Transcript
Reaction Kinetics of Alkaline
Hydrolysis of an Ester
Student name:
Student number:
Lab partners:
CHE 214, Section 02
Group 3A
April 15ft, 2013
TA:
Introduction and Background
Hydrolysis of ester occurs as ester reacts with water molecule or any dilute acid to produce alcohol and anion due to the breaks of chemical bond as it react with the hydroxide anion OH-. However, the alkaline hydrolysis of ester involve the reaction of ester reacts with sodium hydroxide NaOH aqueous solutions to increases the reaction rate since the hydrolysis of ester with water is slow and produce alkaline solution due to the separation of the ions in the reactions. Therefore, the purpose of this lab was to determine experimentally the kinetics of the alkaline and the specific rate constant of the hydrolysis of ethyl acetate by measuring the resistance of the solution at certain time using stop watch and conductivity cell meter. This lab divided into two parts. Firstly, a 0.0200N of NaOH and ethyl acetate placed in a jacketed beaker and mixed together by magnetic stirring to measure the cell resistance at room temperature of 25oC.The hydrolysis of ethyl acetate with sodium hydroxide can be representing by the reaction showing below:
C2H5OOCCH3 + OH- = C2H5OH + CH3COO-
Then, the reading of the cell resistance was taken after three minute from the mixing solution for every two minutes for the next forty minutes to measure the resistivity of the solution since resistance of the solution is increasing proportional with time.
Secondly, the solution mixture was then transferred into graduated cylinder and placed in water bath of 70oC to increase the rate of reaction. Then the solution allowed equilibrating at room temperature to measure it resistivity. After measuring two successive resistance value of the solution as it placed into water bath of the 70oc, the average resistance R and the specific rate constant was calculated by using the formulas below:
The resistance at different time was calculated by using the relation showing below:
R
27622501333500R-R
Where R is the resistance in
R is the average of the two successive resistances within 1% of each other
The rate constant was computed by using the equations shown below:
276225022479000K= slope
a Ro/ (Ro-R)
281940016827500= slope
a (intercept)
Where k is the rate constant
a is the initial reactant concentration in N (mol/L)
In conclusion to all the equations shown above, the result, calculation and graph will demonstrate each one in their own functional way.
Results and Calculations
R was calculated by taking the average of R1 & R2 after heating the solution to 70oC:
R1= 846.0
R2= 850.3
R = (R1+R2) /2= 848.2 R aver = R
Table1. Showing the Calculation of the R/(R-R) at different time in min
Time (min)
Resistance Cell R ()
R/(R-R)
3
503.4
-1.4600
5
525.2
-1.6260
7
544.9
-1.7966
9
562.3
-1.9668
11
578.6
-2.1461
13
593.0
-2.3237
15
606.5
-2.5093
17
618.8
-2.6975
19
630.0
-2.8873
21
640.5
-3.0838
23
650.2
-3.2838
25
659.4
-3.4926
27
667.7
-3.6992
29
675.6
-3.9143
31
683.1
-4.1375
33
689.9
-4.3582
35
696.7
-4.5987
37
702.7
-4.8296
39
708.7
-5.0803
41
714.3
-5.3346
43
719.6
-5.5956
Sample calculation of the R/R-R at time 3 min:
R = 503.4
R= 848.2
R = [503.4 / (503.4 -848.2)] = -1.4600
2476501333500 R-R
The specific rate constant k from equation (12) was calculated as shown below:
16192522479000K= slope
a Ro/ (Ro-R)
16192516764000 = slope
a (intercept)
a is the initial reactant concentration that is ½ the concentration of the stock solution
a = 0.0200N/2 1N= 1 mol L-1
= 0.0100N
y = -0.1027x - 1.0108
k = -0.1027 min-1/ [0.0100N * (-1.0108)]
=10.16 N-1 min-1
Graph1. The Relationship of R/R-R verse the time in min
Discussion and Conclusions
The purpose of this experiment was to measure the resistance cell of the hydrolysis of ester to determine its specific rate constant. In the first part of the lab, the resistance cell of ethyl acetate and sodium hydroxide mixture solution was measured at room temperature of 25oC at different time for 43 minutes to observe the relationship between time and the resistance. The resistance reading was taking for every 2 minutes until 43 minutes of the stirring after adding a (50 mL) of 0.0200N of ethyl acetate to the (50mL) of 0.0200N NaOH in a jacketed beaker. Thus, at the time 3 , 5 and 7 minutes , the resistance cell of the solution were (503.4,525.2and 544.9) respectively which indicate that the resistance of the solution was increased linearly as the time increased due to separation of the ions and hence the ionic conductivity of OH- that is higher than the acetate in the solution.
In the second part of the lab; however, the solution was transferred from the jacketed beaker into a graduated cylinder to place the solution into a water bath of 70oC. The reason for heating the reaction solution to 70oC was to increase the rate of reaction, separate the product easier and hence calculate the average resistance R. Until two successive values of R were within a 1% of each other a (848.2) of R was obtained after heating the reaction solution into water bath of 70oC for two times and taking the average of the cell resistance for the two trials R1= 846.0 and R2= 850.3. As a result the specific rate constant was calculated and found to be 10.16 N-1 min-1.
However, there are a few sources of error that observed in this lab that might cause into inaccurate value of the rate constant. Firstly, the pipette was not dry completely after washing it with distilled water as pipet a 50mL of ethyl acetate into a beaker which might contributed to interaction between the water molecules and the ethyl acetate and thus slow little bit the reaction as it mixed with the NaOH and hence and error in obtain the resistance cell. Secondly, the solution was not heated for exactly 15 minutes into a 70oC of the water bath which in return cause a slightly significant increasing in the resistance.
In conclusion, the resistance of the solution is directly propositional with time. As the time increased the resistance cell of the mixing solution increased too. Also, the resistivity of a solution increased as the temperature of the solution increased as it heated to maximum of 70oC due to the increasing in kinetic energy of substance and the heat transfer between the solution mixture and the surrounding. The specific rate constant of the hydrolysis of ethyl acetate was accurately obtained to be 10.16 N-1 min-1at 25oC as the slop and the intercept of the resistance cell with time were negative from the graph1 and the value of R1 and R2 were within 1% of each other.
References:
Laboratory Manual for Thermodynamics II, Reaction Kinetics of Alkaline Hydrolysis of an Ester, 2013, page 8-10.
Jim Clark, Helping to Understand Chemistry Online, Hydrolysing Esters, analytical Chemistry, 2004
http://www.chemguide.co.uk/organicprops/esters/hydrolysis.html
