Reaction Rate

Thus, chemical kinetics deals with the study of reaction rate. Every chemical reaction occurs at a definite rate under a given set of conditions. Some reactions are very fast and some other reactions are comparatively slow. Reaction rate can be defined as, "The change in the concentration of a reactant or product per unit time and per unit volume (for homogeneous reactions) or per unit area (for heterogeneous reactions)":

where C is change in the concentration of reactant or product during time interval t.

=  rate at which concentration of product increases

= rate at which concentration of reactant decreases

A negative (–) sign placed before a reaction rate symbol signifies a decrease in concentration of the reactant with increase of time and a positive (+) sign before the rate symbol signifies that the concentration of the product increases with increase in time. The concentration change may be positive or negative but the rate of reaction is always positive. The minus sign is always written when required but the plus sign is usually not mentioned.

Thus, chemical reaction speed is the reverse quantity of the reaction time. At certain conditions, the rates are functions of concentrations. Depending on the time interval between measurements, the rates are called:

• average rate: rate measured between long time interval

• instantaneous rate: rate measured between very short interval

• initial rate: instantaneous rate at the beginning of an experiment

However, a more realistic representation for a reaction rate is the change in concentration per unit time, either the decrease of concentration per unit time of a reactant or the increase of concentration per unit time of a product. In this case, the rate is expressed in mol/lsec. For the reaction to be useful, either in the laboratory or in nature, it must occur at a reasonable rate.

Rate of reaction is not uniform. It goes on decreasing from moment to moment due to decrease in the concentration(s) of reactant(s) with the progress of reaction i.e. with time as shown below by rate vs time curve. Thus, the rate defined above is actually the average rate of reaction during the time interval considered.

Molecularity and Order of Reaction

A chemical reaction that takes place in one and only one step i.e., all that occurs in a single step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction. The sequence of steps through which a complicated reaction takes place is called reaction – mechanism. Each step in a mechanism is an elementary step reaction.

The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur and is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction. 

In general, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation. Or the molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.

Chemical Reaction	Molecularity
PCl5  →  PCl3 + Cl2	Unimolecular
2HI  →  H2 + I2	Bimolecular
2SO2 + O2  →  2SO3	Trimolecular
NO + O3  →  NO2 + O2	Bimolecular
2CO + O2  →  2CO2	Trimolecular
2FeCl3 + SnCl2 → SnCl2 + 2FeCl2	Trimolecular

The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product or products is called the molecularity of a reaction. For example, decomposition of H₂O₂ takes place in the following two steps:

Overall Reaction	H2O2 → H2O + 1/2O2
Step 1:	H2O2 → H2O + [O]	Slow
Step 2:	[O] + [O] → O2	Fast

The slowest step is rate-determining. Thus from step 1, reaction appears to be unimolecular.

Reactions of higher molecularity (molecularity > 3) are rare. This is because a reaction takes place by collision between reactant molecules and as number of reactant molecules i.e. molecularity increases the chance of their coming together and colliding simultaneously decreases.

The mathematical expression showing the dependence of rate on the concentration(s) of reactant(s) is known as rate-law or rate-expression of the reaction and sum of the indices (powers) of the concentration terms appearing in the rate law as observed experimentally is called order of reaction.

From the study of the kinetics of many simple reactions, it is observed that for a large number of reactions, the molecularity and order are the same. Some examples are given below to justify this point:

Examples	Reaction	Order	Molecularity
Dissociation of N2O5	N2O5 → N2O4 +  O2	1	1
Dissociation of H2O2	H2O2 → H2O +  1/2O2	1	1
Dissociation of HI	2HI → H2 + I2	2	1
Formation of NO2	2NO + O2 → 2NO2	3	3

Table 1. The Main differences between Molecularity and Order of Reaction

Moleculariy of Reaction	Order of Reaction
It is the total number of reacting species (molecules, atoms or ions) which bring the chemical change.	It is the sum of powers of molar concentration of the reacting species in the rate equation of the reaction.
It is always a whole number.	It may be a whole number, zero, fractional.
It is a theoretical concept.	It is experimentally determined.
It is meaningful only for simple reactions or individual steps of a complex reaction. It is meaningless for overall complex reaction.	It is meant for the reaction and not for its individual steps