Equilibrium is where the rate of the forward and reverse reactions in a chemical reaction are equal, and at this point the concentrations of all of the components in the reaction ( products and reactants) are constant.
There are two different types of equilibrium: homogeneous and heterogeneous.
Homogeneous and Heterogeneous Equilibrium
In the case of homogenous equilibrium, all of the species (components) in the chemical reaction are in the same phase -- the same state, such as a gas, liquid or solid. For further reading, the BBC has produced a revision of the Haber Process, which is an example of homogeneous equilibrium.
In the case of heterogeneous equilibrium, not all of the species are in the same phase.
The Equilibrium Between Different Phases
Different phases are associated with different states, such as those involved in heterogeneous equilibria. There are several examples of this that can used to explain this, such as chromatography and immiscible liquids.
The equilibrium between two different phases depends on the partition coefficient of the two phases, which is another example of an equilibrium constant.
The Partition Equilibrium/ Coefficient
The partition equilibrium is reached because a solute will distribute itself between the two phases in the same ratio (so long as the temperature remains constant).
The Partition Coefficient is not dependant on how much solute there is, or by the amount of the different phases there are but it does matter which solute and which phases are used and it is affected by temperature.
Chromatography
Chromatography is the technique of separating a mixture of components for analysis. The two phases in chromatography are described as being stationary and mobile. Depending on the type of chromatography, the stationary and mobile phases are different, but in general the stationary phase is a liquid such as water held on a solid (such as in paper chromatography) or a solid, and the mobile phase is a gas or liquid (such as the solvent used in paper chromatography), according to ChemGuide.
In all chromatography techniques, it is the distance travelled by both the compound and the solvent that is measured, and is given a value, known as the "Rf value."
This is given by the equation: "Rf = distance travelled by compound / distance travelled by solvent."
The compound is whatever is being separated in the chromatography. Different components of the mixture go at different rates -- they are carried up the paper at different speeds and so go different distances -- some travel a long way, others hardly get off the start point (origin line).
Paper Chromatography
The most inexpensive type of chromatography is paper chromatography, and it it this that is most commonly come across in schools, in both biology and chemistry classrooms. The stationary phase is the water on the chromatography paper, not the paper itself! The solvent that is used is the mobile phase.
The paper is held over a beaker containing the mobile phase, and a spot of the component sample is placed near the base of the paper. When the paper is held in the beaker, the mobile phase is absorbed up through the paper, and carries the components up the paper, with the different components being deposited at different distances, dependant on how far they travel. (Chemguide)
Thin Layer Chromatography
Thin layer chromatography is used as a quicker method as opposed to paper chromatography (though inexpensive, it takes a while for the different components to finish travelling).
Paper is not used at all; instead, a thin layer of gel (hence the name) is painted onto glass or plastic. Again, it is the water held on the gel is the stationary phase, and the mobile phase is the solvent used.
Gas-Liquid Chromatography
Sometimes just called gas chromatography, this is the most expensive type of chromatography and so the least encountered. The stationary phase here is liquid which is on a solid inside a container in an oven. The mobile phase is an inert gas. The sample that is being tested is inserted into the container, and the oven set to a temperature that vapourises (turns to gas) the components of the sample, so these then travel through the container as gases, and it is the speed at which the certain component is moving alongside the mobile phase (the inert gas) instead of being a part of the stationary phase in the container that is measured.
For further reading on chromatography, including more in depth looks at Thin-Layer and Gas-Liquid Chromatography, I recommend visiting Jim Clark's Chemguide Section on Chromatography.
Sources
- Archie Gibb, David Hawley, BrightRed Advanced Higher Chemistry, pg 32, 35.
- BBC Bitesize, The Haber Process, 02/05/2011.
- Chemguide, Chromatography, 02/05/2011.
Hayleigh Hunter - Hayleigh describes herself as the eternal student - she loves to learn. Her articles aim to provide the best quality information ...
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