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-

   

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 if 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 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 CH3CH2CH3?and OH-?are involved in the?rate-determining step?and not bromine (Br2)
• CH3CH2CH3?and OH-?are only involved in the first step of the reaction mechanism, therefore the?rate-determining step?is:
  • CH3CH2CH3?+ OH-?→?CH3CH2CH2-?+ H2O

   

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 a reactant 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+

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