Molecularity of the reaction
Molecularity is defined as the number of atoms or molecules taking part in an elementary step leading to a chemical reaction. The overall chemical reaction may consist of many elementary steps. Each elementary reaction has its own molecularity which is equal to number of atoms or molecules participating in it. If the reaction takes place in more than one step there is no molecularity for the overall reaction. However molecularity and order are identical for elementary reaction (one step).
There are many differences between the concepts of order and molecularity.
Order of a reaction
1. It is the sum of powers raised on concentration terms in the rate expression.
2. Order of a reaction is an experimental value, derived from rate expression.
3. Order of a reaction can be zero, fractional or integer.
4. Order of a reaction may have negative value.
5. It is assigned for overall reaction.
6. It depends upon pressure, temperature and concentration (for pseudo order)
Molecularity of a reaction
1. It is the number of molecules of reactants taking part in elementary step of a reaction.
2.It is a theoretical concept.
3.Molecularity can neither be zero nor fractional.
4.Molecularity can never be negative.
5.It is assigned for each elementary step of mechanism.
6.It is independent of pressure and temperature.
Rate determining step
Most of the chemical reactions occur by multistep reactions. In the sequence of steps it is found that one of the steps is considerably slower than the others. The overall rate of the reaction cannot be lower in value than the rate of the slowest step. Thus in a multistep reaction the experimentally determined rate corresponds to the rate of the slowest step. Thus the step which has the lowest rate value among the other steps of the reaction is called as the rate determining step (or) rate limiting step.
Consider the reaction,
2A + B -- > C + D going by two steps like,
A + B - K1-- > C + Z - (1)step(slow)
Z + A ---K2 -- > D - (2)Step(fast)
2A+B -- > C+ D
Here, the overall rate of the reaction corresponds to the rate of the first step which is the slow step and thus, the first step is called as the rate determining step of the reaction. In the above reaction, the rate of the reaction depends upon the rate constant k1 only. The rate of 2nd step doesn't contribute experimentally determined overall rate of the reaction.
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