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18.1 Phase equilibrium

18.1.1

A liquid in an enclosed chamber will form an equilibrium with it's own vapor. Fast moving particle in the liquid will escape from the surface and become part of the vapor, but slow moving particles in the vapor will be 'captured' by the liquid and become part of it. At a certain vapor 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.

18.1.2

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 vapor pressure will also increase. This can be shown graphically with a graph of pressure against temperature, where, as temperature increases, so does pressure.

18.1.3

Liquids with a high boiling point will have high intermolecular forces. 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 vapor pressure.

18.1.4

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.

18.1.5

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 being condensed (like in reflux)...the more volatile liquid will escape from the top of the fractionating column (the condenser type bit with the water running through it), while the less volatile component will be condensed and returned to the boiling flask. In this way, it is possible to (in theory) completely separate the two components of the mixture.

18.2 The equilibrium law

18.2.1

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

as

,

ie 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, K_c can be calculated, or given K_c and all but one of the concentrations, the final concentration can be calculated.

(It should be noted, however, that concentrations are linked, and so it may not always be necessary to have all the concentrations, or in the second case all but one...but the quadratic formula is not required...so I guess they have to be simple.)

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

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