LS 1a Fall 2014
Calculating Keq: What do we mean by “multiplicative product”?
When we define the equilibrium constant Keq as the “multiplicative product of the
concentrations of the products over the multiplicative product of the reactants” when the
system is at equilibrium, we often use this symbolic short-hand:
Keq=
[Products]
[Reactants]
By this we mean that Keq is calculated by multiplying the concentration of EACH product, and
then dividing it by the concentration of EACH reactant.
Some examples:
[B]
1) If the reactionis
, then the Keq will equal [A] when the system is at equilibrium.
2) If the reaction is
, then the Keq will equal [A][B] when the system is at
[C][D]
equilibrium.
3) If the reaction is
[D][E][F}
, then the Keq will equal [A][B][C] when the system is at
equilibrium.
And so on...
What you may have picked up from the notes, but is worth emphasizing, is that equilibrium
constants do not have units. This is because each concentration value in the Keq expression is
divided by “1 unit standard concentration,” so that what we are actually using “activities” of
each chemical rather than “concentrations” (an exceptionally minor point that is not worth
dwelling on, or even knowing really).
The “standard unit of concentration” is 1 mole/liter. Thus (X moles/liter) divided by (1
mole/liter) leaves you with the value X and no units. For example, if one of the components of a
reaction at equilibrium existed at 5 mM, then it would be included in the Keq calculatuion as
“0.005” rather than “0.005 mM.”