Influence of the concentration of reactants on the rate. The rate of a chemical reaction: conditions, examples. Factors affecting the rate of a chemical reaction. Molecularity and reaction order
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.
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.
Tasks with comments and solutions
Example 23. The increase in the rate of the reaction, the equation of which 2CO + O 2 = 2CO 2, contributes
1) increase in CO concentration
2) decrease in the concentration of O 2
3) pressure drop
4) lowering the temperature
It is known that the rate of a chemical reaction depends on the following factors:
The nature of the reacting substances (ceteris paribus, more active substances react faster);
The concentration of reactants (the higher the concentration, the higher the reaction rate);
Temperatures (an increase in temperature leads to an acceleration of reactions);
The presence of a catalyst (the catalyst speeds up the process);
Pressure (for reactions involving gases, an increase in pressure is equivalent to an increase in concentration, so the rate of reactions increases with increasing pressure);
The degree of grinding of solids (the greater the degree of grinding, the greater the surface area of contact of the solid reagents, and the higher the reaction rate).
Considering these factors, we analyze the proposed answers:
1) an increase in the concentration of CO (initial substance) will indeed lead to an increase in the rate of a chemical reaction;
2) a decrease in the concentration of O 2 will lead not to an increase, but to a decrease in the reaction rate;
3) a decrease in pressure is essentially the same as a decrease in the concentration of reagents, therefore, the reaction rate will also decrease;
4) a decrease in temperature always leads to a decrease in the rate of a chemical reaction.
Example 24. The rate of reaction between iron and hydrochloric acid is increased by
1) adding an inhibitor
2) lowering the temperature
3) pressure increase
4) increase in HCl concentration
First of all, we write the reaction equation:
Let's analyze the proposed answers. It is known that the addition of an inhibitor reduces the reaction rate, and a decrease in temperature also has a similar effect. A change in pressure does not affect the rate of this reaction (because there are no gaseous substances among the reagents). Therefore, to increase the reaction rate, the concentration of one of the reactants, namely hydrochloric acid, should be increased.
Example 25. The rate of the reaction between acetic acid and ethanol is not affected
1) catalyst
2) temperature
3) concentration of starting substances
4) pressure
Acetic acid and ethanol are liquids. Therefore, the change in pressure does not affect the rate of reaction between these substances, because this factor affects only reactions involving gaseous substances.
Example 26. Reacts with hydrogen at the highest rate
4) carbon
Carbon and sulfur are low-active non-metals. When heated, their activity increases markedly; at high temperatures, gaseous hydrogen will interact with solid sulfur (sulfur melting point 444 ° C) and solid carbon. The chemical activity of halogens is much greater than other non-metals (ceteris paribus). The most active of the halogens is fluorine. As you know, even such stable substances as water and fiberglass burn in a fluorine atmosphere. Indeed, hydrogen and chlorine interact either when heated or in bright light, while fluorine and hydrogen explode under any conditions (even at very low temperatures).
Tasks for independent work
79. Hydrochloric acid reacts most rapidly with
2) sodium hydroxide (solution)
3) iron
4) iron(II) carbonate
80. The reaction rate increases with
1) increasing the concentration of CO
2) lowering the temperature
3) pressure increase
4) rise in temperature
5) grinding reagents
81.
A. The interaction of nitrogen with hydrogen is faster at high pressure.
B. The reaction rate depends on the temperature.
1) only A is true
2) only B is true
3) both statements are true
4) both judgments are wrong
82. interact with the highest rate at room temperature
83.
Speed reaction 
will increase at
1) increase in the concentration of sulfur dioxide
2) rise in temperature
3) lowering the temperature
4) increase in pressure
5) decrease in oxygen concentration
84. The rate of chemical reaction between sulfuric acid solution and iron is not affected
1) increase in acid concentration
2) change in vessel volume
3) increase in reaction temperature
4) pressure increase
5) iron grinding
85. The reaction between water and
1) sodium
2) calcium
3) magnesium
86. interact with the fastest
87. The rate of a reaction whose scheme increases with
1) increasing the concentration of iron ions
2) a decrease in the concentration of iron ions
3) lowering the temperature
4) increasing the concentration of acid
5) grinding iron
88. Are the following statements about the rate of a chemical reaction correct?
A. The rate of interaction of zinc with oxygen depends on the pressure of oxygen in the system.
B. With an increase in temperature by 10 ° C, the rate of most reactions increases by 2-4 times.
1) only A is true
2) true, only B
3) both statements are true
4) both judgments are wrong
89. The rate of reaction is not affected by the change
1) concentration of hydrochloric acid
2) pressure
3) concentration of sodium chloride
4) concentration of sodium sulfite
5) temperature
90. Under normal conditions, the reaction proceeds at the highest rate, the equation / scheme of which
91. Are the following statements about the rate of a chemical reaction correct?
A. The interaction of oxygen with zinc proceeds at a higher rate than with copper.
B. The reaction rate in solution depends on the concentration of the reactants.
1) only A is true
2) only B is true
3) both statements are true
4) both judgments are wrong
92. Interact with the lowest rate at room temperature
1) copper sulfate (solution) and sodium hydroxide (solution)
2) sodium and water
3) oxygen and zinc
4) sulfuric acid (solution) and calcium carbonate (tv)
93. Are the following statements about the rate of a chemical reaction correct?
A. The interaction of zinc with hydrochloric acid proceeds at a higher rate than with orthophosphoric acid of the same concentration.
B. The rate of a reaction in a solution depends on the volume of the vessel in which the reaction is carried out.
1) only A is true
2) only B is true
3) both statements are true
4) both judgments are wrong
We are constantly confronted with various chemical interactions. The combustion of natural gas, the rusting of iron, the souring of milk are far from all the processes that are studied in detail in a school chemistry course.
Some reactions take fractions of seconds, while some interactions take days or weeks.
Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. In the new educational standard, a minimum amount of study time is allocated for this issue. In the tests of the unified state exam, there are tasks on the dependence of the reaction rate on temperature, concentration, and even calculation tasks are offered. Many high school students experience certain difficulties in finding answers to these questions, so we will analyze this topic in detail.
The relevance of the issue under consideration
Information about the reaction rate is of great practical and scientific importance. For example, in a specific production of substances and products, the productivity of equipment and the cost of goods directly depend on this value.
Classification of ongoing reactions
There is a direct relationship between the state of aggregation of the initial components and products formed in the course of heterogeneous interactions.
In chemistry, a system is usually understood as a substance or a combination of them.
Homogeneous is such a system that consists of one phase (the same state of aggregation). As an example, we can mention a mixture of gases, several different liquids.
A heterogeneous system is a system in which the reactants are in the form of gases and liquids, solids and gases.
There is not only a dependence of the reaction rate on temperature, but also on the phase in which the components involved in the analyzed interaction are used.
For a homogeneous composition, the process is characteristic throughout the entire volume, which significantly improves its quality.
If the initial substances are in different phase states, then the maximum interaction is observed at the interface. For example, when an active metal is dissolved in an acid, the formation of a product (salt) is observed only on the surface of their contact.
Mathematical relationship between process speed and various factors
What is the equation for the rate of a chemical reaction as a function of temperature? For a homogeneous process, the rate is determined by the amount of a substance that interacts or is formed during the reaction in the volume of the system per unit time.
For a heterogeneous process, the rate is determined through the amount of a substance that reacts or is obtained in the process per unit area for a minimum period of time.
Factors affecting the rate of a chemical reaction
The nature of the reactants is one of the reasons for the different rates of processes. For example, alkali metals form alkalis with water at room temperature, and the process is accompanied by intense evolution of gaseous hydrogen. Noble metals (gold, platinum, silver) are not capable of such processes either at room temperature or when heated.
The nature of the reactants is a factor that is taken into account in the chemical industry in order to increase the profitability of production.
The relationship between the concentration of reagents and the speed of the chemical reaction is revealed. The higher it is, the more particles will collide, therefore, the process will proceed faster.
The law of mass action in mathematical form describes a directly proportional relationship between the concentration of the initial substances and the speed of the process.
It was formulated in the middle of the nineteenth century by the Russian chemist N. N. Beketov. For each process, a reaction constant is determined, which is not related to temperature, concentration, or the nature of the reactants.
In order to speed up the reaction in which a solid is involved, it is necessary to grind it to a powder state.
In this case, an increase in the surface area occurs, which positively affects the speed of the process. For diesel fuel, a special injection system is used, due to which, when it comes into contact with air, the rate of the combustion process of a mixture of hydrocarbons increases significantly.
Heating
The dependence of the rate of a chemical reaction on temperature is explained by molecular kinetic theory. It allows you to calculate the number of collisions between the molecules of the reagents under certain conditions. Armed with such information, under normal conditions, all processes should proceed instantly.
But if we consider a specific example of the dependence of the reaction rate on temperature, it turns out that in order to interact, it is necessary to first break the chemical bonds between atoms so that new substances are formed from them. This requires a significant amount of energy. What is the dependence of the reaction rate on temperature? The activation energy determines the possibility of rupture of molecules, it characterizes the reality of the processes. Its units of measurement are kJ/mol.
With an insufficient energy index, the collision will be ineffective, so it is not accompanied by the formation of a new molecule.
Graphical representation
The dependence of the rate of a chemical reaction on temperature can be represented graphically. When heated, the number of collisions between particles increases, which contributes to the acceleration of interaction.
What is a graph of reaction rate versus temperature? The energy of molecules is plotted horizontally, and the number of particles with a high energy reserve is indicated vertically. A graph is a curve by which one can judge the speed of a particular interaction.
The greater the energy difference from the average, the further the curve point is from the maximum, and a smaller percentage of molecules have such an energy reserve.
Important Aspects
Is it possible to write an equation for the dependence of the reaction rate constant on temperature? Its increase is reflected in the increase in the speed of the process. Such a dependence is characterized by a certain value, called the temperature coefficient of the process rate.
For any interaction, the dependence of the reaction rate constant on temperature was revealed. In the case of its increase by 10 degrees, the process speed increases by 2-4 times.
The dependence of the rate of homogeneous reactions on temperature can be represented mathematically.
For most interactions at room temperature, the coefficient is in the range from 2 to 4. For example, with a temperature coefficient of 2.9, a temperature increase of 100 degrees speeds up the process by almost 50,000 times.
The dependence of the reaction rate on temperature can be easily explained by different values of the activation energy. It has a minimum value during ionic processes, which are determined only by the interaction of cations and anions. Numerous experiments testify to the instantaneous occurrence of such reactions.
At a high value of the activation energy, only a small number of collisions between particles will lead to the implementation of the interaction. With an average activation energy, the reactants will interact at an average rate.
Tasks on the dependence of the reaction rate on concentration and temperature are considered only at the senior level of education, and often cause serious difficulties for children.
Measuring the speed of the process
Those processes that require a significant activation energy involve an initial break or weakening of bonds between atoms in the original substances. In this case, they pass into a certain intermediate state, called the activated complex. It is an unstable state, rather quickly decomposes into reaction products, the process is accompanied by the release of additional energy.
In its simplest form, the activated complex is a configuration of atoms with weakened old bonds.
Inhibitors and Catalysts
Let us analyze the dependence of the enzymatic reaction rate on the temperature of the medium. Such substances act as process accelerators.
They themselves are not participants in the interaction, their number after the completion of the process remains unchanged. If catalysts increase the reaction rate, then inhibitors, on the contrary, slow down this process.
The essence of this lies in the formation of intermediate compounds, as a result of which a change in the speed of the process is observed.
Conclusion
Various chemical interactions take place every minute in the world. How to establish the dependence of the reaction rate on temperature? The Arrhenius equation is a mathematical explanation of the relationship between the rate constant and temperature. It gives an idea of those activation energies at which the destruction or weakening of bonds between atoms in molecules, the distribution of particles into new chemical substances is possible.
Thanks to the molecular kinetic theory, it is possible to predict the probability of interactions between the initial components, to calculate the rate of the process. Among those factors that affect the reaction rate, of particular importance is the change in the temperature index, the percentage concentration of interacting substances, the contact surface area, the presence of a catalyst (inhibitor), as well as the nature of the interacting components.
DEFINITION
Chemical kinetics- the study of the rates and mechanisms of chemical reactions.
The study of the rates of reactions, obtaining data on the factors affecting the rate of a chemical reaction, as well as the study of the mechanisms of chemical reactions is carried out experimentally.
DEFINITION
The rate of a chemical reaction- change in the concentration of one of the reactants or reaction products per unit time with a constant volume of the system.
The rate of homogeneous and heterogeneous reactions are determined differently.
The definition of a measure of the rate of a chemical reaction can be written in mathematical form. Let - the rate of a chemical reaction in a homogeneous system, n B - the number of moles of any of the substances resulting from the reaction, V - the volume of the system, - time. Then in the limit:
This equation can be simplified - the ratio of the amount of substance to volume is the molar concentration of the substance n B / V \u003d c B, from where dn B / V \u003d dc B and finally:
In practice, the concentrations of one or more substances are measured at certain time intervals. The concentrations of the initial substances decrease with time, while the concentrations of the products increase (Fig. 1).
Rice. 1. Change in the concentration of the starting substance (a) and reaction product (b) with time
Factors affecting the rate of a chemical reaction
Factors affecting the rate of a chemical reaction are: the nature of the reactants, their concentrations, temperature, the presence of catalysts in the system, pressure and volume (in the gas phase).
The influence of concentration on the rate of a chemical reaction is associated with the basic law of chemical kinetics - the law of mass action (LMA): the rate of a chemical reaction is directly proportional to the product of the concentrations of reactants raised to the power of their stoichiometric coefficients. The PDM does not take into account the concentration of substances in the solid phase in heterogeneous systems.
For the reaction mA + nB = pC + qD, the mathematical expression of the MAP will be written:
K × C A m × C B n
K × [A] m × [B] n ,
where k is the rate constant of a chemical reaction, which is the rate of a chemical reaction at a concentration of reactants of 1 mol/l. Unlike the rate of a chemical reaction, k does not depend on the concentration of reactants. The higher k, the faster the reaction proceeds.
The dependence of the rate of a chemical reaction on temperature is determined by the van't Hoff rule. Van't Hoff's rule: with every ten degrees increase in temperature, the rate of most chemical reactions increases by about 2 to 4 times. Math expression:
(T 2) \u003d (T 1) × (T2-T1) / 10,
where is the van't Hoff temperature coefficient, showing how many times the reaction rate increased with an increase in temperature by 10 o C.
Molecularity and reaction order
The molecularity of the reaction is determined by the minimum number of molecules that simultaneously interact (participate in the elementary act). Distinguish:
- monomolecular reactions (decomposition reactions can serve as an example)
N 2 O 5 \u003d 2NO 2 + 1 / 2O 2
K × C, -dC/dt = kC
However, not all reactions obeying this equation are monomolecular.
- bimolecular
CH 3 COOH + C 2 H 5 OH \u003d CH 3 COOC 2 H 5 + H 2 O
K × C 1 × C 2 , -dC/dt = k × C 1 × C 2
- trimolecular (very rare).
The molecularity of a reaction is determined by its true mechanism. It is impossible to determine its molecularity by writing the reaction equation.
The order of the reaction is determined by the form of the kinetic equation of the reaction. It is equal to the sum of the exponents of the degrees of concentration in this equation. For example:
CaCO 3 \u003d CaO + CO 2
K × C 1 2 × C 2 - third order
The order of the reaction can be fractional. In this case, it is determined experimentally. If the reaction proceeds in one stage, then the order of the reaction and its molecularity coincide, if in several stages, then the order is determined by the slowest stage and is equal to the molecularity of this reaction.
Examples of problem solving
EXAMPLE 1