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Exercise 5
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INTRODUCTION
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Rate constant
If k1 > k3, formation B is favored since higher k means higher rate
Equilibrium constant
If k3 / k4 > k1 / k2 , formation of C is favored sincethe Keq of C is greater
Note: k3 / k4 = [C]eq / [A]eq and k1 / k2 = [B]eq/[A]eq
C A Bk4
k3 k2
k1
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INTRODUCTION
Reaction to be studied
(yellow)kinetically-
controlled product
(orange)thermodynamically -controlled product
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INTRODUCTION
Objectives
1. to construct progress curves for the reaction schemedemonstrating kinetic vs. thermodynamic control;
2.to calculate equilibrium constants and Gibbs freeenergies for the reaction scheme using the MonteCarlo method; and
3.to observe the effect of reaction time and reactant
concentration on the kinetics and thermodynamics of the isomerization of mercuric iodide.
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METHODOLOGY
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METHODOLOGY- Monte Carlo Method
The program forMonte CarloMethod for
Kinetics was used
The reaction schemeselected in the dialog
box was the kinetic vs.thermodynamic
control button (or
reaction 5).
The necessary values were inputted namely:
activation energies,initial number of
molecules, samplingparameters, number of cycles and temperature
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METHODOLOGY- Experimental Method
50 ml of each of thefollowing solutions were
prepared: 0.05 M and0.00625 M ; 0.10 M and
0.0125 M KI.
25 mL of the solutions was placed (concentratedand dilute) separately in
100 mL beakers andstirred using a magnetic
stirrer.
Each of the KI solutions was added (concentrated
and dilute) to thesolutions.
The colors of the mixtures
at the start of mixing andafter 30 minutes of mixing were noted.
The observations wererecorded.
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RESULTS AND DISCUSSION
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RESULTS AND DISCUSSION
Isomerization of mercuric iodide
(yellow)kinetically-
controlled product
(orange)thermodynamically -controlled product
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tetragonal structure - thermodynamically-controlledstructure more ordered structure
requires a longer time for the molecules to rearrange this way,
takes a longer time for it to be formed
also the arrangement with lower energy
rhombic structure–
kinetically-controlled structure has a more random arrangement of molecules
easier to form when molecules collide
takes less time for it to be formed
Structure with the higher energy
RESULTS AND DISCUSSION
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Monte Carlo method
refers to a broad class of algorithms that solve problems through theuse of random numbers
a computational method which employs probability to study
kinetics of a reaction
Progress curves were generated at differentconditions
Lower temperature, short reaction time (a)
Higher temperature, short reaction time (b)
Lower temperature, long reaction time (c)
RESULTS AND DISCUSSION
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Progress Curves – plots of number of molecules vs. number of cycles or time
(a)(b)
(c)
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Number of molecules become constant over time
Corresponds to equilibration
Can be used to obtain equilbrium concentrations of A
, B, and C and be used to calculate the equlibriumconstants
RESULTS AND DISCUSSION
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RESULTS AND DISCUSSION
Table 5.2 Values of and at different conditions for the reaction schemeof mercuric iodide.
1st condition – K12
>K34
; kinetically controlled product is favored2nd condtion – K34 > K12; thermodynamically controlled product isfavored3rd condition - K34 > k12; thermodynamically controlled product isfavored
Condition
Lower temp, shortrxn time
113.09 31.91
Higher temp, shortrxn time
3.47 5.85
Lower temp, longrxn time
177.20 1822.20
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Calculation of rate constants using Arrheniusequation
Repeat steps for k3 / k4
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Working equation
Values were compared with the values obtainedusing the progress curves
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Condition
From progress curves Using Arrhenius eqn. Percent difference (%)
Lower
temp, short
rxn time
113.09 31.91 132.97 42045.17 14.95 99.92
Higher
temp, short
rxn time
3.47 5.85 132.97 42045.17 97.39 99.99
Lower temp, long
rxn time
177.20 1822.20 132.97 42045.17 33.27 95.67
Table 5.3 Values of the ratios of rate constants and comparison with theequilibrium constants
Percent differences may be accounted by the inherent errors of the Monte Carlomethod
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Values of Gibbs free energy were also obtained
Gibbs free energy
allows us to represent the spontaneity of a process in terms of the
properties of the system.
Working equation
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Table 5.4 Values of the Gibbs energy calculated using the progress curves andthe Arrhenius equation.
Condition
From progresscurves
Using Arrhenius eqn. Percent difference (%)
Lower temp,short rxn time
-31448
-23033
-32525 -70812 3.31 67.47
Higher temp,
short rxn time -8283 -11754 -32525 -70812 74.53 83.40
Lower temp,long rxn time
-34435
-49936
-32525 -70812 5.87 29.48
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The values of G calculated agree with the values of K obtained earlier
1st condition – G12 < G34; reaction towards the
kinetically-controlled products is more spontaneous 2nd condition and 3rd condition – G34 < G12; the
reaction towards the thermodynamically-controlledproduct is more spontaneous.
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Experimental Method
Experimental study of the effect of concentration and time on theisomerization of mercuric iodide
Table 5.5. Data table for the observations in the experimentalmethod.
MIXTURE
COLOR OF MIXTURE
Immediately AfterMixing
After 30 minutes
Conc. KI + Peach Dark orange
Dilute KI + Yellow Lighter shade of dark
orange
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(a) (b)
(c) (d)
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Effect of Concentration
Mixture of concentrated solutions→ orange
thermodynamically-controlled product was formed
Mixture of dilute solutions→ yellow kinetically-controlled product was formed
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• Effect of Temperature
• Immediately after mixing yellow
• kinetically-controlled product was formed
• 3o minutes after mixing orange
• thermodynamically-controlled product was formed
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• Generalizations
• formation of the kinetically-controlled product is favored by short reaction time and low concentration
• Can be accounted by its lower Ea
• formation of the thermodynamically-controlled product is favored by high temperature, long reaction time and high concentration
• Can be accounted by its more negative G
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Source of error/ recommendations
The percent differences in the calculations can be
accounted by the inherent errors of the Monte Carlomethod. Hence, no simple correction can berecommended.
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