Determination of Equilibrium Constants
Many chemical reactions, such as the reaction of acetic acid with
isopropanol shown below are reversible.
The forward and reverse reactions each proceed at a rate governed by
their rate law, the concentrations of reactants and the magnitude of the rate
constant for each reaction, k1 and k-1
for the forward and reverse reactions, respectively.
When the rate of the forward reaction equals the rate of the reverse
reaction, the reaction is at equilibrium.
At equilibrium, the concentrations of reactants and products remain
constant and are determined by the equilibrium constant, KC for the reaction. The
equilibrium constant is the ratio of product concentrations (raised to the
powers given by their coefficients in the balanced chemical equation) over the
reactant concentrations (also raised to the powers given by their coefficients
in the chemical equation). For the
reaction between acetic acid and isopropanol,
KC = [CH3CO2CH(CH3)2][H2O]
reaction between acetic acid (a carboxylic
acid) and isopropanol (a secondary
alcohol) is an example of an esterification reaction. The product isopropyl acetate is an
ester. Esters contribute to the odors
and flavors of many foods and beverages.
For example, it is believed that as wine ages the distribution of esters
slowly changes, improving the quality of the wine. To reach equilibrium between
acetic acid and isopropanol in a reasonable period of time, a trace of sulfuric
acid will be added to our reaction as a catalyst.
To determine the
equilibrium constant, we need to measure the equilibrium concentrations of the
reactants and products. However, if we
start with equal amounts of the reactants
then the equilibrium concentration of products and remaining reactant
are determined by the stoichiometry of the reaction. For example, consider the case where a
solution that is initially 10.0 M in acetic acid and 10.0 M isopropanol is
allowed to come to equilibrium. If the
equilibrium concentration of acetic acid is 3.0 M, then the equilibrium
concentration of isopropanol is also 3.0 M.
Since one mole of acetic acid
reacts with one mole of isopropanol to produce one mole each of isopropyl
acetate and water, [CH3CO2CH(CH3)2] = [H2O]
= 7.0 M and KC = 5.6
We can measure the concentration of acetic acid remaining at
equilibrium by titrating it with 0.200M NaOH allowing us to find KC
for the reaction. Since acetic acid reacts with NaOH in a 1:1 mol ratio, the
concentration of acetic acid can be determined from:
= (corrected volume of 0.200M NaOH used)(0.200 M NaOH)
equilibrated solution used)
The presence of even a small amount of sulfuric acid, however,
requires that we correct the volume of NaOH used in the titration to neutralize
the H2SO4 by titrating a blank containing the same amount
of sulfuric acid as our reaction mixture.
We can also substitute a different alcohol for isopropanol or a
different carboxylic acid for acetic acid and apply the same method for
determination of the equilibrium constant. In practice, many simple carboxylic acids
have noxious odors so we will restrict ourselves to testing different alcohols.
In this experiment we will
measure the equilibrium constant for three reactions:
· a mixture of acetic acid and isopropanol
· a mixture of isopropyl acetate and water and
· a mixture of acetic acid and an unknown alcohol
WEAR SAFETY GLASSES AT ALL
TIMES!! Concentrated sulfuric acid and glacial acetic acid are corrosive and
dangerous. If you spill any on yourself,
rinse the exposed area with water immediately and inform the instructor.
This experiment will take two weeks to
complete so read the directions carefully to determine where you should stop. In this experiment you will become proficient
at titrations as numerous titrations are needed to obtain a decent data
set. Developing your titration skills is
important, as you will use them in future experiments to identify unknowns.