14.4.1 Defining the Molecularity of a Reaction Flashcards
Defining the Molecularity of a Reaction
• A reaction mechanism is a series of elementary steps that describe how an overall reaction progresses on the
atomic-molecular level.
• The molecularity of elementary steps is the number of particles that collide in each elementary step.
note
- A reaction mechanism is a series of elementary steps that describe how an overall reaction progresses on the
atomic-molecular level. - For example, for the reaction between nitrogen dioxide (NO 2 ) and fluorine gas (F 2 ), a possible mechanism is shown. In this mechanism, one NO 2 molecule reacts with one F 2 molecule, forming one FNO 2 molecule and a fluorine atom. This fluorine atom then reacts with another NO 2 molecule, forming another FNO 2 molecule. These two steps are called elementary steps. Since the fluorine atom is created in the first elementary step and consumed in the second, it does not appear in the overall balanced reaction. The fluorine atom is a reaction intermediate.
- Summing the elementary steps gives the overall balanced reaction.
- A reaction mechanism does not prove what is going on at the atomic-molecular level. It is only consistent with what is happening.
- The molecularity of elementary steps is the number of particles that collide in each elementary step.
- If only one particle reacts, the elementary step is
unimolecular. - If two particles collide, the elementary step is bimolecular.
- It is unlikely for three reactants to collide simultaneously. For example, it would be extremely difficult to cause three pool balls to collide simultaneously in the center of a pool table. Therefore, termolecular reactions are improbable. Reaction mechanisms with three elementary steps are probably not an accurate description of reality.
Which of the following statements best describes the relationship between the rate law and the events at the atomic-molecular level?
The rate law is consistent with what is happening at the atomic-molecular level.
The molecularity of an elementary step of a reaction refers to:
the number of molecules that collide in the elementary step of a reaction.
Why is the reaction 2NO2(g) + F2(g) → 2FNO2(g) extremely unlikely to proceed as a single elementary step?
Termolecular reactions are exceedingly unlikely.
What is the molecularity of most common type of reaction?
Bimolecular, such as when two atoms, ions, or molecules collide.
Why are the units of the rate constants different for the elementary steps in the following proposed mechanism for the conversion of ozone into atmospheric oxygen?
2O3(g) → 3O2(g)
proposed mechanism
O3 → O2 + O
O3 + O → 2O2
They are likely to be different because of their different molecularities.
Which of the following is the best description of a reaction mechanism?
A series of elementary steps that explain the overall reaction.
Which of these reactions has a molecularity of one? Step 1a: Cl2 → 2Cl Step 1b: 2Cl → Cl2 Step 2: Cl + CHCl3 → HCl + CCl3 Step 3: Cl + CCl3 → CCl3
Step 1a
Which of these (unbalanced) reactions is an example of a unimolecular reaction?
H2O → H2 + O2
What is the molecularity of each step in the following reaction sequence? Step 1a: Cl2 → 2Cl
Step 1b: 2Cl → Cl2
Step 2: Cl + CHCl3 → HCl + CCl3
Step 3: Cl + CCl3 → CCl3
Step 1a: Unimolecular, Step 1b: Bimolecular, Step 2: Bimolecular, Step 3: Bimolecular
Why are termolecular reactions relatively rare?
The probability of three reactants species colliding at exactly the same time and with enough energy to reach the activated state is very low.