Hydrogen Bonding in Water and Ice

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It is a common observation that density of ice is less than that of water because of which ice always floats over water. Moreover, it is a known fact that density of water is maximum at 4°C.

• Ice is lighter than water:

X-ray studies have shown that in ice, four hydrogen atoms tetrahedrally surround each oxygen atom, in such a way that two hydrogen atoms are linked to oxygen atom by covalent bonds (These lie closer to the oxygen atom at a distance of 1.00 Å.) and the other two hydrogen atoms are linked to the central oxygen atom by hydrogen bonds. (These lie away from oxygen atom at a distance of 1.76Å.) The hydrogen atoms of water molecules are located at the corners of tetrahedron.

Fig: 6.21 - The tetrahedral arrangement of water

As a result of tetrahedral arrangement of H₂O molecules in the solid state, the ice structure extends in three dimensions. Further the molecules of H₂O are not packed closely as hydrogen bonds between oxygen and H atoms are longer than covalent bonds between them. This results in vacant spaces being present in the crystal structure. It gives rise to an open cage like structure for ice, having a larger volume for the given mass of water. Thus, the density of ice is less than water

• Water has maximum density at 4°C:

When ice is melted by increasing its temperature, the open cage structure starts collapsing due to partial break down of the hydrogen bonds. H₂O molecules start to come closer; also the water molecules separated from ice structure fill the vacant spaces in open-cage structure. This results into concentration of volume. Hence, the density of water starts increasing on heating above the melting point of ice and becomes maximum at 4°C.

On further rise in temperature, the kinetic energy of molecules increases and they once again start moving away from one another. This causes expansion, which is much more than the concentration caused by breaking of hydrogen bonds. Therefore, above 4°C the density of water once again starts decreasing. Thus, the maximum density of water is at 4°C.

Problem

Why does KHF₂ exist but KHCl₂ does not?

Solution

Due to hydrogen bonding in HF as HF.......... HF......... HF,

But there is no hydrogen bonding in HCl and so HCl^-₂ ion cannot be formed. Therefore, KHCl₂ does not exist.