Reaction order, Zero-order reactions, First-order reactions, Second-order reactions, Third-order reactions - Physical and colloid chemistry

Reaction order

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 + ....

How to ...

Choose! The largest selection of informative articles.

Tired of looking for a solution?

Get Your ESSAY Now!

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).

Table 15.1

Dimension of the rate constant versus reaction order

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)

Zero-order reactions

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.

First-order reactions

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


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.

Ready to make your order?

Get your great paper now

Second-order reactions

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

Third-order reactions

For third-order reactions:


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

Price Plan & Our Features

$18.00 per page
Due date 14 days or longer
  • Free Outline
  • Free Formatting
  • Free Title page
  • Free Reference Page
  • Free Revisions
  • Free Submission by Chapters
  • Free Plagiarism Report
  • Free Customer Service
Additional Services
  • 1 – page abstract
  • VIP Support
  • Expedited delivery
  • Essay outline
  • $25.25
  • $12.99
  • $8.99
  • $25.25
Order Now

Also We Can Offer!

Other services that we offer

If you don’t see the necessary subject, paper type, or topic in our list of available services and examples, don’t worry! We have a number of other academic disciplines to suit the needs of anyone who visits this website looking for help.

How to ...

We made your life easier with putting together a big number of articles and guidelines on how to plan and write different types of assignments (Essay, Research Paper, Dissertation etc)