Equilibria

summary on chapter 3 and 4 of george facer's a2 edexcel chemistry second edition textboon

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Equilibruim - Revision
Dynamic equilibruim occurs when there is no further change in the concentrations of the reactants
and products because the forward and backwards reactions are happening at the same rate.
The equilibrium constant, Kc
In general, for a reaction,
xA + yB nC + mD
where x, y, n and m are stoichiometric amounts in the equation,
Kc = [C]n[D]m
HOWEVER, this only works for homogeneous equations (where all the reactants and products are in
the same state). You do not include any term for a solid in the equilibrium equation.
This only applies when the reaction is at equilibrium. The only things which will affect Kc are
temperature and the equation itself.
If the equation is written the other way around, then Kc will be different.
Kc = 1
Units of Kc ­ you have to cancel out the concentration's units, in the fraction of the equation. If the
concentration is raised to a power, then the units also have to be raised to that power also.
The equilibrium constant, Kp
This is in terms of partial pressure, which is the contributions of each gas in a mixture to the total
pressure. The partial pressure of a particular gas in a mixture is the same pressure it would exert if it
were alone.
Partial pressure = mole fraction x total pressure
In general, for a reaction,

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A + yB nC + mD
where x, y, n and m are stoichiometric amounts in the equation,
Kp = (pC)n(pD)m
Temperature, pressure and the reaction itself affect Kp. The Kp equation never includes anything but
gases.
Relationship between the equilibrium constant and the total entropy change
Stotal = RlnK
Where R is the gas constant. If s-total increases, the value of K also increases. The magnitude of H/T
always decreases as the temperature is increased.…read more

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