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Equilibrium Shift
Name: Anna
Status: educator
Grade: n/a
Country: Canada
Date: Spring 2011
Question:
Is it possible that sometimes adding a pure liquid (like
water) or a solid to a system in equilibrium will produce a shift in
the equilibrium? (adding or removing solids or liquids is not
supposed to change equilibrium according to theory...)
Replies:
Yes, it can - I am afraid you may have an incomplete understanding
of the theory. A good source to consult is Physical Chemistry by
Atkins and DePaula (any edition).
Equilibrium constant expressions
do not usually include solvents because the
standard states are chosen to have unit activity for pure solids
and liquids. So changing the amount of a pure solid or pure
liquid does not enter into the equilibrium constant expression...
*unless* the liquid is a *solvent* in the reaction.
In that situation, it matters whether the solution
is dilute or concentrated. If the solution is very dilute we can
treat the solvent as a "pure liquid" and neglect it from the
equilibrium constant expression. However, if the solution is very
concentrated then the activity of the solvent is no longer equal
to one, and then it needs to be explicitly included in the
equilibrium constant expression.
Here is an example. The reaction of acetic acid with water is
HAc + H2O <-> Ac- + H3O+. When the acid is dilute we can get away with
writing the equilibrium constant as Ka = [Ac-][H3O+]/[Hac].
However, when the acid is very concentrated, reaction of the acid
with the water it is dissolved in measurably changes the concentration
of water in the solution, and so the full equilibrium constant formula
K = [Ac-][H3O+]/[Hac][H2O] must be used.
Hope this helps!
best, dr. topper
Anna,
Remember that the equilibrium constant is equal to the concentrations of the substances raised to their stoichiometric coefficients. Since pure liquids and solids do not have concentrations, they do not enter into the equation. So, as reactants and products, pure liquids and solids do not affect the chemical equilibrium.
However, water can act as a solvent. So if there are more moles of reactant in aqueous solution form tan there are of the products, than adding water will preferentially dilute more moles of reactants, this would preferentially slow down the forward reaction, and more reactants are produced than products.
I cannot think of a special case for solids.
Greg (Roberto Gregorius)
Canisius College
I am not sure whose theory you are citing, that," adding or removing solids
or liquids is not supposed to change equilibrium according to theory..."
That simply is not true. There are many counter-examples, but here are just
a few.
(1) Add acetone, or most any other pure water soluble organic
compound to an aqueous solution of a water soluble ionic salt greatly
reduces the water solubility of the salt.
(2) Conversely, adding a pure
water soluble ionic salt to an aqueous solution of an organic compound
reduces its water solubility. In the "chemical" jargon, this is called the>"salting out effect". Starting with a false premise will lead to incorrect
consequences. You need to correct your original premise.
Vince Calder
Anna,
You can definitely change the equilibrium of a reaction by adding water
or a solid. The equilibrium is based on the ratio of concentrations for
each component in the reaction vessel. This is the Keq value for the
system. So if the addition of water or solid changes the concentration
ratio the equilibrium will adjust. This is shown by using Le Chatelier's
principle in chemistry classes. When the concentration is adjusted the
equilibrium will shift in predictable a manner to react to the change.
The times when this addition would not change the equilibrium would be
when the solid/liquid does not interact with the solution itself or does
not change the ratios between concentrations. If the addition of water
simply dilutes all of the concentrations the same then the overall
equilibrium will not shift as the overall ration does not change.
Brad Sieve
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