AP Chemistry 5.5: Reaction Mechanisms

A balanced chemical equation tells you the overall reactants and products, but it does not necessarily tell you how the reaction actually happens.

A reaction mechanism is the sequence of individual elementary steps that together produce the overall reaction.

• Many chemical reactions do not occur in one giant collision. Instead, they happen through several smaller steps.
• For example, an overall reaction might look simple:
• A + B → D
• but the actual mechanism could be:
• Step 1: A + B → C
• Step 2: C → D
• When the two steps are added together, C cancels.
• Therefore the overall reaction is:
• A + B → D

A reaction intermediate is a substance that is produced in one step and consumed in a later step.

• Consider:
• Step 1:
• A + B → C
• Step 2:
• C + D → E
• Add:
• A + B + C + D → C + E

Cancel C:

• A + B + D → E
• C is an intermediate because it forms during the mechanism but does not appear in the overall equation.
• Intermediates can appear in mechanisms, but they should cancel when the elementary steps are added.

A catalyst behaves differently from an intermediate.

A catalyst is usually consumed in an early step and regenerated in a later step.

• For example:
• Step 1:
• A + X → AX
• Step 2:
• AX + B → AB + X
• X disappears in Step 1 and is regenerated in Step 2.
• When the equations are added, X cancels.
• Therefore X is a catalyst.
• Compare:
• Intermediate: formed first, then consumed
• Catalyst: consumed first, then regenerated
• This distinction is frequently tested.

In a multistep mechanism, one step may be much slower than the others.

The slowest important step is called the rate-determining step.

• Think of a checkout line. Even if several parts of the process are fast, one slow step can control how quickly the entire process finishes.
• If the slow step is:
• A + B → C
• then the elementary-step rate law is:
• Rate = k[A][B]
• This may become the overall observed rate law if the slow step directly contains the appropriate reactants.

A proposed mechanism must satisfy at least two major requirements.

First, when all elementary steps are added together and intermediates/catalysts are canceled, the result must equal the overall balanced reaction.

Second, the mechanism must be consistent with the experimentally observed rate law.

• A mechanism that gives the correct overall equation but predicts the wrong rate law is not consistent with the experimental evidence.

Suppose the experimental reaction is:

• 2A + B → D
• and the observed rate law is:
• Rate = k[A][B]

Proposed mechanism:

• Step 1 (slow): A + B → C
• Step 2 (fast): A + C → D

Add the steps:

• A + B + A + C → C + D

Cancel C:

• 2A + B → D
• So the mechanism gives the correct overall equation.

Because Step 1 is slow and elementary:

• Rate = k[A][B]
• This matches the experimental rate law.
• Therefore the proposed mechanism is consistent with both pieces of evidence.

Sometimes the slow-step rate law contains an intermediate.

• Suppose:
• Step 1: A + B ⇌ C fast equilibrium
• Step 2: C + D → product slow
• The slow-step rate law would initially be:
• Rate = k[C][D]
• But C is an intermediate, and an overall experimental rate law should generally be written using measurable reactants rather than an intermediate.

If the first step establishes a fast equilibrium, the concentration of C can be related to concentrations of A and B. This is the idea behind the pre-equilibrium approximation.

• At the AP level, you may be asked to use an earlier fast equilibrium to substitute for an intermediate and obtain a rate law containing only overall reactants.
• The key idea is not to leave an intermediate in the final experimentally meaningful rate law if it can be replaced using the mechanism.

Do not assume the overall balanced reaction is automatically elementary.

Do not use overall coefficients as rate-law exponents unless there is evidence that the reaction occurs in one elementary step.

Do not confuse intermediates and catalysts.

• Remember:
• intermediate = created, then consumed
• catalyst = consumed, then regenerated

A valid mechanism must produce both the correct overall equation and the correct observed rate law.

A reaction mechanism answers:

• “What individual steps actually happen between reactants and products?”

The slowest important step often controls the overall rate.