AQA A Level Chemistry

Revision Notes

5.2.8 Rate Determining Step

Rate Determining Step

Rate-determining step & intermediates

  • A chemical reaction can only go as fast as the slowest part of the reaction
    • So, the rate-determining step is the slowest step in the reaction
  • If a reactant appears in the rate-determining step, then the concentration of that reactant will also appear in the rate equation
  • For example, the rate equation for the reaction below is rate = k [CH3Br] [OH]

CH3Br + OH → CH3OH + Br

    • This suggests that both CH3Br and OH take part in the slow rate-determining step
  • This reaction is a bimolecular reaction
    • Unimolecular: one species involved in the rate-determining step
    • Bimolecular: two species involved in the rate-determining step
  • The intermediate is derived from substances that react together to form it in the rate-determining step
    • For example, for the reaction above the intermediate would consist of CH3Br and OH

Reaction Kinetics - Intermediate, downloadable AS & A Level Chemistry revision notes

The intermediate is formed from the species that are involved in the rate-determining step (and thus appear in the rate equation)

Predicting the reaction mechanism

  • The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction
    • This shows the individual reaction steps which are taking place
  • 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 and zero molecules of CO (g)
  • 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 Kinetics - Reaction Mechanism Bromination Propane, downloadable AS & A Level Chemistry revision notes

Reaction mechanism for 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 OHare 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:

Kinetics of Multi-Step Reactions equation

  • The reaction mechanism is:

 

Reaction Kinetics - Reaction Mechanism Halogenation Butanone, downloadable AS & A Level Chemistry revision notes

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)

Reaction Kinetics - Intermediate Butanone, downloadable AS & A Level Chemistry revision notes

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

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