chemistry 1011 slot 51 chemistry 1011 topic rate of reaction text reference masterton and hurley...

Chemistry 1011 Slot 5 1

Chemistry 1011

TOPICRate of Reaction

TEXT REFERENCEMasterton and Hurley Chapter 11


11.7 Reaction Mechanisms

YOU ARE EXPECTED TO BE ABLE TO:

• Define reaction mechanism and show how the reaction order is dependent upon the mechanism by which a reaction takes place.

• For a reaction taking place in more than one step, identify the rate determining step and identify reaction intermediates.

• Determine if a proposed reaction mechanism is consistent with experimental rate data.


Reaction Mechanism

• Description of path or sequence of steps by which a reaction occurs

• Simplest case: single collision• Frequently more than one step• Rate expression and order of reaction

depend on mechanism• This is why rate law and order must be

determined experimentally


Reaction of CO(g) with NO2(g)

CO(g) + NO2(g) NO(g) + CO2(g)

• At high temperatures, reaction is single step

• Rate = k[CO] x [NO2]

• At low temperatures, reaction is two step process

NO2(g) + NO2(g) NO3(g) + NO(g)

CO(g) + NO3(g) CO2(g) + NO2(g)

CO(g) + NO2(g) NO(g) + CO2(g)

• Rate = k [NO2]2


Elementary Steps

• The individual steps that make up a reaction pathway are called elementary steps

• Steps may be unimolecular, bimolecular, etc

• The molecularity is the number of molecules that are involved in an elementary process


Rate Law for Elementary Steps

• For a complete reaction, the rate law and the order must be determined experimentally.

• This is because the mechanism may be simple or complex, and this determines the rate law and order

• For elementary reactions, however, the rate law can be determined from the equation


Rate Law for Elementary StepsElementary Step Molecularity Rate Law

A product unimolecular Rate = k[A]A + B product bimolecular Rate = k[A][B]A + A product bimolecular Rate = k[A]2

2A + B product termolecular Rate = k[A]2[B]

• For the elementary step:NO2(g) + NO2(g) NO3(g) + NO(g)

• The rate law is:

Rate = k[NO2]2


Slow and Fast Steps• Frequently, one step in a mechanism will be

slower than the others

• This is the rate determining step

• Imagine a two step process:Step 1: A + B X + I SLOW

Step 2: A + I Y FAST

Total: 2A + B X + Y

• The first step is the rate determining step


Overall Rate Law and Order

• Rate(First Step) = k1[A][B]

• This slow first step determines the overall rate

• This is the rate law for the overall reaction:

2A + B X + Y

which is therefore second order


Mechanisms and Rate Laws

• The rate law expression is determined by experiment

• The mechanism is a hypothesis about the way the reaction occurs

• The proposed mechanism must yield a rate law expression that is consistent with experiment


Mechanism with a Fast Initial Step

• Sometimes the first step in a reaction mechanism, which results in the creation of a reaction intermediate, will be FAST

• The second step, where the reaction intermediate is a reactant, may be SLOW

• When this happens, the rate determining step will be the second step

• The rate law expression should then include the concentration of the reaction intermediate


Reaction Intermediates and Rate Law

• The concentration of an intermediate cannot be measured

• The concentration of an intermediate cannot be included in a rate law expression if this is to be compared to experiment

• The final rate law expression can only include species occurring in the balanced equation


Reaction Intermediates and Rate Law - an Example

Step 1: NO(g) + Cl2(g) NOCl2(g) FAST

Step 2: NOCl2(g) + NO(g) 2NOCl(g) SLOW

Overall: 2 NO(g) + Cl2(g) 2NOCl(g)

• Rate of overall reaction = rate of step 2

• Rate = k2[NOCl2][NO]

• This is unsatisfactory;

[NOCl2] cannot be measured


Eliminating Intermediates from the Rate Law Expression

• The first (fast) step in the reactionNO(g) + Cl2(g) NOCl2(g) FAST

is reversible• The reactants and products are in equilibrium

rate forward = rate reverse

k1[NO][Cl2] =k[NOCl2]


Eliminating Intermediates from the Rate Law Expression

[NOCl2] = k1[NO][Cl2]

k

Substitute in overall rate law expression

Rate of reaction = rate of step 2

Rate = k2k1 [NO]2[Cl2]

k


Limitations of Mechanism Studies

• Mechanisms are suggested in order to explain observed rate laws and orders of reaction

• Often more than one mechanism can explain experimental results

chemistry 1011 slot 51 chemistry 1011 topic rate of reaction text reference masterton and hurley...

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