Reaction order: concept, types

In chemical kinetics, one of the important tasks is to find the reaction order. Although this quantity is formal, it allows one to best reflect the experimental dependence of the rate of a reaction on concentration. As a rule, to find the reaction rate, the concentrations of the starting compounds taken in degrees corresponding to their stoichiometric coefficients are used. But this is true only for very simple reactions.

The order of a chemical reaction in a substance is a value of degree n, in which the concentration of this compound is included in the formula for finding the reaction rate obtained experimentally. But the general order is the sum of all orders of matter: n = n ₁ + n ₂ . The values ​​of n ₁ and n ₂ correspond to stoichiometric coefficients in the equations of one-stage reactions. In fact, they can have positive or negative values, be integer or fractional numbers.

For example, for the equation of interaction H ₂ + Ι ₂ -> 2HΙ, which corresponds to the formula for determining the velocity v = kC _H C _I , the orders of matter are n _H = 1 and n _I = 1, the general reaction order is n = n _H + n _I = 1 + 1 = 2.

Zero order

Some reactions have a zero order of reaction. As a rule, they are not affected by the concentration of the starting compounds. This happens in the following cases:

• if any starting material is taken in a substantial excess;
• if the speed is regulated by the activation energy of the molecules involved in the reaction, for example, in photosynthesis.

As an example, let us consider the reaction of the interaction of ethyl acetate with water, i.e. its saponification.

Η _{3 2} Η ₅ + Η ₂ -> Η ₃ Η + ₂ Η ₅ Η

Since the mutual solubility of the starting materials is extremely small, their bulk is in different phases. When ether is consumed in a chemical reaction, a new portion of it arrives in time from the ether layer, i.e. its concentration in the solution does not decrease.

First order reactions

Such an interaction of substances can be arbitrarily written by the equation: ―> . An example is the decomposition of dimethyl ether:

Η ₃ Η ₃ -> Η ₄ + Η ₂ +

For which the reaction rate is defined as v = kC _C2H6O . In this case, the order of matter and the general order are the same and equal to unity.

Of practical importance for first-order reactions is the time (τ) for determining a certain given concentration C, if the initial concentration C _is known, as well as the half-life τ _1/2 , this is the time during which half of the starting material has time to react.

Second order reactions

Such interactions include reactions of the type A + B -> products. An example is the reaction of obtaining iodine hydrogen or the alkaline saponification of ethyl acetate, already cited above:

Η _{3 2} H ₅ + Η- -> ₃ - + ₂ H ₅ Η, v = kC _{482 -} .

Also (k) second order have separate decomposition reactions of the type: 2A -> products. Examples of these include:

• 2NOCl -> 2NO + Cl ₂ , v = kC ² _NOCl.
• 2 _{3 ->} 3O ₂ , v = kC ² _3.
• 2NO ₂ -> 2NO + O2, v = kC ² _NO2.

Subsequent reactions

Reactions of the third and subsequent orders are less common than previous options. This is due to the low probability of simultaneous encounter in space of three or more particles. However, an example of such interactions can be the formation of nitrogen and carbon dioxide from their monoxides:

• 2NΟ + Ο ₂ → 2NΟ ₂ , v = kC ² _{NO 2.}
• 2CO + O ₂ → 2CO ₂ , v = kC ² _CO WITH _O2.

For such reactions, there is also a dependence of the concentrations of reagents on the time of its occurrence. The formulas for finding the half-transformation period and reaction rate constants are unified by introducing the index n equal to the order of these same reactions.

Reaction molecular

It is not necessary to confuse the order of the reaction with its molecularity, which is determined precisely by the number of molecules that perform the act of chemical transformation. In contrast to the experimentally determined order, the molecularity of the chemical reaction has a theoretical basis. To determine it, you need to understand the essence of the process, how exactly the molecules interact with each other, through what stages of transformation they go through.

There are several reasons why the order and molecularity do not coincide for the same reaction:

• if one of the reagents is taken in large excess, as already mentioned above;
• for many heterogeneous reactions, the order can change during their implementation, especially if the conditions of their course change;
• catalytic reactions have a multistage mechanism, the essence of which is not always reflected by the stoichiometric equation;
• in complex multi-stage reactions, only one of the intermediate ones can affect the total value of the rate, which, as a result, will determine the order of the entire transformation.

Monomolecular reactions include the decomposition of molecules:

I ₂ -> 2I

In bimolecular reactions, two molecules collide. Moreover, these can be molecules of different substances or of the same:

H ₂ + Ι ₂ -> 2HΙ

Trimolecular reactions are those reactions for which three molecules of the starting materials are needed:

2NΟ + H ₂ -> N ₂ Ο + H ₂ O

H ₂ + O ₂ -> 2H ₂ O