Multi-Step Reactions (CIE A Level Chemistry)

• The reaction mechanism of a reaction describes how many steps are involved in the making and breaking of bonds during a chemical reaction
• It is the slowest step in a reaction and includes the reactants that have an impact on the reaction rate when their concentrations are changed
  • Therefore, all reactants that appear in the rate equation will also appear in the rate-determining step
  • This means that reactants that have a zero-order and intermediates will not be present in the rate-determining step

Predicting the reaction mechanism

• The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction
• For example, nitrogen dioxide (NO₂) and carbon monoxide (CO) react to form nitrogen monoxide (NO) and carbon dioxide (CO₂)
• The overall reaction equation is:

NO₂ (g) + CO (g) → NO (g) + CO₂ (g)

• The rate equation is:

Rate = k [NO₂]²

• From the rate equation it can be concluded that the reaction is zero order with respect to CO (g) and second order with respect to NO₂ (g)
• This means that there are two molecules of NO₂ (g) involved in the rate-determining step
• A possible reaction mechanism could therefore be:

• Step 1:

2NO₂ (g) → NO (g) + NO₃ (g)     slow (rate-determining step)

• Step 2:

NO₃ (g) + CO (g) → NO₂ (g) + CO₂ (g)     fast

• Overall:

2NO₂ (g) + NO₃ (g) + CO (g) → NO (g) + NO₃ (g) + NO₂ (g) + CO₂ (g)

=     NO₂ (g) + CO (g) → NO (g) + CO₂ (g)

Predicting the reaction order & deducing the rate equation

• The order of a reactant and thus the rate equation can be deduced from a reaction mechanism given that the rate-determining step is known
• For example, the reaction of nitrogen oxide (NO) with hydrogen (H₂) to form nitrogen (N₂) and water

2NO (g) + 2H₂ (g) → N₂ (g) + 2H₂O (l)

• The reaction mechanism for this reaction is:

• Step 1:

NO (g) + NO (g) → N₂O₂ (g)     fast

• Step 2:

N₂O₂ (g) + H₂ (g) → H₂O (l) + N₂O (g)     slow (rate-determining step)

• Step 3:

N₂O (g) + H₂ (g) → N₂ (g) + H₂O (l)     fast

• The second step in this reaction mechanism is the rate-determining step
• The rate-determining step consists of:
  • N₂O₂ which is formed from the reaction of two NO molecules
  • One H₂ molecule

• The reaction is, therefore, second order with respect to NO and first order with respect to H₂
• So, the rate equation becomes:

Rate = k [NO]² [H₂]

• The reaction is, therefore, third order overall

Identifying the rate-determining step

• The rate-determining step can be identified from a rate equation given that the reaction mechanism is known
• For example, propane (CH₃CH₂CH₃) undergoes bromination under alkaline solutions
• The overall reaction is:

CH₃CH₂CH₃ + Br₂ + OH^- → CH₃CH₂CH₂Br + H₂O + Br^-

• The reaction mechanism is:

Reaction mechanism of the bromination of propane under alkaline conditions

• The rate equation is:

Rate = k [CH₃CH₂CH₃] [OH^-]

• From the rate equation, it can be deduced that only CH₃COCH₃ and OH^- are involved in the rate-determining step and not bromine (Br₂)
• Since only in step 1 of the reaction mechanism are CH₃COCH₃ and OH^- involved, the rate-determining step is step 1 is the case for step 1 of the reaction mechanism

Identifying intermediates & catalyst

• When a rate equation includes a species that is not part of the chemical reaction equation then this species is a catalyst
• For example, the halogenation of butanone under acidic conditions
• The reaction mechanism is:

• The reaction mechanism is:

Reaction mechanism of the halogenation of butanone under acidic conditions

• The rate equation is:

Rate = k [CH₃CH₂COCH₃] [H⁺]

• The H⁺ is not present in the chemical reaction equation but does appear in the rate equation
  • H⁺ must therefore be a catalyst

• Furthermore, the rate equation suggest that CH₃CH₂COCH₃ and H⁺ must be involved in the rate-determining (slowest) step
• The CH₃CH₂COCH₃ and H⁺ appear in the rate-determining step in the form of an intermediate (which is a combination of the two species)

Intermediate is formed in the rate-determining step from the reaction of CH₃CH₂COCH₃ and H⁺