D12.5 Multi-step Reactions
A reaction mechanism is the process, or sequence of elementary reaction steps, by which a reaction occurs. When there are two or more steps in a mechanism it corresponds to a multi-step reaction.
For example, ozone in the stratosphere protects Earth’s surface from harmful ultraviolet radiation. Ultraviolet photons cause ozone molecules to decompose to oxygen molecules. The overall reaction equation is:
3 O2(g)However, at the molecular level, this reaction does not involve collision between two O3 molecules, simultaneous absorption of an ultraviolet photon, and reaction. Rather, there are two steps that occur one after the other:
| step 1: | O3(g) | |
O2(g) + O(g) |
| step 2: | O(g) + O3(g) | ⟶ | 2 O2(g) |
| overall: | 2 O3(g) | ⟶ | 3 O2(g) |
In step 1, upon absorption of an UV photon, a bond breaks in an O3 molecule, producing an O2 molecule and an O atom:
This step is a unimolecular elementary reaction.
In step 2, the O atom formed in step 1 reacts with a second O3 molecule, producing two O2 molecules:
This step is a bimolecular elementary reaction.
The overall reaction is the sum of these two steps. To sum the steps, write reactants from all steps to the left of a reaction arrow, write products from all steps to the right of the arrow, then cancel species that appear on both sides of the arrow. This gives:
which is the overall reaction equation given above: 2 O3(g) ⟶ 3 O2(g). (Summing the reactions is the same as the process employed by Hess’s law; however, the steps for a Hess’s law calculation need not be the actual reaction mechanism steps.) Note that O(g) is cancelled here, and does not appear in the overall reaction equation. It is an example of a reaction intermediate, an atom or molecule that is a product in an earlier step and reacts away in a later step of a reaction mechanism.
The reaction equation for an elementary reaction specifies exactly which atoms or molecules are involved in that reaction. For example, step 2 in the ozone decomposition reaction mechanism states that one O atom reacts with one O3 molecule to form two O2 molecules. That is, for this elementary reaction to occur, one O atom must collide with one O3 molecule.
In contrast, the overall reaction equation does not necessarily specify which atoms or molecules collide and react. In our example, even though the overall ozone reaction is 2 O3(g) ⟶ 3 O2(g), there is no need for two O3 molecules to collide in order for products to form.
Because each elementary reaction has a single transition state, in an overall reaction that consists of several sequential elementary reaction steps, there is a series of transition states, one for each step in the mechanism. The figure below shows the overall reaction energy diagram for the ozone decomposition reaction.
Note that although one of the O3 molecules does not react until step 2, you still need to include it as a reactant in the reaction energy diagram. In other words, in a reaction energy diagram, all the atoms and molecules that are involved in the reaction are accounted for from the very beginning to the very end. This is because there is a significant quantity of energy associated with each atom or molecule; to omit one, or to suddenly add one in the middle of the reaction energy diagram, would significantly change the energy (y-value) associated with that point along the reaction progress, and the resulting reaction energy diagram becomes nonsensical.
Therefore, writing out the reaction energy diagram in stepwise fashion, we have:
| Kinetics | Thermodynamics | ||||
| step 1: | 2 O3(g) | ⟶ | O2(g) + O(g) + O3(g) | Ea,1 | ΔrG°(1) > 0 |
| step 2: | O2(g) + O(g) + O3(g) | ⟶ | 3 O2(g) | Ea,2 | ΔrG°(2) < 0 |
| overall: | 2 O3(g) | ⟶ | 3 O2(g) | ΔrG° = ΔrG°(1) + ΔrG°(2) | |
Exercise: Reaction Mechanism
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