TERM PAPER

CHE-101

“pseudo order ”

DOA:-26/8/10

DOR:-28/9/10

DOS: 6/11/10

SUBMITTED TO:- SUBMITTED BY:-

Ms. GEETIKA ARORA Mr. Nishant Kumar garg

Department Of chemistry, Roll no :- RE6001B35

Reg. No:-11006867

Section:-E6001

ACKNOWLEDGMENTS

This is a humble effort to express our sincere gratitude towards those who have guided and helped us to complete this project.

A project reported is major milestone during the study period of a student. We could have faced many problems but our teachers’ kind response to our needs and requirement, their patient approach and their positive criticism helped us in making our project. Very warm thanks to our project-in-charge “MS. GEETIKA ARORA MAM” with her support and constant encouragement AND LPU LIBRARY it was not very easy without whose support to finish our project.

With the motivation of our parent it was very easy to finish our project successfully and satisfactorily in short span of time.

NISHANT KUMAR GARG

ABSTRACT

In physical chemistry,we study about the speed and the progress of chemical reaction in CHEMICAL KINECTICS.It tells us about the concentration of the reactants at different time with the reaction progress.Sometimes some reaction looks like the reaction of first and second order but they actually not.These are termed as pseudo order.

Further this term paper also highlights few experiments with their complete mechanism for complete understanding that what happens at pseudo order level.

CONTENTS

1. Introduction2. Rates, rate laws and rate constant3. Rate of reaction4. Rate laws and rate constants

simple rate laws First order Second order reaction

5. Importance of rate law6. Pseudo chemical reaction7. Kinetic experiments related to first order.8. Collision theory of reaction rate (arrhenius theory of reaction rate)9. Refrences

INTRODUCTION

Kinetics is the study of the rates of chemical processes in an effort to understand what it is that influence these rates and to develop theories which can be used to predict them.

CHEMICAL KINECTICS is the area of chemistry concerned with the speeds,or rates,at which a chemical reaction occurs.Kinectics refers to the rates of the reaction,or chemical reaction rate,which is the change in the concentration of the reactant or a product with time.

RATES, RATE LAWS AND RATE CONSTANTS

In this Section we will introduce the language and terms used to describe

the rates of chemical reactions. At this stage we will not be concerned withthe theory of reactions or mechanisms, but just stand back and describe theoverall rates.

Rate of reactionThe rate is defined as change in concentration of the reactant and product with respect to time.Generally, Reactants=products Assume, A+B=C

We can talk about the rate of formation or loss of any species – reactant,intermediate or product. It is, however, important to specify which specieswe are talking about. The rate can be positive or negative: a positive ratemeans that the concentration is increasing with time e.g. a product; anegative rate means that the concentration is falling with time e.g. areactant.The rate may vary with time (and concentration), so it is usual to definethe rate over a very small time, . We think of the rate as the derivative ofconcentration with respect to time

Rate laws and rate constants

Experimentally it is found that rates depend on the concentrations of thespecies involved in the reaction equation (and sometimes on theconcentrations of species which do not appear at first sight to be involved!).The relation between the rate and these concentrations can often beexpressed mathematically in the form of an equation called a rate law.

Some rate laws are very simple and some are very complicated. A rate lawmay be determined experimentally (Section 4) or may be the result of atheoretical prediction, or both.THE rates law expresses the relationship of the rate of a reaction to the rate constant and the concentration of the reactants raised to some powers .

aA+Bb=Cc+Dd rate=k[A]a[B]b

where k is constant, characteristic of a particular reaction, called the rateconstant or the rate coefficient. The powers a, b ... are also constants: a isthe order with respect to A, b is the order with respect to B.

Simple rate laws

First orderA first order rate law is one in which the rate is proportional to theconcentration raised to the power 1 (hence "first") A=PRODUCTS

Rate=k[A] is called a first order rate constant. The rate equation, and the reactionit describes, is said to be first order in A or it is said that the order withrespect to A is one. As the units of r are concentration time–1 and the unitsof [A] are concentration, the units of RATE in first order reaction are found as time–1.

Second order reaction A=PRODUCTS

Rate=k[A]2 A second order reaction has the concentration raised to the power of 2is called a second order rate constant. The rate equation, and thereaction it describes, is said to be second order in A, or the order withrespect to A is two. The units of k2nd are found to be conc.–1 time–1.

IMPORTANCE OF RATE LAW.

1.If we know the rate law and the constants in it we can use this to predict the rate for any set of conditions (concentrations). The rate law is thus a very succinct and practical way of expressing the rate. You might use this, for example, in a model of the atmosphere or in predicting the rate of an enzyme catalysed reaction.

2. The form of the rate law can tell us something about the mechanism of the reaction. This is a point which we will consider in more detail below.

3. Knowing the rate law enables us to separate the concentration dependence from the underlying, fundamental effect which is the size of the rate constant.

PSEUDO CHEMICAL REACTION Consider the general reactions... A+B=PRODUCTS In which order with respect to each reactants is 1 that the overall order of reaction is 2.The rate law is Rate=k[A][B]

however one of the react is present in large excess.,e.g.,one of the reactant may be solvent ,its concentration will remain almost constant.In that case,the rate of the reaction depend only on the concentration of the reaction present in smallar amount.Theus,the reaction becomes a first order reaction.

There are some reactions which appear to be second order but in fact these are first order reaction with respect to the different reactants i.e. Now, if one of the reactant is present in high concentration (solvent) then there is very little change in its concentration. In other words the concentration of that reactant remains practically constant during the reaction. For example, If [A] = 0.01 M and that of solvent water [B] = 55.5 M, the concentration of B changes only from 55.5 to 55.49 M even after the completion of the reaction. Under such conditions, the rate of the given reaction will be according to first order reaction.

The reaction therefore, behaves as a first order reaction not a second order reaction.

Some examples are1) Pseudo reaction can be explained by taking an example of hydrolysis

of ethyl acetate: The molecularity of the reaction is two because it involves two reacting species, ethyl acetate and water. But the concentration of ethyl acetate changes during the reaction while the concentration of water remains constant since it is present in such large excess.

CH3COOC2H5 + H2O= CH3COOH+C2H5OH

According to reaction equation the absolute changes [A] and [B] have the same magnitude. Since the initial amount of A is smaller, the effect on the concentration and the reaction rate is much greater for A as for B

You can illustrate this with some numerical values:Let initial concentartions be [A]₀ = 1 and [B]₀=100At the instant where half of A has reacted away:[A] = 0.5 and [B]= 99.5So [B] has changed only by 0.5% while [A] changed by 50%. Compare reaction rate to intial rater = k·0.5·[A]₀·0.995·[B]₀ = 0.4975·k·[A]₀·[B]₀ = 0.4975·r₀ You can see, that the drop in reaction rate is almost entirely determined by the change of [A], because the relative change in concentration of A is much larger.

Therefore you can ingnore the change of concentration in [B] thorughout the reaction, if B is in large excess: [B] ≈ [B]₀The rate equation simplifies to:r = k'·[A] where k' = k·[B]₀So you can describe reaction kinetics by a pseudo 1st order rate law.

Kinetic Experiments RELATED TO FIRST ORDER.• Bromination of Reactive Aromatics

• Fading of Triphenylmethane Dyes

• Fading of Phenolphtalein in Alkaline Solution

The bromination of acetanilide, 4-nitrophenol and diphenylether is based on a second order reaction:

=k.[Ar][Br]

If the initial concentration of the aromatic reactant (Ar) is much larger than the bromine concentration, the concentration of Ar will not change appreciably during the course of the reaction The concentration of the aromatic reactant in excess remains (essentially) constant. Thus the rate’s dependence on bromine can be isolated and the rate law can be written

=k’[Br] where k’=k[Ar] ……….(2)

Equation (2) represents the differential form of the rate law. Integration of this equation and determination of the integration constant C produces the corresponding integrated law.Substituting [Br] = c into equation (2) yields:

=k ’.dt …………..(3)

Integrating equation (3) gives:

lnc = −k'⋅t +C ……………. (4)

The constant of integration C can be evaluated using boundary conditions. At t = 0 the concentration of bromine is C0.Therefore: C = lnC0 …………….. (5)

Accordingly is obtained:

lnc = lnc0− k'⋅ t …………… (6)

or C=C0 e-k’.t ………… ⋅ (7)

The decrease in the bromine concentration can be followed by photometric measurement.

COLLISION THEORY OF REACTION RATE (ARRHENIUS THEORY OF REACTION RATE)

A collision between high energy molecules overcomes the forces of repulsion and brings the formation of an unstable molecule cluster, called the activated complex. The life span of an activated complex is very small. Thus, the activated complex breaks either into reactants again or new substances, i.e., products. The activation energy (Ea) depends upon the nature of chemical bonds undergoing rupture and is independent of enthalpies of reactants and products. The energy changes during exothermic and endothermic reactions versus the progress of the reaction

REFRENCES

1. WIKIPEDIA ;pseudo order search

2. PARDEEPS; new course chemistry OF P.N.KAPIL

3. Mc GRAW HILLS ;text book of chemistry OF RAYMOND CHANG