Reaction Mechanisms
- A balanced chemical equation for a chemical reaction indicates what is present at the beginning and the end of a reaction
- However, it provides no details about the steps that occur at the molecular level as the reactants are turned into products
What is a reaction mechanism?
- A reaction mechanism is a description of a path, or a sequence of steps, by which a reaction occurs at the molecular level
- The equation that describes each step involved in a reaction at a molecular level is called an elementary reaction
- In the simplest case, a reaction mechanism involves only one step and is represented by a single equation
- For example, at above 600 K, the reaction between carbon monoxide, CO and nitrogen dioxide, NO2, has been shown to occur in a single step
CO (g) + NO2 (g) → CO2 (g) + NO (g) (elementary reaction)
- Some reactions involve more than one step and are represented by two, or more, equations:
- For example, at low temperatures (less than 600K), the reaction between carbon monoxide, CO and nitrogen dioxide, NO2, takes place by a different mechanism, involving two elementary reactions:
NO2 (g) + NO2 (g) → NO3 (g) + NO (g) (elementary reaction)
NO3 (g) + CO (g) → CO2 (g) + NO2 (g) (elementary reaction)
- The sum of the equations representing the steps involved in a reaction mechanism must always be equal to the overall balanced equation
- The elementary equations for the above two step mechanism can be shown to add up to the overall balanced chemical equation for the reaction between CO and NO2
Step 1: NO2 (g) + NO2 (g) → NO3 (g) + NO (g)
Step 2: NO (g) + CO (g) → CO2 (g) + NO (g)
Overall equation:
NO2 (g) + NO2 (g)+ NO3 (g)+ CO (g) → NO3 (g) + NO (g) + CO2 (g) + NO2 (g)
- Reaction mechanisms are difficult to observe directly and are theoretical attempts to explain experimental observations of a reaction
- Therefore, we propose reaction mechanisms that are possible
- Since multiple mechanisms can be proposed, experimental evidence is required for a mechanism to be proven
- In general, when reaction mechanisms are proposed, they must satisfy the following criteria:
- The equations for the elementary steps must combine to give the equation for the overall reaction
- The proposed elementary steps must be reasonable
- For example, elementary steps involving three or more molecules are typically unlikely because the likelihood of having three more molecules being aligned with sufficient energy is low
- The mechanism must support the experimentally determined rate law