Kinetics Question- Rate Laws

[virtualmaster999]

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Hi all!

Quick question on rate laws. So just to clarify (off of something I learned from Chad's videos):

Rate law= k(reactants) for slow step, but it will always include catalysts/ intermediates (for our purposes right)? I think that's what Chad was saying. Does this always hold true?

Thanks in advance!

 

[DAT Destroyer]

Hi all!

Quick question on rate laws. So just to clarify (off of something I learned from Chad's videos):

Rate law= k(reactants) for slow step, but it will always include catalysts/ intermediates (for our purposes right)? I think that's what Chad was saying. Does this always hold true?

Thanks in advance!

Yes, a catalyst can be included in a rate law. That's because, most reactions occur in a series of steps and the rate is based on the "rate determining step", which is the slowest step. A catalyst may be a reactant in the rate determining step, and a product in a subsequent step. Therefore, the catalyst is not included in the overall reaction. The rate law expression would NOT contain intermediates, however for the slow step refer to a text book such as Brown and LeMay or Raymond Chang for more clarity.

Hope this helps.

Dr. Jim Romano

 

[virtualmaster999]

Yes, a catalyst can be included in a rate law. That's because, most reactions occur in a series of steps and the rate is based on the "rate determining step", which is the slowest step. A catalyst may be a reactant in the rate determining step, and a product in a subsequent step. Therefore, the catalyst is not included in the overall reaction. The rate law expression would NOT contain intermediates, however for the slow step refer to a text book such as Brown and LeMay or Raymond Chang for more clarity.

Hope this helps.

Dr. Jim Romano

Yes, thank you for the clarification! The reason why I wanted to double check was because Chad's vids stated that for pre health exam purposes, the intermediate would be left in the rate law expression (this is not how it is for a typical gen chem class, but chad stated that it would require too much time/work for the exam to ask you the "correct" rate law w/o the intermediate, thus, the exam would just have the intermediate included). Have you seen a case where the DAT asks you to find the "correct" rate law, or if it's in the Destroyer by chance?

 

[DAT Destroyer]

Yes, thank you for the clarification! The reason why I wanted to double check was because Chad's vids stated that for pre health exam purposes, the intermediate would be left in the rate law expression (this is not how it is for a typical gen chem class, but chad stated that it would require too much time/work for the exam to ask you the "correct" rate law w/o the intermediate, thus, the exam would just have the intermediate included). Have you seen a case where the DAT asks you to find the "correct" rate law, or if it's in the Destroyer by chance?

The questions in the DESTROYER accurately reflect the DAT exam. What you are referring to is DERIVING a rate law given the slow step of a mechanism...along with the fast steps and equilibrium steps. This is long and sometimes tedious work, not seen on multiple question type exams. For the DAT....be able to look at data and write the rate law from numerical data, find the rate constants, and find the rates at various concentrations. I have included this in problem #193, 238, of the 2015 book.

Dr. Romanp

 

[cc609]

Pertaining to number 193 on Destroyer 2015:

Above the table of experiments given, they give the reaction 2NO+O2--> 2NO2
Would the coefficients of the equation given always be the same as the rate constant? As in, could we derive the rate= K[NO]^2[O2] from that equation instead of solving it out with the numbers given on the table? I know rate laws must be experimentally proven but all the rate laws I've solved out always matched the equation :/

The questions in the DESTROYER accurately reflect the DAT exam. What you are referring to is DERIVING a rate law given the slow step of a mechanism...along with the fast steps and equilibrium steps. This is long and sometimes tedious work, not seen on multiple question type exams. For the DAT....be able to look at data and write the rate law from numerical data, find the rate constants, and find the rates at various concentrations. I have included this in problem #193, 238, of the 2015 book.

Dr. Romanp

 

[virtualmaster999]

The questions in the DESTROYER accurately reflect the DAT exam. What you are referring to is DERIVING a rate law given the slow step of a mechanism...along with the fast steps and equilibrium steps. This is long and sometimes tedious work, not seen on multiple question type exams. For the DAT....be able to look at data and write the rate law from numerical data, find the rate constants, and find the rates at various concentrations. I have included this in problem #193, 238, of the 2015 book.

Dr. Romanp

Wonderful, I always appreciate your feedback and explanations. Thank you 🙂

 

[FeralisExtremum]

P
Above the table of experiments given, they give the reaction 2NO+O2--> 2NO2
Would the coefficients of the equation given always be the same as the rate constant? As in, could we derive the rate= K[NO]^2[O2] from that equation instead of solving it out with the numbers given on the table? I know rate laws must be experimentally proven but all the rate laws I've solved out always matched the equation :/

The only way you could assume that the rate law would match the coefficients of the equation would be if you knew it was an elementary reaction. Always solve from the experimental data unless you are explicitly told the equation you are given is an elementary reaction, or if you are given the mechanism of a reaction and told that specific part of it is the slow step.