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Chemistry LibreTexts

RATE DETERMTING STEP

  1. Introducation

  2. Speed of Reaction

  3. Problem workout /Answer

  4. Sources

Introducation:

Rate determing step is the slowest step within a chemical reaction. The slowest step determines the rate of chemical reaction.The slowest step of a chemical reaction can be determined by seting up a reaction mechanisims. Many reactions don't happen in a single reaction but they happen in multiple elementary steps.

Example 1: 

               Overall Reaction: 2NO2 +F2 -> 2NO2 F

               elementary step 1: NO2 + F2 ->NO2F +F (slow)

               elementary step 2: NO2+F -> NO2F (fast)

For elemenatry step 1 has a rate constant of k1 and for elementary step 2 it has a rate constant of k2. The slowest step in this mechanism is elementary step 1 which is our rate determing step. Looking at this mechanism I see Intermidates. Intermidates are molecules or elements that are found on the product of one step but are also located in the reactent of another step. In this case we have two intermidates NO2 and F.

The rate equation is dervied by the slowest step in the reaction. When writing a rate equation you set up the equation by writing rate is equal to the rate constant of the slowest step times the concentrations of the reactent or reactents raised to there reaction order. Lets look at elementary step one.

 elementary step one: NO2 +F2 -> NO2F + F

 Here in this example rate=k1[NO2][F2].

Speed of Reaction

How fast a reaction happens depends on the slowest step in a reaction.

 

Problems to Work Out:

Problem 1:

Overall reaction: 2NO + O2 -> 2NO2

elementary step one: NO + NO <-> N2O2 (fast equilibrium)

elementary step two: N2O2 + O2 -> 2 NO2 (slow)

 

Question 1: What are the intermedates ?

Question 2: What is the rate equation?

Answer 1: N2Ois found on the porduct side and the rectant side.

Answer 2:rate= k2[N2O2][O2]; N2O2 gets cancled out leaving the overall reaction rate.

 

Prbolem 2:

elementary step one : Br2 + M <-> Br + Br +M (fast equilibrium)

elementary step two : Br +H2 -> HBr +H         (slow)

elementary step three : H + Br2 -> HBR + Br   (fast)

 

Question 1: What is the overall reaction?

Question 2: What is the rate equation?

Question 3: Are there intermedaites if so what are they?

Question 4: What is the rate deteming step?

 

Answer 1: H2+Br2 -> 2HBr ; Br and H will be cancled out and therefore they won't appear in the overall reaction.

Answer 2: rate=k2 [Br][H2]

Answer 3: Br, H are the intermedates in the reaction.

Answer 4: elementary step 2 is the slowest step in the mechanism.

 

 

Done by: Filmon  Tewolde

Sources

Oxoby , Gillis , Nachtrieb(2002). Princples of Modern Chemistry 5th edition.

Chang, R (2004). Physical chemistry for Biosciences. Sausaltio, Ca: Univeristy Science Books