The Equilibrium Constant of an Ester Hydrolysis Reaction

CHM 152

Shaina Moon
Lucas Bodine
Professer James Giles
10/29/2014

Abstract: A series of five reaction mixtures were prepared and titrated to a pale pink endpoint using a standardized NaOH solution in order to find the equilibrium constant. The average equilibrium constant was calculated to be Kc=0.157.

Introduction: The purpose of this experiment was to determine the equilibrium constant (Kc), for the acid-catalyzed reaction between an unknown ester and water to produce an unknown alcohol and an unknown carboxylic acid. The equilibrium constant was found by using the equilibrium concentrations of the reactants and products that was determined by a combination of measurement and calculation.
Experimental: This experiment was conducted over a period of 3 lab classes. The first step was to obtain an unknown ester sample from the stock room. The unknown ester sample used was ethyl acetate (C4H8O2). The next step was to determine the density of the deionized water and the density of the 3 M HCl solution. The molar mass and the density were also determined for the ester and the alcohol. (Refer to table for density and molar mass). The reaction mixtures were prepared for five different numbered bottles with the volumes measured and recorded to 0.01mL precision. (Refer to table for reaction mixtures). Once the mixtures were prepared, the bottles were closed and the mixtures were shaken vigorously and stored in a lab drawer for one week in order for the mixtures to come to chemical equilibrium.
Once the mixtures were at chemical equilibrium, the next part of this experiment was to calculate the mass of sodium hydroxide needed to prepare 0.5L of a 0.7 M NaOH solution. The mass was calculated to be 14 g NaOH. Using a trip balance, NaOH pellets were roughly weighed out to the mass and the pellets were transferred into a polyethylene bottle. Deionized water was added to bring the total volume of the solution to about 500 mL and the NaOH pellets were then dissolved in the solution. The bottle was capped tightly to prevent atmospheric CO2 from entering and reacting with the NaOH.
The mass of potassium hydrogen phthalate (KHP) needed to neutralize 35 mL of 0.7 M NaOH solution was calculated to be 5.0 g KHP. A digital single-pan balance was used to weigh out the mass of KHP. The KHP was added to a 150 mL Erlenmeyer flask and was dissolved in 50 mL of DI water. Once the KHP was dissolved, two drops of phenolphthalein indicator was added to titrate the NaOH solution to a pale pink endpoint. This process was completed two more times and the average molarity of the NaOH solution was calculated to 0.692 M as the solution was now standardized.
For the final part of this experiment, the contents of the reaction mixtures of each numbered bottle was transferred to an Erlenmeyer flask, rinsing any remaining solution into the flask with deionized water. Once the reaction mixtures were transferred, deionized water was added to the flask to bring a total volume of the solution to 50 mL. Two drops of phenolphthalein indicator was added to the solution. The reaction mixtures were then titrated to a pale pink end point using the standardized NaOH solution that was prepared in the previous week.

Results:
Titration of NaOH
Trial
Titrated Amount

1
35.20 mL
2
35.50 mL
3
35.45 mL

Average
35.38 mL

Average Molarity
0.692 M NaOH

Density
Deionized Water
3 M HCl solution
Ester
Alcohol

0.9970 g/mL
1.07 g/mL
0.9003 g/mL
0.7893 g/mL

Molar Mass
Ester
Alcohol

88.11 g
46.07 g

Reaction Mixtures
Bottle #
3 M HCl (mL)
H2O(mL)
Ester (mL)
Alcohol (mL)

1
5
5
0
0
2
5
0
5
0
3
5
1
4
0
4
5
3
2
0
5
5
2
2
1

Moles in Reaction Mixtures
Bottle #
3 M HCl (mL)
H2O(mL)
Ester (mL)
Alcohol (mL)

1
0.0135 mol
0.277 mol
0
0
2
0.0135 mol
0
0.0511 mol
0
3
0.0135 mol
0.0554 mol
0.0409 mol
0
4
0.0135 mol
0.166 mol
0.0204 mol
0
5
0.0135 mol
0.111 mol
0.0204 mol
0.0171 mol

Titration of Reaction Mixtures