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# What is order of rxn?

The order of reaction is defined as the dependence of the concentration of all reactants in a chemical reaction on the rate law expression. For example, in a first order chemical reaction, the rate of reaction is entirely dependent on the concentration of one reactant in the reaction.

Order of a chemical reaction can be defined as the sum of power of concentration of reactants in the rate law expression is called the order of that chemical reaction. Reactions can be first order reaction, second order reaction, pseudo first order reaction etc. depending on the concentration of the reactants. Order of a reaction is an experimental value. It means it is an experimentally determined parameter. It can have fractional value as well.

There are some characteristics of order of reaction that are enumerated as follows:

If experimental rate law expression is given for a reaction, then we can deduce the order of that reaction as well. For example, consider a reaction –

aA + bB \ P

and rate law is given as –

rate = k(Ax)(By)

order of reaction for the above reaction on the basis of given rate law can be written as follows –

order of reaction = x + y

Order of reaction is determined by experiment. Although if we know rate law expression determined experimentally then we can determine order of reaction using rate law. Order of reaction can be an integer or fractional value. Following orders of reactions are possible –

Molecularity and order of reaction both give information about the chemical reaction but are very different from each other as one tells about the number of molecules taking part in reaction while another one tells about the relationship between rate of reaction and concentration of reactants. For your better understanding we are providing you here pointwise difference between molecularity and order of reaction-

In these reactions the rate of reaction doesn’t depend upon the concentration of reactants. It means change in concentration of reactants doesn't affect the rate of reaction.

Example -

\

In these reactions the rate of reaction depends on the concentration of one reactant only. There can be many reactants in the reaction but concentration of only one reactant will affect the rate of reaction. Concentration of other reactants will have no effect on order of reaction.

Example – \

Rate = k

In these reactions the rate of reaction depends on the concentration of two different reactants or square of concentration of one reactant.

Example – \

Rate = k2

\ \ \

Rate = k

Those reactions which are not of 1st order but approximated or appear to be of 1st order due to higher concentration of the reactant/s than other reactants are known as pseudo first order reactions.

Example – Hydration of alkyl halide

\ \ \

Rate of reaction = k

As methyl iodide is also used in aqueous solution form so the concentration of water is far higher than methyl iodide.

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So, concentration of water doesn’t change much and can be approximated as no change or constant.

Now we can write – Rate of reaction = k’

Where k’ = k

Thus, the reaction appears to be first order, but it is actually of second order that’s why it is known as pseudo first order reaction.

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Once the rate equation is obtained, the entire composition of the mixture of all the species in the reaction can be understood.

The order of reaction can be defined as the power dependence of rate on the concentration of all reactants. For example, the rate of a first-order reaction is dependent solely on the concentration of one species in the reaction. Some characteristics of the reaction order for a chemical reaction are listed below.

In order to determine the reaction order, the power-law form of the rate equation is generally used. The expression of this form of the rate law is given by r = kxy.

In the expression described above, ‘r’ refers to the rate of reaction, ‘k’ is the rate constant of the reaction, and are the concentrations of the reactants. The exponents of the reactant concentrations x and y are referred to as partial orders of the reaction. Therefore, the sum of all the partial orders of the reaction yields the overall order of the reaction.

There are several different methods which can be followed in order to determine the reaction order. Some of these methods are described in this subsection.

Apart from these methods, there exist other ways to obtain the reaction order, such as the method of flooding in which the concentration of a single reactant is measured when all the other reactants are present in huge excess.

As discussed earlier, the value of the order of reaction may be in the form of an integer, zero, or a fraction. A graph detailing the reaction rates for different reaction orders can be found below.

Chemical reactions can be classified into the following types based on the dependence of the rate on the concentration.

The molecularity of a reaction refers to the number of atoms, molecules, or ions which must undergo a collision with each other in a short time interval for the chemical reaction to proceed. The key differences between molecularity and reaction order are tabulated below.

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The order of reaction can be defined as the power dependence of rate on the concentration of all reactants. For example, the rate of a first-order reaction is dependent solely on the concentration of one species in the reaction.

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Maintenance Of Way

For the reaction, aA + bB → cC + dD If the rate of the reaction is given as rate = kx y. Then, the sum x + y gives overall order of the reaction.

e.g. For the reaction

H2(g) + I2(g) → 2HI(g) is

rate = k.

The reaction is of first-order in H2 and I2 each and hence overall order is second order.

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