Activation Energy of the Reaction between Br— and BrO3— in Acid Solution

Kwok Tak Seng Catholic Secondary School
S.6 Chemistry
Experiment [9] – To Determine the Activation Energy of the Reaction between Br— and BrO3— in Acid Solution

S.6 ( )
Name: [ ] ( )

Time allowed: 55 minutes

Introduction:
The reaction can be represented by 5Br—(aq) + BrO3—(aq) + 6H+(aq) → 3Br2(aq) + 3H2O(l)
The progress of the reaction may be followed by adding a fixed amount of phenol together with some methyl red indicator. The bromine produced during the reaction reacts very rapidly with phenol. Once all the phenol is consumed, any further bromine bleaches the indicator immediately. So, the time for the reaction to proceed to a given point may be determined.

Requirements:
Apparatus: Beaker Burettes Boiling tubes Stop-watch Thermometer Bunsen burner Tripod and gauze
Chemicals: 0.01 M phenol solution 0.083 M KBr / 0.017 M KBrO3 solution 0.5 M sulphuric acid Methyl red indicator

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Experimental procedures:
1. Place 10 cm3 of 0.01 M phenol solution, 10 cm3 of 0.083 M KBr / 0.017 M KBrO3 mixture, and 10 drops of methyl red indicator into the FIRST boiling tube.
2. Place 5 cm3 of 0.5 M sulphuric acid in the SECOND boiling tube.
3. Place BOTH boiling tubes into a large beaker of water which is maintained between 49 ℃ — 51 ℃ by careful warming. Suitably clamp the FIRST boiling tube and place a thermometer in the solution.
Allow the contents of the tubes to reach the temperature of the water bath (+ 1 ℃).
4. Pour the 5 cm3 of sulphuric acid into the clamped boiling tube, and swirl gently.
5. Start the stop-watch as quickly as possible.
6. The clamped tube should remain in the water bath throughout the experiment. Record the time, to the nearest second, from the moment of mixing to that of complete disappearance of the red colour.
7. Record also the temperature, to the nearest degree, of the content of the clamped tube at the end of the experiment.
8. Repeat the steps 1 – 7, at about 45 ℃, 40 ℃, 35 ℃, and 30 ℃.
Record your results in Table 1.

Results:
Table 1:-
Experiment
Temperature / ℃
Temperature / T
Time (t) / seconds ln 1/t
1/T
1
(49 ℃ — 51 ℃)

2
(45 ℃)

3
(40 ℃)

4
(35 ℃)

5
(30 ℃)

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Data treatment:
1. Write an equation for the reaction between bromine and phenol.

C6H5OH(aq) + 3Br2(aq) → C6H2Br3OH(aq) + 3H+(aq) + 3Br-(aq)

(Phenol = C6H5OH, bromine = Br2)

2. Arrhenius equation is given as:- k = A e—Ea/RT k represents to _______rate constant ________________________. Ea represents to ______activation energy________________________.

3. 1/t can replace k in the Arrhenius equation. And so derive another equation to relate ln k and 1/t.

when T increase, -Ea/RT increase , k increase , rate increase

k direcrly proportional to 1/t (????????)

4. Plot a suitable graph to calculate Ea. Show your calculation steps clearly.
[Given: R = 8.3 J K—1 mol—1]

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Discussion:
1. What function does the methyl red play in the experiment?
[Hint: It is not acting as an indicator in the accepted sense.]

In an acid medium, Br-(aq) and BrO3-(aq) react to form Br2(aq).
5Br-(aq) + BrO3-(aq) + 6H+(aq) → 3Br2(aq) + 3H2O(l) ...... (*)

The fixed and small amount of phenol rapidly removes the Br2(aq) formed in reaction (*). When all phenol is completely reacted, further Br2 formed would then bleach the methyl red rapidly.

Therefore, the fade of the red colour of methyl red indicates that a fixed amount Br2 is formed in reaction (*).

2. Why is it unsatisfactory to measure the reaction rate at high temperature such as 80 ℃?

This is because the reaction rate is very fast under a high temperature, in which the reaction time is too short (i.e. may be about 1 second or even less) to be measured.
And this would cause a large error in the time reading.
This is why it is unsatisfactory to measure the reaction rate at high temperature such as 80C.

3. Explain how temperatures can affect reaction rates.

Raising the temperature of chemicals increases the rate of their reaction. The activation energy for a reaction is the minimum energy two molecules need in order to collide together with enough force to react. As the temperature rises, molecules move more vigorously, and more of them have the required activation energy, increasing the rate of the reaction. The rate of a reaction doubles for every 10 degrees Celsius rise in temperature.