The order of the chemical reaction is a formal concept. The physical meaning of the reaction order for elementary ( one-stage ) reactions is as follows : the reaction order is equal to the number of simultaneously changing concentrations.
Depending on the kind of kinetic equation connecting the reaction rate with the concentration of reacting substances, the reactions zero, first , second /i> and of the third order.
If the reaction rate does not depend on the concentration of the reactants, the reaction has zero order. If the reaction rate depends on the concentration of the substance in the first degree, First order ; if in the second degree, then this is the second-order reaction, etc.
The indicator of the degree of concentration of the reacting substance (a, p, 5) in the kinetic equation of the reaction is called the order of reaction for a given substance (A, B and D, respectively).
The general order of the chemical reaction, or simply the order of the reaction, is a quantity equal to the sum of the indices of the degree of concentration of reagents in the kinetic equation of the reaction. The general order of the reaction = = a + β + σ + ....
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The indicator of the degree of concentration of a given substance in the kinetic equation, as a rule, does not coincide with its stoichiometric coefficient in the reaction equation. Only for elementary, simple reactions, ie. reactions occurring in one stage, the exponents in the kinetic equation coincide with the stoichiometric coefficients of the reactants in the reaction equation.
The units for measuring the rate constant k depend on the sum of the exponents for reactant concentrations ( reaction order) in expression of the law of acting masses and the type of reaction ( homogeneous or heterogeneous reaction).
In Table. 15.1 shows examples of the dimensionality of the reaction rate constant depending on the order of the reaction, which are easily obtained, remembering that, for example, for a heterogeneous second-order reaction described by the equation
the expression of the law of acting masses will be written in the form of the dependence
From such a record, it is seen that this is a second-order reaction (the exponent of the reacting substance В is equal to 2), therefore, the dimension of the rate constant of this reaction is
units of measurement k =
= units of reaction speed/(units of concentration).
After substituting the units of measurement, we get
units of measure k = mol/(l • cm • with • ( mole/l )) = l/(mole • cm • c).
Dimension of the rate constant versus reaction order
The dimension of k for a homogeneous reaction
The dimension k for a heterogeneous response
Mole/(l · s · cm)
1/(s · cm)
l/(mol • s • cm)
For zero-order reactions, the kinetic equation has the following form:
The rate of the zero order reaction is constant in time and does not depend on the concentrations of the reacting substances. This is typical for those processes whose speed is less than the speed of delivery of reacting substances to the place of behavior of the reaction. This is often the case in heterogeneous reactions at the phase interface.
The zero order is also the reaction, the rate of which is limited by the supply of energy necessary to activate the reacting molecules (for example, photochemical reactions, where the determining factor is, for example, the amount of absorbed light, and not the concentration of the substance). In addition, often in catalytic reactions, the rate is determined by the concentration of the catalyst and does not depend on the concentration of the reactants.
Consider the time dependence of the concentration of the initial substance A for the first-order reaction
First-order reactions are characterized by a kinetic equation of the form(15.6) The first-order equations can describe the rates of elementary monomolecular reactions (isomerization, thermal decomposition, etc.), as well as reactions with a more complex mechanism, for example hydrolysis of sucrose with the formation of glucose and fructose. This reaction is bimolecular, however, due to the presence of a large excess of water, the rate depends only on the sucrose concentration.
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For second-order reactions, the kinetic equation has the trace
An example of second-order reactions is the formation and decomposition of hydrogen iodide, i.e. direct and reverse reactions in the system
as well as the decomposition of nitrogen dioxide
For third-order reactions:(15.9)
In the simplest case, when (15.10)
The third order is, for example, the reaction of oxidation of nitric oxide to dioxide:
2NO + 0 2 - & gt; 2N0 2
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