http://www.ck12.org Chapter 19. Chemical Equilibrium
FIGURE 19.4
The brown color is from nitrogen dioxide
gas. Nitrogen dioxide is a byproduct from
the combustion of fossil fuels like coal and
gasoline.Calculating Equilibrium Constants
Experiments are very useful for determining equilibrium constants for various reactions. We are going to give a
simple example of an experiment that could be done to determine the equilibrium constant for this reaction.
The equilibrium constant expression for the reaction above has the following form:
Kc=[[NNO^2 O 2 ]^42 ]
In order to calculate the value of Kc, we need to determine the concentrations of both the reactant and the product
for any system that has reached equilibrium. One possible method for achieving this is as follows:
- First, we would put a known amount of our reactant (NO 2 ) into a rigid container of fixed volume.
Remember: The equilibrium constant expression only applies for a specific constant temperature, so we want to
keep the temperature consistent throughout course of the reaction. Most equilibrium constants are given
for a standard temperature of 25°C.- After putting our reactant into the container, we would allow the system to reach equilibrium. We know that
the system is at equilibrium once the concentrations of each component are no longer changing over time.
Because N 2 O 4 is colorless but NO 2 is brown, the concentration of NO 2 can be determined by colorimetry,
which relates the amount of visible light absorbed by a given sample to its concentration. - Once at equilibrium, we will determine the concentration of the remaining NO 2 , and the concentration of
N 2 O 4 can be calculated by knowing the total amount of material present in our reaction vessel. - Finally, these values can then be plugged into the equilibrium constant expression to determine the value of
the equilibrium constant.
Example 19.4
0.2000 moles of pure NO 2 is placed in a rigid 1-liter container at a fixed temperature. The system is allowed to reach