Advertisements
Advertisements
Question
Why is molecularity applicable for only elementary reactions whereas order of a reaction is applicable for elementary and complex reactions? Explain with suitable examples.
Solution
- Reactions occuring in a single step which cannot be broken down further into simpler reactions are called elementary reactions. Molecularity refers to how many reactant molecules are involved in reactions. Hence, the molecularity of each elementary reaction is fixed.
- A number of chemical reactions are complex. They take place as a series of elementary steps. The molecularity of each step in a complex reaction may be different. Hence, the concept of molecularity is meaningless for complex reactions.
- The slowest step in a complex reaction determines the rate of the overall reaction. Hence, the order of the reaction is applicable for both; elementary and complex reactions.
Example:
Consider the reaction: \[\ce{2NO2Cl_{(g)} -> 2NO_{2(g)} + Cl_{2(g)}}\]
The reaction takes place in two steps:
| 1. | \[\ce{NO_2Cl_{(g)} ->[k1] NO_{2(g)} + Cl_{(g)}}\] | (slow) |
| 2. | \[\ce{NO_2Cl_{(g)} + Cl ->[k2] NO_{2(g)} + Cl_{2(g)}}\] | (fast) |
| Overall: \[\ce{2NO_2Cl_{(g)} -> 2NO_{2(g)} + Cl_{2(g)}}\] | ||
The molecularity of elementary reaction (1) is 1 and that of elementary reaction (2) is 2. The first step being slower than the second and is the rate determining step. Hence, the reaction is of first order.
RELATED QUESTIONS
Distinguish between Order and Molecularity of reaction.
Answer the following in one or two sentences.
What is the rate-determining step?
A reaction takes place in two steps:
- \[\ce{NO_{(g)} + Cl2_{(g)} -> NOCl2_{(g)}}\]
- \[\ce{NOCl2_{(g)} + NO_{(g)} -> 2NOCl_{(g)}}\]
- Write the overall reaction.
- Identify the reaction intermediate.
- What is the molecularity of each step?
Which of the following is a unimolecular reaction?
Define molecularity.
Write an expression for instantaneous rate of reaction:
2N2O(g) → 4NO2(g) + O2(g).
What is the order of reaction?
For a zero-order reaction, molecularity can never be equal to zero. Explain.
Give one example of the reaction where order and molecularity are the same.
Mention any two factors that influence the rate of chemical reaction.
If for the reaction A → products, a straight line graph passing through origin is obtained between the rate of reaction against concentration of A, what would be the order of reaction? Why?
The rate determining step of a reaction is the step ____________.
For the elementary reaction, \[\ce{3H2_{(g)} + N2_{(g)} -> 2NH3_{(g)}}\] identify the correct relation among the following relations.
The reaction \[\ce{2NO2Cl_{(g)} -> 2NO2_{(g)} + Cl2_{(g)}}\] takes place in two steps as
(i) \[\ce{NO2Cl_{(g)} -> NO2_{(g)} + Cl_{(g)}}\]
(ii) \[\ce{NO2Cl_{(g)} + Cl_{(g)} -> NO2_{(g)} + Cl2_{(g)}}\]
Identify the reaction intermediate.
A chemical species that is formed in one elementary step in the mechanism of complex reaction and consumed in the subsequent step is called ____________.
Consider the following elementary reaction;
\[\ce{2AB_{(g)} -> A2_{(g)} + B2_{(g)}}\]
The molecularity of the reaction is ____________.
A reaction involving two different reactants can never be a ______
The reaction takes place in two steps as
(i) \[\ce{NO2Cl(g) ->[k1] NO2(g) + Cl(g)}\]
(ii) \[\ce{NO2Cl(g) + Cl(g) ->[k2] NO2(g) + Cl2(g)}\]
Identify the reaction intermediate.
What is an elementary reaction?
How does a catalyst differ from reaction intermediate?
Find out the reaction intermediate and molecularity of the following reactions.
\[\ce{NO_{(g)} + Cl2_{(g)} -> NOCl_{2(g)}}\]
\[\ce{NOCl_{2(g)} + NO_{(g)} -> 2NOCl_{(g)}}\]
Draw the structure of BrF5 and HOCl.
A complex chemical reaction takes place in two steps.
Step I: NO(g) + O3(g) → NO3(g) + O(g)
Step II: NO3(g) + O(g) → NO2(g) + O2(g)
The predicted rate law is rate = k[NO][O3]
- Identify the rate determining step.
- Name the reaction intermediate/s. Why is/are it/these intermediate/s?
Define molecularity of reaction.
