## Wednesday, August 27, 2008

### Chemical Kinetics - July December Revision

Jee Syllabus

Chemical kinetics:
Rates of chemical reactions;
Order of reactions;
Rate constant;
First order reactions;
Temperature dependence of rate constant (Arrhenius equation).
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The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.

Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.

Rate of reaction

The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.

So this can be expressed as Δ concentration/Δ time. That is change in concentration divided by time taken for the change.

Molar concentration i.e., moles per liter (M), is used in these equations.

The brackets, [ ] are always used to to indicate molar concentrations.

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Law of Mass Action

In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.

For the reaction aA + bB → cC (when it is a rate determining step)

Rate of reaction is proportional to [A]^a[B]^b

The above proportionality can be written as an equation, by putting in a proportionality constant k.

Rate = k *[A]^a[B]^b

K is called the specific rate constant

Order of Reaction

From the rate law for a reaction order of reaction can be determined.

For a particular species or reactant, the order is equal to the exponent for that species in the rate law.

For example for Rate = k *[A][B]^2
for B the order of reaction is 2. For A it is 1.

The overall order of reaction is equal to the sum of all the individual orders of reactants.

Temperature

As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.

As a general approximation, the rate roughly doubles for each 10°C rise in temperature.

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Molecularity

I recently read the lesson in NCERT Book Part I for Class XII.

There is a section on Molecularity of a Reaction

The number of reacting species which much collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

In the case of reaction

NH4NO2 --> N2 + 2H20

Only one molecule of the reactant decomposes. It is a unimolecular reaction. Its molecularity is one.

In case of the reaction

2HI --> H2 + I2

Two molecules of HI are involved in the reaction. It is a bimolecular reaction and its molecularity is two.

In case of the reaction

2NO + O2 --> 2NO2

2 molecules of NO and one molecule of O2 are involved. Hence it is a trimolecular reaction. The probability that more than three molecules can collide and react simultaneously is very small.

Molecularity above three is not observed. Hence reactions involving many molecules take place in steps.

Hence rate determining step will be there. Law of mass action is applicable to that step.