The following notes were written for the previous IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

Equilibrium (hl)

17.1 - Liquid - vapour equilibrium

A liquid in an enclosed chamber will form an equilibrium with it's own vapour.

Fast moving particle in the liquid will escape from the surface and become part of the vapour, but slow moving particles in the vapour will be 'captured' by the liquid and become part of it. At a certain vapour pressure, the number of particles escaping (or evaporating) from the liquid will exactly equal the number being captured by it, and so a dynamic equilibrium is formed between the two.

Effect of temperature on vapour pressure

As the temperature increases, the average speed of particles is higher. As a result, more particles will have sufficient speed to escape the liquid, and fewer will be slow enough to be recaptured by the liquid. This means that as temperature increases, the equilibrium vapour pressure will also increase. This can be shown graphically with pressure against temperature, where, as temperature increases, so does the vapour pressure.

Enthalpy of vaporisation and boiling point

The boiling point (temperature) is reached when the vapour pressure is equal to atmospheric pressure. Liquids with a high boiling point have high intermolecular forces. The Enthalpy of vaporisation is a measure of the energy change when 1 mol of liquid is converted to gas at standard pressure. As a result a lower enthalpy of vaporisation implies that less energy is required to break the intermolecular bonds, and so a lower enthalpy of vaporisation will result in a higher vapour pressure.


enthalpy of vapourisation intermolecular forces boiling point vapour pressure
high strong high low
low weak low high

Mixtures of liquids

When two liquids are in a mixture, particles from both liquids escape, forming a partial pressure for each above the mixture. The partial pressure of each liquid will be directly related to its volatility and to the mole fraction of it compared to the total number of mols in the liquid.

Separation of liquid mixtures

When it is necessary to separate the components of a mixture in which there is more than one volatile component, fractional distillation must be used. (simple distillation is when there is only one volatile fraction, and it is boiled off and the condensed).

Fractional distillation is achieved by continuous boiling of the two liquids while they are simultaneously being condensed (like in reflux) The more volatile liquid will escape from the top of the fractionating column side arm and be condensed in the cooling tube, while the less volatile component will be condensed in the fractionating tube of the column and returned to the boiling flask. In this way, it is possible to (in theory) completely separate the two components of the mixture.

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17.2 - The equilibrium law

The value of Kc can be expressed for a general reaction:

A + 2B 3C + D

Kc = ([C]3[D])/([A][B]2)

The concentrations of the products (each to the power of their coefficient) over the concentrations of the reactants (each to the power of their coefficient).

All concentrations are taken when the system has reached equilibrium, and so given all concentrations, Kc can be calculated, or given Kc and all but one of the concentrations, the final concentration can be calculated.

The units for Kc can also be calculated by replacing each concentration with mols dm-3 (remembering to take exponents into account) and cancelling out.

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