Download Reaction rate and activation energy of the acidolysis

LEC
05.02
Reaction rate and activation energy of the acidolysis
of ethyl acetate
Related concepts
Reaction rate, reaction rate constant, rate law for first and second order reactions, reactions with pseudo order, Arrhenius
equation, activation energy.
Principle
In acid solution, ethyl acetate is hydrolysed to equivalent quantities of ethanol and acetic acid according to a pseudo-first order
rate law. The alkalimetric determination of the acetic acid formed
enables conclusions to be drawn on the temporal concentration
of ester.
Tasks
Determine the reaction rate constant for the acidolysis of ethyl
acetate at two (or more) temperatures. Calculate the activation
energy of the reaction from the temperature dependence of the
measured rate constants.
Equipment
Immersion thermostat, 100°C
Accessory set for immersion thermostat
Bath for thermostat, 6 l, Makrolon
Rubber tubing, di = 6 mm
Hose clip, d = 8…12 mm
Digital thermometer
Immersion probe NiCr-Ni
Stopwatch, digital, 1/100 s
Magnetic heating stirrer
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Magnetic stirrer bar, l = 15 mm
Magnetic stirrer bar, l = 30 mm
Support rod, l = 500 mm, M10 thread
Retort stand, h = 750 mm
Burette clamp, roller mounting
Right angle clamp
Universal clamp
Burette, 50 ml, with Schellbach line
Graduated cylinder, 100 ml
Volumetric flask, 1000 ml
Volumetric pipette, 5 ml
Volumetric pipette, 100 ml
Pipettor
Pipette dish
Pasteur pipettes
Rubber bulbs
Crystallisation dish, 1000 ml
Erlenmeyer flask, 250 ml, wide neck
Erlenmeyer flask, 250 ml, narrow neck, SB 29
Rubber stopper, 17/22 mm
Glass beaker, 250 ml, short
Funnel, glass, do = 55 mm
Wash bottle, 500 ml
Ethyl acetate, 250 ml
Hydrochloric acid, 1 M, 1000 ml
Sodium hydroxide solution, 1 M, 1000 ml
Phenolphthalein solution, 1%, 100 ml
Water, distilled, 5 l
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Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3050201
1
LEC
05.02
Reaction rate and activation energy of the acidolysis
of ethyl acetate
Set-up and procedure
Set up the experiment as shown in Fig. 1.
Prepare 0.2 molar NaOH solution by pipetting 200 ml of
1.0 molar sodium hydroxide solution into a 1000 ml volumetric
flask and filling up to the calibration mark with water.
Fill the burette with 0.2 molar NaOH solution.
Pipette 100 ml of 0.1 molar hydrochloric acid solution into an
Erlenmeyer flask, seal it with a stopper, and temperature equilibrate it for approximately 15 minutes at 25 °C (measure the
exact temperature T1). Start the reaction by adding 5 ml of ethyl
acetate (room temperature). Shake the flask briefly, then replace
it in the temperature controlled bath. After 10 minutes, and at
further intervals of 10 minutes, take 5 ml samples and transfer
them into a wide neck Erlenmeyer flask containing 100 ml of
cold water. This will stop the reaction immediately. Titrate the
solutions with as little delay as possible with the 0.2 molar sodium hydroxide solution, using phenolphthalein as indicator.
Terminate the measurement series after a reaction time of
50 minutes. Repeat the above procedure at a temperature of
45 °C (T2).
The volumes of NaOH at time t0 (VNaOH; 0, neutralisation of the
constant quantity of HCl) and subsequent to complete conversion (VNaOH; ∞) are required for the evaluation. They can either be
calculated (see ‘Theory and evaluation’) or be determined experimentally as follows.
To determine VNaOH; ∞, after concluding the first measurement
series, heat the solution which was converted to the greatest
extent to approximately 70°C in a water bath on the magnetic
stirrer. The reaction will go to completion at this temperature.
Allow the solution to cool, then titrate it with 0.2 molar NaOH
solution as described above.
To determine the initial consumption VNaOH; 0 titrate 5 ml of the
0.1 molar hydrochloric acid solution used, whereby the volume
must be corrected by a factor of 100/105 for the ester portion
which is absent here.
The ester concentrations cE; 0 and cE at time t0 and t can be
replaced by the volumes of NaOH required for neutralisation of
the samples at the start (vNaOH; 0), during the reaction (VNaOH)
and after complete conversion (VNaOH; ∞):
ln
VNaOH,q VNaOH,0
VNaOH,q VNaOH
ln Q k' t
(1.3)
The volumes VNaOH; 0 and VNaOH; ∞ can be experimentally determined (see ‘Set-up and procedure’) or be calculated using relationships (2.1) and (2.2):
VNaOH,0 where
cHCl
cNaOH
V1
(2.1)
Concentration of the HCl solution (= 1.0 mol/l)
Concentration of the NaOH solution (= 0.2 mol/l)
Sample volumes (= 5 ml)
VNaOH,q where
rE
ME
VE
cHCl V1 100
·
cNaOH 105
rE VE V1
VNaOH,0
ME VS cNaOH
(2.2)
Density of ethyl acetate at T = 298 K (= 0.895 g/ml)
Molar mass of ethyl acetate (= 88.12 g/mol)
Volumes of ethyl acetate contained in the volume of
the total system VS = 105 ml at time t0 (= 5 ml)
In accordance with equation (1.3), the plot of the expression
ln [(VNaOH; ∞ - VNaOH; 0 / VNaOH; ∞ - VNaOH)] as a function of time
results in a rising straight line with a slope of k’ (Fig. 2).
Theory and evaluation
The acid ester hydrolysis is described by the equilibrium
[H3O+]
CH3COOC2H5 + H2O
CH3COOH + C2H5OH
Under the given experimental conditions, equilibrium is shifted
quantitatively towards the reaction products. The reaction velocity (rate) vR of this reaction is given by the rate law:
vR where
k
cE, cW, cK
dcE
k cEcWcK
dt
Fig. 2: Graphic determination of the reaction rate constants for
the acid hydrolysis of ethyl acetate at two temperatures
(x: T1 = 299.5 K, o: T2 = 314,15 K; c(H3O+) = 1.0 mol · l-1;
ln Q = ln[(VNaOH; ∞ - VNaOH; 0)])
(1)
Reaction rate constant
Concentration of ester, water and catalyst at time t
The rate of the reaction investigated is a function of the acid concentration and can be controlled by it.
As a result of the practical constancy of the concentrations of
H2O (stoichiometric excess) and H3O+ (catalyst), this reduces to
dcE
k' cE
dt
(1.1)
The rate of hydrolysis thus conforms to a pseudo-first-order time
rule whose integration results in the following :
ln
2
P3050201
cE,0
cE
k' t
(1.2)
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEC
05.02
Reaction rate and activation energy of the acidolysis
of ethyl acetate
The constant k includes the dependence of the reaction velocity on the binding conditions of the participating molecules, the
type of reaction and the temperature. For two molecules to
react, they must not only collide, but also have a sufficient energy content. The activation energy EA is the difference between
the average energy content prior to reaction and the energy
required for reaction. The molecules obtain the energy that is
needed for activation from heat supplied, from light and from the
exchange of energy when collisions occur. Such take-up of
energy activates the molecules (loosens bonds, polarisation etc.)
so that they can react. The portion of molecules with this
increased energy content increases with increasing temperature.
The greater the portion of the molecules capable of reaction, the
more molecules that will react, and so the higher the reaction
velocity.
The activation energy can be determined using the empirical
Arrhenius equation:
k’ = kmax · e
where
R
kmax
EA
RT
kmax is the velocity constant which would be given when every
every collision resulted in reaction, i.e. when the activation energy was 0.
For two known pairs of values having the rate constants k’1 and
k’2 and the temperatures T1 and T2, using
EA
ln kmax
RT
ln k'1 EA
ln kmax
R T1
(3.11)
ln k'2 EA
ln kmax
R T2
(3.12)
from which, by subtraction
EA R ·
T1 T2
k'2
· ln
T2 T1
k'1
(3.2)
If further data regarding k’ and T are available (i.e., measurements at a number of temperatures), then the activation energy
can alternatively be determined from the slope of the linear relation between ln k’ and 1/T according to equation (3.1).
(3)
Universal gas constant ( = 8.31441 J · K-1 · mol-1)
Maximum rate constant at infinite temperature
(frequency factor)
ln k' the following concrete relationships result:
Data and results
The linear relationships presented in Fig.2 confirm the validity of
a pseudo-first-order time rule. The slopes of the straight lines,
which are determined by regression analysis correspond to the
rate constants of k’1 = 7.80 · 10-3 min-1 at T1 = 299.15 K and
k’2 = 2.86 · 10-2 min-1 at T2 = 314.15 K. From these values,
using equation (3.2), an activation energy of EA = 67.7 kJ · mol-1
is obtained.
Literature values:
k’ = 6.3 · 10-3 min-1 (T = 293.15 K) ; EA = 67.7 kJ · mol-1.
(3.1)
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3050201
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LEC
05.02
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P3050201
Reaction rate and activation energy of the acidolysis
of ethyl acetate
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen