Problem P7. Determination of Rate Constants for n-Butyl Acetate Hydrolysis

Hydrolysis, in other words splitting with water, is one of the most important reaction types that an ester can undergo. The hydrolysis of ester can be catalyzed either by acids or by bases and when the catalyst is a base, the reaction yields a carboxylate salt and an alcohol.

In this experiment, you are asked to determine rate constants of the alkaline hydrolysis of nbutyl acetate using sodium hydroxide, which is a typical second-order reaction.

The rate of the reaction can be calculated by the following equation:

Then Reaction rate = k[CH3COOC4H9][OH−]

1 [A] 1 [A]0 = k × ��,

where [A] is the concentration of reactant at time t ([A]= [B]), [A]0 is the initial concentration and k is the second-order constant, which has dimension of concentration–1 time–1 (L mol–1 s–1). In this case, a characteristic plot that will produce a linear function is 1/ [A] vs. time t, with a slope of k.

Chemicals Substance Name State GHS Hazard Statement

n-Butyl acetate Aqueous solution H226, H336, P210

NaOH Sodium hydroxide Aqueous solution H290, H314, P280, P301 + P330 + P331, P303 + P361 + P353, P305 + P351 + P338 + P310

HCl Hydrochloric acid Aqueous solution H290, H314, H335, P260, P280, P303 + P361 + P353, P304 + P340 + P310, P305 + P351 + P338 + P310

Phenolphthalein Solution H226, H319, P210, P280, P303 + P361 + P353, P337 + P313, P370 + P378, P403 + P235

Glassware and equipment

 1 Laboratory stand with burette clamp  5 Volumetric pipettes, 20 mL  5 Pipette pump  5 Titration flasks, 250 mL  10 Volumetric flasks, 250 mL  1 Burette, 50 mL  Stop watch

Procedure

1. Fill the burette with the solution of hydrochloric acid (HCl) (0.02 M).

2. Transfer 60 mL of the solution of n-butyl acetate (0.02 M) into the volumetric flask and 60 mL of the solution of sodium hydroxide (0.02 M) into another volumetric flask at room temperature. Mix the two solutions in titration flask.

3. Five minutes after mixing, pipette 20 mL of reaction mixture into a titration flask. Add 4 drops of phenolphthalein indicator to the solution.

4. Titratethe sample solution with HCl (0.02 M) until the solution become as colorless. Record the amount of HCl used. Hint: you can add 6 ml of HCl solution immediately and then carry out the rest of titration with more care.

5. Repeat steps 3 and 4 for 10, 15, 20, and 25 minutes from the moment of mixing. Fill in the table below.

Hint: You can repeat each step several times to increase the accuracy of data.

Calculations & Analysis:

Fill in the blanks in the following table with the data measured during the experiment.

Time (min) VHCl (mL)

5 10 15 20 25

P7.1. Calculate the concentration of [OH−] at each time.

P7.2. Plot [ 1 OH−] vs. time .

P7.3. Calculate the rate constant.

P7.4. Calculate the reaction rate.

P7.5. Calculate the initial half-life for the reaction with initial conditions.

Solution:

Filled table with the measured data in the experiment.

P7.1. Time (min) VHCl (mL)

5 10.8 10 10 15 9.4 20 9 25 8.6

t (minutes) t (s) Remaining NaOH (mol) Remaining [NaOH] (mol/L) 1/[NaOH]

5 300 0.000216 10 600 0.0002 20 1200 0.000188 15 900 0.00018 25 1500 0.000172 0.0108 92.5925926 0.01 100 0.0094 106.382979 0.009 111.111111 0.0086 116.27907

P7.2.

P7.3.

1 [A] 1 [A]0 = k.��,

k = 0.0195 L mol–1 s–1

P7.4.

Reaction rate = k[CH3COOC4H9][NaOH] Reaction rate = 0.0195 ��������−1�� −1×0.01 ��������−1×0.01 ��������−1=1.95×����−����������−���� −��

P7.5. The half-life (t1/2) equation for a second-order reaction is given by:

��1/2 =

��1/2 =

1 k × [A]o

1 0.0195 ��������−1�� −1 × 0.01 ��������−1