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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.
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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 Chemical Methodfor Studying the Variation of[A]withTime
sampling, quenching, and titration p.02 Chemical Method = … Monitoring the change of conc. along with time by:
p.03 Example 1: O O + + R’OH OH- O- R C OR’ R C (aq) (aq) (aq) (aq) Step1: Known amounts of ester and NaOH in beakers are placed in a thermostatic bath at known temperature. Step2: Start the reaction by mixing the reactants rapidly. The time is noted. Step3: At regular time intervals, a fixed volume of the mixture is pipetted into about 5 volumes of ice-water to quench the reaction.
p.04 Example 1 (continued): O O + + R’OH OH- O- R C OR’ R C (aq) (aq) (aq) (aq) Step4: Remaining [OH-] in the reaction mixture is determined by titration against standard HCl using phenolphthalein. Step5: Repeat steps 3 and 4 to obtain variation of [OH-] against t.
p.05 Example 2: MnO2(s) O2(g) 2H2O(l) 2H2O2(aq) + Step1: Known amounts of H2O2 is mixed with a small amount of MnO2(s) catalyst in a beaker placed in a thermostatic bath at known temperature. Step2: At regular time intervals, a fixed volume of the mixture is pipetted into excess dilute H2SO4(aq) to quench the reaction by removing the MnO2(s) catalyst.
p.06 Example 2 (continued): MnO2(s) O2(g) 2H2O(l) 2H2O2(aq) + Step4: Remaining [H2O2] in the reaction mixture is determined by titration against standard KMnO4(aq). 5H2O2 + 2MnO4- + 6H+ 2Mn2+ + 8H2O + 5O2 Step5: Repeat steps 3 and 4 to obtain variation of [H2O2] against t.
Examples: Br2 + HCOOH 2Br- + 2H+ + CO2 H2O2 + 2I- + 2H+ I2 + 2H2O p.07 Physical Methodfor Studying the Variation of[A]withTime < COLORIMETRY > < COLORIMETRY >
measuring the variation of absorbance by colorimeter p.08 Colorimetry = … Monitoring the change of conc. along with time by: absorbance [coloured substance] [colour intensity]
p.09 Procedures: Calibration of Colorimeter + Measurement (I) Calibration of Colorimeter Step1: Use distilled water as sample, the absorbance is set to zero. Step2: A standard solution of Br2(aq) [e.g. 1.0M] is put into the sample cell in the colorimeter. Step3: Record the absorbance of 1.0M Br2(aq). Step4: Repeat steps 2 and 3 with different [Br2(aq)].
Absorbance [Br2(aq)] p.10 Calibration of Colorimeter (continue) Step5: Plot a Calibration Curve of “Absorbance” versus “[Br2(aq)]”.
Absorbance time p.11 (II) Measurement Br2 + HCOOH 2Br- + 2H+ + CO2 Put the Br2 containing reaction mixture in the sample cell and put into the colorimeter. Step1: Start the stop watch to monitor the variation of absorbance with time. Step2:
Absorbance Absorbance time t1 [Br2]t1 [Br2(aq)] p.12 Monitoring the change of conc. along with time … Measurement Calibration A1 A1 [Br2]t1 t1
measuring the volume of gas formed by syringe p.13 Another Physical Method … Monitoring the change of conc. along with time by:
Example: p.14 Mg + 2HCl MgCl2 + H2 (no colour change!)
p.15 Rate Eqn. & Order of Rxn
order of rxn w.r.t. [A] order of rxn w.r.t. [B] p.16 Rate Equation (Rate Law) the mathematical equation relating the rate of rxn to the [reactants]. Examples: 3A + 2B C rate [A]x[B]y
rate constant p.17 k[A]x[B]y rate= (varies with temp.!!) * usually, order of reaction = 0, 1 or 2. Therefore, the rate equation shows that: rate of rxn is affected by [reactants] rate of rxn is affected by temperature
How to do it? [A] [A]03 [A]02 [A]01 kept constant t p.18 If [B] and temp. are kept constant, when [A] doubles, rate of reaction increases 4 times. initial rate method Repeat expt. with constant [A] and different [B], find n!
rate m = 1 m = 2 m = 0 [A] Determination of k, m and n by graphical method p.19 Plot rate vs [A] : rate = k’ [A]m Only m could be found!
log (rate) m = 2 m = 1 m = 0 log [A] p.20 rate = k’ [A]m log (rate) = m log [A] + log k’ y = mx + c log k’ i.e. Both k (=k’/[B]) and m could be found!
p.21 Key Points Rate Equation shows that the rate of rxn is affected by conc. (order) & temp (k). Find the order of rxn (m, n) by Initial Rate Method (many combinations of [A] & [B]!) Plot log (rate) = m log[A] + log k’, both m, k’ and k could be found.
p.22 Integrated Rate Equation
[A] [A]0 t p.23 Zeroth OrderRxn Monitor the variation of conc. along with time by Chemical / Physical Method, [A] = - k’t + [A]0 slope = - k’
p.24 To study Integrated Equation, e.g. : [A] = -k’t + [A]0(zeroth order) During the reaction, both [A] and [B] decrease! In order to ensure that the decreasing rate is due to decreasing [A], not [B] … large excess of [B]should be used. i.e. keep [B] as “effectively constant”.
First OrderRxn ln [A] ln [A]0 t p.25 ln [A] = - k’t + ln [A]0 slope = - k’
Second OrderRxn [A]-1 [A]0-1 t p.26 [A]-1 = k’t + [A]0-1 slope = k’
m = 0 m = 1 m = 2 p.27 Summary … 3 Integrated Rate Eqns [A] = - k’t + [A]0 ln [A] = - k’t + ln [A]0 [A]-1 = k’t + [A]0-1
10cm3 1.00 mol dm-3 H2O2 50cm3 borate buffer 10cm3 diluted KMnO4 Titrate against dilute KMnO4 10cm3 sample Flask A (150cm3 water) (around 5 mins) 10cm3 1.0M H2SO4 Flask B start stop watch! p.28 Kinetics Expt.:Decomposition of H2O2
× 3 × 2 MnO2 2H2O2 O2 + 2H2O Flask A Flask B p.29 What happens in Flasks A and B …? H2O2 + 2OH- O2 + 2H2O + 2e-` MnO4- + 2H2O + 3e- MnO2 + 4OH- 3H2O2 + 2MnO4- 3O2 + 2H2O + 2OH- + 2MnO2 MnO2 is killed by H2SO4. 2MnO4- + 5H2O2 + 6H+ 2Mn2+ + 8H2O + 5O2
For 1st order rxn, ln [A] = - k t + ln [A]0 ln ([A]0/[A]) = k t p.30 Date Treatment … As vol. of MnO4- used [A], ln (V0/V) = k t If a straight line is plotted [ln(V0/V) vs t] 1st order, and slope = k!
Chemical Kinetics Colorimetry Chemical Method Physical Method Volume measurement Differential Rate Equation Integrated Rate Equation Differential / Integrated Rate Eqn? One Combination? (Continuous Method) Many Combinations? (Initial Rate Method) Physical/Chemical Method? p.31 Set Conc. + Collect Data + Plotting Order of Rxn + “k”