Properties of Liquids: MDCAT Chemistry notes
Properties of Liquids MDCAT notes: intermolecular forces, London forces and polarizability, dipole forces, boiling point trends, and water as a solvent.
The liquid state
In a liquid the molecules are close together but free to move past one another. A liquid therefore has a definite volume but no definite shape, is only slightly compressible, and shows diffusion, evaporation, surface tension, viscosity, a boiling point and a freezing point. Solutions of liquids also show osmotic pressure. Melting is a property of solids, not liquids.
Most of these properties depend on the strength of the intermolecular forces: boiling point, heat of vaporization, heat of sublimation, surface tension and viscosity all rise as the forces get stronger. Quantities such as the number of moles or molar mass do not depend on these forces.
Intermolecular versus intramolecular forces
Intramolecular forces (ionic, covalent, metallic bonds) hold atoms together inside a molecule and involve the valence electrons. Intermolecular forces act between whole molecules; they are much weaker and do not involve sharing or transfer of valence electrons.
Types of intermolecular forces
| Force | Between | Example |
|---|---|---|
| Dipole-dipole | Two polar molecules | $\mathrm{HCl}$, $\mathrm{SO_2}$, acetone |
| Dipole-induced dipole (Debye forces) | Polar and non-polar molecule | $\mathrm{HCl}$ with $\mathrm{Ar}$ |
| Instantaneous dipole-induced dipole (London dispersion) | Any molecules, the only force in non-polar ones | $\mathrm{He}$, $\mathrm{Cl_2}$, $\mathrm{I_2}$, alkanes |
| Hydrogen bonding | H bonded to F, O or N | $\mathrm{H_2O}$, $\mathrm{HF}$, alcohols |
Strength order: London dispersion (weakest) < dipole-dipole < hydrogen bonding.
London dispersion forces and polarizability
In a non-polar molecule the electron cloud is momentarily lopsided, creating an instantaneous dipole that induces a dipole in a neighbouring non-polar molecule. Polarizability is the ease with which an electron cloud is distorted. It increases with:
- more electrons and larger atomic or molecular size, so $\mathrm{F_2} \lt \mathrm{Cl_2} \lt \mathrm{Br_2} \lt \mathrm{I_2}$ (fluorine and chlorine are gases, bromine a liquid, iodine a solid)
- longer chains, so boiling points of alkanes rise: $\mathrm{C_2H_6} \lt \mathrm{C_4H_{10}} \lt \mathrm{C_6H_{14}} \lt \mathrm{C_{10}H_{22}}$
Polar molecules and boiling point
- $\mathrm{SO_2}$ is bent and polar, while $\mathrm{CO_2}$ is linear and non-polar. So $\mathrm{SO_2}$ has dipole-dipole forces and a higher heat of vaporization.
- Diethyl ether has no O-H hydrogen, so it cannot hydrogen bond with itself. Its forces are weak and it boils at about 35 °C, far below water, alcohols and phenols.
- Water is hydrogen bonded, so it has an unusually high boiling point, a high surface tension and a high specific heat of $4.18\ \mathrm{J\,g^{-1}\,{}^\circ C^{-1}}$.
Water as a solvent
Water is called the universal solvent because it is polar (large electronegativity difference between O and H) and forms hydrogen bonds. Its dipoles surround and separate the ions of ionic compounds (hydration), and it hydrogen bonds with polar molecules such as sugars and alcohols.
Common MDCAT traps
- Iodine, chlorine and helium are non-polar: only London forces act between their molecules, even in the solid.
- HCl molecules are polar, so they have dipole-dipole forces as well as London forces.
- Polarizability is highest for the largest molecule, e.g. hexane among small alkanes.
- Melting is not a property of a liquid; diffusion and freezing are.
- Intermolecular forces are the ones not related to valence electrons.
Quick revision
- London dispersion forces are the weakest intermolecular forces.
- Larger molecules have more polarizable electron clouds and higher boiling points.
- Water has the highest surface tension among common liquids like benzene, ether and alcohol.
- Specific heat of water is $4.18\ \mathrm{J\,g^{-1}\,K^{-1}}$.
- Water dissolves ionic solids because it is polar.