Using Gas Chromatography to Measure Chemical Equilibrium
From Modern Chemical Technology (Vol 2)
By Harry G. Hajian and Robert L. Pecsok
Copyright 1990, New Jersey: Prentice Hall

In this experiment you will run an equilibrium reaction in solution.  Observable changes in concentration will occur initially, but eventually the concentrations of all products and reactants will remain unchanged. The reaction is:

  Ethyl acetate + Methyl alcohol   Methyl acetate + Ethyl alcohol

Some students should run the forward reaction using ethyl acetate and methyl alcohol as reactants;  others should run the reverse reaction between methyl acetate and ethyl alcohol.

Part A: Attainment of Equilibrium

In the first part of this experiment, you will run a reaction between two organic substances.  You will monitor the decrease in concentration of reactants, as well as the increase in product concentration by gas chromatography.

SAFETY PRECAUTIONS  The organic reagents used in the experiment are flammable;  be sure that no flames are used in the laboratory.  Methyl alcohol is toxic and is easily absorbed through the skin.  Take precautions to keep this substance off your hands.  Observe the standard precautions when handling concentrated hydrochloric acid.
 

PROCEDURE

1.  Prepare the following table in your notebook.  Calculate the weight of 0.5 mole of each substance.  Use the value for the density of each substance to calculate the volume of 0.5 mole of each.  Record the values you calculate in the table.
 

3.  Determine the retention times of ethyl acetate, methyl acetate, methyl alcohol, and ethyl alcohol by injecting standards onto your column.

4.  Measure out a 0.50 sample of ethyl acetate in a clean dry, 250 mL flask.  Similarly, measure out and add 0.50 mole methyl alcohol to the flask.  Loosely stopper the flask with a cork stopper and swirl the contents.  (If you are assigned the reverse reaction, you will add 0.50 mole of methyl acetate plus 0.50 mole ethyl alcohol to the flask.)

5.  Inject a 0.5 µL sample of the reaction mixture into the GC to obtain a chromatogram of the starting materials.  How many peaks (other than air) are present?  Compare the retention times of the peaks with the standard peaks obtained in Step 3.  Do the peaks correspond to the starting material.

6.  Add 0.3 mL of concentrated hydrochloric acid with a pipet to the reaction mixture and swirl the flask gently.  Place a cork stopper loosely in the flask.  (The hydrochloric acid serves as a catalyst for the reaction.  If it were not added, the reaction would perhaps require weeks to reach a state of equilibrium.)

7.  Set up a water bath and bring its temperature to 45^o C.  Place the loosely stoppered flask containing the reaction mixture into the water bath.  Maintain the temperature at 45^o for 15 minutes.  Periodic stirring of the water bath with a glass stirring rod will keep the temperature of the bath uniform.

8.  After the flask has been in the water bath for 15 minutes, transfer a 1.0 mL sample from the reaction mixture with a pipet into a clean flask.

9.  Inject 0.5 µL of the reaction mixture into the GC.  Label the chromatogram.  Is there any difference between this chromatogram and the first one you ran on the mixture?  Using the standard retention times from Step 3, identify the peaks present.  Has a reaction occurred?  What is the basis for your answer?

10.  After an additional 15 minutes have elapsed, take another 1.0 mL sample and inject 0.5 µL into the GC to obtain another chromatogram of the mixture.  Are there any changes in the chromatogram?  Compare the relative areas of the peaks that are present in this chromatogram with the previous two.  Label this chromatogram.

11.  Repeat Step 10 two more times at 15 minute intervals.

12.  Remove the reaction flask from the water bath.  After the contents have reached room temperature, obtain another chromatogram of the mixture.

13.  Stopper the flask tightly, label it, and store it.  During your next laboratory period obtain one more chromatogram of the mixture.

14.  Save your reaction mixture for Part B.
 

What The Experiment Shows

After comparing results with other students you will see that the products of the forward reaction are the same substances are the reactants of the reverse reaction and vice versa.  That is, the reaction is clearly reversible.

Let us now consider only the forward reaction between ethyl acetate and methyl alcohol;  our conclusions will also apply to the reverse reaction.  When ethyl acetate and methyl alcohol are first mixed, no reaction occurs.  However, when the acid catalyst is added and the solution warmed, products begin to form.  Gradually the concentration of the products increases.  The forward reaction is faster than the reverse reaction at this time.  As more and more products form, the reverse reaction increases at a faster rate.  (This is what you should expect from our discussion of rates in the last chapter.)  Also, as the concentration of reactants decreases, the forward reaction becomes progressively slower.  Eventually - and this probably required 24 hours - the relative concentrations of all reactants and products remain constant, because the rates of the forward and reverse reactions become equal.

It is interesting that the factor of concentration, important in determining rates of reactions, also determines the position of equilibrium of a system.  Let us investigate this relationship further.          

Part B:  Shifting The Equilibrium

In this part of the experiment you will change the position of the equilibrium by adding one of the substances involved in the reaction to your equilibrium mixture of Part A.

PROCEDURE

1.  Obtain a chromatogram of your mixture from Part A.  It should be at equilibrium.

2.  Divide your reaction mixture from Part A in half.  Label with "Part A Reaction" and "Part B Reaction."

3.  Calculate the volume of 0.2 mole of any one of the substances involved in the reaction.

4.  Add the required volume of the substance using a pipet you have chosen to the Part B reaction mixture.  Immediately inject a sample of the new mixture into your GC.  Obtain a chromatogram, label, and measure the areas of the peaks.

5.  Place the Part B mixture flask in a 45oC water bath, and stir the mixture periodically.

6.  Analyze the mixture by GC after 15 minutes.

7.  Remove the reaction flask from the water bath.  After the contents have reached room temperature, obtain another chromatogram of both mixtures.

8.  Obtain another chromatogram of both mixtures (A and B).  If the relative areas of the peaks have changed, save the mixtures for the next lab period and repeat.  If no change is seen, no further chromatograms need to be taken.
 

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