1.1 What is Chemistry?

19.2. Applications of Equilibrium Constants http://www.ck12.org

0 mol+ 2 x= 0 .01966 mol

x= 0 .00983 mol

Then, use the value ofxto determine how much N 2 O 4 is present at equilibrium:

0 .8450 mol−x= 0 .8450 mol− 0 .00983 mol= 0 .83517 mol

At equilibrium, our reaction mixture contains 0.01966 moles of NO 2 and 0.83517 moles of N 2 O 4. Because the
reaction is taking place in a 1-L container, the concentrations of these two components will be 0.01966 M and
0.83517 M, respectively. Plugging these values into the equilibrium constant expression, we get the following:

Kc=

[N 2 O 4 ]

[NO 2 ]^2

Kc=

[ 0. 83517 ]

[ 0. 01966 ]^2

Kc= 2. 16 × 103

Again, the equilibrium constant for this reaction has a value of 2.16× 103. The measured equilibrium constant for a
given reaction at a set temperature is always the same, regardless of whether we start with all reactants, all products,
or some combination of the two. Even though the final concentrations depend on the initial conditions (the final
concentrations are different in the previous two example problems), the ratio defined by the equilibrium constant
expression will have the same value once the system reaches equilibrium. This shows that the equilibrium constant
is the same for any given chemical reaction at a specified temperature, regardless of the initial amounts of reactants
and products.

Interpreting the Value of an Equilibrium Constant

What does the value of Kctell us about the relative position of the equilibrium? Let’s look at the results of
the previous example problem. At equilibrium, we had a product concentration of 0.83517 M and a reactant
concentration of 0.01966 M. Because there are more products than reactants at equilibrium, we say that this reaction
isproduct-favoredat the given temperature. Similarly, reactions in which the reactants are more prevalent than the
products at equilibrium are said to bereactant-favored.

How can we tell whether a reaction is product-favored or reactant-favored without actually calculating equilibrium
amounts? Remember that in the equilibrium constant expression, the product concentrations are in the numerator,
whereas the reactant concentrations are in the denominator. In a product-favored equilibrium, the value of the
numerator would be much larger than the value of the denominator, so Keq>1 means that the products are favored
over the reactants. Conversely, a Keq<1 would suggest that the reactants are favored over the products. Finally, a
Keqvalue that is approximately equal to 1 means that significant amounts of both the reactants and the products are
present at equilibrium.

Lesson Summary

• The equilibrium constant is a ratio that describes the relative equilibrium concentrations of products and
  reactants for a given chemical process.


• An equilibrium constant can be determined from experimental data.