ORDER OF REACTION
The sum of the exponents of the concentration terms in the rate expression of the chemical reaction.
Consider a reaction aA + bB →P
Rate of reaction = dx/dt = k[A]a[B]b
¥ the value of a is the order of reaction with respect to A, and b is the order of reaction with respect B.while the sum of a+b is the overall order of reaction.
¥ The order of reaction is not necessarily equal to the number in the co efficient of balance chemical equation.It is an experimental value.
¥ Order of reaction may be zero, 1st, 2nd or 3rd or may be in fraction.
¥ order of reaction is in fact the number of species ( atoms , molecules, ions) involve in the rate of chemical reaction.
¥ If a reaction is completed in more than one steps, then order of reaction is determined from slow step i.e. rate determining step
1; ZERO ORDER REACTION;
A reaction is zero order with respect to a reactant when changing the concentration of that reactant has no effect on the reaction rate.
¥ Metal catalytic reactions like decomposition of NH3 in the presence of Mo or W.
For such reaction dx/dt = k[NH3]0 is zero order reaction.
¥ photochemical reactions are usually zero order.
2; Fractional order;
in some of the reaction the involvement of some reactants are not in whole number but in fraction, called fractional order.
¥ para hydrogen is converted into ortho hydrogen at high temperature.
H2(p) +H2(O/p)→ H2(O) + H2(O/P)
For this reaction the rate expression is dx/dt = k[H2(P)]1[H2(P/O)]1/2
Order of reaction is 1+ ½= 1.5
3; first order reactions;
the reaction in which one specie is involved in the rate expression.
That particle is responsible for the progress of chemical reaction.
A; Br2 → 2Br dx/dt = [Br2]1
B; H2O2 → H2O 1/2O2 dx/dt = [H2O2]1
C; Nuclear dissentegration are also first order like
88Ra226 → 86Rn222 + 2He4 dx/dt = [Ra]1
4; second order kinetics
The reaction in which the rate of reaction is proportional to the product of the concentration of two reactants each raise to the first power.
1; Thermal decomposition of NO2
2NO2 → N2+ 2O2 dx/dt = k[NO2]2
2; Decomposition of HI in gas phase.
2HI → H2 + I2 dx/dt = k[HI]2
3; oxidation of NO with ozone,
NO + O3 → NO2 + O2 dx/dt = k [NO][O3]
5; third order kinetics;
When three molecules are involved in rate expression
1; Reduction of NO
2NO + H2 → N2O +Cl2 dx/dt = k[NO]2[H2]
2;SnCl2 + 2FeCl3 → SnCl4 + 2 FeCl2 dx/dt = k[SnCl][FeCl3]2 3rd order
3; Formation of nitrosyl chloride
2NO + Cl2 → 2NOCl dx/dt = k[NO][Cl2]
MOLECULARITY OF REACTION
♣ The total number of molecules taking part in the rate determining step of a chemical reaction is called "molecularity" of the reaction.
♣ A reaction is unimolecular if only one molecule is involved in its rate-determining step.
♣ A reaction is bimolecular if two molecules take part in its rate-determining step.
For example: C2H5OH → C2 H4 + H2O Is a unimolecular reaction because only one molecule is involved.
♣ The molecularity of a reaction can not always be predicted by the chemical equation alone, that every reaction must follow the rules of chemical kinetics.
♣ Molecularity is always in whole number
How can we determine order of reaction experimentally?
In order to determine the order of reaction,
1; First determine the rate of chemical reaction. From this we will be enable to determine the involvement of each reactants in chemical reaction;
Keeping the concentration of one reactant constant, observe the rate of reaction by changing the concentration of other reactants.
Similarly observe the rate of reaction with the change of concentration of all reactants
From this we will be enable to find out the order of reaction by involvement of the molecules of each reactants in chemical reactio
Suppose If the rate of reaction become double by double the concentration of reactants, It means the the order of reaction is first.( First order reaction)
If the rate of reaction increase four times by double the concentration, It means the order is second (second order reaction).
If the rate of reaction increase nine to times by double the concentration, It means the order is third (third order reaction).
If the rate not effect by increase in concentration, It means the order is zero with respect to that reactants