Chemistry (university) · Part 03
Intermolecular forces, gases and solutions
Forces between molecules, the gas laws, real gases, solubility and colligative properties.
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- Intermolecular forcesIntermolecular forces are attractions between separate molecules. They are much weaker than bonds inside molecules. The main types are London dispersion forces, dipole-dipole forces and hydrogen bonds.
- Hydrogen bondingA hydrogen bond is a strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F. It is the strongest type of intermolecular force, but still far weaker than a covalent bond.
- Vapour pressureVapour pressure is the pressure of vapour in balance with its liquid in a closed container. It rises with temperature. A liquid boils when its vapour pressure equals the outside pressure.
- Ideal gas lawThe ideal gas law is PV = nRT. P is pressure, V is volume, n is moles, T is temperature in kelvin, and R is the gas constant, 0.08206 L·atm/(mol·K). It works well at low pressure and high temperature.
- Dalton's law of partial pressuresIn a mixture of gases, each gas adds its own pressure as if it were alone. The total pressure is the sum of the partial pressures. Each partial pressure equals the mole fraction times the total pressure.
- Kinetic molecular theoryKinetic molecular theory models a gas as tiny particles in constant random motion with no attraction between them. Temperature measures average kinetic energy. At the same temperature, lighter molecules move faster.
- Real gases and van der Waals equationReal gases differ from ideal gases at high pressure and low temperature because their molecules take up space and attract each other. The van der Waals equation adds two corrections: a for attraction and b for molecule size.
- Solubility and 'like dissolves like'A substance dissolves when attractions between solute and solvent are about as strong as the attractions it must break. Polar solvents dissolve polar and ionic solutes. Nonpolar solvents dissolve nonpolar solutes.
- Colligative propertiesColligative properties depend on the number of dissolved particles, not on what they are. Adding a solute lowers the vapour pressure and freezing point and raises the boiling point. ΔTf = i·Kf·m.