Elementary ReactionsTable of contentsAn elementary reaction is a single step reaction having a single transition state and no intermediates. IntroductionElementary reactions add up to complex reactions. In other words, non-elementary reactions can be explained in multiple elementary reaction steps. A set of elementary reactions comprises a reaction mechanism, which predicts the elementary steps involved in a complex reaction. Below are two reaction coordinates of two reactions. One describes an elementary reaction while the other describes a non-elementary reaction.
Types of Elementary ReactionsThe molecularity of a reaction refers to the number of reactant particles involved in the reaction. As there can only be discrete number of particles, the molecularity must be an integer. Molecularity can be described as either unimolecular, bimolecular, or termolecular. There are no known elementary reactions involving four or more molecules.1The following table summarizes the three known types of elementary reactions:
Practice Problems1. 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 is a termolecular reaction?
a. \(A + 2B + C \rightarrow D\)
b. \(A + B + B \rightarrow C\)
c. all of the above
d. \(2A + 2B + 2C \rightarrow 2D\)
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).
Solutions1. Non-elementary steps, or complex reactions, are sets of elementary reactions. That is to say, 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 as it involves A + B + B + C, or 4 molecules
"b" is a termolecular reaction as it involves 3 particles: A + B + B
"c" is incorrect because "a" is incorrect
"d" is a termolecular reaction as the coefficients cancel out to give the reaction: \(A + B + C \rightarrow D\), which involves 3 particles (A + B + C)
"e" is the correct answer as "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 \rightarrow Products") }}
"c" is incorrect because "a" is incorrect
"d" is a possible rate law for the bimolecular reaction: \(A + A \rightarrow Products") }}
"e" is the correct answer as "b" and "d" are correct
5. Impossible. The molecularity of a reaction MUST be an integer as there cannot be "a half of a particle" producing a reaction.
6. False, we cannot conclude anything. While a termolecular reaction requires the collision of three particles, the reverse logic is not necessarily true. That is, having three collisions is not sufficient for a termolecular reaction.
1. For example, particles A + A + B collide with each other at the same place and time. However, particle B was in the wrong orientation, so no reaction occurred. Instead, the two A particles were in the correct orientation and produced a reaction, which is a bimolecular reaction.
2. Consider a second example: two collisions between particles A + A + B occured, but there was not enough energy to produce a reaction. Instead, a third collision between A and B had the sufficient energy and correct orientation to produce a reaction. Such a reaction is, again, only bimolecular.
3. A last example: particle A collides twice with a wall, and then once with B to produce a reaction. Such a reaction involving three collisions at different places and different time is only a bimolecular reaction. Further Readings on Related Topics
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