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Chemical Equilibria 205
Quick Tip
Don’t Forget!
At equilibrium the concentrations of the chemical species are constant, but not
necessarily equal. There may be a lot of C and D and a little A and B or vice versa.
The concentrations are constant, unchanging, but not necessarily equal.
If the temperature is constant and the reaction is at equilibrium, then the ratio
of the two reactions, the forward and reverse, should become a constant. This
constant is the reaction quotient, Q, and has the following form:
Q c [C]c [D] d
[A] a [B] b
This reaction quotient is a fraction. The numerator is the product of the chemical
species on the right hand side of the equilibrium arrow, each one raised to
the power of that species’ coefficient in the balanced chemical equation. The
denominator is the product of the chemical species on the left hand side of the
equilibrium arrow, each one raised to the power of that species’ coefficient in
the balanced chemical equation. It is called Q c, in this case, since molar concentrations
are being used. If this was a gas phase reaction, gas pressures could
be used and it would become a Q p.
Remember: products over reactants.
14-2 Equilibrium Constants (K)
We can write a reactant quotient at any point during the reaction, but the most
meaningful point is when the reaction has reached equilibrium. At equilibrium,
the reaction quotient becomes the equilibrium constant, K c (or K p if gas pressures
are being used). We usually express this equilibrium constant simply as a
number without units since it is a ratio of concentrations or pressures. In addition,