Order of Reaction - Overview, Structure, Properties & Uses

Order of Reaction - Overview, Structure, Properties & Uses

Define order of reaction/ What is order of reaction

Order of reaction definition: A chemical reaction's Order of reaction is determined by the relationship between the rate of a reaction and the concentrations of the species involved. An expression (or rate equation) of the reaction in question must be obtained to get the reaction order. Order of reaction is decided by the mechanism of the reaction. After obtaining the rate equation, it can be determined what the composition of the mixture of all the species in the reaction is. Out of order meaning the reaction is not in order

Order of reaction: What does it mean?

A chemical reaction's order can be defined as the product of the power of the concentrations of the reactants in the rate law expression. Reactants' concentration determines the order in which they react. You may have a first-order reaction, second-order reaction, pseudo-first-order reaction, etc. Experimentally determined order determines how a reaction proceeds. Therefore, it implies that it is an experimental parameter. Additionally, it can have a fractional value.

A reaction can be characterized by its experimental rate law if the order of that reaction can be drawn from the experimental rate law. For example, consider a reaction –

P = aA + bB

The rate law is given as –

Rate = k [A]x[B]y

For the above reaction, an order of reaction can be written as follows on the basis of the rate law:

order of reaction = x + y

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How to find order of reaction:

Identifying the order in which reactions occur

The order of reactions is determined experimentally. The order of reactions can be used to determine rate law expression if we know it from experimental results.

It is possible for the order of reaction to be integer or fractional. Reactions can be ordered in the following manner:

  1. Order of reaction can be zero - In an order of zero reaction, the concentration of reactants doesn't have an effect on for the reaction rate.

  2. A negative integer value for order of reaction indicates that the concentration of the reactants affects the rate for the reaction in the inverse direction.

  3. An integer order of reaction indicates that the concentration of reactants directly influences reaction rate.

  4. Intricate relationships between the concentration of reactants and rate of reaction can be shown by fractional values of the order of reaction. Reactions with fractional order are usually complex.

As described above, 'R' refers to the reaction rate, 'k' to the reaction rate constant, and [A] and [B] indicate the concentrations of the reactants. As partial orders for the reaction, x and y are exponents expressing their reaction concentrations. Because of this, the sum of all the partial orders for the reaction determines the overall order for the reaction.

Order of reaction determination can be made using the following techniques

Graphical Method

This method is used when only one reactant is involved in the reaction. This method assumes that the reaction occurs in a first-order if we plot a line between log AA (where A is a reactant and AA is its concentration) and t (time). In a similar way, if we examine a graph between 1AA and t we get a straight line, which is evidence of a second-order reaction. In contrast, if we draw a straight line between 1AA2 and t, the reaction is termed a third-order reaction.

Method of initial rates

An expression for the power law is first finitely transformed into the natural logarithm. As follows: ln r = ln k + x. ln[A] + y. ln[B] + ....

Now, partial orders related to each reactant are calculated by performing the reaction while varying the concentration of the specific reaction product while keeping the concentration of the other reactants constant.

Based on the partial order of A, the power-law expression for the rate equation becomes

lnr = x. ln[A] + C, where C is an infinitesimal.

It is now possible to plot the graph by taking 'ln r' as a function of ln[A], and the corresponding slope is the partial order.

Method of Differentiation

  • The order of reaction can be determined easily through this method

  • Firstly, the rate expression for the reaction is written (r = k[A]x[B]y...)

  • It can be determined from the exponents x + y+.... the final value for the reaction order.

As well as these methods there are other ways to obtain the reaction order, such as the flooding method, whereby the concentration of a single reactant is measured when the concentrations of all other reactants are greatly excessive.

Molecularity vs. order of reaction: what is the difference?

Chemical reactions are described through molecularity and order of reaction, both of which provide information about the chemical reaction, but they are very different as one relates to the number of molecules taking part in it and the other to the rate of reaction and the concentration of reactants. The following table will help you to appreciate the difference between molecularity and order of reaction

Sr. No.


Order of reaction


The number of molecules involved in determining the rate for the reaction.

This graph depicts the relationship between the concentration of reactants and rate of reaction.


A molecularity determination uses only rates determining steps.

Order of reaction may be derived from taking into account all steps for the reaction.


It doesn't depend on pressure or temperature.

Pressure, temperature, and concentration all affect it.


Numbers are always whole numbers.

Whether it is zero, an integer, or a fractional value is entirely up to you.


The mechanism of a reaction can provides information about its molecular structure.

Experimental data can be used to determine reaction order.


It is not possible to have a molecular weight that is negative.

It is possible for the order of reaction to be a negative number.

Reaction order: Different values

Reaction with zero-order

  1. In the presence of reactants, neither concentration nor rate of reaction is influenced.

  2. There is no effect of changing the concentration of reactants on the reaction rate.

  3. Some examples of these types of reactions include the enzyme-catalysed oxidation of CH3CH2OH (ethanol) to CH3CHO (acetaldehyde).

Reaction at first order

One reactant concentration is all that is needed to control the rate of reaction in these reactions. Reaction rates are affected by the concentration of one reactant but cannot be controlled if there are many reactants present. The order for the reaction will not be affected by the concentration of the other reactants.

Example – N2O5 = N2O3 + O2

Rate = k[N2O5]

Reactions of the second order

These reactions depend on either the concentration of two different reactants or the square of the concentration of one reactant.

Example – 2NO2 = 2NO + O2

Rate = k[NO2]2


Rate = k[CH3COOC2H5] [OH-]

First-order pseudo-reaction

These reactions occur even though they are not of the first order but have the appearance of being the first order due to the higher concentrations of the reactant/s than other reactants and are classified as pseudo-first-order reactions.

Example – Hydration of alkyl halide

CH3I + H2O = CH3OH + H+ + I-

Reaction rate = k [CH3I] [H2O]

Aqueous solutions of methyl iodide are also used, so the concentration of water is much higher than that of the iodine compound.

[CH3I] <<< [ H2O]

Therefore, the concentration of water does not change much and can be approximated as constant or no change.

Now we can write – Rate of reaction = k’ [CH3I]

Where k’ = k [H2O]

The reaction appears to be first-order, however, it is of second-order and thus is known as pseudo-first-order.

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