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Elementary and Complex Reactions - Practice Questions & MCQ
Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main
Quick Facts
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19 Questions around this concept.
Solve by difficulty
In the series reaction given below:
and then the RDS step of the reaction is
Statement I: - Reaction between $\mathrm{H}_2$ and $\mathrm{I}_2$ to form 2 HI.
Statement II:- Those reactions which take place in one step are called elementary reactions.
Which among the following is correct regarding the chemical species corresponding to location I, II and III on the given reaction coordinate diagram?
Assertion (A): For complex reactions molecularity and order are not same
Reason (R): Order of reaction may be zero.
In the following questions two or more options may be correct.
In any unimolecular reaction ______________.
(i) only one reacting species is involved in the rate-determining step.
(ii) the order and the molecularity of the slowest step are equal to one.
(iii) the molecularity of the reaction is one and order is zero.
(iv) both molecularity and order of the reaction are one.
In the following questions two or more options may be correct.
Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?
(i) Order is the same as molecularity.
(ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity.
(iv) Molecularity can never be zero.
Concepts Covered - 2
Complex Reaction
Complex reactions are those reactions when a sequence of elementary reactions or single-step reactions gives us the products. Since the complex reactions occur in multiple steps thus, molecularity for such reactions cannot be determined. It can only be determined for elementary reactions.
For example:
$\mathrm{NO}_2+\mathrm{CO} \rightarrow \mathrm{NO}+\mathrm{CO}_2$
The rate of the reaction is given experimentally as follows:
rate $=\mathrm{k}\left[\mathrm{NO}_2\right]^2$
Now, clearly the order of this reaction is 2 but since it is a complex reaction, thus the molecularity of this reaction cannot be determined.
NOTE: For any complex or elementary reaction, it has been found that:
Molecularity $\leq 3$
Because the probability of simultaneous and effective collision of three molecules is very low. Thus, termolecular or higher molecularity reaction is rarely observed.
Order of Reaction vs Molecularity
Difference between Order of Reaction and Molecularity
| Order of Reaction | Molecularity |
| Experimentally determined | Theoretically determined |
| Defined for elementary as well as complex | Defined only for elementary reactions |
| Can be zero, positive or fractional | Always a positive integer |
| Order of reaction can vary according to the conditions | Molecularity is fixed for any elementary reaction |
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Complex Reaction
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