BSc/MSc/BTech Chemistry Lab Expt: Determination of Arrhenius Parameters in the Kinetic Study of Acid Catalysed Hydrolysis of Ester
Determination of Arrhenius Parameters in the Kinetic Study of Acid Catalysed Hydrolysis of Ester
Introduction:
Chemical reactions and reaction rate
Chemical kinetics is the part of
physical chemistry that studies reaction rates. The reaction rate for a
reactant or product in a particular reaction is intuitively defined as how fast
a reaction takes place. Through the study of chemical kinetics, one can
reasonably get an idea as to how to alter the reaction conditions and improve
the reaction rate, which is necessary to increase the production of chemical
products in the industry perspective. It will also help us to learn how to
suppress or slow down unwanted side reactions.
For a generic reaction: A+B →C
The simple rate equation is of the form:
The concentration is usually in mol
cm-3 and k is the reaction rate coefficient
or rate constant.
Although it is not really a constant,
because it includes everything that affects reaction rate outside
concentration: mainly temperature, ionic strength, surface area of the
adsorbent or light irradiation (in the case of photochemical reactions).
The exponents a & b
are called reaction orders and depend on the reaction mechanism. The
stoichiometric coefficients and reaction orders are very often equal, but only
in one step reactions.
Acid-catalysed Hydrolysis of Methyl
Esters
The hydrolysis of ester is catalysed
by either an acid or base. This can be achieved in a number of ways. The most
common method is to use a Lewis acid or Bronsted acid to form a positively
charged intermediate that is far more reactive and even mild nucleophiles such
as water will react.
Aim of the Experiment:
To determine the rate
constant of the acid-catalysed hydrolysis of ethyl acetate and to calculate the
Arrhenius parameters.
Principle:
The hydrolysis of an
ester occurs according to the equation:
The following rate equation is applicable to the above reaction:
Rate α [Ester] [H2O]
Since [H2O]
remains constant,
where, ‘c’ represents concentration of the
ester at any time t; k1' is the pseudo 1st
order rate coefficient. As the reaction progresses, the accumulation of acetic
acid increases. Drawing a known volume of the reaction mixture at known regular
intervals of time and titrating it against standard sodium hydroxide solution
will indicate the increase in acetic acid presence.
The acid hydrolysis of
ester is a first-order bimolecular reaction, and the reaction follows pseudo
first order kinetics. This is because the amount of water is in large excess so
that its concentration does not change significantly to alter the reaction
rate. The reaction goes practically to completion (the equilibrium shifts to
the right) and the rate is first order with respect to the ester.
Overall, rate coefficient
of a reaction (k) is given by,
Where, A =
pre-exponential factor, Ea = Activation energy of the reaction, T=
Temperature, R = Universal Gas Constant (8.314 JK-1 mol-1)
Requirements:
Reagents and solutions: Ethyl acetate, 0.5(N) HCl, ~0.2(N) NaOH,
Phenolphthalein indicator, Ice cubes, 0.1(N) oxalic acid
Apparatus:
Burette 50mL, Pipettes-5mL, 10mL, Conical flasks, Wash bottle Reaction bottle
250mL.
Procedure for
Standardisation of NaOH using standard 0.1N Oxalic Acid
10mL (VOx)of
given 0.1N standard Oxalic acid (NOx) is pipetted out into a 100mL conical
flask. This solution is titrated against the given unknown concentration of
NaOH using phenolphthalein indicator until the end point is colourless to pale
pink. Note the end point volume as VNaOH. Tabulate the values and
repeat the titration for concurrent readings and determine the unknown concentration
of supplied NaOH solution.
|
|
Initial burette reading (Vinitial) mL |
Final burette reading (Vfinal) mL |
Consumed volume, VNaOH=Vinitial-Vfinal mL |
Average volume of NaOH, (VNaOH)avg mL |
|
Trial-1 |
|
|
|
|
|
Trial-2 |
|
|
|
Calculation:
Find out, concentration
of NaOH, NNaOH = ____________(N).
Procedure for
Determination of Kinetics for Ester Hydrolysis:
Exactly 100 mL of
0.5N-hydrochloric acid solution are taken in a 250 mL conical flask and exactly
5 mL of the ester is added to it. Zero time (t=0) is noted when half the volume
of ester solution in the pipette is transferred into flask. After thorough mixing,
immediately 10 mL of the solution is pipetted out into a clean conical flask
containing ice cubes. It is then titrated against standardised NaOH solution
from the burette using phenolphthalein indicator. The end-point is the first
appearance of a pale permanent pink colour. The volume of at end point is noted
as V0. The same volumes of the reaction mixture are withdrawn at
regular intervals, say 10 minutes and is titrated against sodium hydroxide
solution. The end point volume at each t, is noted as Vt. The
reaction is allowed to go to completion by keeping the reaction mixture over a
hot water bath for about 30 minutes. The final reading is then noted as Vꝏ.
Calculations:
Let V0 be the
volume of alkali used at zero time and Vt. be the volume of alkali
used after the time ‘t’ seconds. Let Vꝏ be the reading when the
reaction is completed. Concentrations of the ester at various time intervals
are expressed in terms of volume of NaOH solution.
a= initial concentration of ester = (Vꝏ
- Vo)
(a-x) = concentration of
ester at any time ‘t’ = (Vꝏ - VO) – (Vt – Vo)
= (Vꝏ - Vo –
Vt + Vo)
= (Vꝏ - Vt)
The specific rate
constant of the reactions is given by
The rate constant values
are calculated at different time intervals which should nearly be the same. A
graph is drawn between log(Vꝏ - Vt) and time ‘t’.
From the slope of the plot, the rate constant is calculated, and it is
compared with the experimental value.
Table-1
Supplied
data: Activation energy of the
reaction, Ea =
29.775 kJ mol-1 for
283-313 K. (Citak et al., J. Inst. Sci. Tech. 2019, 9(1), 382-388). Use this
data and average rate coefficient to calculate the Arrhenius parameters.
Results:
Room temperature = _____°C. =________K.
Vꝏ = ________mL; V0 =________mL.
The results are as
follows: (mention with units)
1.
Calculated rate coefficient value (kavg)
= ___________________
2.
Graphical rate coefficient value (from
Slope, kGraph) = __________________
3.
Intercept on y-axis (=log a)
=__________________
4.
Calculate, a = ____________.
5. Half-life of the reaction,
=________________________.
6.
Arrhenius Pre-Exponential Factor (A) =
_____________________.
7.
Order of the reaction = _________________________
8.
Molecularity of the reaction = ___________________
9.
Comment on the nature of the graph.
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