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PART IIIExperimental Methods
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The rate of reaction cannot be measures directly.
It can only determined from concentration and time data.
1. Volume of gas evolved (refer to kinetic note: KINETIC.pptx)2.
Mass change
3. Colorimetry
4. Titrimetric analysis
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http://localhost/var/www/apps/conversion/tmp/scratch_1/KINETIC.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_1/KINETIC.pptx
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3. Colorimetry1. Use colour changes to measure reaction rate
range by using a spectrophotometer.
2. The amount of light of a particular frequencythat is absorbed
depends on the concentrationof the coloured substance.
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Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g)
time
393 nmlight
Detector D[Br2] aDAbsorption
red-brown
Example: Reaction between bromine and methanoic acid.
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rate a [Br2]
rate = k [Br2]
k =rate
[Br2]= rate constant
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Measure out samples of reactants with known concentration
Mix them together , start a clock and stir the mixture
thoroughly
The reaction is then quenched to prevent further reaction
fromtaking place by :1. rapid cooling in ice, or2. diluting with
cold water, thus lowering the reaction temperature
of the reactants
The quenched solution is then titrated against a suitable
standardsolution.
4. Titrimetric analysis
At regular time intervals, withdraw samples using a pipette
andquench (stop) the reaction
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4. Titrimetric analysisREACTION OF IODINE WITH PROPANE
Iodine and propanone react, in the presence ofacid, in a
substitution reaction to formiodopropanone:
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Example: Reaction of bromine and propanone(Refer George Facers
book page 4)
1. 25ml propanone+25ml dilute sulphuric acid and mix with
25mliodine and start a clock.
2. The samples are run into excess sodium hydrogencarbonate
toquench (stop) the reaction.
3. By titrating with sodium thiosulfate the concentration of
iodine in thereacting mixture can be measured.
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Methods for Measuring Rates of Reaction
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IODINE CLOCK REACTION(time for reaction to finish)
1. The reactants are mixed andthe time taken to produce afixed
amount of product ismeasured.
2. The time t, is measured frommixing until the firstformation
of the blue colour.
3. The rate of reaction isproportional to 1/t.
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4. When all the sodium thiosulfate is used up, a blue colour
issuddenly formed because the iodine forms a complex withthe starch
that has been added.
5. In this reaction, step 2 has no effect on the overall rate-it
ismuch faster than step 1.
6. Step 1 is the rate-determining step.
7. Increasing concentration of H+, iodide or hydrogenperoxide
will shorten the time.
Rate=1/time
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