When defining the concept speed chemical reaction it is necessary to distinguish between homogeneous and heterogeneous reactions. If a reaction occurs in a homogeneous system, for example, in a solution or in a mixture of gases, then it occurs throughout the entire volume of the system. Speed of homogeneous reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit volume of the system. Since the ratio of the number of moles of a substance to the volume in which it is distributed is the molar concentration of the substance, the rate of a homogeneous reaction can also be defined as change in concentration per unit time of any of the substances: the initial reagent or the reaction product. To ensure that the calculation result is always positive, regardless of whether it is based on a reagent or a product, the “±” sign is used in the formula:
Depending on the nature of the reaction, time can be expressed not only in seconds, as required by the SI system, but also in minutes or hours. During the reaction, the magnitude of its speed is not constant, but continuously changes: it decreases as the concentrations of the starting substances decrease. The above calculation gives the average value of the reaction rate over a certain time interval Δτ = τ 2 – τ 1. True (instantaneous) speed is defined as the limit to which the ratio Δ tends WITH/ Δτ at Δτ → 0, i.e., the true speed is equal to the derivative of the concentration with respect to time.
For a reaction whose equation contains stoichiometric coefficients that differ from unity, the rate values expressed for different substances are not the same. For example, for the reaction A + 3B = D + 2E, the consumption of substance A is one mole, the supply of substance B is three moles, and the input of substance E is two moles. That's why υ (A) = ⅓ υ (B) = υ (D) =½ υ (E) or υ (E) . = ⅔ υ (IN) .
If a reaction occurs between substances located in different phases of a heterogeneous system, then it can only occur at the interface between these phases. For example, the interaction between an acid solution and a piece of metal occurs only on the surface of the metal. Speed of heterogeneous reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit interface surface:
.
The dependence of the rate of a chemical reaction on the concentration of reactants is expressed by the law of mass action: at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reacting substances raised to powers equal to the coefficients in the formulas of these substances in the reaction equation. Then for the reaction
2A + B → products
the ratio is valid υ ~ · WITH A 2 · WITH B, and to transition to equality a proportionality coefficient is introduced k, called reaction rate constant:
υ = k· WITH A 2 · WITH B = k·[A] 2 ·[B]
(molar concentrations in formulas can be denoted by the letter WITH with the corresponding index and the formula of the substance enclosed in square brackets). The physical meaning of the reaction rate constant is the reaction rate at concentrations of all reactants equal to 1 mol/l. The dimension of the reaction rate constant depends on the number of factors on the right side of the equation and can be c –1 ; s –1 ·(l/mol); s –1 · (l 2 /mol 2), etc., that is, such that in any case, in calculations, the reaction rate is expressed in mol · l –1 · s –1.
For heterogeneous reactions, the equation of the law of mass action includes the concentrations of only those substances that are in the gas phase or in solution. The concentration of a substance in the solid phase is a constant value and is included in the rate constant, for example, for the combustion process of coal C + O 2 = CO 2, the law of mass action is written:
υ = kI·const··= k·,
Where k= kI const.
In systems where one or more substances are gases, the rate of reaction also depends on pressure. For example, when hydrogen interacts with iodine vapor H 2 + I 2 = 2HI, the rate of the chemical reaction will be determined by the expression:
υ = k··.
If you increase the pressure, for example, by 3 times, then the volume occupied by the system will decrease by the same amount, and, consequently, the concentrations of each of the reacting substances will increase by the same amount. The reaction rate in this case will increase 9 times
Dependence of reaction rate on temperature described by van't Hoff's rule: with every 10 degree increase in temperature, the reaction rate increases by 2-4 times. This means that as the temperature rises in arithmetic progression the rate of a chemical reaction increases exponentially. The base in the progression formula is temperature coefficient of reaction rateγ, showing how many times the rate of a given reaction increases (or, which is the same thing, the rate constant) with an increase in temperature by 10 degrees. Mathematically, Van't Hoff's rule is expressed by the formulas:
or 
where and are the reaction rates, respectively, at the initial t 1 and final t 2 temperatures. Van't Hoff's rule can also be expressed by the following relations:
; 
; 
; 
,
where and are, respectively, the rate and rate constant of the reaction at temperature t; and – the same values at temperature t +10n; n– number of “ten-degree” intervals ( n =(t 2 –t 1)/10), by which the temperature has changed (can be an integer or fractional number, positive or negative).
Examples of problem solving
Example 1. How will the rate of the reaction 2CO + O 2 = 2CO 2, occurring in a closed vessel, change if the pressure is doubled?
Solution:
The rate of this chemical reaction is determined by the expression:
υ start = k· [CO] 2 · [O 2 ].
An increase in pressure leads to a 2-fold increase in the concentration of both reagents. Taking this into account, we rewrite the expression of the law of mass action:
υ 1 = k· 2 · = k·2 2 [CO] 2 ·2[O 2 ] = 8 k·[CO] 2 ·[O 2 ] = 8 υ beginning
Answer: The reaction speed will increase 8 times.
Example 2. Calculate how many times the reaction rate will increase if the temperature of the system is increased from 20 °C to 100 °C, taking the value of the temperature coefficient of the reaction rate equal to 3.
Solution:
The ratio of reaction rates at two different temperatures is related to the temperature coefficient and temperature change by the formula:
Calculation:
Answer: The reaction speed will increase by 6561 times.
Example 3. When studying the homogeneous reaction A + 2B = 3D, it was found that during 8 minutes of the reaction, the amount of substance A in the reactor decreased from 5.6 mol to 4.4 mol. The volume of the reaction mass was 56 l. Calculate the average rate of a chemical reaction for the studied period of time for substances A, B and D.
Solution:
We use the formula in accordance with the definition of the concept “average rate of a chemical reaction” and substitute numerical values, obtaining the average speed for reagent A:
From the reaction equation it follows that, compared with the rate of loss of substance A, the rate of loss of substance B is twice as large, and the rate of increase in the amount of product D is three times as large. Hence:
υ (A) = ½ υ (B) =⅓ υ (D)
and then υ (B) = 2 υ (A) = 2 2.68 10 –3 = 6.36 10 –3 mol l –1 min –1 ;
υ (D) = 3 υ (A) = 3 2.68 10 –3 = 8.04 10 –3 mol l –1 min –1
Answer: υ(A) =2.68·10 –3 mol·l–1 ·min–1; υ (B) = 6.36·10–3 mol·l–1 min–1; υ (D) = 8.04·10–3 mol·l–1 min–1.
Example 4. To determine the rate constant of the homogeneous reaction A + 2B → products, two experiments were carried out at different concentrations of substance B and the reaction rate was measured.
One of the areas of physical chemistry, chemical kinetics, studies the rate of a chemical reaction and the conditions affecting its change. It also examines the mechanisms of these reactions and their thermodynamic validity. These studies are important not only for scientific purposes, but also for monitoring the interaction of components in reactors during the production of all kinds of substances.
The concept of speed in chemistry
The reaction rate is usually called a certain change in the concentrations of the compounds that entered the reaction (ΔC) per unit time (Δt). Mathematical formula The rate of a chemical reaction is as follows:
ᴠ = ±ΔC/Δt.
The reaction rate is measured in mol/l∙s if it occurs throughout the entire volume (that is, the reaction is homogeneous) and in mol/m 2 ∙s if the interaction occurs on the surface separating the phases (that is, the reaction is heterogeneous). The “-” sign in the formula refers to changes in the concentrations of the initial reactants, and the “+” sign refers to changing concentrations of the products of the same reaction.
Examples of reactions at different rates
Interactions chemical substances can be carried out at different speeds. Thus, the growth rate of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some are walking slowly biochemical reactions, such as photosynthesis and protein synthesis. Corrosion of metals occurs at a fairly low rate.
Medium speed can be used to describe reactions that require one to several hours. An example would be cooking, which involves the decomposition and transformation of compounds contained in foods. Synthesis of individual polymers requires heating the reaction mixture for a certain time.
An example of chemical reactions whose speed is quite high are neutralization reactions, the interaction of sodium bicarbonate with a solution of acetic acid, accompanied by the release carbon dioxide. You can also mention the interaction of barium nitrate with sodium sulfate, in which the release of a precipitate of insoluble barium sulfate is observed.
A large number of reactions can occur at lightning speed and are accompanied by an explosion. Classic example- interaction of potassium with water.
Factors affecting the rate of a chemical reaction
It is worth noting that the same substances can react with each other at different rates. For example, a mixture of gaseous oxygen and hydrogen may not show signs of interaction for quite a long time, but when the container is shaken or hit, the reaction becomes explosive. Therefore, chemical kinetics identifies certain factors that have the ability to influence the rate of a chemical reaction. These include:
- the nature of the interacting substances;
- concentration of reagents;
- temperature change;
- presence of a catalyst;
- pressure change (for gaseous substances);
- area of contact of substances (if we are talking about heterogeneous reactions).
Influence of the nature of the substance
So significant difference in the rates of chemical reactions is explained different meanings activation energy (Ea). It is understood as a certain excess amount of energy in comparison with its average value required by a molecule during a collision in order for a reaction to occur. It is measured in kJ/mol and values are usually in the range of 50-250.
It is generally accepted that if E a = 150 kJ/mol for any reaction, then at n. u. it practically does not leak. This energy is spent on overcoming repulsion between the molecules of substances and on weakening the bonds in the original substances. In other words, the activation energy characterizes the strength chemical bonds in substances. Based on the value of activation energy, you can preliminary estimate the rate of a chemical reaction:
- E a< 40, взаимодействие веществ происходят довольно быстро, поскольку почти все столкнове-ния частиц при-водят к их реакции;
- 40-<Е а <120, предполагается средняя реакция, поскольку эффективными будет лишь половина соударений молекул (например, реакция цинка с соляной кислотой);
- E a >120, only a very small part of particle collisions will lead to a reaction, and its speed will be low.
Effect of concentration
The dependence of the reaction rate on concentration is most accurately characterized by the law of mass action (LMA), which states:
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, the values of which are taken in powers corresponding to their stoichiometric coefficients.
This law is suitable for elementary one-stage reactions, or any stage of the interaction of substances characterized by a complex mechanism.
If you need to determine the rate of a chemical reaction, the equation of which can be conditionally written as:
αA+ bB = ϲС, then
in accordance with the above formulation of the law, the speed can be found using the equation:
V=k·[A] a ·[B] b , where
a and b are stoichiometric coefficients,
[A] and [B] are the concentrations of the starting compounds,
k is the rate constant of the reaction under consideration.
The meaning of the rate coefficient of a chemical reaction is that its value will be equal to the rate if the concentrations of the compounds are equal to units. It should be noted that for correct calculation using this formula, it is worth taking into account the state of aggregation of the reagents. The solid concentration is taken to be unity and is not included in the equation because it remains constant during the reaction. Thus, only the concentrations of liquid and gaseous substances are included in the calculations according to the ZDM. Thus, for the reaction of producing silicon dioxide from simple substances, described by the equation
Si (tv) + Ο 2(g) = SiΟ 2(tv) ,
the speed will be determined by the formula:
Typical task
How would the rate of the chemical reaction of nitrogen monoxide with oxygen change if the concentrations of the starting compounds were doubled?
Solution: This process corresponds to the reaction equation:
2ΝΟ + Ο 2 = 2ΝΟ 2.
Let us write down the expressions for the initial (ᴠ 1) and final (ᴠ 2) reaction rates:
ᴠ 1 = k·[ΝΟ] 2 ·[Ο 2 ] and
ᴠ 2 = k·(2·[ΝΟ]) 2 ·2·[Ο 2 ] = k·4[ΝΟ] 2 ·2[Ο 2 ].
ᴠ 1 /ᴠ 2 = (k·4[ΝΟ] 2 ·2[Ο 2 ]) / (k·[ΝΟ] 2 ·[Ο 2 ]).
ᴠ 2 /ᴠ 1 = 4 2/1 = 8.
Answer: increased 8 times.
Effect of temperature
The dependence of the rate of a chemical reaction on temperature was determined experimentally by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases 2-4 times with every 10 degree increase in temperature. There is a mathematical expression for this rule that looks like:
ᴠ 2 = ᴠ 1 ·γ (Τ2-Τ1)/10, where
ᴠ 1 and ᴠ 2 - corresponding speeds at temperatures Τ 1 and Τ 2;
γ - temperature coefficient, equal to 2-4.
At the same time, this rule does not explain the mechanism of the influence of temperature on the rate of a particular reaction and does not describe the entire set of patterns. It is logical to conclude that with increasing temperature, the chaotic movement of particles intensifies and this provokes a greater number of collisions. However, this does not particularly affect the efficiency of molecular collisions, since it depends mainly on the activation energy. Also, their spatial correspondence to each other plays a significant role in the efficiency of particle collisions.
The dependence of the rate of a chemical reaction on temperature, taking into account the nature of the reagents, obeys the Arrhenius equation:
k = A 0 e -Ea/RΤ, where
A o is a multiplier;
E a - activation energy.
An example of a problem using van't Hoff's law
How should the temperature be changed so that the rate of a chemical reaction, whose temperature coefficient is numerically equal to 3, increases by 27 times?
Solution. Let's use the formula
ᴠ 2 = ᴠ 1 ·γ (Τ2-Τ1)/10.
From the condition ᴠ 2 /ᴠ 1 = 27, and γ = 3. You need to find ΔΤ = Τ 2 -Τ 1.
Transforming the original formula we get:
V 2 /V 1 =γ ΔΤ/10.
We substitute the values: 27 = 3 ΔΤ/10.
From this it is clear that ΔΤ/10 = 3 and ΔΤ = 30.
Answer: the temperature should be increased by 30 degrees.
Effect of catalysts
In physical chemistry, the rate of chemical reactions is also actively studied by a section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Substances that can speed up a reaction, but are not consumed in it themselves, are called catalysts.
It has been proven that catalysts change the mechanism of the chemical interaction itself and contribute to the emergence of new transition states, which are characterized by lower energy barrier heights. That is, they help reduce the activation energy, and therefore increase the number of effective particle impacts. A catalyst cannot cause a reaction that is energetically impossible.
Thus, hydrogen peroxide can decompose to form oxygen and water:
N 2 Ο 2 = N 2 Ο + Ο 2.
But this reaction is very slow and in our first aid kits it exists unchanged quite for a long time. When opening only very old bottles of peroxide, you may notice a slight popping sound caused by the pressure of oxygen on the walls of the vessel. Adding just a few grains of magnesium oxide will provoke active gas release.
The same reaction of peroxide decomposition, but under the influence of catalase, occurs when treating wounds. Living organisms contain many different substances that increase the rate of biochemical reactions. They are usually called enzymes.
Inhibitors have the opposite effect on the course of reactions. However, this is not always a bad thing. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example, to prevent the oxidation of fats.
Substance contact area
In the event that the interaction occurs between compounds that have different states of aggregation, or between substances that are not capable of forming a homogeneous environment (immiscible liquids), then this factor also significantly affects the rate of the chemical reaction. This is due to the fact that heterogeneous reactions take place directly at the interface between the phases of interacting substances. Obviously, the wider this boundary, the more particles have the opportunity to collide, and the faster the reaction occurs.
For example, it goes much faster in the form of small chips than in the form of a log. For the same purpose, many solids are ground into a fine powder before being added to the solution. Thus, powdered chalk (calcium carbonate) acts faster with hydrochloric acid than a piece of the same mass. However, in addition to increasing the area, this technique also leads to a chaotic rupture of the crystal lattice of the substance, and therefore increases the reactivity of the particles.
Mathematically, the rate of a heterogeneous chemical reaction is found as the change in the amount of substance (Δν) occurring per unit time (Δt) per unit surface
(S): V = Δν/(S·Δt).
Effect of pressure
A change in pressure in the system has an effect only when gases take part in the reaction. An increase in pressure is accompanied by an increase in the molecules of a substance per unit volume, that is, its concentration increases proportionally. Conversely, a decrease in pressure leads to an equivalent decrease in the concentration of the reagent. In this case, the formula corresponding to the ZDM is suitable for calculating the rate of a chemical reaction.
Task. How will the rate of the reaction described by the equation increase?
2ΝΟ + Ο 2 = 2ΝΟ 2,
if the volume of a closed system is reduced by three times (T=const)?
Solution. As volume decreases, pressure increases proportionally. Let's write down the expressions for the initial (V 1) and final (V 2) reaction rates:
V 1 = k 2 [Ο 2 ] and
V 2 = k·(3·) 2 ·3·[Ο 2 ] = k·9[ΝΟ] 2 ·3[Ο 2 ].
To find how many times the new speed is greater than the initial one, you should separate the left and right sides of the expressions:
V 1 /V 2 = (k 9[ΝΟ] 2 3[Ο 2 ]) / (k [ΝΟ] 2 [Ο 2 ]).
The concentration values and rate constants are reduced, and what remains is:
V 2 /V 1 = 9 3/1 = 27.
Answer: the speed has increased 27 times.
To summarize, it should be noted that the speed of interaction of substances, or more precisely, the quantity and quality of collisions of their particles, is influenced by many factors. First of all, these are the activation energy and the geometry of the molecules, which are almost impossible to correct. As for the remaining conditions, to increase the reaction rate one should:
- increase the temperature of the reaction medium;
- increase the concentrations of the starting compounds;
- increase the pressure in the system or reduce its volume if we are talking about gases;
- bring dissimilar substances to one state of aggregation (for example, by dissolving them in water) or increase the area of their contact.
In life we encounter different chemical reactions. Some of them, like the rusting of iron, can last for several years. Others, such as fermenting sugar into alcohol, take several weeks. Firewood in a stove burns in a couple of hours, and gasoline in an engine burns in a split second.
To reduce equipment costs, chemical plants increase the speed of reactions. And some processes, for example, food spoilage and metal corrosion, need to be slowed down.
Chemical reaction rate can be expressed as change in the amount of matter (n, modulo) per unit of time (t) - compare the speed of a moving body in physics as a change in coordinates per unit of time: υ = Δx/Δt. So that the speed does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of the reacting substances (v), i.e. we get change in the amount of a substance per unit time per unit volume, or change in the concentration of one of the substances per unit time:
n 2 − n 1 Δn
υ = –––––––––– = –––––––– = Δс/Δt (1)
(t 2 − t 1) v Δt v
where c = n / v is the concentration of the substance,
Δ (read “delta”) is a generally accepted designation for a change in value.
If substances have different coefficients in the equation, the reaction rate for each of them calculated using this formula will be different. For example, 2 moles of sulfur dioxide reacted completely with 1 mole of oxygen in 10 seconds in 1 liter:
2SO2 + O2 = 2SO3
The oxygen rate will be: υ = 1: (10 1) = 0.1 mol/l s
Speed for sulfur dioxide: υ = 2: (10 1) = 0.2 mol/l s- this does not need to be memorized and said during the exam, the example is given so as not to be confused if this question arises.
The rate of heterogeneous reactions (involving solids) is often expressed per unit area of contacting surfaces:
Δn
υ = –––––– (2)
Δt S
Reactions are called heterogeneous when the reactants are in different phases:
- a solid with another solid, liquid or gas,
- two immiscible liquids
- liquid with gas.
Homogeneous reactions occur between substances in one phase:
- between well-mixed liquids,
- gases,
- substances in solutions.
Conditions affecting the rate of chemical reactions
1) The reaction speed depends on nature of reactants. Simply put, different substances react at different rates. For example, zinc reacts violently with hydrochloric acid, while iron reacts rather slowly.
2) The higher the reaction speed, the faster concentration substances. Zinc will react much longer with a highly dilute acid.
3) The reaction speed increases significantly with increasing temperature. For example, for fuel to burn, it is necessary to ignite it, i.e., increase the temperature. For many reactions, a 10°C increase in temperature is accompanied by a 2–4-fold increase in rate.
4) Speed heterogeneous reactions increases with increasing surfaces of reacting substances. Solids are usually ground for this purpose. For example, in order for iron and sulfur powders to react when heated, the iron must be in the form of fine sawdust.
Please note that in this case formula (1) is implied! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.
5) The rate of reaction depends on the presence of catalysts or inhibitors.
Catalysts- substances that accelerate chemical reactions, but are not consumed. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:
2H 2 O 2 = 2H 2 O + O 2
Manganese(IV) oxide remains at the bottom and can be reused.
Inhibitors- substances that slow down the reaction. For example, corrosion inhibitors are added to a water heating system to extend the life of pipes and batteries. In cars, corrosion inhibitors are added to brake and coolant fluid.
A few more examples.
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:
- the nature of the reactants,
- the presence of a catalyst,
- temperature (van't Hoff rule),
- pressure,
- surface area of reacting substances.
If we consider the simplest chemical reaction A + B → C, we will notice that instant The speed of a chemical reaction is not constant.
Literature
- Kubasov A. A. Chemical kinetics and catalysis.
- Prigogine I., Defey R. Chemical thermodynamics. Novosibirsk: Nauka, 1966. 510 p.
- Yablonsky G.S., Bykov V.I., Gorban A.N., Kinetic models of catalytic reactions, Novosibirsk: Nauka (Sib. Department), 1983. - 255 p.
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Topics of the Unified State Examination codifier:Speed reaction. 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.
Speed reaction – 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..
Note! 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 at elevated temperatures, 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. The greater the concentration of reacting substances (liquids, gases), the greater the number of effective collisions, and the higher the rate of the chemical reaction.
Based on a large number of 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 reacting substances in powers equal to their coefficients in the equation of the chemical reaction.
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 chemical substances, 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 ensure 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 . The larger the contact surface area 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 very quickly under normal conditions.
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.
