elements. They have 7 electrons in their outer shell with a configuration of (ns2 np5). Fluorine being the first halogen in group 17, is highly reactive. Astatine is a halogen because of its resemblance with iodine despite it being radioactive.

Electronic Configuration:

The general electronic configuration for group 17 elements is ns2np5. This configuration clearly shows that they have 7 electrons in their valence shell. They require one more electron to complete their octet and achieve noble gas configuration.