Chemical Kinetics Presentation u

7/28/2019 Chemical Kinetics Presentation u

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Introduction Chemical kinetics, the branch of physical chemistry that is

concerned with understanding the rates of chemicalreactions.

Chemical kinetics includes investigations of how differentexperimental conditions can influence the speed of achemical reaction and yield information aboutthe reaction's mechanism.

Importance of chemical kinetics is that it provides evidencefor the mechanisms of chemical processes.

Many commercial processes can take place by alternativereaction paths, and knowledge of the mechanisms makes itpossible to choose reaction conditions that favour one path

over others.


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Reaction Rates The Reaction Rates at any instant during the course of

the reaction is the rate at which the concentrations ofthe reactants decreases with time or the concentrationof products increases with time.

For a reaction of the type

aA + bB cC + dD

The Reaction Rate can be given as


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Factors On Which Reaction RatesDepends Nature of the Reactants

Physical State Concentration

Temperature

Catalyst

Pressure


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Order Of Reaction The order of reaction with respect to a certain

reactant, is defined as the power to which itsconcentration term in the rate law is raised.

For a general reaction: aA +bB cC +dD

rate = k[A]p[B]q

Order of the reaction with respect to A is p, that of B is

q and overall order is p+q Reaction order can be determined only by experiment.

The constant of proportionality, k in rate law is calledrate constant


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Molecularity Of A Reaction Molecularity in chemistry is the number of colliding

molecular entities that are involved in a single reactionsteps.

It is a theoretical concept and can only be applied toelementary reactions.

Molecularity of a reaction is always a whole number.

Pseudo Molecular Reactions: when one of the reactantis present in excess then the order of the reaction is 1but molecularity is 2.


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Integrated Rate Equations

Zero Order Reaction:

unit = mol lit-1 time-1

examples: The Photochemical reaction of H2 and Cl2over water (saturated with HCl) surface is zero Order.

First Order Reaction:

unit = time-1

examples: Inversion of cane sugar.

C12

H22

O11

+ H2O H+ ions C

6H

12O

6+ C

6H

12O

6


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Second Order Reactions

1. When concentration of both reactants are equal:

2. When concentration of the reactants are not equal:

unit = lit mole-1 time-1

examples: Oxidation Of Hydrogen Bromide4HBr + O2 2Br + 2H2O


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Half Life Period Half life period of a reaction can be defined as the time

required when half of the reactant is converted intothe products, or it can also be defined as the time

required for the reaction to be half completed

For Zero Order t1/2 = 0.5 a/k

For First Order t1/2 = 0.693/k

For Second Order t1/2 = 1/ka


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Arrhenius Equation K=Ae-Ea/RT is Called the Arrhenius Equation .

K is the specific rate constantA is the frequency factor

R is the molar gas constant

T is the Temperature in Kelvin scale

Ea is the Activation energy


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Activation Energy And Chemical

Reaction The activated complex will decompose at a definite

energy level at a definite way. If it decomposes at ahigher energy level than the then the average energy of

the molecule, the reaction will be endothermic innature otherwise it will be exothermic in nature

If the reaction is reversible and endothermic, theactivation energy required for backward reaction will

always be less than forward reaction but if t hereaction is exothermic the activation energy requiredfor the backward reaction will always be greater thanforward reaction.


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Effect of catalyst on activation

energy and chemical reactionA catalyst alters the rate of the reaction but itself

remain unchanged.

A positive catalyst is used so the activated complex willalways be formed at a lower energy level then theuncatalysed reaction

In case of negative catalyst, the activated complex willbe formed at a higher energy level than theuncatalysed reaction.


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Chemical Kinetics Presentation u