A statement of chemical kinetics

1. May 4, 2015

mooncrater

1. The problem statement, all variables and given/known data
There was a statement in a question which said:
"The order of an elementary chemical reaction step can be determined by examining its stoichiometry. "
And it was given correct.

2. Relevant equations

3. The attempt at a solution
But shouldn't it be wrong? Order of a reaction is found experimentally and nothing else can determine it. So how can we simply see the stoichiometry of a reaction and say " yeah... it has this and this order". Does here "elementary chemical reaction step" make any change to the thing that I am saying?

Last edited: May 4, 2015
2. May 4, 2015

rohini

Order of a rxn is determined by the slowest step of the rxn .so if a rxn is taking place in no.of steps .we need to find out which was the slowest step and then see the coefficient s but for elementary rxn .it is a single step rxn so its mere coincidence that stoichiometric coeff gives us the order of rxn

3. May 4, 2015

Always?

4. May 4, 2015

mooncrater

Okay then does this statement fit here?
"Molecularity refers to the order of rate determining step (the slowest one)"

5. May 4, 2015

rohini

For complex rxn order is given by the slowest step and molecularity of the slowest step is same as the order of overall rxn

6. May 4, 2015

mooncrater

Thanks.. I got it now..

7. May 4, 2015

rohini

But remember molecularity can never be zero or even a fraction but order of rxn can be .as in zero order rxn .rate of rxn is independent of its concentration.
In pseudo first order rxn one of the reactants is in excess so change in it would be like const

8. May 4, 2015

mooncrater

So can we say that the slowest reaction will never have a fractional order?

9. May 5, 2015

rohini

fractional order reactions, the order is a non-integer, which often indicates a chemical chain reaction or other complex reaction mechanism. For example, the pyrolysis ofethanal (CH3-CHO) into methane and carbon monoxide proceeds with an order of 1.5 with respect to ethanal: r = k[CH3-CHO]3/2.[12] The decomposition of phosgene (COCl2) to carbon monoxide and chlorine has order 1 with respect to phosgene itself and order 0.5 with respect to chlorine: r = k[COCl2] [Cl2]1/2.

The order of a chain reaction can be rationalized using the steady stateapproximation for the concentration of reactive intermediates such as free radicals. For the pyrolysis of ethanal, the Rice-Herzfeldmechanism is[12]

Initiation CH3CHO → •CH3 + •CHO
Propagation •CH3 + CH3CHO → CH3CO• + CH4
CH3CO• → •CH3 + CO
Termination 2 •CH3 → C2H6
where • denotes a free radical. To simplify the theory, the reactions of the •CHO to form a second •CH3 are ignored.

In the steady state, the rates of formation and destruction of methyl radicals are equal, so that

,
so that the concentration of methyl radical satisfies

.
The reaction rate equals the rate of the propagation steps which form the main reaction products CH4 and CO:

in agreement with the experimental order of 3/2

10. May 5, 2015

rohini

In chemical kinetics, the order of reactionwith respect to a given substance (such asreactant, catalyst or product) is defined as theindex, or exponent, to which its concentrationterm in the rate equation is raised.