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Chemical reaction rate, reaction order, reaction half-life, etc.
are all macro descriptions of reaction characteristics, and reaction mechanism is a description of the micro process of reaction
.
3.
3.
1 Elementary reaction
Elementary reaction refers to a reaction in which reactant molecules are directly converted into products in one step
.
E.
NO 2 +CO=NO+CO 2
Many chemical reactions, although their reaction equations are very simple, are not elementary reactions, but go through two or more steps
The completed complex reaction
.
For example, the reaction 2NO+O 2 =2NO 2 may have gone through the following elementary steps
2NO=N 2 O 2(fast) (1)
N 2 O 2 =2NO (fast) (2)
N 2 O 2 +O 2 =2NO 2(slow) (3)
To study the reaction mechanism of a complex reaction is to study which elementary steps the reaction has gone through
.
The number of particles required for the reaction in the elementary step of the elementary reaction or the complex reaction is called the number of molecules of the reaction
.
The reaction can be a single-molecule reaction or a bi-molecular reaction caused by the collision of two particles.
For elementary reactions or elementary steps of complex reactions, the rate equation can be written directly from the reaction formula
.
For example, elementary reactions
aA+bB=gG+hH
The reaction rate equation is
r=k[A] a [B] b
That is, the reaction rate of the elementary reaction is proportional to the continuous product of the reactant concentration with its stoichiometric number as an exponential power
.
The reaction NO 2 +CO=NO+CO 2 is the elementary reaction, and the reaction rate equation is
r=k[NO 2 ][CO]
In some reactions, the reaction order measured by the experiment is equal to the stoichiometric number of the reactants in the reaction formula, but the reaction is not necessarily a basic element reaction
.
For example, neither of the following two reactions are elementary reactions
2NO+O 2 =2NO r=k[NO] 2 [O 2 ]
H 2 +I 2 =2HI r=A[H 2 ][I 2 ]
Related Links: The relationship between reactant concentration and reaction time-first-order reaction, second-order reaction and third-order reaction
