Experiment P06
To show that the decomposition of hydrogen peroxide is a first order reaction
Chemicals:
Hydrogen peroxide - 3 volume, (10 cm3)
0.004 M KMnO4, (200 cm3)
0.1 M H2SO4, (70 cm3)
Borate buffer: 3.1 g of boric acid dissolved in 25 cm3 of 1 M sodium hydroxide
solution and diluted with distilled water to 250cm3. (50 cm3)
Apparatus:
Stop-watch,
10.0 cm3 pipette,
250 cm3 conical flasks, (2)
100 cm3 measuring cylinder,
titration apparatus.
Principle:
The rate of a reaction is affected by the concentration of reactants according to the
rate equation which has to be determined experimentally.
If the decomposition of hydrogen peroxide is a first order reaction, then the rate
equation is
Rate = k [ H2O2 ]
Using calculus notation we may write
d [C ]
= k[C]
dt
Where [C] is the concentration of H2O2 at time t.
d [C ]
= kdt
[C ]
t
at d [C ]
-
= k  dt
0
ao [C ]
- [ ln at - ln ao ] = k(t - 0)
a
ln 0 = kt
at
a
1
k=
ln o
t
at
where ao = initial concentration of hydrogen peroxide in mol dm-3 and at =
concentration of hydrogen peroxide in mol dm-3 after time t, k is the rate constant.
P.1
Experiment P06
To show that the decomposition of hydrogen peroxide is a first order reaction
Procedure:
a.
Using a measuring cylinder, introduce into a 250 cm3 conical flask 150 cm3 of distilled
water, 10 cm3 of 3-volume hydrogen peroxide, 50 cm3 of a borate buffer and 10 cm3 of
0.004 M potassium permanganate solution. The potassium permanganate is reduced by
some of the hydrogen peroxide to colloidal manganese(IV) oxide which acts as a catalyst in
the decomposition of hydrogen peroxide.
b.
Now pipette carefully 10.0 cm3 sample of the reaction mixture and run it into another
conical flask containing 10 cm3 of 0.1M sulphuric acid. Note the time at which this is done.
Take this time as to.
c.
Titrate the remaining hydrogen peroxide in the 10 cm3 sample with 0.004 M potassium
permanganate solution.
d.
Withdraw further samples after 5, 10, 15, 20, 25 and 30 minutes and determine the
remaining hydrogen peroxide as before.
e.
Enter the results in the following table.
t/min.
Final burette reading
Initial burette reading
Vol. of KMnO4(cm3)
(cm3)
(cm3)
Vt
0
5
10
15
20
25
30
Notes:
If Vo cm3 of the potassium permanganate solution are required for the oxidation of hydrogen
peroxide at the beginning (to) of the reaction, and Vt cm3 are required after time t, the
concentration of hydrogen peroxide at these times are proportional to Vo and Vt respectively.
P.2
Experiment P06
To show that the decomposition of hydrogen peroxide is a first order reaction
Name:
Seat No.:
Date:
Grade:
a.
Complete the following table:
t / min
Vo
Vt
(cm3)
(cm3)
0
5
10
15
20
25
30
b.
Plot the following graphs:
i.
Vt against t
P.3
ln
Vo
Vt
1 V
k= ln o
t Vt
Experiment P06
To show that the decomposition of hydrogen peroxide is a first order
reaction
ii.
c.
ln
Vo
against t
Vt
Name:
Do the results obtained above indicate that the reaction concerned is a first-order reaction?
Explain your answer.
Find the rate constant k from graph ii.
e.
For first order reaction the time required for the initial concentration to be reduced to half is
given by
0.693
t1 =
= constant
k
2
(Hints: ln
Vo
= kt)
Vt
d.
(It is independent of the initial concentration.)
Read these times from graph i.
Time (min.)
ao to 1/2 ao
1/2 ao to 1/4 ao
1/4 ao to 1/8 ao
What do you observe from these results?
P.4