Q. 21

Use the information and data given below to answer the questions (a) to (c):

• Stronger intermolecular forces result in higher boiling point.

• Strength of London forces increases with the number of electrons in the molecule.

• Boiling point of HF, HCl, HBr and HI are 293 K, 189 K, 206 K and 238 K respectively.

(a) Which type of intermolecular forces are present in the molecules HF, HCl, HBr and HI?

(b) Looking at the trend of boiling points of HCl, HBr and HI, explain out of dipole-dipole interaction and London interaction, which one is predominant here.

(c) Why is boiling point of hydrogen fluoride highest while that of hydrogen chloride lowest?

Answer :

• In halides, due to the presence of electronegative halogen atoms (F, Cl, Br, I), these halogen atoms acquire a delta negative (δ-) and H being very less electronegative acquires a delta positive (δ+) charge.


• Due to the formation of opposite polarities, it makes halide a polar molecule.


• Also intermolecular hydrogen bonding is observed in compound where very high electronegative elements like(O,F,N) are present. Due to intermolecular hydrogen bonding, the stability of the molecule increases along with physical properties like boiling point.


(a) Since the halides are a polar molecule, due to the presence of permanent dipoles, the dipole-dipole interactions along with the London forces are found in these halides (HF, HCl, HBr).


• Along with that, as we discussed earlier that the presence of very high electronegative elements (likeO,F,N), lead to intermolecular hydrogen bonding, in HF due to electronegative atom F, an additional intermolecular hydrogen bonding is present.


(b) It is told that


• Strength of London forces increases with the number of electrons in the molecule.


• In the periodic table, the first element of the group is F, followed by Cl, Br and I.


• So, HI has the most number of electrons in the molecule (as I hasthe highest atomic number amongst F and Cl) and HF has the least number of electrons.


• Thus, London interactions increase from HF to HI.


• But in a polar molecule like halides, more the electronegativity is of the halogen atom, more polar is the molecule (as delta negative charge δ-increases).


• As electronegativity increases from I to F, dipole-dipole interactions increase from HI to HF.


• Boiling point increases from HI to HCl (as I- is a very large ion, it is bonded by Van der Waal’s forces, so more energy is required to break the bonds. As the size of the ions decrease, lesser energy is required).


• From this trend, we can conclude that the London’s forces are predominant because these interactions also increase from HF to HI.


(c) Ad we discussed earlier that the presence of very high electronegative elements (like O,F,N), lead to intermolecular hydrogen bonding, in HF due to electronegative atom F, an additional intermolecular hydrogen bonding is present.


In HCl, size of Cl- ion is very small, due to which the bonded Van der Waal’s forces are very weak, so less energy is required to break the bonds. Thus Boiling point is the least.


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