computational exercise on lewis acid base reactions

7/27/2019 Computational Exercise on Lewis acid base reactions

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Exercise 8Computational Exercise on Lewis

Acid-Base ReactionA oral report in CHEM 120.1 A.Y. 2013-2014

By Anna Esperanza Q. Legaspi


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INTRODUCTION


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INTRODUCTION

Acids and bases are classified in different ways

o Based on taste and feel

o Hydrogens produced in water (Arrhenius)

o Bronsted-Lowry definition (proton transfer)o Lewis theory (interaction of electron pair

acceptor and electron pair donor molecules orions)

o Etc.


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INTRODUCTION

LEWIS CONCEPT

A Lewis acid is a substance that acts as an electronpair acceptor.

A Lewis base is a substance that acts as an electronpair donor.


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Rate constant

If k1 > k3, formation B is favored since higher kmeans 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 ofthe 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 valueswere inputted namely:

activation energies,initial number of

molecules, samplingparameters, number ofcycles 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 solutionswas placed (concentratedand dilute) separately in

100 mL beakers andstirred using a magnetic

stirrer.

Each of the KI solutionswas added (concentrated

and dilute) to thesolutions.

The colors of the mixtures

at the start of mixing andafter 30 minutes ofmixing were noted.

The observations wererecorded.

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RESULTS AND DISCUSSION


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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



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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)



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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



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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

Lowertemp, 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 ofK 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 + YellowLighter 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 byshortreaction time and low concentration

Can be accounted by its lower Ea

formation of the thermodynamically-controlled product is favoredbyhigh 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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computational exercise on lewis acid base reactions

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