Molecular Interactions

Boyle’s Law

Constant temperature volume of a fixed amount of gas is inversely proportional to its pressure

P = 1/v

Charles Law

Volume of a fixed amount of gas, at a constant pressure, is proportional to absolute temperature

v = T

Avogadro’s Law

Equal volumes of gases at constant temperature and pressure contain equal numbers of molecules

v = n

Combined Gas Law

Pv = T

Pv/T = constant

Ideal Gas Law

Pv = nRT

Dalton’s Law

Total pressure exerted by a mixture of gases is the sum of the partial pressures of each individual gas

Partial pressure

Ptotal = Pa + Pb + Pc + …..

Pa

= naRT/V

Partial pressure of a component

Px

Mole fraction

Xa = na/(na + nb + nc + ….)

How do gas molecules move?

In a random (but straight) line

When do molecules interact?

When they collide

Polarizability

An electric field induces a dipole

More electronegative

lower polarisability

Bigger atoms/molecules

higher polarisability

Interactions between ions

Coulomb potential

potential energy associated with the electrostatic interaction between charged particles

Interaction between an ion and a dipole

u = μ1q2/4πε0r²

Dipole-Dipole interaction (Keesom)

𝑉 = − 2𝜇1²𝜇2²/3(4𝜋ε0)𝑟^6𝑘𝑇

Dipole induced dipole interaction (Debye)

A polar molecule has an electric field which can induce a dipole in a neighbouring polarizable molecule - these dipoles attract

𝑉 = − 𝜇1²𝛼2′/4𝜋ε0𝑟6

Induced dipole - induced dipole interaction (London/dispersion)

Non polar molecules attract

Fluctuations in the instantaneous electron density creates a dipole. This dipole can then induce a dipole in a neighbouring molecule

Van der Waals interactions

Sum of all the dipole interactions

V = C/r6

Magnitude of C depends on the molecule and which interaction it has

No dipoles

varying polarisability

equal polarizability

varying dipole strength

When can a molecule leave a liquid

When kinetic energy > strength of attraction

What is boiling point dependent on?

Strength of attraction

Evaporation can occur

below the boiling point

Enthalpy of Vaporisation

Stronger the attractions, the more energy is needed to convert from liquid to gas at the boiling point

ΔHvap/Tbp

ΔSvap

Troutons rule with Hydrogen bonding

Above the line = more structure in liquid

Below the line = more structure in gas

Vapour pressure

Pressure exerted by gas in co-existance with liquid

Surface tension

Molecules at the surface have fewer neighbours so the system tries to minimise the surface area

Attractive interactions between atoms

Van der Waals

Repulsive interactions between atoms

Nuclear + electronic