Question

When the melting points of the Hydrogen halides are measured, it is found that of the three, HCl, HBr and HI, HI has the highest and HC1, the lowest. Why is this unexpected?

Answer 1

The order of Boiling point is like this:
HF > HI > HBr > HCl

But the important thing to know is not the order itself but why that order happened. Let's go to the basics; the periodic table.

The halogens in the periodic table. Halogen literally means 'salt producing' since these compounds form many salts

If you notice, the elements present in the compounds you gave belong to one family: the halogens. Halogens are a class of elements that include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At) in that order. When you progress it in that order, new shells of electrons get added (and so does protons and neutrons) and hence, they get 'heavier'. Hold this information for now.

Since these halogens are bonded with hydrogen, they are known as Hydrogen Halides. (Astatine is not usually considered since it is very radioactive)

The hydrogen halide atoms arranged as HF > HCl > HBr > HI from top to bottom

When the atomic number of the element increases, it must be certain that their attractive force increases also. An analogy: a heavier planet can attract more strongly than a lighter planet. Anyway, when the atomic number of the element increases the number of electrons present in them also increases. These electrons are constantly moving around in the atom. At some point, there are more electrons on one side of the atom than the other. This creates an imbalance of charge, creating a small charge of δ+ and δ−. These small charges induces its neighbours to create additional charges of δ+ and δ− thus creating a long chain of induced dipoles. A dipole is a pair of two opposite but equal charges separated by a small distance. These effects are temporary as the electrons can move about still. Since these dipoles are induced (and hence can be reverted), they are also known as induced dipole force or better known as London Dispersion Forces.

London dispersion forces

A point to be noted is that they aren't really forces but rather attraction between molecules.

Now, it must intuitively be certain that more the number of electrons then more the amount of London dispersion forces. This is true; the more the electrons the atom has, the stronger the amount of London dispersion forces and hence greater the attraction. The greater the attraction, the more energy you have to put to break the bonds apart. If you see the trend above, Iodine has more electrons than Bromine hence has a greater attraction force between its atoms hence has a greater boiling point. Similar for the case of Bromine and Chlorine. Hence, you can place the order of their boiling points as:
I > Br > Cl

Hence,
HI > HBr > HCl

The boiling points of the hydrogen halides

However, you notice an anomaly; HF should have a boiling point below that of HCl, then how come it has the highest boiling point? After all, Fluorine has lesser electrons than Chlorine and hence should have lesser London dispersion forces between them. This is true but there is another reason behind that: Hydrogen Bonding.

Picture fluorine; it sits right above the other halogens. It belongs to Period 2 which does not have many shells to itself. Since it has so less electrons, it must be very small. In fact, it is. Fluorine is so small yet it has a really high charge density. Why does it have a high charge density? Its outer electrons are not as shielded by the inner electrons and it has 7 outer electrons. (read more on: Shielding effect) These seven electrons face tremendous attraction from its nucleus hence it 'shrinks'. This causes the fluorine atom to shrink hence its charge density increases. As such, it can attract other electrons to itself when it is covalently bonded to another element. This property is called electronegativity.