Reaction Mechanisms (College Board AP Chemistry)

• 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

• In the proposed two-step reaction mechanism for the reaction between CO and NO₂, the chemical species, NO₃ which was produced in the first step and consumed in the next is known as an intermediate
  • Reaction intermediates are species produced during an elementary step of a reaction
  • But, they do not appear in the overall equation because they react in a subsequent step in the mechanism
  • Multistep mechanisms typically involve one or more intermediates
• Even though they are not products or reactants in the overall reaction, intermediates are essential for the reaction to take place
  • Intermediates can be stable and, therefore, sometimes be identified and even isolated
  • For example, the NO₃ intermediate can be identified from its visible light spectrum as it reacts quickly with CO to give CO₂ and NO
• Intermediates can be identified from an energy profile diagram of a reaction which occurs in multiple steps
  • Consider the energy profile diagram for the CO/NO₂ reaction below

Energy Profile Diagram

Energy profile diagram of the reaction between carbon monoxide and nitrogen dioxide showing the activated complex and intermediate

• • Unlike intermediates, activated complexes or transition states are always inherently unstable and as such can never be isolated
  • However, they can be characterized by the use of advanced “ultrafast” techniques

• Intermediates are also useful means of providing evidence to support a proposed reaction mechanism