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Introduction to Chemical Kinetics

Chemical kinetics was developed in1864, by Peter Waage and Cato Guldberg Pioneered by formulating thelaw of mass action, which is state that the rate of chemical reactiondirectly proportional to the quantity of the substance reacting.Chemical kinetic is the study of rate of chemical process and is alsoknown as reaction kinetics. Kinetics is termed as study of motion butin Greek kinesis means movement. It is that one of the branch ofchemistry that deals with study and discussion of chemical reactionwith respect to rate of reaction, effect of various variables, atomsrearrangement, and intermediate formation.


At the macroscopic level, we study anddiscuss about the amounts of the reacted, formed and the rates oftheir formation. But in chemical kinetics at molecular or microscopiclevel, we study and discussion about chemical reaction mechanismaccording to the following consideration made.

  1. During chemical reaction,reactants molecular or atoms are colloid with each other.

  2. The molecular must have thesufficient activation energy to initiate the reaction.

  3. In some cases, the orientation ofthe molecules at the time of collision must also be considered.

Chemical kinetics are mainly used todetermine the reaction rate. The main factors that affecting thereaction rates are nature of reactants, physical state of reactants,concentration of reactants, reaction temperature, reaction pressure,and catalyst used for reaction.


Chemical KineticsDefinition

Chemical kinetics is the branch ofchemistry that deals with the study of the speed or rate of thechemicals reactions, yield and formation about mechanism of thereaction, and factors affecting the reaction rate. In other wordschemical kinetics are simply concerned with the chemical process andrate of reactions.


Chemical KineticsEquations

Zero order reaction

When the rate of reaction isindependent of the concentration of the material then that reactionis called as zero order reaction and it has constant rate of reactionthat means its value is always equal to the value of the rateconstant K.

Rate law for zero order reaction

A -------> P

Where,

A- Reactant

P - Product

Rate of reaction(r)= -d[A]/dt = k

d[A] = -Kdt

= -K

[A] = -Kt + C

Integrated form of kinetic equation forzero order reaction

[A] = -Kt + [A] 0

First order reaction

Here, the rate of reaction is directlyproportional to the concentration of one of the reactant.

Rate law for first order reaction

A-----> P--->(1)

r = -d[A]/dt =k[A]--->(2)

d[A]/A=-Kdt--->(3)

= -K --->(4)

In [A] = -Kt + C--->(5)

When t="0," [A] = [A] 0

In [A] 0 = -K(0) + C

In [A] 0 = C--->(6)

Substitute the equation (6) intoequation (5)

In [A] = -Kt + In [A] 0

In [A]/[A]0= -Kt--->(7)

The above equation (7) can also writtenas

[A] = [A] 0.e -Kt


Second order reaction

It is directly proportional to eitherthe concentration of squared of the reactant, or the product of theconcentration of two reactants.

A+B ------->P

r = d[A]/dt = -K[A][B] --->(1)

Initially the two reactants are havingthe equal concentration, so the equation (1) becomes,

-d[A]/dt = K[A]2--->(2)

--->(3)

1/[A] = Kt+C--->(4)

When t="o," [A] = [A]0, C becomes equal to 1/[A]

1/[A] =Kkt + 1/[A]0

1/[A] = 1/[A]0+ Kt--->(5)


ChemicalKinetics Problems

  1. What % of A remains after 150s? ithas a rate constant about K = 2.95 X 10-3 s-1

A---->P

We know that theintegrated rate law for first order reaction,

[A] = [A]0.e-Kt

Factor remaining [A]/[A]0 = e-Kt

= exp {-2.95 X10-3 s-1) (150s)}

= 0.642=64.2%

  1. Find the rate constant for secondorder reaction initially [A]0 = 200M and after 50min [A]= 0.50M

A+B -------> P

We know that theintegrated rate law for first order reaction,

1/[A] = 1/[A]0 + Kt

1/[0.50M] = 1/[2.00M] + k [50 min]

K = 3.0 x 10-2 M-1 min-1