9.
DETERMINATION OF REACTION RATE CONSTANT FIRST ORDER
AIM: To determine the reaction rate constant and half life of ester (methyl acetate or number of globules)
at 0.5M HCL at room temperature
APPARATUS AND CHEMICALS REQUIRED:
Conical flask (250 ml)
10 ml volumetric pipette
Burette
Ethyl acetate or methyl acetate
Test tube
0.5 N HCL solution
0.25 N NAOH solution
Phenolphthalein indicator
Ice cold water
PROCEDURE:
Preparation of hydrochloric acid solution (0.5N) IP
Solutions of any normality XN may be prepared by diluting 85 x ml of HCL to 1000ml with water.
Measure 850 ml of distilled water into a 1000ml volumetric flask. Add 42.5 ml of conc. HCL and
slowly added. Finally make up the water in 1000ml.
Preparation of NAOH solution (0.25 N) IP:
Solutions of any normally XN may be prepared by dissolving 40 x gm of NAOH in water and diluting
to 1000ml. weigh 10gm of NAOH and transferred into 1000ml volumetric flask. Add water slowly
with stirring finally makeup the water into 1000ml.
PROCEDURE:
KINETIC METHOD
100ml of 0.5N HCL solution is measured and transferred into a 250ml conical flask
It should be kept in the water bath for equilibrium (do not heat)
10ml of the given ester it transfer into the test tube and kept in the water bath for
equilibrium.Normally it takes 10minutes
The acid solution its mix ester sample thoroughly and kept in water bath
Immediately after mixing 5ml of the mixer is withdrawn using the pipette and
transferred in to a conical flask containing 10ml ice water
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The few drops of phenolphthalein indicator is added to the mixture
The reaction mixer is titrated against 0.25N NAOH solution. This value of alkaliconsumed
represents V0.
5ml samples by periodically at 10, 20, 30, 40, 50, 60, 75minutes. the volumeconsumed
at each time interval represent Vt
The reaction mixture is heated at water bath at the 600c at 20 minutes
The mixture cooled to room temperature
5ml of sample by withdrawn at transfer into the conical flask containing 10 ml ice cold
water.The titration is repeated and this value represents
OBSERVATIONS :
1. Water bath temperature =
2. Titre reading at t=0, (Vo)= ml 0.1 N NaOH
3. Infinity reading ml 0.1 N NaOH.
4. Initial Concentration of ester= a =Vo- = ml 0.1 N NaOH.
OBSERVATION TABLE :
Time (t) (min) Vt (ml) X= (Vt-V0) (a-x) = Log (a-x) K=2.303/t.
(ml) - Vo) (ml) log10a/a-x
0 0
10
20
30
40
50
Vt = Volume (ml) of 0.1 N NaOH required at
time t.k= Reaction rate constant
x= Change in concentration of methyl acetate
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a-x = Concentration of methyl acetate remaining at time (t).
Calculations:
1. Determination of k by substitution: Calculate values of k by substituting titre readings at
differenttime interval in equation of reaction rate constant for first order reaction.
k=2.303/t log10a/a-x
2. Determination of k from graph: Find values of k from slope of the graph as given
below. Inlogarithmic form, the rate constant equation for first order is written as
Log(a-x) = -k/2.303t + log a
Plot the graph of log (a-x) vs. time for first order reaction.
Precautions:
1. Maintain temperature of reaction mixture constant during reaction.
2. Shake reaction mixture after every withdrawal.
3. Use dry pipette to withdraw reaction mixture samples.
4. Withdraw samples at right time interval and use ice or cold water to arrest reaction.
5. Judge the end point accurately.
REPORT:
The reaction rate constant (k) of the given data (methyl acetate or ethyl acetate) in 0.5NHCL acid
is: From graphical method= ______minutes and From substitution method=
_________minutes.
The half -life (t1/2) of the given ester (methyl acetate or ethyl acetate) in 0.5N HCL acid is
From graphical method=
From substitution method=
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