C. Y. Yeung (CHW, 2009). Physical Chemistry: Chemical Kinetics. Colorimetry. Chemical Method. Physical Method. Volume measurement. Differential Rate Equation. Integrated Rate Equation. AL CHEM REVIEW. C. Y. Yeung (CHW, 2009). p.01. - PowerPoint PPT Presentation
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IntegratedIntegratedRate EquationRate Equation
VolumeVolumemeasurementmeasurement
ColorimetryColorimetry
Physical Method
Physical Chemistry:Physical Chemistry: Chemical KineticsChemical Kinetics
Chemical MethodChemical Method for Studying for Studying
the Variation ofthe Variation of [A][A] withwith TimeTime
C. Y. Yeung (CHW, 2009) p.01
Chemical Method = …Chemical Method = …
Monitoring the change of Monitoring the change of conc. along with time by:conc. along with time by:
sampling,sampling,
quenching, andquenching, and
titrationtitration
p.02
Example 1:Example 1:
RR CCOO
OR’OR’(aq)(aq) ++ OHOH--(aq)(aq) RR CC
OOOO--
(aq)(aq) ++ R’OHR’OH(aq)(aq)
Step1:Step1: Known amountsKnown amounts of ester and NaOH in be of ester and NaOH in beakers are placed in a thermostatic bath at akers are placed in a thermostatic bath at known temperatureknown temperature..
Step2:Step2: Start the reaction by mixing the reactants Start the reaction by mixing the reactants rapidly. The rapidly. The timetime is noted. is noted.
Step3:Step3: At At regular time intervalsregular time intervals, , a fixed volume a fixed volume of tof the mixture is pipetted into about 5 volumes he mixture is pipetted into about 5 volumes of ice-water to of ice-water to quenchquench the reaction. the reaction.
p.03
Example 1 (continued):Example 1 (continued):
RR CCOO
OR’OR’(aq)(aq) ++ OHOH--(aq)(aq) RR CC
OOOO--
(aq)(aq) ++ R’OHR’OH(aq)(aq)
Step4:Step4: Remaining [OHRemaining [OH--]] in the reaction mixture i in the reaction mixture is determined by titration against s determined by titration against standarstandard HCld HCl using using phenolphthaleinphenolphthalein..
Step5:Step5: Repeat steps 3 and 4 to obtain Repeat steps 3 and 4 to obtain variation of variation of [OH[OH--] against t] against t..
p.04
Example 2:Example 2:
2H2H22OO22(aq)(aq) ++ OO22(g)(g)
Step1:Step1: Known amountsKnown amounts of H of H22OO22 is mixed with a is mixed with a small amount of MnOsmall amount of MnO22(s) catalyst in a (s) catalyst in a beaker placed in a thermostatic bath at beaker placed in a thermostatic bath at known temperatureknown temperature..
Step2:Step2: At At regular time intervalsregular time intervals, , a fixed volume a fixed volume of tof the mixture is pipetted into excess dilute Hhe mixture is pipetted into excess dilute H22SSOO44(aq) to (aq) to quenchquench the reaction by the reaction by removing tremoving the MnOhe MnO22(s) catalyst(s) catalyst..
MnOMnO22(s)(s)2H2H22O(l)O(l)
p.05
Example 2 (continued):Example 2 (continued):
Step4:Step4: Remaining [HRemaining [H22OO22]] in the reaction mixture in the reaction mixture is determined by titration against is determined by titration against standarstandard KMnOd KMnO44(aq)(aq)..
Step5:Step5: Repeat steps 3 and 4 to obtain Repeat steps 3 and 4 to obtain variation of variation of [H[H22OO22] against t] against t..
(I) Calibration of Colorimeter(I) Calibration of Colorimeter
Step1:Step1: Use distilled water as sample, the Use distilled water as sample, the absorbance absorbance is set to zerois set to zero..
p.09
Calibration of Colorimeter + MeasurementCalibration of Colorimeter + Measurement
Step2:Step2: A A standard solutionstandard solution of Br of Br22(aq) [e.g. 1.0M] is put i(aq) [e.g. 1.0M] is put into the sample cell in the colorimeter.nto the sample cell in the colorimeter.
Step3:Step3: Record the Record the absorbanceabsorbance of 1.0M Br of 1.0M Br22(aq).(aq).
Step4:Step4: Repeat steps 2 and 3 with Repeat steps 2 and 3 with different [Brdifferent [Br22(aq)](aq)]..
Step5:Step5: Plot a Plot a Calibration CurveCalibration Curve of “Absorbance” v of “Absorbance” versus “[Brersus “[Br22(aq)]”.(aq)]”.
Calibration of ColorimeterCalibration of Colorimeter (continue) (continue)
[Br[Br22(aq)](aq)]
AbsorbanceAbsorbance
p.10
Put the BrPut the Br22 containing reaction mixture in containing reaction mixture in the sample cell and put into the colorimeter.the sample cell and put into the colorimeter.
p.11(II) Measurement(II) Measurement
Step1:Step1:
Start the stop watch to monitor the variation Start the stop watch to monitor the variation of absorbance with time.of absorbance with time.
Monitoring the change of Monitoring the change of conc. along with time by:conc. along with time by:
measuring the volume of measuring the volume of gas formed by syringegas formed by syringe
p.14
Mg + 2HCl Mg + 2HCl MgCl MgCl22 + + HH22
Example:Example:
(no colour change!)(no colour change!)
Rate Eqn.Rate Eqn. & & Order of RxnOrder of Rxnp.15
Rate Equation Rate Equation (Rate Law)(Rate Law)
the mathematical equation relating ththe mathematical equation relating the e rate of rxnrate of rxn to the to the [reactants][reactants]..
3A + 2B 3A + 2B C C
p.16
Examples:Examples:
raterate [A][A]xx[B][B]yy
order of rxn w.r.t. [A]order of rxn w.r.t. [A]order of rxn w.r.t. [B]order of rxn w.r.t. [B]
p.17
raterate = =rate constantrate constant
(varies with temp.!!)(varies with temp.!!)
kk[A][A]xx[B][B]yy
Therefore, the rate equation shows that:Therefore, the rate equation shows that:
rate of rxn is affected byrate of rxn is affected by [reactants] [reactants]
rate of rxn is affected byrate of rxn is affected by temperature temperature
* usually, order of reaction = * usually, order of reaction = 0, 1 0, 1 oror 2 2..
p.18
If [B] and temp. are kept constant, when [A] If [B] and temp. are kept constant, when [A] doubles, rate of reaction increases 4 times. doubles, rate of reaction increases 4 times.
How to do it?How to do it?[A] [B] Initial
rateExpt. 1 [A]01 [B] Rate1
Expt. 2 [A]02 [B] Rate2
Expt. 3 [A]03 [B] Rate3
….. ….. ….. …..kept constantkept constant
tt
[A][A]
[A][A]0303
[A][A]0202
[A][A]0101
initial rate methodinitial rate method
Repeat expt. with constant [A] and different [B], find n!Repeat expt. with constant [A] and different [B], find n!
Plot Plot raterate vs vs [A] [A] ::
p.19Determination of k, m and n Determination of k, m and n
by by graphical methodgraphical method
rate = k’ [A]rate = k’ [A]mm
[A] [B] Initial rate
Expt. 1 [A]1 [B] Rate1
Expt. 2 [A]2 [B] Rate2
Expt. 3 [A]3 [B] Rate3
….. ….. ….. …..
raterate
[A][A]
m = 0m = 0
m = 1m = 1
m = 2m = 2
Only m could be found!Only m could be found!
p.20rate = k’ [A]rate = k’ [A]mm
log (rate) = m log [A] + log k’log (rate) = m log [A] + log k’
log (rate)log (rate)
log [A]log [A]
y = mx + cy = mx + c
m = 0m = 0
m = 1m = 1
m = 2m = 2
log k’log k’
i.e. Both k (=k’/[B]) and m could be found!i.e. Both k (=k’/[B]) and m could be found!
Key PointsKey Pointsp.21
Rate Equation shows that the rate of rxn Rate Equation shows that the rate of rxn is affected by is affected by conc. (order)conc. (order) & & temp (k)temp (k)..
Find the order of rxn (m, n) by Find the order of rxn (m, n) by Initial RatInitial Rate Method e Method (many combinations of [A] & [B]!)(many combinations of [A] & [B]!)
Plot Plot log (rate) = m log[A] + log k’log (rate) = m log[A] + log k’, , both both mm, , k’k’ and and kk could be found. could be found.
Monitor the variation of Monitor the variation of conc.conc. along with along with timetime by Chemical / Physical Method, by Chemical / Physical Method,
Time t1 t2 t3 t4
[A] [A]1 [A]2 [A]3 [A]4
[A][A]
tt
[A][A]00 slope = - k’slope = - k’
To study Integrated Equation, To study Integrated Equation, e.g. :e.g. :
During the reaction, both [A] and [B] During the reaction, both [A] and [B] decrease!decrease!
p.24
large excess of [B]large excess of [B] should be used.should be used.
In order to ensure that the decreasing In order to ensure that the decreasing rate is due to decreasing [A], not [B] …rate is due to decreasing [A], not [B] …
i.e. keep [B] as “effectively constant”.i.e. keep [B] as “effectively constant”.
p.25
Time t0 t1 t2 t3 ….[A] [A]0 [A]1 [A]2 [A]3 ….ln[A] ln [A]0 ln [A]1 ln [A]2 ln [A]3 ….
ln [A] = - k’t + ln [A]ln [A] = - k’t + ln [A]00
ln [A]ln [A]
tt
ln [A]ln [A]00 slope = - k’slope = - k’
First OrderFirst Order RxnRxn
p.26
Time t0 t1 t2 t3 ….[A] [A]0 [A]1 [A]2 [A]3 ….[A]-1 [A]0