The dependence of the rate of a chemical reaction. The rate of chemical reactions
Systems. But given value does not reflect real opportunity reaction progress, her 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.
From point of view 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 acting masses 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. energy thermal motion.
- 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.
The rate of a chemical reaction
The topic "The rate of a chemical reaction" is perhaps the most complex and controversial in the school curriculum. This is due to the complexity of chemical kinetics itself, one of the branches of physical chemistry. The very definition of the concept of "rate of a chemical reaction" is already ambiguous (see, for example, the article by L.S. Guzey in the newspaper "Chemistry", 2001, No. 28,
with. 12). More more problems arises when trying to apply the law of mass action for the reaction rate to any chemical systems, because the range of objects for which a quantitative description of kinetic processes is possible within the school curriculum, very narrow. I would like to emphasize the incorrectness of using the law of mass action for the rate of a chemical reaction at chemical equilibrium.
At the same time, it would be wrong to refuse to consider this topic in school at all. Ideas about the rate of a chemical reaction are very important in the study of many natural and technological processes; without them, it is impossible to talk about catalysis and catalysts, including enzymes. Although when discussing the transformations of substances, mainly qualitative ideas about the rate of a chemical reaction are used, the introduction of the simplest quantitative ratios is still desirable, especially for elementary reactions.
In the published article, the issues of chemical kinetics are considered in sufficient detail, which can be discussed at school lessons chemistry. Exclusion from the course school chemistry controversial and controversial points of this topic is especially important for those students who are going to continue their chemical education at the university. After all, knowledge acquired at school often conflicts with scientific reality.
Chemical reactions can vary significantly in time. A mixture of hydrogen and oxygen at room temperature can remain virtually unchanged for a long time, but on impact or ignition, an explosion will occur. The iron plate slowly rusts, and a piece of white phosphorus ignites spontaneously in air. It is important to know how fast a particular reaction proceeds in order to be able to control its progress.
Basic concepts
quantitative characteristic of how fast a given reaction proceeds is the rate of a chemical reaction, that is, the rate of consumption of reagents or the rate of appearance of products. In this case, it does not matter which of the substances involved in the reaction is in question, since they are all interconnected through the reaction equation. By changing the amount of one of the substances, one can judge the corresponding changes in the amounts of all the others.
The rate of a chemical reaction () called the change in the amount of substance of the reactant or product () per unit of time () per unit volume (V):
= /(V ).
Reaction rate in this case usually expressed in mol/(l s).
The above expression refers to homogeneous chemical reactions occurring in a homogeneous medium, for example between gases or in solution:
2SO 2 + O 2 \u003d 2SO 3,
BaCl 2 + H 2 SO 4 \u003d BaSO 4 + 2HCl.
Heterogeneous chemical reactions take place on the contact surfaces solid and gas, solid and liquid, etc. Heterogeneous reactions include, for example, reactions of metals with acids:
Fe + 2HCl \u003d FeCl 2 + H 2.
In e that case the rate of a reaction is the change in the amount of a reactant or product () per unit of time() per unit area (S):
= /(S ).
The rate of a heterogeneous reaction is expressed in mol/(m 2 s).
To control chemical reactions, it is important not only to be able to determine their speed, but also to find out what conditions affect them. Branch of chemistry that studies the rate of chemical reactions and the effect on it various factors, is called chemical kinetics.
Collision frequency of reacting particles
The most important factor, which determines the rate of a chemical reaction, - concentration.
As the concentration of the reactants increases, the rate of the reaction usually increases. In order to enter into a reaction, two chemical particles must approach each other, so the reaction rate depends on the number of collisions between them. An increase in the number of particles in given volume leads to more frequent collisions and to an increase in the reaction rate.
For homogeneous reactions, increasing the concentration of one or more reactants will increase the rate of the reaction. With a decrease in concentration, the opposite effect is observed. The concentration of substances in a solution can be changed by adding or removing reactants or a solvent from the reaction sphere. In gases, the concentration of one of the substances can be increased by introducing an additional amount of this substance into the reaction mixture. Concentrations of all gaseous substances can be increased simultaneously by decreasing the volume occupied by the mixture. In this case, the reaction rate will increase. Increasing volume has the opposite effect.
The rate of heterogeneous reactions depends on surface area of contact of substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents, as well as on the state of crystalline structures solids. Any disturbance in the crystal structure causes an increase reactivity solids, because to destroy a strong crystal structure additional energy is required.
Consider the combustion of wood. A whole log burns relatively slowly in air. If you increase the surface of contact of wood with air, splitting the log into chips, the burning rate will increase. At the same time, wood burns much faster in pure oxygen than in air, which contains only about 20% oxygen.
For a chemical reaction to occur, particles must collide - atoms, molecules, or ions. As a result of collisions, atoms rearrange and new chemical bonds arise, which leads to the formation of new substances. The probability of a collision of two particles is quite high, the probability of a simultaneous collision of three particles is much less. A simultaneous collision of four particles is extremely unlikely. Therefore, most reactions proceed in several stages, at each of which no more than three particles interact.
The oxidation reaction of hydrogen bromide proceeds at a noticeable rate at 400–600 °C:
4HBr + O 2 \u003d 2H 2 O + 2Br 2.
According to the reaction equation, five molecules must collide at the same time. However, the probability of such an event is practically zero. Moreover, experimental studies have shown that increasing the concentration - either oxygen or hydrogen bromide - increases the reaction rate by the same number of times. And this despite the fact that four molecules of hydrogen bromide are consumed for each molecule of oxygen.
A detailed examination of this process shows that it proceeds in several stages:
1) HBr + O 2 = HOOVr (slow reaction);
2) HOOVr + HBr = 2NOVr (fast reaction);
3) NOVr + HBr = H 2 O + Br 2 (fast reaction).
These reactions, the so-called elementary reactions, reflect reaction mechanism oxidation of hydrogen bromide with oxygen. It is important to note that only two molecules are involved in each of the intermediate reactions. Adding the first two equations and twice the third gives summary equation reactions. The overall reaction rate is determined by the slowest intermediate reaction, in which one molecule of hydrogen bromide and one molecule of oxygen interact.
The rate of elementary reactions is directly proportional to the product of molar concentrations with (with is the amount of substance per unit volume, with = /V) reagents taken in powers equal to their stoichiometric coefficients ( law of mass action for the rate of a chemical reaction). This is true only for reaction equations that reflect the mechanisms of real chemical processes when the stoichiometric coefficients in front of the formulas of the reagents correspond to the number of interacting particles.
According to the number of molecules interacting in the reaction, reactions are distinguished as monomolecular, bimolecular and trimolecular. For example, the dissociation of molecular iodine into atoms: I 2 \u003d 2I - a monomolecular reaction.
The interaction of iodine with hydrogen: I 2 + H 2 \u003d 2HI - a bimolecular reaction. The law of mass action for chemical reactions of different molecularity is written in different ways.
Monomolecular reactions:
A = B + C,
= kc A ,
where k is the reaction rate constant.
Bimolecular reactions:
= kc A c AT.
Trimolecular reactions:
= kc 2A c AT.
Activation energy
clash chemical particles leads to a chemical interaction only if the colliding particles have an energy exceeding a certain certain value. Consider the interaction of gaseous substances consisting of molecules A 2 and B 2:
A 2 + B 2 \u003d 2AB.
In the course of a chemical reaction, the rearrangement of atoms occurs, accompanied by a break chemical bonds in the starting materials and the formation of bonds in the reaction products. When reacting molecules collide, the so-called activated complex, in which the electron density is redistributed, and only then the final reaction product is obtained:
The energy required for the transition of substances to the state of an activated complex is called activation energy.
Activity chemical substances manifests itself in the low activation energy of reactions with their participation. 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 proceed almost instantly. If the activation energy is high, then a very small part of the collisions leads to the formation of new substances. Thus, the rate of reaction between hydrogen and oxygen at room temperature is practically zero.
So, the reaction rate is affected by the nature of the reactants. Consider, for example, the reactions of metals with acids. If we put identical pieces of copper, zinc, magnesium and iron into test tubes with dilute sulfuric acid, we can see that the intensity of the release of hydrogen gas bubbles, which characterizes the reaction rate, differs significantly for these metals. In a test tube with magnesium, a rapid evolution of hydrogen is observed, in a test tube with zinc, gas bubbles are released somewhat calmer. The reaction proceeds even more slowly in a test tube with iron (Fig.). Copper does not react at all with dilute sulfuric acid. Thus, the reaction rate depends on the activity of the metal.
When replacing sulfuric acid (strong acid) with acetic ( weak acid) the reaction rate slows down significantly in all cases. It can be concluded that the nature of both reactants, both the metal and the acid, affects the rate of the reaction of a metal with an acid.
Raise temperature leads to an increase in the kinetic energy of chemical particles, i.e. increases the number of particles having an energy higher than the activation energy. As the temperature rises, the number of particle collisions also increases, which increases the rate of the reaction to some extent. However, increasing the efficiency of collisions by increasing the kinetic energy has a greater effect on the reaction rate than increasing the number of collisions.
When the temperature rises by ten degrees, the speed increases by a factor equal to the temperature coefficient of speed:
= T+10 /T .
When the temperature rises from T before T"
reaction rate ratio T" and T equals
temperature coefficient of velocity in the power ( T"
– T)/10:
T" /T = (T"–T)/10.
For many homogeneous reactions, the temperature coefficient of the rate is 24 (van't Hoff's rule). The dependence of the reaction rate on temperature can be traced by the example of the interaction of copper(II) oxide with dilute sulfuric acid. At room temperature, the reaction proceeds very slowly. When heated, the reaction mixture quickly turns blue due to the formation of copper(II) sulfate:
CuO + H 2 SO 4 \u003d CuSO 4 + H 2 O.
Catalysts and inhibitors
Many reactions can be accelerated or slowed down by the introduction of certain substances. The added substances do not participate in the reaction and are not consumed during its course, but they have a significant effect on the reaction rate. These substances change the reaction mechanism (including the composition of the activated complex) and lower the activation energy, which ensures the acceleration of chemical reactions. Substances that accelerate reactions are called catalysts, and the very phenomenon of such an acceleration of the reaction - catalysis.
Many reactions proceed very slowly or not at all in the absence of catalysts. One of these reactions is the decomposition of hydrogen peroxide:
2H 2 O 2 \u003d 2H 2 O + O 2.
If immersed in a vessel with aqueous solution hydrogen peroxide a piece of solid manganese dioxide, then a rapid release of oxygen will begin. After removal of manganese dioxide, the reaction practically stops. By weighing, it is easy to verify that manganese dioxide is not consumed in this process - it only catalyzes the reaction.
Depending on whether the same or different states of aggregation there is a catalyst and reactants, distinguish between homogeneous and heterogeneous catalysis.
In homogeneous catalysis, the catalyst can accelerate the reaction by forming intermediates through interaction with one of the starting reactants. For example:
In heterogeneous catalysis, the chemical reaction usually takes place on the surface of the catalyst:
Catalysts are widely distributed in nature. Almost all transformations of substances in living organisms proceed with the participation of organic catalysts - enzymes.
Catalysts are used in chemical production to speed up certain processes. In addition to them, substances that slow down chemical reactions are also used, - inhibitors. With the help of inhibitors, in particular, they protect metals from corrosion.
Factors affecting the rate of a chemical reaction
| Increase speed | Reduce speed |
|---|---|
| The presence of chemically active reagents | The presence of chemically inactive reagents |
| Increasing the concentration of reagents | Reducing the concentration of reagents |
| Increasing the surface of solid and liquid reagents | Reducing the surface of solid and liquid reagents |
| Temperature increase | Temperature drop |
| The presence of a catalyst | The presence of an inhibitor |
TASKS
1. Define the rate of a chemical reaction. Write an expression kinetic law acting masses for the following reactions:
a) 2C (tv.) + O 2 (g.) \u003d 2CO (g.);
b) 2НI (g.) \u003d H 2 (g.) + I 2 (g.).
2. What determines the rate of a chemical reaction? Give a mathematical expression for the dependence of the rate of a chemical reaction on temperature.
3. Indicate how it affects the reaction rate (at constant volume):
a) increasing the concentration of reagents;
b) grinding of a solid reagent;
c) lowering the temperature;
d) introduction of a catalyst;
e) decrease in the concentration of reagents;
e) temperature increase;
g) introduction of an inhibitor;
h) decrease in the concentration of products.
4. Calculate the rate of a chemical reaction
CO (g) + H 2 O (g) \u003d CO 2 (g) + H 2 (g)
in a vessel with a capacity of 1 liter, if after 1 min 30 s after it began, the amount of hydrogen substance was 0.32 mol, and after 2 min 10 s it became 0.44 mol. How will an increase in CO concentration affect the rate of reaction?
5. As a result of one reaction, 6.4 g of hydrogen iodide was formed over a certain period of time, and in another reaction under the same conditions, 6.4 g of sulfur dioxide. Compare the rates of these reactions. How will the rates of these reactions change with increasing temperature?
6. Determine the reaction rate
CO (g.) + Cl 2 (g.) \u003d COCl 2 (g.),
if 20 s after the start of the reaction, the initial amount of carbon monoxide (II) substance decreased from 6 mol by 3 times (the reactor volume is 100 l). How will the reaction rate change if less active bromine is used instead of chlorine? How will the reaction rate change with the introduction
a) a catalyst b) an inhibitor?
7. In which case is the reaction
CaO (tv.) + CO 2 (g.) \u003d CaCO 3 (tv.)
runs faster: when using large pieces or calcium oxide powder? Calculate:
a) the amount of the substance; b) the mass of calcium carbonate formed in 10 s, if the reaction rate is 0.1 mol/(l s), the volume of the reactor is 1 liter.
8. The interaction of a sample of magnesium with hydrochloric acid HCl allows you to get 0.02 mol of magnesium chloride 30 s after the start of the reaction. Determine how long it takes to get 0.06 mol of magnesium chloride.
E) from 70 to 40 °C, the reaction rate decreased by 8 times;
g) from 60 to 40 °C, the reaction rate decreased by 6.25 times;
h) from 40 to 10 °C, the reaction rate decreased by 27 times.
11. The owner of the car painted it new paint, and then found that according to the instructions it should dry for 3 hours at 105 ° C. How long will the paint dry at 25 ° C if temperature coefficient the polymerization reaction underlying this process is: a) 2; b) 3; at 4?
ANSWERS TO TASKS
1. a) = kc(O 2); b) = kc(HI) 2 .
2. T+10 = T .
3. The reaction rate increases in cases a, b, d, f; decreases - c, e, g; does not change -
4. 0.003 mol/(l s). As the concentration of CO increases, the rate of the reaction increases.
5. The rate of the first reaction is 2 times lower.
6. 0.002 mol/(l s).
7. a) 1 mol; b) 100 g.
9. The rates of reactions e, g, h will increase by 2 times; 4 times - a, b, e; 8 times - in, city.
10. Temperature coefficient:
2 for reactions b, f; = 2.5 – c, g; = 3 – e, h; = 3.5 – a, d.
a) 768 hours (32 days, i.e. more than 1 month);
b) 19,683 hours (820 days, i.e. more than 2 years);
c) 196,608 hours (8192 days, i.e. 22 years).
Physical chemistry: lecture notes Berezovchuk A V
2. Factors affecting the rate of a chemical reaction
For homogeneous, heterogeneous reactions:
1) concentration of reacting substances;
2) temperature;
3) catalyst;
4) inhibitor.
Only for heterogeneous:
1) the rate of supply of reactants to the interface;
2) surface area.
The main factor - the nature of the reacting substances - the nature of the bond between the atoms in the molecules of the reagents.
NO 2 - nitric oxide (IV) - fox tail, CO - carbon monoxide, carbon monoxide.
If they are oxidized with oxygen, then in the first case the reaction will go instantly, it is worth opening the stopper of the vessel, in the second case the reaction is extended in time.
The concentration of reactants will be discussed below.
Blue opalescence indicates the moment of precipitation of sulfur, the higher the concentration, the higher the rate.
Rice. ten
The greater the concentration of Na 2 S 2 O 3, the less time the reaction takes. On the graph (Fig. 10) is shown directly proportional dependence. The quantitative dependence of the reaction rate on the concentration of the reactants is expressed by the MMA (law of mass action), which states: the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.
So, basic law of kinetics is an experimentally established law: the reaction rate is proportional to the concentration of the reactants, example: (i.e. for the reaction)
For this reaction H 2 + J 2 = 2HJ - the rate can be expressed in terms of a change in the concentration of any of the substances. If the reaction proceeds from left to right, then the concentration of H 2 and J 2 will decrease, the concentration of HJ will increase in the course of the reaction. For instantaneous speed reactions can be written as:
square brackets indicate concentration.
physical meaning k– molecules are in continuous motion, collide, scatter, hit the walls of the vessel. In order for the chemical reaction of HJ formation to occur, the H 2 and J 2 molecules must collide. The number of such collisions will be the greater, the more H 2 and J 2 molecules are contained in the volume, i.e., the greater will be the values of [Н 2 ] and . But the molecules move at different speeds, and the total kinetic energy two colliding molecules will be different. If the fastest H 2 and J 2 molecules collide, their energy can be so high that the molecules break into iodine and hydrogen atoms, which fly apart and then interact with other H 2 + J 2 molecules ? 2H+2J, then H + J 2 ? HJ + J. If the energy of the colliding molecules is less, but high enough to weaken the H - H and J - J bonds, the reaction of formation of hydrogen iodine will occur:
For the majority of colliding molecules, the energy is less than necessary to weaken the bonds in H 2 and J 2 . Such molecules "quietly" collide and also "quietly" disperse, remaining what they were, H 2 and J 2 . Thus, not all, but only a part of the collisions leads to a chemical reaction. The coefficient of proportionality (k) shows the number of effective collisions leading to the reaction at concentrations [H 2 ] = = 1 mol. Value k–const speed. How can the speed be constant? Yes, speed uniform rectilinear motion called a constant vector quantity equal to the ratio of the displacement of the body for any period of time to the value of this interval. But the molecules move randomly, so how can the speed be const? But constant speed can only be at constant temperature. As the temperature rises, the proportion of fast molecules whose collisions lead to a reaction increases, i.e., the rate constant increases. But the increase in the rate constant is not unlimited. At a certain temperature, the energy of the molecules will become so large that almost all collisions of the reactants will be effective. When two fast molecules collide, a reverse reaction will occur.
A moment will come when the rates of formation of 2HJ from H 2 and J 2 and decomposition will be equal, but this is already chemical equilibrium. The dependence of the reaction rate on the concentration of the reactants can be traced using the traditional reaction of the interaction of a sodium thiosulfate solution with a sulfuric acid solution.
Na 2 S 2 O 3 + H 2 SO 4 \u003d Na 2 SO 4 + H 2 S 2 O 3, (1)
H 2 S 2 O 3 \u003d S? + H 2 O + SO 2?. (2)
Reaction (1) proceeds almost instantaneously. The rate of reaction (2) depends at a constant temperature on the concentration of the reactant H 2 S 2 O 3 . It is this reaction that we observed - in this case, the rate is measured by the time from the beginning of the pouring of solutions to the appearance of opalescence. In the article L. M. Kuznetsova the reaction of interaction of sodium thiosulfate with hydrochloric acid is described. She writes that when the solutions are drained, opalescence (turbidity) occurs. But this statement by L. M. Kuznetsova is erroneous, since opalescence and clouding are different things. Opalescence (from opal and Latin escentia- suffix meaning weak action) - light scattering by turbid media due to their optical inhomogeneity. light scattering- deviation of light rays propagating in the medium in all directions from the original direction. colloidal particles are able to scatter light (Tyndall-Faraday effect) - this explains the opalescence, slight turbidity of the colloidal solution. When conducting this experiment, it is necessary to take into account the blue opalescence, and then the coagulation of the colloidal suspension of sulfur. The same density of the suspension is noted by the apparent disappearance of any pattern (for example, the grid at the bottom of the cup), observed from above through the solution layer. Time is counted by a stopwatch from the moment of draining.
Solutions Na 2 S 2 O 3 x 5H 2 O and H 2 SO 4.
The first is prepared by dissolving 7.5 g of salt in 100 ml of H 2 O, which corresponds to a 0.3 M concentration. To prepare a solution of H 2 SO 4 of the same concentration, it is necessary to measure 1.8 ml of H 2 SO 4 (k), ? = = 1.84 g / cm 3 and dissolve it in 120 ml of H 2 O. Pour the prepared solution of Na 2 S 2 O 3 into three glasses: in the first - 60 ml, in the second - 30 ml, in the third - 10 ml. Add 30 ml of distilled H 2 O to the second glass, and 50 ml to the third. Thus, in all three glasses there will be 60 ml of liquid, but in the first the salt concentration is conditionally = 1, in the second - ½, and in the third - 1/6. After the solutions are prepared, pour 60 ml of H 2 SO 4 solution into the first glass with a salt solution and turn on the stopwatch, etc. Considering that the reaction rate decreases with dilution of the Na 2 S 2 O 3 solution, it can be determined as a value inversely proportional to time v= one/? and build a graph by plotting the concentration on the abscissa and the rate of the reaction on the ordinate. From this conclusion - the reaction rate depends on the concentration of substances. The data obtained are listed in table 3. This experiment can be performed using burettes, but this requires the performer great practice because the schedule is wrong.
Table 3
Speed and reaction time
The Guldberg-Waage law is confirmed - professor of chemistry Gulderg and the young scientist Waage).
Consider next factor– temperature.
As the temperature increases, the rate of most chemical reactions increases. This dependence is described by the van't Hoff rule: "When the temperature rises for every 10 ° C, the rate of chemical reactions increases by 2-4 times."
where ? – temperature coefficient, showing how many times the reaction rate increases with an increase in temperature by 10 ° C;
v 1 - reaction rate at temperature t 1 ;
v 2 - reaction rate at temperature t2.
For example, the reaction at 50 °C proceeds in two minutes, how long will the process end at 70 °C if the temperature coefficient ? = 2?
t 1 = 120 s = 2 min; t 1 = 50 °С; t 2 = 70 °C.
Even a slight increase in temperature causes a sharp increase in the reaction rate of active molecular collisions. According to the activation theory, only those molecules participate in the process, the energy of which is greater than the average energy of the molecules by a certain amount. This excess energy is the activation energy. Its physical meaning is the energy that is necessary for the active collision of molecules (rearrangement of orbitals). The number of active particles, and hence the reaction rate, increases with temperature according to an exponential law, according to the Arrhenius equation, which reflects the dependence of the rate constant on temperature
where BUT - Arrhenius proportionality factor;
k– Boltzmann's constant;
E A - activation energy;
R- gas constant;
T- temperature.
A catalyst is a substance that speeds up the rate of a reaction but is not itself consumed.
Catalysis- the phenomenon of a change in the reaction rate in the presence of a catalyst. Distinguish between homogeneous and heterogeneous catalysis. Homogeneous- if the reactants and the catalyst are in the same state of aggregation. Heterogeneous– if the reactants and the catalyst are in different states of aggregation. About catalysis see separately (further).
Inhibitor A substance that slows down the rate of a reaction.
The next factor is surface area. The larger the surface of the reactant, the greater the speed. Consider, for example, the influence of the degree of dispersity on the reaction rate.
CaCO 3 - marble. We lower the tiled marble into hydrochloric acid HCl, wait five minutes, it will dissolve completely.
Powdered marble - we will do the same procedure with it, it dissolved in thirty seconds.
The equation for both processes is the same.
CaCO 3 (tv) + HCl (g) \u003d CaCl 2 (tv) + H 2 O (l) + CO 2 (g) ?.
So, when adding powdered marble, the time is less than when adding tile marble, with the same mass.
With an increase in the interface between phases, the rate of heterogeneous reactions increases.
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Chemical Methods
Methods for measuring the reaction rate
In the example above, the reaction rate between calcium carbonate and acid was measured by studying the volume of gas evolved as a function of time. Experimental data on reaction rates can be obtained by measuring other quantities.
If the reaction changes total gaseous substances, then its flow can be observed by measuring the pressure of the gas at a constant volume. In cases where one of the starting materials or one of the reaction products is colored, the progress of the reaction can be monitored by observing the change in color of the solution. Other optical method is the measurement of the rotation of the plane of polarization of light (if the starting substances and reaction products have different rotational abilities).
Some reactions are accompanied by a change in the number of ions in the solution. In such cases, the reaction rate can be studied by measuring electrical conductivity solution. AT next chapter some other electrochemical methods, which can be used to measure reaction rates.
The progress of the reaction can be monitored by measuring the concentration of one of the participants in the reaction over time using a variety of methods. chemical analysis. The reaction is carried out in a thermostated vessel. At certain intervals, a sample of the solution (or gas) is taken from the vessel and the concentration of one of the components is determined. To obtain reliable results, it is important that no reaction occurs in the sample taken for analysis. This is achieved by chemical binding of one of the reagents, rapid cooling or dilution of the solution.
Experimental studies show that the reaction rate depends on several factors. Let us first consider the influence of these factors at a qualitative level.
1.The nature of the reactants. From laboratory practice, we know that the neutralization of an acid by a base
H + + OH - ® H 2 O
interaction of salts with the formation of a sparingly soluble compound
Ag + + Cl – ® AgCl
and other reactions in electrolyte solutions occur very quickly. The time required for such reactions to complete is measured in milliseconds and even microseconds. This is quite understandable, because the essence of such reactions is the approach and combination of charged particles with charges of the opposite sign.
In contrast ionic reactions interaction between covalently bound molecules usually proceeds much more slowly. Indeed, in the course of the reaction between such particles, the bonds in the molecules of the starting substances must break. To do this, colliding molecules must have a certain amount of energy. In addition, if the molecules are complex enough, in order for a reaction to occur between them, they must be oriented in space in a certain way.
2. Reactant concentration. The rate of a chemical reaction, with other equal conditions, depends on the number of collisions of reacting particles per unit time. The probability of collisions depends on the number of particles per unit volume, i.e. from concentration. Therefore, the reaction rate increases with increasing concentration.
3. The physical state substances. In homogeneous systems, the reaction rate depends on the number of particle collisions in solution volume(or gas). In heterogeneous systems chemical interaction going on at the interface. An increase in the surface area of a solid during its grinding facilitates the access of the reacting particles to the particles of the solid, which leads to a significant acceleration of the reaction.
4. Temperature has a significant effect on the rate of various chemical and biological processes. With an increase in temperature, the kinetic energy of the particles increases, and, consequently, the fraction of particles whose energy is sufficient for chemical interaction increases.
5. Steric factor characterizes the need for mutual orientation of the reacting particles. The more complex the molecules, the lower the probability of their proper orientation, the lower the efficiency of collisions.
6. Availability of catalysts.Catalysts are substances that change the rate of a chemical reaction. Introduced into the reaction system in small amounts and remaining unchanged after the reaction, they are capable of extremely changing the rate of the process.
The main factors on which the reaction rate depends will be discussed in more detail below.
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 speed of homogeneous and heterogeneous reactions are defined differently.
The definition of a measure of the rate of a chemical reaction can be written as 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 molar concentration substances n B /V = c B , whence dn B / V = 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 kinetic equation reactions. He is equal to the sum indicators of 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
| 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. |
| Decision | Let's write a formula for calculating average speed chemical reaction:
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