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Topic 4.2


Dynamic equilibria
Equilibrium constants
Changing the position of equilibrium


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Consider a reversible reaction A + B C + D

As the reaction proceeds, the rate of the forward reaction decreases and the rate of the reverse reaction increases. Eventually, the reaction will
reach a stage where both forward and backward reactions are proceeding at the same rate:…

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This reaction would not be expected to proceed significantly under normal conditions, since water is more stable than steam at normal
temperatures. However puddles will disappear completely if left for long enough. This is because the water vapour is removed by wind currents
as soon as it is produced, and…

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In other reactions, even where the system is closed, the reverse reaction is insignificant:
Eg H+(aq) + OH(aq) H2O(l)
As the reverse reaction is not significant, the reaction is represented by single arrow.

Reactions are therefore only represented by equilibrium signs if
the system is closed
the reverse reaction is…

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kf/ kr is also constant at a given temperature. It is therefore normally regarded as a single constant Kc and is known as the equilibrium constant
of the reaction.

Kc = [C][D]2



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2. Units for equilibrium constants (Kc)

The units of the equilibrium constant vary, depending on the number of species involved. The units can be deduced by multiplying out the units of
concentration and cancelling as appropriate. The units of the above equilibrium constant are mol1dm3.

Eg H2(g) + I2(g) 2HI(g)…

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The equilibrium constants of the forward and reverse reactions are not the same, but are
related by a simple relationship:

Eg for the reaction 2SO2(g) + O2(g) 2SO3(g)

Kc(f) = [SO3]2 Kc(r) = [SO2]2[O2]
[SO2]2[O2] [SO3]2

The two equilibrium constants can be related by the relationship Kc(f)= 1/Kc(r)


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3. Calculations of equilibrium constants (Kc)

If the concentrations of all the reacting species are known, the equilibrium constant can be calculated by putting the values into the expression for
the equilibrium constant and computing the value.

Often, however, the concentration of only one of the species at equilibrium is…

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If the conditions are changed after equilibrium has been established, the system may no longer be at equilbrium and may move in one direction
or another to reestablish equilibrium. The direction in which the system will move to reestablish equilibrium can be predicted by Le…


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