The rate of a chemical reaction

The rate of a chemical reaction- change in the amount of one of the reacting substances per unit of time in a unit of reaction space. It is a key concept of chemical kinetics. The rate of a chemical reaction is always positive, therefore, if it is determined by the initial substance (the concentration of which decreases during the reaction), then the resulting value is multiplied by −1.

For example for a reaction:

the expression for speed will look like this:

For elementary reactions, the exponent at the concentration value of each substance is often equal to its stoichiometric coefficient; for complex reactions, this rule is not observed. In addition to concentration, the following factors influence the rate of a chemical reaction:

• the nature of the reactants,
• the presence of a catalyst
• temperature (van't Hoff rule),
• pressure,
• the surface area of ​​the reactants.

If we consider the simplest chemical reaction A + B → C, then we notice that instant the rate of a chemical reaction is not constant.

Literature

• Kubasov A. A. Chemical kinetics and catalysis.
• Prigogine I., Defey R. Chemical thermodynamics. Novosibirsk: Nauka, 1966. 510 p.
• Yablonsky G. S., Bykov V. I., Gorban A. N., Kinetic models of catalytic reactions, Novosibirsk: Nauka (Siberian Branch), 1983.- 255 p.

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Systems. But this value does not reflect the real possibility of the reaction, its speed and mechanism.

For a complete representation of a chemical reaction, one must have knowledge of what temporal patterns exist during its implementation, i.e. chemical reaction rate and its detailed mechanism. The rate and mechanism of the reaction studies chemical kinetics the science of chemical process.

In terms of chemical kinetics, reactions can be classified into simple and complex.

simple reactions- processes occurring without the formation of intermediate compounds. According to the number of particles participating in it, they are divided into monomolecular, bimolecular, trimolecular. The collision of more than 3 particles is unlikely, so trimolecular reactions are quite rare, and four-molecular ones are unknown. Complex reactions- processes consisting of several elementary reactions.

Any process proceeds with its inherent speed, which can be determined by the changes that occur over a certain period of time. middle chemical reaction rate expressed as a change in the amount of a substance n consumed or received substance per unit volume V per unit time t.

υ = ± dn/ dt· V

If the substance is consumed, then we put the sign "-", if it accumulates - "+"

At constant volume:

υ = ± DC/ dt,

Reaction rate unit mol/l s

In general, υ is a constant value and does not depend on which substance we are following in the reaction.

The dependence of the concentration of the reagent or product on the reaction time is presented as kinetic curve, which looks like:

It is more convenient to calculate υ from experimental data if the above expressions are converted into the following expression:

The law of active masses. Order and rate constant of reaction

One of the wording law of mass action sounds like this: The rate of an elementary homogeneous chemical reaction is directly proportional to the product of the concentrations of the reactants.

If the process under study is represented as:

a A + b B = products

then the rate of a chemical reaction can be expressed kinetic equation:

υ = k [A] a [B] b or

υ = k C a A C b B

Here [ A] and [B] (C A andC B) - concentration of reagents,

a andb are the stoichiometric coefficients of a simple reaction,

k is the reaction rate constant.

The chemical meaning of the quantity k- This speed reaction at single concentrations. That is, if the concentrations of substances A and B are equal to 1, then υ = k.

It should be taken into account that in complex chemical processes the coefficients a andb do not match the stoichiometric ones.

The law of mass action is fulfilled under a number of conditions:

• The reaction is thermally activated, i.e. thermal motion energy.
• The concentration of reagents is evenly distributed.
• The properties and conditions of the environment do not change during the process.
• Environment properties should not affect k.

For complex processes law of mass action cannot be applied. This can be explained by the fact that a complex process consists of several elementary stages, and its speed will not be determined by the total speed of all stages, but only by one of the slowest stages, which is called limiting.

Each reaction has its own order. Determine private (partial) order by reagent and general (full) order. For example, in the expression for the rate of a chemical reaction for a process

a A + b B = products

υ = k·[ A] a·[ B] b

a– order by reagent BUT

b — order by reagent AT

General order a + b = n

For simple processes the reaction order indicates the number of reacting particles (coincides with stoichiometric coefficients) and takes integer values. For complex processes the order of the reaction does not coincide with the stoichiometric coefficients and can be any.

Let us determine the factors influencing the rate of a chemical reaction υ.

1. The dependence of the reaction rate on the concentration of reactants

   determined by the law of mass action: υ = k[ A] a·[ B] b

Obviously, with increasing concentrations of reactants, υ increases, because the number of collisions between the substances participating in the chemical process increases. Moreover, it is important to consider the order of the reaction: if it n=1 for some reagent, then its rate is directly proportional to the concentration of this substance. If for any reagent n=2, then doubling its concentration will lead to an increase in the reaction rate by 2 2 \u003d 4 times, and increasing the concentration by 3 times will speed up the reaction by 3 2 \u003d 9 times.

Speed ​​reaction is determined by the change in the molar concentration of one of the reactants:

V \u003d ± ((C 2 - C 1) / (t 2 - t 1)) \u003d ± (DC / Dt)

Where C 1 and C 2 are the molar concentrations of substances at times t 1 and t 2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the original substance).

Reactions occur when molecules of reactants collide. Its speed is determined by the number of collisions and the likelihood that they will lead to a transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors affecting the rate of chemical reactions.
1. The nature of the reactants. An important role is played by the nature of chemical bonds and the structure of the molecules of the reagents. Reactions proceed in the direction of the destruction of less strong bonds and the formation of substances with stronger bonds. Thus, high energies are required to break bonds in H 2 and N 2 molecules; such molecules are not very reactive. To break bonds in highly polar molecules (HCl, H 2 O), less energy is required, and the reaction rate is much higher. Reactions between ions in electrolyte solutions proceed almost instantaneously.
Examples
Fluorine reacts explosively with hydrogen at room temperature; bromine reacts with hydrogen slowly even when heated.
Calcium oxide reacts vigorously with water, releasing heat; copper oxide - does not react.

2. Concentration. With an increase in concentration (the number of particles per unit volume), collisions of reactant molecules occur more often - the reaction rate increases.
The law of active masses (K. Guldberg, P. Waage, 1867)
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

AA + bB + . . . ® . . .

• [A] a [B] b . . .

The reaction rate constant k depends on the nature of the reactants, temperature, and catalyst, but does not depend on the concentrations of the reactants.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the reaction rate expression.

3. Temperature. For every 10°C increase in temperature, the reaction rate increases by a factor of 2-4 (Van't Hoff's rule). With an increase in temperature from t 1 to t 2, the change in the reaction rate can be calculated by the formula:

(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, respectively; g is the temperature coefficient of this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:

• e-Ea/RT

where
A is a constant depending on the nature of the reactants;
R is the universal gas constant;

Ea is the activation energy, i.e. the energy that colliding molecules must have in order for the collision to result in a chemical transformation.
Energy diagram of a chemical reaction.

A - reagents, B - activated complex (transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.

4. The contact surface of the reactants. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them.

5. Catalysis. Substances that participate in reactions and increase its rate, remaining unchanged by the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. At homogeneous catalysis the reagents and the catalyst constitute one phase (they are in the same state of aggregation), with heterogeneous catalysis- different phases (they are in different states of aggregation). In some cases, the course of undesirable chemical processes can be drastically slowed down by adding inhibitors to the reaction medium (the phenomenon negative catalysis").

7.1. Homogeneous and heterogeneous reactions

Chemical substances can be in different states of aggregation, while their chemical properties in different states are the same, but the activity is different (which was shown in the last lecture using the example of the thermal effect of a chemical reaction).

Consider various combinations of aggregate states in which two substances A and B can be.

A phase is a region of a chemical system within which all the properties of the system are constant (the same) or continuously change from point to point. Separate phases are each of the solids, in addition, there are phases of solution and gas.

Homogeneous is called chemical system, in which all substances are in the same phase (in solution or in gas). If there are several phases, then the system is called

heterogeneous.

Respectively chemical reaction called homogeneous if the reactants are in the same phase. If the reactants are in different phases, then chemical reaction called heterogeneous.

It is easy to understand that since a chemical reaction requires the contact of reagents, a homogeneous reaction occurs simultaneously in the entire volume of the solution or reaction vessel, while a heterogeneous reaction occurs at a narrow boundary between the phases - at the interface. Thus, purely theoretically, a homogeneous reaction occurs faster than a heterogeneous one.

Thus, we pass to the concept chemical reaction rate.

The rate of a chemical reaction. The law of active masses. chemical balance.

7.2. The rate of a chemical reaction

The branch of chemistry that studies the rates and mechanisms of chemical reactions is a branch of physical chemistry and is called chemical kinetics.

The rate of a chemical reaction is the change in the amount of a substance per unit time per unit volume of the reacting system (for a homogeneous reaction) or per unit surface area (for a heterogeneous reaction).

The ratio of the change in the amount of a substance to the volume of the system can be interpreted as a change in the concentration of a given substance.

Note that for reagents in the expression for the rate of a chemical reaction, a minus sign is put, since the concentration of the reagents decreases, and the rate of the chemical reaction is actually a positive value.

Further conclusions are based on simple physical considerations that consider a chemical reaction as a consequence of the interaction of several particles.

Elementary (or simple) is a chemical reaction that occurs in one stage. If there are several stages, then such reactions are called complex, or compound, or gross reactions.

In 1867, to describe the rate of a chemical reaction, was proposed law of mass action: the rate of an elementary chemical reaction proportional to the concentrations of reactants in powers of stoichiometric coefficients.n A +m B P,

A, B - reagents, P - products, n ,m - coefficients.

W =k n m

The coefficient k is called the rate constant of a chemical reaction,

characterizes the nature of the interacting particles and does not depend on the particle concentration.

The rate of a chemical reaction. The law of active masses. chemical balance. The quantities n and m are called reaction order by substance A and B, respectively, and

their sum (n + m) - reaction order.

For elementary reactions, the reaction order can be 1, 2, and 3.

Elementary reactions with order 1 are called monomolecular, with order 2 - bimolecular, with order 3 - trimolecular according to the number of molecules involved. Elementary reactions higher than the third order are unknown - calculations show that the simultaneous meeting of four molecules at one point is too incredible an event.

Since a complex reaction consists of a certain sequence of elementary reactions, its rate can be expressed in terms of the rates of the individual stages of the reaction. Therefore, for complex reactions, the order can be any, including fractional or zero (the zero order of the reaction indicates that the reaction occurs at a constant rate and does not depend on the concentration of the reacting particles W = k).

The slowest of the stages of a complex process is usually called the limiting stage (rate-limiting stage).

Imagine that a large number of molecules went to a free cinema, but there is a inspector at the entrance who checks the age of each molecule. Therefore, a stream of matter enters the cinema door, and the molecules enter the cinema one at a time, i.e. So slow.

Examples of elementary reactions of the first order are the processes of thermal or radioactive decay, respectively, the rate constant k characterizes either the probability of breaking a chemical bond, or the probability of decay per unit time.

There are a lot of examples of elementary reactions of the second order - this is the most familiar way for us to proceed reactions - particle A flew into particle B, some kind of transformation took place and something happened there (note that products in theory do not affect anything - all attention given only to reacting particles).

On the contrary, there are quite a few elementary reactions of the third order, since it is quite rare for three particles to meet at the same time.

As an illustration, consider the predictive power of chemical kinetics.

The rate of a chemical reaction. The law of active masses. chemical balance.

First order kinetic equation

(illustrative additional material)

Let us consider a homogeneous first-order reaction, the rate constant of which is equal to k , the initial concentration of substance A is equal to [A]0 .

For the purposes of our course, this conclusion is for informational purposes only, in order to demonstrate to you the use of the mathematical apparatus for calculating the course of a chemical reaction. Therefore, a competent chemist cannot fail to know mathematics. Learn math!

The rate of a chemical reaction. The law of active masses. chemical balance. A graph of the concentration of reactants and products versus time can be qualitatively depicted as follows (using the example of an irreversible first-order reaction)

Factors that affect the rate of reaction

1. Nature of the reactants

For example, the reaction rate of the following substances: H2 SO4, CH3 COOH, H2 S, CH3 OH - with hydroxide ion will vary depending on the strength of the H-O bond. To assess the strength of this bond, you can use the value of the relative positive charge on the hydrogen atom: the larger the charge, the easier the reaction will go.

2. Temperature

Life experience tells us that the reaction rate depends on temperature and increases with increasing temperature. For example, the process of souring milk occurs faster at room temperature, and not in the refrigerator.

Let us turn to the mathematical expression of the law of mass action.

W =k n m

Since the left side of this expression (the reaction rate) depends on temperature, therefore, the right side of the expression also depends on temperature. At the same time, the concentration, of course, does not depend on temperature: for example, milk retains its fat content of 2.5% both in the refrigerator and at room temperature. Then, as Sherlock Holmes used to say, the remaining solution is the right one, no matter how strange it may seem: the rate constant depends on the temperature!

The rate of a chemical reaction. The law of active masses. chemical balance. The dependence of the reaction rate constant on temperature is expressed using the Arrhenius equation:

− E a

k = k0 eRT ,

wherein

R = 8.314 J mol-1 K-1 - universal gas constant,

E a is the activation energy of the reaction (see below), it is conditionally considered independent of temperature;

k 0 is the pre-exponential factor (i.e., the factor that stands before the exponent e ), the value of which is also almost independent of temperature and is determined, first of all, by the order of the reaction.

Thus, the value of k0 is approximately 1013 s-1 for a first-order reaction, and 10 -10 l mol-1 s-1 for a second-order reaction,

for a third-order reaction - 10 -33 l2 mol-2 s-1. These values ​​do not have to be memorized.

The exact values ​​of k0 for each reaction are determined experimentally.

The concept of activation energy becomes clear from the following figure. In fact, the activation energy is the energy that the reacting particle must have in order for the reaction to occur.

Moreover, if we heat the system, then the energy of the particles increases (dotted graph), while the transition state (≠) remains at the same level. The difference in energy between the transition state and the reactants (activation energy) is reduced, and the reaction rate according to the Arrhenius equation increases.

The rate of a chemical reaction. The law of active masses. chemical balance. In addition to the Arrhenius equation, there is the van't Hoff equation, which

characterizes the dependence of the reaction rate on temperature by means of the temperature coefficient γ:

The temperature coefficient γ shows how many times the rate of a chemical reaction will increase when the temperature changes by 10o.

Van't Hoff equation:

T 2 − T 1

W (T 2 )= W (T 1 )× γ10

Typically, the coefficient γ is in the range from 2 to 4. For this reason, chemists often use the approximation that a 20o increase in temperature leads to an increase in the reaction rate by an order of magnitude (i.e., 10 times).