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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:
At this stage, a dynamic equilibrium has been reached. The reaction has not stopped it is simply moving in both directions at the same
rate.…read more

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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 so the reverse reaction is not allowed to take place.…read more

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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 significant
Eg H2(g) + I2(g) 2HI(g), closed system, reverse reaction significant reaction represented by equilibrium sign.
1.…read more

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It is therefore normally regarded as a single constant Kc and is known as the equilibrium constant
of the reaction.…read more

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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)
Kc = [HI] It has no units.
Eg PCl5(g) PCl3(g) + Cl2(g)
Kc = [PCl3][Cl2] It has units of moldm3.…read more

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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)
8…read more

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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 given, and the others must be deduced.…read more

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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 Chatelier's
"If a constraint is imposed on a system at equilibrium, then the system will respond in such a way as to counteract the effect of that constraint.…read more


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