D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
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Influence of concentrations on the velocity. Additional material.

Reactions seldom proceed as they are written down on a paper. As a rule, reaction consists of several stages. For example, reaction

4HBr + O2 = 2Br2 + 2H2O

consist of three consecutive stages:

1) HBr + O2 = HOOBr
2) HOOBr + HBr = 2 HOBr
3) HOBr + HBr = H2O + Br2

The first stage is the slowest. It determines the velocity of entire reaction (so called limiting stage). Dependence of velocity of reaction of HBr oxidation from concentration will be written down as:

v = k[HBr][O2],

the overall order of reaction is 2 (compare it with the sum of stoichiometric coefficients!).

As usually one stage of reaction is carried out due to collision of two particles, in extremely rare cases - of three, and never more. Therefore the overall order of reaction above 3 does not meet.

There can be more complex mechanisms of reactions. If it is impossible to distinguish limiting stage orders of reactions can be fractional or negative.


Expression for the law of mass action

Order of reaction

H2 + Cl2 = 2HCl

v = k[Cl2]1/2[H2]

n(Cl2) = 0,5; overall order 1,5

2CO + O2 = 2CO2

v = k[O2]/[CO]

n(CO) = -1; overall order 0

The most simple and investigated cases are reactions with the overall order equal to 1, 2 or 3:

H2S2O3 = SO2 + S + H2O

v = k[H2S2O3]

overall order 1

H2 + I2 = 2HI

v = k[H2][I2]

overall order 2

2NO + Cl2 = 2NOCl

v = k[NO]2[Cl2]

overall order 3

Radioactive decay obeys the rule of reactions of first order, though it is not chemical reaction. For the description of kinetics is important not the nature of described process, but how many particles participate in one act of this process.



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