Intermolecular forces and hydrogen bonding

Chemistry · Liquids and solids. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Intermolecular forces are the weak attractions between molecules. They are much weaker than the covalent bonds inside a molecule, but they decide melting point, boiling point and solubility. Stronger forces mean a higher boiling point.

Polar molecules attract each other by dipole-dipole forces. Non-polar molecules and noble gas atoms attract only by London dispersion forces, which come from temporary dipoles. These grow stronger as the electron cloud gets larger and easier to distort.

Hydrogen bonding is an extra strong attraction. It forms when hydrogen is bonded to a small, highly electronegative atom (F, O or N) and is attracted to a lone pair on such an atom in a neighbouring molecule.

Rules to remember

Dipole-dipole forces: between the permanent dipoles of polar molecules, e.g. HCl, CHCl₃
Dipole-induced dipole (Debye) forces: a polar molecule induces a dipole in a non-polar molecule or atom, e.g. HCl and argon
London forces: an instantaneous dipole in one molecule induces a dipole in the next. They act between all atoms and molecules
London forces increase with polarisability, that is with the size of the electron cloud (boiling points rise from F₂ to I₂ and from He to Rn), with the number of atoms in the molecule, and with surface area (a straight chain boils higher than a branched isomer)
Hydrogen bonding makes the boiling points of NH₃, H₂O and HF much higher than those of the other hydrides in their groups
Each water molecule can form up to four hydrogen bonds. In ice these give an open tetrahedral structure with empty spaces, so ice is less dense than water and floats
Water has its maximum density at 4 °C

Common mistake

Students say that boiling water breaks the O−H bonds. Boiling only separates the molecules by overcoming hydrogen bonds between them; the covalent bonds inside each molecule stay intact.

Worked example

Which of the hydrogen halides HF, HCl, HBr and HI has the lowest boiling point?

HF molecules are joined by hydrogen bonds, because fluorine is small and highly electronegative. So HF has the highest boiling point of the four.
HCl, HBr and HI have no hydrogen bonding. Their boiling points depend mainly on London forces.
London forces increase with the size of the electron cloud: HCl < HBr < HI.
So the smallest molecule without hydrogen bonding has the weakest forces.

Answer: HCl has the lowest boiling point. The order is HCl < HBr < HI < HF.

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Chapter 4: Liquids and Solids

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