Reaction speed is determined by a change in the molar concentration of one of the reactants:
V = ± ((C 2 - C 1) / (t 2 - t 1)) = ± (DC / Dt)
Where C 1 and C 2 are the molar concentrations of substances at times t 1 and t 2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the starting substance).
Reactions occur when molecules of reacting substances collide. Its speed is determined by the number of collisions and the likelihood that they will lead to transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors influencing the rate of chemical reactions.
1. The nature of the reacting substances. The nature of the chemical bonds and the structure of the reagent molecules play an important role. Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, breaking bonds in H 2 and N 2 molecules requires high energies; such molecules are slightly reactive. Breaking bonds in highly polar molecules (HCl, H 2 O) requires less energy, and the reaction rate is much higher. Reactions between ions in electrolyte solutions occur almost instantly.
Examples
Fluorine reacts with hydrogen explosively at room temperature; bromine reacts with hydrogen slowly when heated.
Calcium oxide reacts with water vigorously, releasing heat; copper oxide - does not react.
2. Concentration. With increasing concentration (the number of particles per unit volume), collisions of molecules of reacting substances occur more often - the reaction rate increases.
Law of mass action (K. Guldberg, P. Waage, 1867)
Speed chemical reaction is directly proportional to the product of the concentrations of the reactants.
AA + bB + . . . ® . . .
The reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the concentrations of the reactants.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the expression of the reaction rate.
3. Temperature. For every 10°C increase in temperature, the reaction rate increases by 2-4 times (van't Hoff's rule). As the temperature increases from t 1 to t 2, the change in reaction rate can be calculated using the formula:
| |
|
(t 2 - t 1) / 10 |
| Vt 2 / Vt 1 | = g | |
(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, respectively; g is the temperature coefficient of this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:
Where
A is a constant depending on the nature of the reactants;
R is the universal gas constant;
Ea is the activation energy, i.e. the energy that colliding molecules must have in order for the collision to lead to a chemical transformation.
Energy diagram of a chemical reaction.
| Exothermic reaction | Endothermic reaction |
A - reagents, B - activated complex (transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.
4. Contact surface of reacting substances. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction occurs. The surface area of solids can be increased by grinding them, and for soluble substances by dissolving them.
5. Catalysis. Substances that participate in reactions and increase its speed, remaining unchanged at the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. At homogeneous catalysis the reagents and the catalyst constitute one phase (are in the same state of aggregation), with heterogeneous catalysis- different phases (are in different states of aggregation). In some cases, the occurrence of undesirable chemical processes can be sharply slowed down by adding inhibitors to the reaction medium (the "phenomenon" negative catalysis").
7.1. Homogeneous and heterogeneous reactions
Chemical substances can be in different states of aggregation, while their chemical properties in different states are the same, but the activity is different (which was shown in the last lecture using the example of the thermal effect of a chemical reaction).
Let us consider various combinations of states of aggregation in which two substances A and B can exist.
A (g.), B (g.) |
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A (TV), B (TV) | A (w.), B (tv.) | mix | A(tv), B(g) | A (f.), B (g.) |
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mix | (solution) | |||||||||||||||||||||||||||||
heterogeneous | heterogeneous | heterogeneous | homogeneous | heterogeneous | heterogeneous | homogeneous |
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Hg(l) + HNO3 | H2O + D2O | Fe + O2 | H2S + H2SO4 | CO+O2 |
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A phase is a region of a chemical system within which all the properties of the system are constant (identical) or continuously change from point to point. Each of the solids is a separate phase, and there are also solution and gas phases.
Called homogeneous chemical system, in which all substances are in one phase (in solution or gas). If there are several phases, then the system is called
heterogeneous.
Respectively chemical reaction called homogeneous if the reactants are in the same phase. If the reagents are in different phases, then chemical reaction called heterogeneous.
It is not difficult to understand that since contact of reagents is required for a chemical reaction to occur, a homogeneous reaction occurs simultaneously throughout the entire volume of a solution or reaction vessel, while a heterogeneous reaction occurs at a narrow boundary between phases - at the interface. Thus, purely theoretically, a homogeneous reaction occurs faster than a heterogeneous one.
Thus we come to the concept rate of chemical reaction.
The rate of a chemical reaction. Law of mass action. Chemical equilibrium.
7.2. Chemical reaction rate
The branch of chemistry that studies the rates and mechanisms of chemical reactions is a branch of physical chemistry and is called chemical kinetics.
Speed of chemical reaction is the change in the amount of a substance per unit time per unit volume of the reacting system (for a homogeneous reaction) or per unit surface area (for a heterogeneous reaction).
Thus, if the volume | or area | interfaces |
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do not change, then the expressions for the rates of chemical reactions have the form: |
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hom o | |||||||||||
The ratio of a change in the amount of a substance to the volume of the system can be interpreted as a change in the concentration of a given substance.
Note that for reagents, the expression for the rate of a chemical reaction is written with a minus sign, since the concentration of the reagents decreases, and the rate of the chemical reaction is actually a positive value.
Further conclusions are based on simple physical considerations that consider a chemical reaction as a consequence of the interaction of several particles.
Elementary (or simple) is a chemical reaction that occurs in one stage. If there are several stages, then such reactions are called complex, or composite, or gross reactions.
In 1867, it was proposed to describe the rate of a chemical reaction law of mass action: the rate of an elementary chemical reaction is proportional to the concentrations of reactants in powers of stoichiometric coefficients.n A +m B P,
A, B – reactants, P – products, n, m – coefficients.
W =k n m
The coefficient k is called the rate constant of a chemical reaction,
characterizes the nature of interacting particles and does not depend on the particle concentration.
The rate of a chemical reaction. Law of mass action. Chemical balance. The quantities n and m are called reaction order by substance A and B respectively, and
their sum (n +m) – reaction order.
For elementary reactions, the reaction order can be 1, 2 and 3.
Elementary reactions with order 1 are called monomolecular, with order 2 - bimolecular, with order 3 - trimolecular, based on the number of molecules involved. Elementary reactions above the third order are unknown - calculations show that the simultaneous meeting of four molecules at one point is too incredible an event.
Since a complex reaction consists of a certain sequence of elementary reactions, its rate can be expressed in terms of the rates of individual stages of the reaction. Therefore, for complex reactions, the order can be any, including fractional or zero (zero order of a reaction indicates that the reaction occurs at a constant rate and does not depend on the concentration of reacting particles W = k).
The slowest of stages complex process usually called the rate-limiting stage.
Imagine that large number molecules went to a free cinema, but at the entrance there is a controller who checks the age of each molecule. Therefore, a flow of matter enters the cinema doors, and molecules enter the cinema hall one at a time, i.e. very slowly.
Examples of elementary first-order reactions are processes of thermal or radioactive decay, accordingly, the rate constant k characterizes either the probability of breaking a chemical bond or the probability of decay per unit time.
There are a lot of examples of elementary second-order reactions - this is the most familiar way of reactions for us - particle A collided with particle B, some kind of transformation occurred and something happened there (note that products in theory do not affect anything - all attention is given only to reacting particles).
On the contrary, there are quite a few elementary third-order reactions, since it is quite rare for three particles to meet simultaneously.
As an illustration, let's look at the predictive power of chemical kinetics.
The rate of a chemical reaction. Law of mass action. Chemical balance.
First order kinetic equation
(illustrative supplementary material)
Let us consider a homogeneous first-order reaction, the rate constant of which is equal to k, the initial concentration of substance A is equal to [A]0.
By definition, the rate of a homogeneous chemical reaction is equal to |
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K[A] | change in concentration per unit time. Once substance A – |
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reagent, put a minus sign. | ||||||||||||||||||
Such an equation is called differential (there is |
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derivative) | ||||||||||||||||||
[A] | To solve it, we transfer the quantities to the left side |
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concentrations, and on the right - time. | ||||||||||||||||||
If the derivatives of two functions are equal, then the functions themselves |
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should differ by no more than a constant. | ||||||||||||||||||
To solve this equation, take the integral of the left side (over |
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concentration) and the right side (in time). So as not to scare |
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ln[ A ] = −kt +C | listeners, we will limit ourselves to the answer. | |||||||||||||||||
ln icon – natural logarithm, i.e. number b such that | ||||||||||||||||||
= [A],e = 2.71828… | ||||||||||||||||||
ln[ A ]- ln0 = - kt | The constant C is found from the initial conditions: | |||||||||||||||||
at t = 0 the initial concentration is [A]0 | ||||||||||||||||||
[A] | Times logarithm – | this is a power of a number, we use the properties of powers |
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[A]0 | e a− b= | |||||||||||||||||
Now let's get rid of the nasty logarithm (see definition |
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logarithm 6-7 lines higher), | Why do we raise the number? | |||||||||||||||||
to the power of the left side of the equation and the right side of the equation. | ||||||||||||||||||
[A] | E−kt | Multiply by [A]0 | ||||||||||||||||
[A]0 | ||||||||||||||||||
First order kinetic equation. | ||||||||||||||||||
[ A ]= 0 × e − kt | Based on | the obtained kinetic equation of the first |
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order maybe | calculated | substance concentration | ||||||||||||||||
at any time | ||||||||||||||||||
For the purposes of our course, this conclusion is for informational purposes in order to demonstrate to you the use of mathematical apparatus for calculating the progress of a chemical reaction. Consequently, a competent chemist cannot but know mathematics. Learn math!
The rate of a chemical reaction. Law of mass action. Chemical balance. A graph of the concentration of reagents and products versus time can be qualitatively depicted as follows (using the example irreversible reaction first order)
Factors that affect reaction speed
1. Nature of reactants
For example, the reaction rate of the following substances: H2 SO4, CH3 COOH, H2 S, CH3 OH - with hydroxide ion will vary depending on the strength H-O bonds. To assess the strength of a given bond, you can use the relative positive charge on the hydrogen atom: the greater the charge, the easier the reaction will be.
2. Temperature
Life experience tells us that the rate of reaction depends on temperature and increases with increasing temperature. For example, the process of milk souring occurs faster at room temperature rather than in the refrigerator.
Let us turn to the mathematical expression of the law of mass action.
W =k n m
Once left side This expression (reaction rate) depends on temperature, therefore, the right side of the expression also depends on temperature. In this case, the concentration, of course, does not depend on temperature: for example, milk retains its fat content of 2.5% both in the refrigerator and at room temperature. Then, as Sherlock Holmes used to say, the remaining solution is the correct one, no matter how strange it may seem: the rate constant depends on temperature!
The rate of a chemical reaction. Law of mass action. Chemical balance. The dependence of the reaction rate constant on temperature is expressed using the Arrhenius equation:
− Ea
k = k0 eRT,
in which
R = 8.314 J mol-1 K-1 – universal gas constant,
E a is the activation energy of the reaction (see below), it is conventionally considered independent of temperature;
k 0 is the pre-exponential factor (i.e. the factor that comes before the exponentiale), the value of which is also almost independent of temperature and is determined, first of all, by the order of the reaction.
Thus, the value of k0 is approximately 1013 s-1 for a first-order reaction, 10 -10 l mol-1 s-1 for a second-order reaction,
for a third order reaction – 10 -33 l2 mol-2 s-1. It is not necessary to remember these values.
The exact values of k0 for each reaction are determined experimentally.
The concept of activation energy becomes clear from the following figure. In fact, activation energy is the energy that a reacting particle must have in order for a reaction to occur.
Moreover, if we heat the system, then the energy of the particles increases (dashed graph), while the transition state (≠) remains at the same level. The energy difference between the transition state and the reactants (activation energy) decreases, and the reaction rate according to the Arrhenius equation increases.
The rate of a chemical reaction. Law of mass action. Chemical balance. In addition to the Arrhenius equation, there is the Van't Hoff equation, which
characterizes the dependence of the reaction rate on temperature through the temperature coefficient γ:
The temperature coefficient γ shows how many times the rate of a chemical reaction will increase when the temperature changes by 10o.
Van't Hoff equation:
T 2− T 1
W (T 2 )= W (T 1 )× γ10
Typically, the coefficient γ is in the range from 2 to 4. For this reason, chemists often use the approximation that an increase in temperature by 20o leads to an increase in the reaction rate by an order of magnitude (i.e., 10 times).
Topics of the Unified State Examination codifier:Reaction speed. Its dependence on various factors.
The rate of a chemical reaction shows how quickly a particular reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur at every collision, but only when the particle has the appropriate energy.
Reaction speed – the number of elementary collisions of interacting particles ending in a chemical transformation per unit of time.
Determining the rate of a chemical reaction is related to the conditions under which it is carried out. If the reaction homogeneous– i.e. products and reagents are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:
υ = ΔC / Δt.
If the reactants or products are in different phases, and the collision of particles occurs only at the phase boundary, then the reaction is called heterogeneous, and its speed is determined by the change in the amount of substance per unit time per unit of reaction surface:
υ = Δν / (S·Δt).
How to make particles collide more often, i.e. How increase the rate of a chemical reaction?
1. The easiest way is to increase temperature . As you probably know from your physics course, temperature is a measure of the average kinetic energy of motion of particles of a substance. If we increase the temperature, then particles of any substance begin to move faster and, therefore, collide more often.
However, as the temperature increases, the rate of chemical reactions increases mainly due to the fact that the number of effective collisions increases. As the temperature rises, the number of active particles that can overcome the energy barrier of the reaction sharply increases. If we lower the temperature, the particles begin to move more slowly, the number of active particles decreases, and the number of effective collisions per second decreases. Thus, When the temperature increases, the rate of a chemical reaction increases, and when the temperature decreases, it decreases..
Pay attention! This rule works the same for all chemical reactions (including exothermic and endothermic). The reaction rate is independent of the thermal effect. The rate of exothermic reactions increases with increasing temperature, and decreases with decreasing temperature. The rate of endothermic reactions also increases with increasing temperature and decreases with decreasing temperature.
Moreover, back in the 19th century, the Dutch physicist Van't Hoff experimentally established that most reactions increase their speed approximately equally (about 2-4 times) when the temperature increases by 10 o C. Van't Hoff's rule sounds like this: an increase in temperature by 10 o C leads to an increase in the rate of a chemical reaction by 2-4 times (this value is called the temperature coefficient of the rate of a chemical reaction γ). The exact value of the temperature coefficient is determined for each reaction.
Here v 2 - reaction rate at temperature T 2, v 1 - reaction rate at temperature T 1, γ — temperature coefficient of reaction rate, Van't Hoff coefficient.
In some situations, it is not always possible to increase the reaction rate using temperature, because some substances decompose when the temperature rises, some substances or solvents evaporate when elevated temperature etc., i.e. the conditions of the process are violated.
2. Concentration. You can also increase the number of effective collisions by changing concentration reactants . usually used for gases and liquids, because in gases and liquids, particles move quickly and actively mix. How more concentration reacting substances (liquids, gases), those larger number effective collisions, and the higher the rate of chemical reaction.
Based on large number experiments in 1867 in the works of Norwegian scientists P. Guldenberg and P. Waage and, independently of them, in 1865 by Russian scientist N.I. Beketov derived the basic law of chemical kinetics, establishing the dependence of the rate of a chemical reaction on the concentration of the reactants:
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants in powers equal to their coefficients in the chemical reaction equation.
For a chemical reaction of the form: aA + bB = cC + dD the law of mass action is written as follows:
here v is the rate of the chemical reaction,
C A And C B — concentrations of substances A and B, respectively, mol/l
k – proportionality coefficient, reaction rate constant.
For example, for the reaction of ammonia formation:
N 2 + 3H 2 ↔ 2NH 3
The law of mass action looks like this:
The reaction rate constant shows at what speed substances will react if their concentrations are 1 mol/l, or their product is equal to 1. The rate constant of a chemical reaction depends on temperature and does not depend on the concentration of the reacting substances.
The law of mass action does not take into account the concentrations of solids, because They react, as a rule, on the surface, and the number of reacting particles per unit surface does not change.
In most cases, a chemical reaction consists of several simple steps, in which case the equation of a chemical reaction shows only the summary or final equation of the processes occurring. In this case, the rate of a chemical reaction depends in a complex way (or does not depend) on the concentration of reactants, intermediates or catalyst, therefore the exact form of the kinetic equation is determined experimentally, or based on an analysis of the proposed reaction mechanism. Typically, the rate of a complex chemical reaction is determined by the rate of its slowest step ( limiting stage).
3. Pressure. For gases, the concentration directly depends on pressure. As pressure increases, the concentration of gases increases. The mathematical expression of this dependence (for an ideal gas) is the Mendeleev-Clapeyron equation:
pV = νRT
Thus, if among the reactants there is a gaseous substance, then when As pressure increases, the rate of a chemical reaction increases; as pressure decreases, it decreases. .
For example. How will the reaction rate of the fusion of lime with silicon oxide change:
CaCO 3 + SiO 2 ↔ CaSiO 3 + CO 2
when pressure increases?
The correct answer would be - not at all, because... there are no gases among the reagents, and calcium carbonate is a solid salt, insoluble in water, silicon oxide is a solid. The product gas will be carbon dioxide. But the products do not affect the rate of the direct reaction.
Another way to increase the rate of a chemical reaction is to direct it along a different path, replacing the direct interaction, for example, of substances A and B with a series of sequential reactions with a third substance K, which require much less energy (have a lower activation energy barrier) and occur at given conditions faster than the direct reaction. This third substance is called catalyst .
- This chemicals, participating in a chemical reaction, changing its speed and direction, but non-consumable during the reaction (at the end of the reaction, they do not change either in quantity or composition). An approximate mechanism for the operation of a catalyst for a reaction of type A + B can be chosen as follows:
A+K=AK
AK + B = AB + K
The process of changing the reaction rate when interacting with a catalyst is called catalysis. Catalysts are widely used in industry when it is necessary to increase the rate of a reaction or direct it along a specific path.
Based on the phase state of the catalyst, homogeneous and heterogeneous catalysis are distinguished.
Homogeneous catalysis – this is when the reactants and the catalyst are in the same phase (gas, solution). Typical homogeneous catalysts are acids and bases. organic amines, etc.
Heterogeneous catalysis - this is when the reactants and the catalyst are in different phases. As a rule, heterogeneous catalysts are solid substances. Because interaction in such catalysts occurs only on the surface of the substance; an important requirement for catalysts is a large surface area. Heterogeneous catalysts are characterized by high porosity, which increases the surface area of the catalyst. Thus, the total surface area of some catalysts sometimes reaches 500 square meters per 1 g of catalyst. Large area and porosity provide effective interaction with reagents. Heterogeneous catalysts include metals, zeolites - crystalline minerals of the aluminosilicate group (compounds of silicon and aluminum), and others.
Example heterogeneous catalysis – ammonia synthesis:
N 2 + 3H 2 ↔ 2NH 3
Porous iron with Al 2 O 3 and K 2 O impurities is used as a catalyst.
The catalyst itself is not consumed during the chemical reaction, but other substances accumulate on the surface of the catalyst, binding the active centers of the catalyst and blocking its operation ( catalytic poisons). They must be removed regularly by regenerating the catalyst.
In biochemical reactions, catalysts are very effective - enzymes. Enzymatic catalysts act highly efficiently and selectively, with 100% selectivity. Unfortunately, enzymes are very sensitive to increased temperature, acidity of the environment and other factors, so there are a number of limitations for the implementation of processes with enzymatic catalysis on an industrial scale.
Catalysts should not be confused with initiators process and inhibitors. For example, ultraviolet irradiation is necessary to initiate the radical reaction of methane chlorination. This is not a catalyst. Some radical reactions are initiated by peroxide radicals. These are also not catalysts.
Inhibitors- These are substances that slow down a chemical reaction. Inhibitors can be consumed and participate in a chemical reaction. In this case, inhibitors are not catalysts, on the contrary. Reverse catalysis is impossible in principle - the reaction will in any case try to follow the fastest path.
5. Contact area of reacting substances. For heterogeneous reactions, one way to increase the number of effective collisions is to increase reaction surface area . How larger area contact surface of the reacting phases, the greater the rate of the heterogeneous chemical reaction. Powdered zinc dissolves much faster in acid than granular zinc of the same mass.
In industry, to increase the contact surface area of reacting substances, they use fluidized bed method. For example, in the production of sulfuric acid by the boiling donkey method, pyrites are fired.
6. Nature of reactants . The rate of chemical reactions, other things being equal, is also influenced by chemical properties, i.e. nature of the reacting substances. Less active substances will have a higher activation barrier and react more slowly than more active substances. More active substances have a lower activation energy, and enter into chemical reactions much easier and more often.
At low activation energies (less than 40 kJ/mol), the reaction occurs very quickly and easily. A significant part of collisions between particles ends in a chemical transformation. For example, ion exchange reactions occur when normal conditions very quickly.
At high activation energies (more than 120 kJ/mol), only a small number of collisions result in a chemical transformation. The rate of such reactions is negligible. For example, nitrogen practically does not interact with oxygen under normal conditions.
At average activation energies (from 40 to 120 kJ/mol), the reaction rate will be average. Such reactions also occur under normal conditions, but not very quickly, so that they can be observed with the naked eye. Such reactions include the interaction of sodium with water, the interaction of iron with hydrochloric acid etc.
Substances that are stable under normal conditions usually have high activation energies.
Like any processes, chemical reactions occur over time and are therefore characterized by one or another speed.
The branch of chemistry that studies the rate of chemical reactions and the mechanism of their occurrence, called chemical kinetics. Chemical kinetics operates with the concepts of “phase” and “system”. Phase – it is a part of a system separated from its other parts by an interface.
Systems can be homogeneous or heterogeneous. Homogeneous systems consist of single phase. For example, air or any mixture of gases, salt solution. Heterogeneous systems consist of two or more phases. For example, liquid water– ice – steam, salt solution + sediment.
Reactions occurring in a homogeneous system, are called homogeneous. For example, N 2 (g) + 3H 2 (g) = 2NH 3 (g). They flow throughout. Reactions occurring in a heterogeneous system, are called heterogeneous. For example, C (k) + O 2 (g) = CO 2 (g). They flow at the phase interface.
Chemical reaction rate determined the amount of substance that reacts or is formed during a reaction per unit time per unit volume(for homogeneous reaction) or per unit interface(for a heterogeneous system).
The reaction rate depends on the nature of the reactants, their concentration, temperature, and the presence of catalysts.
1. The nature of the reacting substances.
Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, breaking bonds in H 2 and N 2 molecules requires high energies; such molecules are slightly reactive. Breaking bonds in highly polar molecules (HCl, H 2 O) requires less energy, and the reaction rate is much higher. Reactions between ions in electrolyte solutions occur almost instantly.
2. Concentration.
As the concentration increases, collisions of molecules of reacting substances occur more often - the reaction rate increases.
The dependence of the rate of a chemical reaction on the concentration of reactants is expressed law of mass action (LMA): at constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances.
IN general case For homogeneous reactions
nA (g) + mB (g) = pAB (g)
the reaction rate dependence is expressed by the equation:
where C A and C B are the concentrations of reactants, mol/l; k is the reaction rate constant. For a specific reaction 2NO (g) + O 2 (g) = 2NO 2 (g), the mathematical expression for the ZDM is:
υ = k∙∙
The reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the concentrations of the reactants. The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions (when substances are in different states of aggregation), the reaction rate depends only on the concentration of gases or dissolved substances, and the concentration of the solid phase is not included in the mathematical expression of EDM:
nA (k) + mB (g) = pAB (g)
For example, the rate of combustion of carbon in oxygen is proportional only to the oxygen concentration:
C (k) + O 2 (g) = CO 2 (k)
3. Temperature.
As the temperature increases, the speed of movement of molecules increases, which in turn leads to an increase in the number of collisions between them. For a reaction to take place, the colliding molecules must have a certain excess energy. The excess energy that molecules must have before their collision can lead to the formation of a new substance, called activation energy. Activation energy ( E a) are expressed in kJ/mol. Its value depends on the nature of the reacting substances, i.e. Each reaction has its own activation energy. Molecules with activation energy, called active. Increasing the temperature increases the number of active molecules, and therefore increases the rate of the chemical reaction.
The dependence of the rate of a chemical reaction on temperature is expressed van't Hoff's rule: for every 10 °C increase in temperature, the reaction rate increases by 2-4 times.
where υ 2 and υ 1 are reaction rates at temperatures t 2 and t 1,
γ is the temperature coefficient of the reaction rate, showing how many times the reaction rate increases when the temperature increases by 10 0 C.
4. Contact surface of reacting substances.
For heterogeneous systems, the larger the contact surface, the faster the reaction occurs. The surface area of solids can be increased by grinding them, and for soluble substances by dissolving them.
5. Catalysts.
Substances that participate in reactions and increase its speed, remaining unchanged at the end of the reaction, are called catalysts. The change in reaction rate under the influence of catalysts is called catalysis. There are catalysis homogeneous And heterogeneous.
TO homogeneous These include processes in which the catalyst is in the same state of aggregation as the reactants.
2SO 2 (g) + O 2 (g) 2SO 3 (g)
The action of a homogeneous catalyst is to form more or less strong intermediate active compounds, from which it is then completely regenerated.
TO heterogeneous Catalysis refers to processes in which the catalyst and reactants are in different states of aggregation, and the reaction occurs on the surface of the catalyst.
N 2(g) + 3H 2(g) 2NH 3(g)
The mechanism of action of heterogeneous catalysts is more complex than homogeneous ones. A significant role in these processes is played by the phenomena of absorption of gaseous and liquid substances on the surface of a solid substance - the phenomenon of adsorption. As a result of adsorption, the concentration of reacting substances increases, their chemical activity increases, which leads to an increase in the reaction rate.
Chemical reaction rate
Chemical reaction rate- change in the amount of one of the reacting substances per unit of time in a unit of reaction space. Is a key concept in chemical kinetics. The rate of a chemical reaction is always a positive value, therefore, if it is determined by the starting substance (the concentration of which decreases during the reaction), then the resulting value is multiplied by −1.
For example for the reaction:
the expression for speed will look like this:
. The rate of a chemical reaction at any given time is proportional to the concentrations of the reactants raised to powers equal to their stoichiometric coefficients.For elementary reactions, the exponent of the concentration of each substance is often equal to its stoichiometric coefficient; for complex reactions this rule is not observed. In addition to concentration, the following factors influence the rate of a chemical reaction:
If we consider the simplest chemical reaction A + B → C, we will notice that instant The speed of a chemical reaction is not constant.
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RATE OF CHEMICAL REACTION- the basic concept of chemical kinetics. For simple homogeneous reactions, the rate of a chemical reaction is measured by the change in the number of moles of the reacted substance (at a constant volume of the system) or by the change in the concentration of any of the starting substances... Big Encyclopedic Dictionary
RATE OF CHEMICAL REACTION- the basic concept of chemistry. kinetics, expressing the ratio of the amount of reacted substance (in moles) to the period of time during which the interaction occurred. Since the concentrations of reactants change during interaction, the rate is usually ... Big Polytechnic Encyclopedia
rate of chemical reaction- a quantity characterizing the intensity of a chemical reaction. The rate of formation of a reaction product is the amount of this product as a result of a reaction per unit time per unit volume (if the reaction is homogeneous) or per... ...
rate of chemical reaction- the basic concept of chemical kinetics. For simple homogeneous reactions, the rate of a chemical reaction is measured by the change in the number of moles of the reacted substance (at a constant volume of the system) or by the change in the concentration of any of the starting substances... Encyclopedic Dictionary
Chemical reaction rate- a quantity characterizing the intensity of a chemical reaction (See Chemical reactions). The rate of formation of a reaction product is the amount of this product resulting from a reaction per unit time per unit volume (if... ...
RATE OF CHEMICAL REACTION- basic concept of chemistry kinetics. For simple homogeneous reactions of S. x. r. measured by the change in the number of moles reacted in va (at a constant volume of the system) or by the change in the concentration of any of the initial va or reaction products (if the volume of the system ...
MECHANISM OF CHEMICAL REACTION- For complex reactions consisting of several. stages (simple or elementary reactions), a mechanism is a set of stages, as a result of which the starting materials are converted into products. Molecules can act as intermediates in these reactions... ... Natural science. Encyclopedic Dictionary
Nucleophilic substitution reactions- (eng. nucleophilic substitution reaction) substitution reactions in which the attack is carried out by a nucleophilic reagent carrying a lone electron pair. The leaving group in nucleophilic substitution reactions is called a nucleofuge. Everything... Wikipedia
Chemical reactions- transformation of some substances into others, different from the original ones chemical composition or building. Total number atoms of each given element, as well as themselves chemical elements, the constituent substances, remain in R. x. unchanged; this R. x... Great Soviet Encyclopedia
drawing speed- linear speed of metal movement at the exit from the die, m/s. On modern drawing machines, the drawing speed reaches 50–80 m/s. However, even when drawing wire, the speed, as a rule, does not exceed 30–40 m/s. At… … Encyclopedic Dictionary of Metallurgy
