Structure & Properties of Matter Lesson # 8: Intermolecular Forces

Definitions Intramolecular forces – intra means “within” – so this means atom to atom bonds within a molecule. Intermolecular forces – inter means “between” – so this means interactions between two or more molecules. Johannes van der Waals suggested that molecules don’t act exactly as we assume they will (especially ideal gases) because of forces exerted between molecules. He called these van der Waals forces.

Intermolecular Forces & Water Consider the changes of state in water from solid to liquid to gas. The water molecules do not change upon melting and boiling. There is always 1 oxygen and 2 hydrogens covalently bonded together in a water molecule. Changes in state are due to changes in the intermolecular forces between molecules, not the bonds themselves.

Heating Water (continued) When you supply energy to ice through heating, the molecules vibrate and absorb energy – enough to break away from a molecule near it and the ice melts. When you supply more energy, the added energy allows some water molecules to overcome their attraction to other water molecules, so the molecules escape into the gas phase – hence, boiling. Intermolecular forces also explain things like solubility, viscosity, etc.

Change of State in Water

Dipole-Dipole Forces In a liquid where many polar molecules are present, the molecules arrange themselves in the most stable arrangement possible – minimizing repulsive forces and maximizing attractive forces. This arrangement of positives and negative ends aligning gives the structure strength, and are called dipole-dipole forces. We often discuss strength in terms of boiling point. Polar molecules that have these dipole-dipole forces tend to have higher boiling points than non-polar, where there are no dipole interactions possible.

Hydrogen Bonding Unusually strong dipole-dipole forces exist when hydrogen is bonded to a very electronegative atom such as N, O, or F. The difference in electronegativity between these atoms is so large it is almost as if the hydrogen atom loses electrons altogether and so neighbouring negative atom can get very close to its positive nucleus. The result is an unusually large dipole-dipole force of attraction – a hydrogen bond. Hydrogen bonding does not happen with all hydrogen-containing molecules – some atoms either have a small electronegativity or are too large to move inside hydrogen’s atomic radius.

Hydrogen Bonding

London Dispersion Forces Even molecules without dipoles can exert forces on each other, though weaker. London dispersion forces arise when electrons move about the nucleus, and at any moment the charge distribution may be uneven, and a temporary dipole is created. A slightly negative charge would be at the location where the most electrons exist, and a slightly positive charge where electrons are lacking.

London Dispersion (continued) Since electrons are constantly moving, these temporary dipoles are very short-lived and constantly changing. Still, this is enough to explain why chemicals can exist in the liquid and solid state, even if non-polar. London dispersion forces tend to be the strongest in large molecules, as larger molecules contain more atoms, which means more possibilities for temporary dipoles. This is another homologous series – as the molar mass increases, London dispersion forces are stronger.

London Dispersion

Surface Tension in Liquids The property describing the fact that water can bead up on a solid surface is called surface tension. It is defined as the resistance of a liquid to increase its surface area. The energy of the water is lowest when the surface of the water is minimized. Thus water tends to form spherical drops on surfaces.

Capillary Action in Liquids Polar liquids also exhibit capillary action – the spontaneous rising of a liquid in a narrow tube. Hence the meniscus in a graduated cylinder. Cohesive forces (intermolecular forces among molecules) and adhesive forces (forces between the liquid and their container) are responsible. In a graduated cylinder, the glass itself contains oxygen atoms with partial negative charges that attract the positive end of polar molecules and pull them towards them.

Viscosity in Liquids Viscosity is a liquid’s resistance to flow. Liquids with large intermolecular forces are highly viscous, as the forces are holding all the molecules close together. The larger the liquid, the more viscous the liquid tends to be. VIDEO – Properties of Liquids