Rate-Determining Steps from Equations (Edexcel International A Level Chemistry)

Revision Note

Richard

Author

Richard

Last updated

Rate-Determining Steps from Equations

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

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)

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 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

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.