Speed chemical reaction - change in the amount of one of the reacting substances per unit of time in a unit of reaction space.
The speed of a chemical reaction is influenced by the following factors:
- the nature of the reacting substances;
- concentration of reactants;
- contact surface of reacting substances (in heterogeneous reactions);
- temperature;
- action of catalysts.
Active collision theory allows us to explain the influence of certain factors on the rate of a chemical reaction. The main provisions of this theory:
- Reactions occur when particles of reactants that have a certain energy collide.
- The more reactant particles there are, the closer they are to each other, the more likely they are to collide and react.
- Only effective collisions lead to a reaction, i.e. those in which “old connections” are destroyed or weakened and therefore “new” ones can be formed. To do this, the particles must have sufficient energy.
- The minimum excess energy required for effective collision of reactant particles is called activation energy Ea.
- The activity of chemicals is manifested in the low activation energy of reactions involving them. The lower the activation energy, the higher the reaction rate. For example, in reactions between cations and anions, the activation energy is very low, so such reactions occur almost instantly
The influence of the concentration of reactants on the reaction rate
As the concentration of reactants increases, the reaction rate increases. In order for a reaction to occur, two chemical particles must come together, so the rate of the reaction depends on the number of collisions between them. An increase in the number of particles in a given volume leads to more frequent collisions and an increase in the reaction rate.
An increase in the rate of reaction occurring in the gas phase will result from an increase in pressure or a decrease in the volume occupied by the mixture.
Based on experimental data in 1867, Norwegian scientists K. Guldberg and P. Waage, and independently of them in 1865, Russian scientist N.I. Beketov formulated the basic law of chemical kinetics, establishing dependence of the reaction rate on the concentrations of the reactants -
Law of mass action (LMA):
The rate of a chemical reaction is proportional to the product of the concentrations of the reacting substances, taken in powers equal to their coefficients in the reaction equation. (“effective mass” is a synonym modern concept"concentration")
aA +bB =cС +dD, Where k– reaction rate constant
ZDM is performed only for elementary chemical reactions occurring in one stage. If a reaction proceeds sequentially through several stages, then the total speed of the entire process is determined by its slowest part.
Expressions for the rates of various types of reactions
ZDM refers to homogeneous reactions. If the reaction is heterogeneous (reagents are in different states of aggregation), then the ZDM equation includes only liquid or only gaseous reagents, and solid ones are excluded, affecting only the rate constant k.
Molecularity of the reaction is the minimum number of molecules involved in an elementary chemical process. Based on molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.
Rate of heterogeneous reactions
- Depends on surface area of contact between substances, i.e. on the degree of grinding of substances and the completeness of mixing of reagents.
- An example is wood burning. A whole log burns relatively slowly in air. If you increase the surface of contact between wood and air, splitting the log into chips, the burning rate will increase.
- Pyrophoric iron is poured onto a sheet of filter paper. During the fall, the iron particles become hot and set fire to the paper.
Effect of temperature on reaction rate
In the 19th century, the Dutch scientist Van't Hoff experimentally discovered that with an increase in temperature by 10 o C, the rates of many reactions increase by 2-4 times.
Van't Hoff's rule
For every 10 ◦ C increase in temperature, the reaction rate increases by 2-4 times.
Here γ (Greek letter "gamma") - the so-called temperature coefficient or van't Hoff coefficient, takes values from 2 to 4.
For each specific reaction, the temperature coefficient is determined experimentally. It shows exactly how many times the rate of a given chemical reaction (and its rate constant) increases with every 10 degree increase in temperature.
Van't Hoff's rule is used to approximate the change in the reaction rate constant with increasing or decreasing temperature. A more precise relationship between the rate constant and temperature was established by the Swedish chemist Svante Arrhenius:
How more E a specific reaction, so less(at a given temperature) will be the rate constant k (and rate) of this reaction. An increase in T leads to an increase in the rate constant, this is explained by the fact that an increase in temperature leads to a rapid increase in the number of “energetic” molecules capable of overcoming the activation barrier Ea.
Effect of catalyst on reaction rate
You can change the rate of a reaction by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy.
Catalysts- these are substances that participate in a chemical reaction and increase its speed, but at the end of the reaction they remain unchanged qualitatively and quantitatively.
Inhibitors– substances that slow down chemical reactions.
Changing the rate of a chemical reaction or its direction using a catalyst is called catalysis .
Main concepts studied:
Rate of chemical reactions
Molar concentration
Kinetics
Homogeneous and heterogeneous reactions
Factors affecting the rate of chemical reactions
Catalyst, inhibitor
Catalysis
Reversible and irreversible reactions
Chemical equilibrium
Chemical reactions are reactions as a result of which one substance produces another (from starting materials new substances are formed). Some chemical reactions take place in a fraction of a second (explosion), while others take minutes, days, years, decades, etc.
For example: the combustion reaction of gunpowder occurs instantly with ignition and explosion, and the reaction of darkening of silver or rusting of iron (corrosion) occurs so slowly that its result can be monitored only after a long time.
To characterize the speed of a chemical reaction, the concept of chemical reaction speed - υ is used.
Chemical reaction rate is the change in the concentration of one of the reactants of a reaction per unit time.
Formula for calculating the rate of a chemical reaction:
| υ = | from 2 – from 1 | = | ∆s |
| t 2 – t 1 | ∆t |
c 1 – molar concentration of the substance at the initial time t 1
c 2 – molar concentration of the substance at the initial time t 2
since the rate of a chemical reaction is characterized by a change in the molar concentration of the reacting substances (starting substances), then t 2 > t 1, and c 2 > c 1 (the concentration of the starting substances decreases as the reaction proceeds).
Molar concentration (s)– is the amount of substance per unit volume. The unit of measurement for molar concentration is [mol/l].
The branch of chemistry that studies the rate of chemical reactions is called chemical kinetics. Knowing its laws, a person can control chemical processes and set them a certain speed.
When calculating the rate of a chemical reaction, it is necessary to remember that reactions are divided into homogeneous and heterogeneous.
Homogeneous reactions– reactions that occur in the same environment (i.e. the reactants are in the same state of aggregation; for example: gas + gas, liquid + liquid).
Heterogeneous reactions– these are reactions occurring between substances in a heterogeneous medium (there is a phase interface, i.e. the reacting substances are in different states of aggregation; for example: gas + liquid, liquid + solid).
The above formula for calculating the rate of a chemical reaction is valid only for homogeneous reactions. If the reaction is heterogeneous, then it can only occur at the surface of the reactants.
For a heterogeneous reaction, the rate is calculated using the formula:
∆ν – change in the amount of substance
S – interface area
∆ t – time period during which the reaction took place
The rate of chemical reactions depends on various factors: the nature of the reactants, the concentration of the substances, temperature, catalysts or inhibitors.
Dependence of reaction rates on the nature of the reactants.
Let's analyze this dependence of the reaction rate using an example: let’s drop metal granules of equal area into two test tubes containing the same amount of hydrochloric acid (HCl) solution: an iron (Fe) granule into the first test tube, and a magnesium (Mg) granule into the second. As a result of observations, based on the rate of hydrogen release (H 2), it can be noted that the highest rate is c hydrochloric acid Magnesium reacts more than iron. The rate of this chemical reaction is influenced by the nature of the metal (i.e. magnesium is a more reactive metal than iron, and therefore reacts more vigorously with the acid).
Dependence of the rate of chemical reactions on the concentration of reactants.
The higher the concentration of the reacting (starting) substance, the faster the reaction proceeds. Conversely, the lower the concentration of the reactant, the slower the reaction.
For example: pour a concentrated solution of hydrochloric acid (HCl) into one test tube, and a dilute solution of hydrochloric acid into the other. Let's put a zinc granule (Zn) in both test tubes. We will observe, by the rate of hydrogen evolution, that the reaction will proceed faster in the first test tube, because the concentration of hydrochloric acid in it is greater than in the second test tube.
To determine the dependence of the rate of a chemical reaction, use law of action of (acting) masses : the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers that are equal to their coefficients.
For example, for a reaction proceeding according to the scheme: nA + mB → D, the rate of a chemical reaction is determined by the formula:
υ h.r. = k · C (A) n · C (B) m , Where
υ x.r - rate of chemical reaction
C (A) – A
C (B) – molar concentration of a substance IN
n and m – their coefficients
k – rate constant of a chemical reaction (reference value).
The law of mass action does not apply to substances in a solid state, because their concentration is constant (due to the fact that they react only on the surface, which remains unchanged).
For example: for reaction 2 Cu + O 2 = 2 CuO the reaction rate is determined by the formula:
υ h.r. = k C(O 2)
PROBLEM: The rate constant for the reaction 2A + B = D is 0.005. calculate the reaction rate at the molar concentration of substance A = 0.6 mol/l, substance B = 0.8 mol/l.
Dependence of the rate of a chemical reaction on temperature.
This dependence is determined van't Hoff rule (1884): with every 10°C increase in temperature, the rate of a chemical reaction increases on average by 2–4 times.
Thus, the interaction of hydrogen (H 2) and oxygen (O 2) at room temperature almost does not occur, the rate of this chemical reaction is so low. But at a temperature of 500 C o this reaction takes place in 50 minutes, and at a temperature of 700 C o it occurs almost instantly.
Formula for calculating the rate of a chemical reaction according to the Van't Hoff rule:
where: υ t 1 and υ t 2 - rates of chemical reactions at t 2 and t 1
γ is the temperature coefficient, which shows how many times the reaction rate increases with an increase in temperature by 10 C o.
Changing reaction speed:
2. Substitute the data from the problem statement into the formula:
Dependence of reaction rates on special substances - catalysts and inhibitors.
Catalyst- a substance that increases the rate of a chemical reaction, but does not itself participate in it.
Inhibitor- a substance that slows down a chemical reaction, but does not itself participate in it.
Example: into a test tube with a solution of 3% hydrogen peroxide (H 2 O 2), which has been heated, add a smoldering splinter - it will not light up, because the reaction rate of the decomposition of hydrogen peroxide into water (H 2 O) and oxygen (O 2) is very low, and the resulting oxygen is not enough to carry out a high-quality reaction to oxygen (sustaining combustion). Now let’s add a little black powder of manganese (IV) oxide (MnO 2) into the test tube and see that the rapid release of gas bubbles (oxygen) has begun, and the smoldering splinter brought into the test tube flares up brightly. MnO 2 is the catalyst for this reaction; it accelerated the rate of the reaction, but did not participate in it itself (this can be proven by weighing the catalyst before and after the reaction - its mass will not change).
The concept of “speed” is found quite often in the literature. It is known from physics that the greater the distance a material body (a person, a train, spacecraft) for a certain period of time, the higher the speed of this body.
How to measure the speed of a chemical reaction that “goes nowhere” and does not cover any distance? In order to answer this question, you need to find out what Always changes in any chemical reaction? Since any chemical reaction is a process of changing a substance, the original substance disappears in it, turning into reaction products. Thus, during a chemical reaction, the amount of a substance always changes, the number of particles of the starting substances decreases, and therefore its concentration (C).
Unified State Examination task. The rate of a chemical reaction is proportional to the change:
- concentration of a substance per unit time;
- amount of substance per unit volume;
- mass of a substance per unit volume;
- volume of substance during the reaction.
Now compare your answer with the correct one:
the rate of a chemical reaction is equal to the change in the concentration of the reactant per unit time
Where C 1 And From 0- concentrations of reactants, final and initial, respectively; t 1 And t 2- the time of the experiment, the final and initial period of time, respectively.
Question. Which value do you think is greater: C 1 or From 0? t 1 or t 0?
Since reactants are always consumed in a given reaction, then
Thus, the ratio of these quantities is always negative, and speed cannot be a negative quantity. Therefore, a minus sign appears in the formula, which simultaneously indicates that the speed any reactions over time (under constant conditions) are always decreases.
So, the rate of the chemical reaction is:
The question arises: in what units should the concentration of reactants (C) be measured and why? In order to answer it, you need to understand what condition is main for any chemical reaction to occur.
In order for particles to react, they must at least collide. That's why the higher the number of particles* (number of moles) per unit volume, the more often they collide, the higher the probability of a chemical reaction.
* Read about what a “mole” is in lesson 29.1.
Therefore, when measuring the rates of chemical processes, they use molar concentration substances in reacting mixtures.
The molar concentration of a substance shows how many moles of it are contained in 1 liter of solution
So, the greater the molar concentration of the reacting substances, the more particles there are per unit volume, the more often they collide, and the higher (all other things being equal) the rate of the chemical reaction. Therefore, the basic law of chemical kinetics (this is the science of the rate of chemical reactions) is law of mass action.
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.
For a reaction of type A + B →... mathematically this law can be expressed as follows:
If the reaction is more complex, for example, 2A + B → or, which is the same, A + A + B → ..., then
Thus, an exponent appeared in the speed equation « two» , which corresponds to the coefficient 2 in the reaction equation. For more complex equations As a rule, large exponents are not used. This is due to the fact that the probability of a simultaneous collision of, say, three molecules A and two molecules B is extremely small. Therefore, many reactions occur in several stages, during which no more than three particles collide, and each stage of the process proceeds at a certain speed. This speed and the kinetic equation of speed for it are determined experimentally.
The above chemical reaction rate equations (3) or (4) are valid only for homogeneous reactions, i.e. for such reactions when the reacting substances are not separated by the surface. For example, a reaction occurs in an aqueous solution, and both reactants are highly soluble in water or any mixture of gases.
It's another matter when it happens heterogeneous reaction. In this case, there is an interface between the reacting substances, for example, carbon dioxide gas reacts with water solution alkalis. In this case, any gas molecule is equally likely to react, since these molecules move quickly and chaotically. What about particles of liquid solution? These particles move extremely slowly, and those alkali particles that are “at the bottom” have virtually no chance of reacting with carbon dioxide unless the solution is constantly stirred. Only those particles that “lie on the surface” will react. So for heterogeneous reactions -
the reaction rate depends on the size of the interface surface, which increases with grinding.
Therefore, very often the reacting substances are crushed (for example, dissolved in water), the food is thoroughly chewed, and during the cooking process - ground, passed through a meat grinder, etc. A food product that is not crushed is practically not digestible!
Thus, with maximum speed(other things being equal) homogeneous reactions take place in solutions and between gases (if these gases react at ambient conditions), and in solutions where the molecules are located “nearby”, and the grinding is the same as in gases (and even more !), - the reaction speed is higher.
Unified State Examination task. Which of the reactions occurs with the most higher speed at room temperature:
- carbon with oxygen;
- iron with hydrochloric acid;
- iron with acetic acid solution
- solutions of alkali and sulfuric acid.
IN in this case we need to find which process is homogeneous.
It should be noted that the rate of a chemical reaction between gases or a heterogeneous reaction in which a gas participates also depends on pressure, since with increasing pressure the gases are compressed and the concentration of particles increases (see formula 2). The rate of reactions in which gases are not involved is not affected by changes in pressure.
Unified State Examination task. The rate of chemical reaction between the acid solution and iron is not affected
- acid concentration;
- iron grinding;
- reaction temperature;
- increase in pressure.
Finally, the speed of a reaction also depends on the reactivity of the substances. For example, if oxygen reacts with a substance, then, other things being equal, the reaction rate will be higher than when the same substance interacts with nitrogen. The fact is that the reactivity of oxygen is noticeably higher than that of nitrogen. We will look at the reason for this phenomenon in the next part of the Self-Teacher (Lesson 14).
Unified State Examination task. The chemical reaction between hydrochloric acid and
- copper;
- iron;
- magnesium;
- zinc
It should be noted that not every collision of molecules leads to their chemical interaction (chemical reaction). In a gas mixture of hydrogen and oxygen, under normal conditions, several billion collisions occur per second. But the first signs of the reaction (water droplets) will appear in the flask only after a few years. In such cases they say that the reaction practically doesn't work. But she possible, otherwise how to explain the fact that when this mixture is heated to 300 °C, the flask quickly fogs up, and at a temperature of 700 °C a terrible explosion will occur! It’s not for nothing that a mixture of hydrogen and oxygen is called “explosive gas.”
Question. Why do you think the reaction rate increases so sharply when heated?
The reaction rate increases because, firstly, the number of particle collisions increases, and secondly, the number of active collisions. It is the active collisions of particles that lead to their interaction. In order for such a collision to occur, the particles must have a certain amount of energy.
The energy that particles must have in order for a chemical reaction to occur is called activation energy.
This energy is spent on overcoming the repulsive forces between outer electrons atoms and molecules and the destruction of “old” chemical bonds.
The question arises: how to increase the energy of reacting particles? The answer is simple - increase the temperature, since with increasing temperature the speed of movement of particles increases, and, consequently, their kinetic energy.
Rule van't Hoff*:
With every 10 degree increase in temperature, the reaction rate increases by 2–4 times.
VANT-HOFF Jacob Hendrik(08/30/1852–03/1/1911) - Dutch chemist. One of the founders of physical chemistry and stereochemistry. Nobel Prize in chemistry No. 1 (1901).
It should be noted that this rule (not a law!) was established experimentally for reactions that were “convenient” for measurement, that is, for such reactions that proceeded neither too quickly nor too slowly and at temperatures accessible to the experimenter (not too high and not too low).
Question. What do you think is the fastest way to cook potatoes: boil them or fry them in a layer of oil?
In order to properly understand the meaning of the described phenomena, you can compare the reacting molecules with a group of students who are about to jump high. If they are given a barrier 1 m high, then the students will have to run up (increase their “temperature”) in order to overcome the barrier. Nevertheless, there will always be students (“inactive molecules”) who will not be able to overcome this barrier.
What to do? If you adhere to the principle: “A smart person won’t climb a mountain, a smart person will bypass a mountain,” then you should simply lower the barrier, say, to 40 cm. Then any student will be able to overcome the barrier. At the molecular level this means: in order to increase the reaction rate, it is necessary to reduce the activation energy in a given system.
In real chemical processes, this function is performed by a catalyst.
Catalyst is a substance that changes the rate of a chemical reaction while remaining unchanged towards the end of the chemical reaction.
Catalyst participates in a chemical reaction, interacting with one or more starting substances. In this case, intermediate compounds are formed and the activation energy changes. If the intermediate is more active (active complex), then the activation energy decreases and the reaction rate increases.
For example, the reaction between SO 2 and O 2 occurs very slowly under normal conditions practically doesn't work. But in the presence of NO, the reaction rate increases sharply. First NO very quickly reacts with O2:
resulting nitrogen dioxide fast reacts with sulfur(IV) oxide:
Task 5.1. Using this example, show which substance is a catalyst and which is an active complex.
Conversely, if more passive compounds are formed, the activation energy may increase so much that the reaction practically does not occur under these conditions. Such catalysts are called inhibitors.
In practice, both types of catalysts are used. So special organic catalysts - enzymes- participate in absolutely all biochemical processes: food digestion, muscle contraction, breathing. Life cannot exist without enzymes!
Inhibitors are necessary to protect metal products from corrosion, fat-containing food products from oxidation (rancidity). Some medications also contain inhibitors that inhibit the vital functions of microorganisms and thereby destroy them.
Catalysis can be homogeneous or heterogeneous. An example of homogeneous catalysis is the effect of NO (this is a catalyst) on the oxidation of sulfur dioxide. An example of heterogeneous catalysis is the action of heated copper on alcohol:
This reaction occurs in two stages:
Task 5.2. Determine which substance is the catalyst in this case? Why is this type of catalysis called heterogeneous?
In practice, heterogeneous catalysis is most often used, where the catalysts are solids: metals, their oxides, etc. On the surface of these substances there are special points (crystal lattice nodes), where, in fact, the catalytic reaction occurs. If these points are covered with foreign substances, then catalysis stops. This substance, detrimental to the catalyst, is called catalytic poison. Other substances - promoters- on the contrary, they enhance catalytic activity.
A catalyst can change the direction of a chemical reaction, that is, by changing the catalyst, you can obtain different reaction products. Thus, from the alcohol C 2 H 5 OH in the presence of zinc and aluminum oxides, butadiene can be obtained, and in the presence of concentrated sulfuric acid, ethylene can be obtained.
Thus, during a chemical reaction, the energy of the system changes. If during the reaction energy is released in the form of heat Q, such a process is called exothermic:
For endo thermal processes heat is absorbed, i.e. thermal effect Q< 0 .
Task 5.3. Determine which of the proposed processes is exothermic and which is endothermic:
The equation of a chemical reaction in which thermal effect, is called the thermochemical equation of the reaction. In order to create such an equation, it is necessary to calculate the thermal effect per 1 mole of the reactant.
Task. When 6 g of magnesium is burned, 153.5 kJ of heat is released. Write a thermochemical equation for this reaction.
Solution. Let's create an equation for the reaction and indicate ABOVE the formulas that are given:
Having made up the proportion, we find the desired thermal effect of the reaction:
The thermochemical equation for this reaction is:
Such tasks are given in the assignments majority Unified State Exam options! For example.
Unified State Examination task. According to the thermochemical reaction equation
the amount of heat released when burning 8 g of methane is equal to:
Reversibility of chemical processes. Le Chatelier's principle
* LE CHATELIER Henri Louis(8.10.1850–17.09.1936) - French physical chemist and metallurgist. Formulated common law shifts of equilibrium (1884).
Reactions can be reversible or irreversible.
Irreversible These are reactions for which there are no conditions under which the reverse process is possible.
An example of such reactions are reactions that occur when milk sours, or when burnt delicious cutlet. How impossible to miss minced meat back through the meat grinder (and get a piece of meat again), it is also impossible to “reanimate” the cutlet or make fresh milk.
But let’s ask ourselves a simple question: is the process irreversible?
In order to answer this question, let's try to remember, is it possible to carry out the reverse process? Yes! The decomposition of limestone (chalk) to obtain quicklime CaO is used on an industrial scale:
Thus, the reaction is reversible, since there are conditions under which both process:
Moreover, there are conditions under which the speed of the forward reaction is equal to the speed of the reverse reaction.
Under these conditions, chemical equilibrium is established. At this time, the reaction does not stop, but the number of particles obtained is equal to the number of decomposed particles. That's why able chemical equilibrium the concentrations of reacting particles do not change. For example, for our process at the moment of chemical equilibrium
sign means equilibrium concentration.
The question arises, what will happen to the equilibrium if the temperature is increased or decreased or other conditions are changed? This question can be answered by knowing Le Chatelier's principle:
if you change the conditions (t, p, c) under which the system is in a state of equilibrium, then the equilibrium will shift towards the process that resists change.
In other words, an equilibrium system always resists any influence from the outside, just as a capricious child who does “the opposite” resists the will of his parents.
Let's look at an example. Let equilibrium be established in the reaction producing ammonia:
Questions. Is the number of moles of reacting gases the same before and after the reaction? If a reaction occurs in a closed volume, when is the pressure greater: before or after the reaction?
It is obvious that this process occurs with a decrease in the number of gas molecules, which means pressure decreases during the direct reaction. IN reverse reactions - on the contrary, the pressure in the mixture increases.
Let us ask ourselves what will happen if in this system increase pressure? According to Le Chatelier’s principle, the reaction that “does the opposite” will proceed, i.e. lowers pressure. This is a direct reaction: fewer gas molecules - less pressure.
So, at increase pressure, the equilibrium shifts towards the direct process, where the pressure drops, as the number of molecules decreases gases
Unified State Examination task. At increase pressure balance shifts right in the system:
If as a result of the reaction number of molecules gases does not change, then a change in pressure does not affect the equilibrium position.
Unified State Examination task. A change in pressure affects the shift in equilibrium in the system:
The equilibrium position of this and any other reaction depends on the concentration of the reacting substances: by increasing the concentration of the starting substances and decreasing the concentration of the resulting substances, we always shift the equilibrium towards the direct reaction (to the right).
Unified State Examination task.
will shift to the left when:
- increased blood pressure;
- decrease in temperature;
- increasing CO concentration;
- decreasing CO concentration.
The process of ammonia synthesis is exothermic, that is, accompanied by the release of heat, that is temperature rise in the mixture.
Question. How will the equilibrium shift in this system when temperature drop?
Arguing similarly, we do conclusion: when decreasing temperature, the equilibrium will shift towards the formation of ammonia, since in this reaction heat is released, and the temperature rises.
Question. How does the rate of a chemical reaction change as the temperature decreases?
Obviously, as the temperature decreases, the rate of both reactions will sharply decrease, i.e., you will have to wait a very long time for the desired equilibrium to be established. What to do? In this case it is necessary catalyst. Although he does not affect the equilibrium position, but accelerates the onset of this state.
Unified State Examination task. Chemical equilibrium in the system
shifts towards the formation of the reaction product when:
- increased blood pressure;
- temperature increase;
- decrease in pressure;
- use of a catalyst.
Conclusions
The rate of a chemical reaction depends on:
- the nature of the reacting particles;
- concentration or interface area of reactants;
- temperature;
- presence of a catalyst.
Equilibrium is established when the rate of the forward reaction is equal to the rate of the reverse process. In this case, the equilibrium concentration of the reactants does not change. The state of chemical equilibrium depends on conditions and obeys Le Chatelier's principle.
DEFINITION
Chemical kinetics– the study of the rates and mechanisms of chemical reactions.
The study of reaction rates, obtaining data on factors influencing the rate of a chemical reaction, as well as the study of the mechanisms of chemical reactions are carried out experimentally.
DEFINITION
Chemical reaction rate– change in the concentration of one of the reactants or reaction products per unit time with a constant volume of the system.
The rates of homogeneous and heterogeneous reactions are defined differently.
The definition of a measure of the rate of a chemical reaction can be written in mathematical form. Let be the rate of a chemical reaction in a homogeneous system, n B be the number of moles of any of the substances resulting from the reaction, V be the volume of the system, and be time. Then in the limit:
This equation can be simplified - the ratio of the amount of a substance to the volume is the molar concentration of the substance n B / V = c B, from which dn B / V = dc B and finally:
In practice, the concentrations of one or more substances are measured at certain time intervals. Concentrations of starting substances decrease over time, and concentrations of products increase (Fig. 1).
Rice. 1. Change in the concentration of the starting substance (a) and the reaction product (b) over time
Factors affecting the rate of a chemical reaction
Factors that influence 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 the reacting substances raised to the power of their stoichiometric coefficients. The ZDM 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 ZDM 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 the 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: for every ten degrees increase in temperature, the rate of most chemical reactions increases by about 2 to 4 times. Mathematical expression:
(T 2) = (T 1) × (T2-T1)/10,
where is the van’t Hoff temperature coefficient, showing how many times the reaction rate increases when the temperature increases by 10 o C.
Molecularity and reaction order
The molecularity of a reaction is determined by the minimum number of molecules that simultaneously interact (participate in an elementary act). There are:
- monomolecular reactions (an example is decomposition reactions)
N 2 O 5 = 2NO 2 + 1/2O 2
K × C, -dC/dt = kC
However, not all reactions that obey this equation are monomolecular.
- bimolecular
CH 3 COOH + C 2 H 5 OH = 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 equation of a reaction.
The order of the reaction is determined by the form of the kinetic equation of the reaction. He equal to the sum indicators of degrees of concentration in this equation. For example:
CaCO 3 = 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
| Exercise | The reaction proceeds according to the equation 2A + B = 4C. The initial concentration of substance A is 0.15 mol/l, and after 20 seconds it is 0.12 mol/l. Calculate the average reaction rate. |
| Solution | Let's write the formula for calculating the average rate of a chemical reaction: Purpose of the work: study of the rate of a chemical reaction and its dependence on various factors: nature of reactants, concentration, temperature. Chemical reactions occur at different rates. Speed of chemical reaction is called the change in the concentration of a reactant per unit time. It is equal to the number of interaction events per unit time per unit volume for a reaction occurring in a homogeneous system (for homogeneous reactions), or per unit interface surface for reactions occurring in a heterogeneous system (for heterogeneous reactions). Average speed reactions v avg. in the time interval from t 1 to t 2 is determined by the relation: Where C 1 And C 2– molar concentration of any reaction participant at time points t 1 And t 2 respectively. The “–” sign before the fraction refers to the concentration of the starting substances, Δ WITH < 0, знак “+” – к концентрации продуктов реакции, ΔWITH > 0. The main factors influencing the rate of a chemical reaction: the nature of the reactants, their concentration, pressure (if gases are involved in the reaction), temperature, catalyst, interface area for heterogeneous reactions. Most chemical reactions are complex processes occurring in several stages, i.e. consisting of several elementary processes. Elementary or simple reactions are reactions that occur in one step. For elementary reactions, the dependence of the reaction rate on concentration is expressed by the law of mass action. At constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers equal to the stoichiometric coefficients. For reaction in general view a A + b B… → c C, according to the law of mass action v expressed by the ratio v = К∙с(А) а ∙ с(В) b, Where c(A) And s(B)– molar concentrations of reactants A and B; TO– rate constant of this reaction, equal to v, If c(A)a=1 and c(B)b=1, and depending on the nature of the reactants, temperature, catalyst, and interface area for heterogeneous reactions. The expression of the reaction rate as a function of concentration is called the kinetic equation. In the case of complex reactions, the law of mass action applies to each individual stage. For heterogeneous reactions, the kinetic equation includes only the concentrations of gaseous and dissolved substances; yes, for burning coal C (k) + O 2 (g) → CO 2 (g) the velocity equation has the form v = K∙s(O 2) A few words about the molecularity and kinetic order of the reaction. Concept "molecularity of reaction" apply only to simple reactions. The molecularity of a reaction characterizes the number of particles participating in an elementary interaction. There are mono-, bi- and trimolecular reactions, in which one, two and three particles participate, respectively. The probability of three particles colliding at the same time is small. The elementary process of interaction of more than three particles is unknown. Examples of elementary reactions: N 2 O 5 → NO + NO + O 2 (monomolecular) H 2 + I 2 → 2HI (bimolecular) 2NO + Cl 2 → 2NOCl (trimolecular) The molecularity of simple reactions coincides with the general kinetic order of the reaction. The order of the reaction determines the nature of the dependence of rate on concentration. The general (total) kinetic order of a reaction is the sum of exponents at the concentrations of reactants in the reaction rate equation, determined experimentally. As temperature increases, the rate of most chemical reactions increases. The dependence of the reaction rate on temperature is approximately determined by the Van't Hoff rule. For every 10-degree increase in temperature, the rate of most reactions increases by 2–4 times. where and are the reaction rate, respectively, at temperatures t 2 And t 1 (t 2 >t 1); γ is the temperature coefficient of the reaction rate, this is a number showing how many times the rate of a chemical reaction increases when the temperature increases by 10 0. Using Van't Hoff's rule, it is only possible to approximately estimate the effect of temperature on the reaction rate. A more accurate description of the dependence of the temperature reaction rate is feasible within the framework of the Arrhenius activation theory. One of the methods of accelerating a chemical reaction is catalysis, which is carried out using substances (catalysts). Catalysts- these are substances that change the rate of a chemical reaction due to repeated participation in intermediate chemical interactions with reaction reagents, but after each cycle of intermediate interaction they restore their chemical composition. The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. reducing the difference between the average energy of active molecules (active complex) and the average energy of molecules of the starting substances. The rate of the chemical reaction increases. |