Molecularity of the following elementary reaction is 2 NO + O2 ---->2 NO2
A) 0.5 B) 1 C) 2 D) 3
Answers
option D ==> 3
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The molecularity of the reaction is 3 because reactant side 3 (2NO+ 1 O2) molecules are taking part in the reaction.
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Answer:
The molecularity of a reaction refers to the number of reactant particles involved in the reaction. Because there can only be discrete numbers of particles, the molecularity must take an integer value. Molecularity can be described as unimolecular, bimolecular, or termolecular. There are no known elementary reactions involving four or more molecules.1
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Unimolecular Reaction
Bimolecular Reaction
Termolecular Reaction
Unimolecular Reaction
A unimolecular reaction occurs when a molecule rearranges itself to produce one or more products. An example of this is radioactive decay, in which particles are emitted from an atom. Other examples include cis-trans isomerization, thermal decomposition, ring opening, and racemization. The rate at which a substance decomposes is dependent on its concentration. Unimolecular reactions are often first-order reactions as explained by Frederick Alexander Lindemann, which is referred to as the Lindemann mechanism.
Bimolecular Reaction
A bimolecular reaction involves the collision of two particles. Bimolecular reactions are common in organic reactions such as nucleophilic substitution. The rate of reaction depends on the product of the concentrations of both species involved, which makes bimolecular reactions second-order reactions.
Termolecular Reaction
A termolecular reaction requires the collision of three particles at the same place and time. This type of reaction is very uncommon because all three reactants must simultaneously collide with each other, with sufficient energy and correct orientation, to produce a reaction. There are three ways termolecular reactions can react, and all are third order.
Table 1: The three known types of elementary reactions:
Unimolecular
A→Products
rate=k[A]
N2O4(g)→2NO2(g)
Bimolecular
A+A→Products
rate=k[A]2
2NOCl→2NO(g)+CO2(g)
A+B→Products
rate=k[A][B]
CO(g)+NO3(g)→NO2(g)+CO2(g)
Termolecular
A+A+A→Products
rate=k[A]3
A+A+B→Products
rate=k[A]2[B]
2NO(g)+O2(g)→2NO2(g) 2
A+B+C→Products
rate=k[A][B][C]
H+O2(g)+M→HO2(g)+M 3
Practice Problems
1. How are non-elementary steps and elementary steps related?
2. Choose the correct statements.
a. An elementary step has 0 intermediates.
b. An elementary step has 1 intermediate.
c. An elementary step has 2 intermediates.
d. An elementary step has 0 transition states.
e. An elementary step has 1 transition state.
f. An elementary step has 2 transition states.
3. Which of the following elementrary reactions is a termolecular reaction?
a. A+2B+C→D
b. A+B+B→C
c. all of the above
d. 2A+2B+2C→2D
e. b and d
f. none of the above
4. Which rate law corresponds to a bimolecular reaction?
a. rate=k[A][A]2
b. rate=k[A][B]
c. all of the above
d. rate=k[A]2
e. b and d
f. none of the above
5. Give an example of a reaction with a molecularity of 1/2.
6. True or False: Given species A and B inside a container, instruments detect that three (3) collisions occured before product was formed. That is, we know a reaction occured after detecting three collisions in a box. We can conclude that the reaction is a termolecular reaction (as the reaction could have been produced from A+A+B or A+B+B).
Solutions
1. Non-elementary steps, or complex reactions, are sets of elementary reactions. The addition of elementary steps produces complex, non-elementary reactions.
2. The correct statements are "a" and "e". By definition of elementary reactions they have 0 intermediates because they cannot be broken down. Again by definition of an elementary reaction, a single-step reaction will have 1 transition state. There is no reaction with 0 transition states. Having 2 transition states implies having 1 intermediate, making the reaction non-elementary.
3. "e" is the answer.
"a" is not a termolecular reaction because it involves A + B + B + C, or 4 molecules
"b" is a termolecular reaction because it involves 3 particles: A + B + B
"c" is incorrect because "a" is incorrect
"d" is a termolecular reaction, simplifying to the reaction: A+B+C→D , which involves 3 particles (A + B + C)
"e" is the correct answer because "b" and "d" are correct
4. "e" is the answer.
"a" is incorrect because the rate law describes a third-order reaction, which is true for termolecular reactions
"b" is a possible rate law for the bimolecular reaction: A+B→Products
"c" is incorrect because "a" is incorrect
"d" is a possible rate law for the bimolecular reaction: A+A→Products
"e" is the correct answer because "b" and "d" are correct
5.