Speed reaction 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. Character plays a big role chemical bonds and the structure of reagent molecules. 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 reacting substances.
AA + bB + . . . ® . . .
- [A] a [B] b . . .
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:
- e -Ea/RT
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), how more surface contact, 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").
Some chemical reactions occur almost instantly (explosion of an oxygen-hydrogen mixture, ion exchange reactions in an aqueous solution), others quickly (combustion of substances, interaction of zinc with acid), and others slowly (rusting of iron, rotting of organic residues). Reactions are known to be so slow that a person simply cannot notice them. For example, the transformation of granite into sand and clay occurs over thousands of years.
In other words, chemical reactions can occur with different speed.
But what is it speed reaction? What is the exact definition of this quantity and, most importantly, its mathematical expression?
The rate of a reaction is the change in the amount of a substance per unit of time in one unit of volume. Mathematically, this expression is written as:
Where n 1 Andn 2 – amount of substance (mol) at time t 1 and t 2, respectively, in a system of volume V.
Which plus or minus sign (±) will appear in front of the speed expression depends on whether we are looking at a change in the amount of a substance - a product or a reactant.
Obviously, during the reaction, reagents are consumed, that is, their quantity decreases, therefore, for reagents, the expression (n 2 - n 1) always has a value less than zero. Since speed cannot be a negative value, in this case you need to put a minus sign in front of the expression.
If we look at the change in the amount of the product, and not the reactant, then the minus sign is not required before the expression for calculating the speed, since the expression (n 2 - n 1) in this case is always positive, because the amount of product as a result of the reaction can only increase.
Substance quantity ratio n to the volume in which this amount of substance is located is called molar concentration WITH:
Thus, using the concept of molar concentration and its mathematical expression, we can write another option for determining the reaction rate:
The reaction rate is the change in the molar concentration of a substance as a result of a chemical reaction occurring in one unit of time:
Factors affecting reaction speed
It is often extremely important to know what determines the speed of a particular reaction and how to influence it. For example, the oil refining industry literally fights for every additional half a percent of product per unit of time. After all, considering great amount of refined oil, even half a percent results in a large financial annual profit. In some cases, it is extremely important to slow down some reaction, in particular the corrosion of metals.
So what does the reaction rate depend on? It depends, oddly enough, on many different parameters.
In order to understand this issue, first of all, let's imagine what happens as a result of a chemical reaction, for example:
A + B → C + D
The equation written above reflects the process in which molecules of substances A and B, colliding with each other, form molecules of substances C and D.
That is, undoubtedly, in order for the reaction to take place, at a minimum, a collision of molecules is necessary starting materials. Obviously, if we increase the number of molecules per unit volume, the number of collisions will increase in the same way that the frequency of your collisions with passengers on a crowded bus will increase compared to a half-empty one.
In other words, the reaction rate increases with increasing concentration of reactants.
In the case where one or more of the reactants are gases, the reaction rate increases with increasing pressure, since the pressure of a gas is always directly proportional to the concentration of its constituent molecules.
However, the collision of particles is a necessary, but not at all sufficient condition for the reaction to occur. The fact is that, according to calculations, the number of collisions of molecules of reacting substances at their reasonable concentration is so great that all reactions must occur in an instant. However, in practice this does not happen. What's the matter?
The fact is that not every collision of reactant molecules will necessarily be effective. Many collisions are elastic—the molecules bounce off each other like balls. In order for a reaction to take place, the molecules must have sufficient kinetic energy. The minimum energy that the molecules of the reacting substances must have in order for the reaction to take place is called the activation energy and is denoted as E a. In a system consisting of large quantity molecules, there is a distribution of molecules by energy, some of them have low energy, some have high and medium energy. Of all these molecules, only a small fraction of the molecules have an energy exceeding the activation energy.
As you know from a physics course, temperature is actually a measure of the kinetic energy of the particles that make up a substance. That is, the faster the particles that make up a substance move, the higher its temperature. Thus, obviously, by increasing the temperature we essentially increase the kinetic energy of molecules, as a result of which the proportion of molecules with energy exceeding E a increases and their collision will lead to a chemical reaction.
Fact positive influence Temperature on the rate of reaction was empirically established by the Dutch chemist Van't Hoff back in the 19th century. Based on his research, he formulated a rule that still bears his name, and it goes like this:
The speed of any chemical reaction increases 2-4 times with an increase in temperature by 10 degrees.
The mathematical representation of this rule is written as:
Where V 2 And V 1 is the speed at temperatures t 2 and t 1, respectively, and γ is the temperature coefficient of the reaction, the value of which most often lies in the range from 2 to 4.
Often the speed of many reactions can be increased using catalysts.
Catalysts are substances that speed up the course of a reaction without being consumed.
But how do catalysts increase the rate of a reaction?
Let's remember about the activation energy E a. Molecules with an energy lower than the activation energy in the absence of a catalyst cannot interact with each other. Catalysts change the path along which a reaction proceeds, just as an experienced guide will route an expedition not directly through a mountain, but with the help of detour paths, as a result of which even those companions who did not have enough energy to climb a mountain will be able to move to another her side.
Despite the fact that the catalyst is not consumed during the reaction, it nevertheless takes an active part in it, forming intermediate compounds with the reagents, but by the end of the reaction it returns to its original state.
In addition to the above factors affecting the reaction rate, if there is an interface between the reacting substances (heterogeneous reaction), the reaction rate will also depend on the contact area of the reactants. For example, imagine a granule of aluminum metal that is dropped into a test tube containing an aqueous solution of hydrochloric acid. Aluminum is an active metal that can react with non-oxidizing acids. WITH hydrochloric acid The reaction equation is as follows:
2Al + 6HCl → 2AlCl 3 + 3H 2
Aluminum is a solid, which means that the reaction with hydrochloric acid occurs only on its surface. Obviously, if we increase the surface area by first rolling the aluminum granule into foil, we will thereby provide a larger number of aluminum atoms available for reaction with the acid. As a result, the reaction rate will increase. Similarly, increasing the surface solid can be achieved by grinding it into powder.
Also, the rate of a heterogeneous reaction in which a solid reacts with a gaseous or liquid substance is often positively influenced by stirring, which is due to the fact that as a result of stirring, the accumulated molecules of reaction products are removed from the reaction zone and a new portion of reactant molecules is “brought in.”
Lastly, it should also be noted the enormous influence on the rate of reaction and the nature of the reagents. For example, the lower an alkali metal is in the periodic table, the faster it reacts with water, fluorine, among all halogens, reacts most quickly with hydrogen gas, etc.
Summarizing all of the above, the speed of the reaction depends on the following factors:
1) concentration of reagents: the higher, the more speed reactions
2) temperature: with increasing temperature, the rate of any reaction increases
3) contact area of the reactants: the larger the contact area of the reagents, the higher the reaction rate
4) stirring, if a reaction occurs between a solid and a liquid or gas, stirring can speed it up.
A chemical reaction is the transformation of one substance into another.
Whatever type of chemical reactions they are carried out at different rates. For example, geochemical transformations in the bowels of the Earth (formation of crystalline hydrates, hydrolysis of salts, synthesis or decomposition of minerals) take place for thousands, millions of years. And reactions such as the combustion of gunpowder, hydrogen, saltpeter, and berthollet salt occur within fractions of seconds.
The rate of a chemical reaction refers to the change in the amounts of reacting substances (or reaction products) per unit time. The most commonly used concept average speed reactions (Δc p) in the time interval.
v av = ± ∆C/∆t
For products ∆С > 0, for starting substances -∆С< 0. Наиболее употребляемая единица измерения - моль на литр в секунду (моль/л*с).
The rate of each chemical reaction depends on many factors: the nature of the reacting substances, the concentration of the reacting substances, changes in the reaction temperature, the degree of grinding of the reacting substances, changes in pressure, and the introduction of a catalyst into the reaction medium.
Nature of reactants significantly affects the rate of a chemical reaction. As an example, consider the interaction of some metals with a permanent component - water. Let's define the metals: Na, Ca, Al, Au. Sodium reacts with water at ordinary temperatures very violently, releasing a large amount of heat.
2Na + 2H 2 O = 2NaOH + H 2 + Q;
Calcium reacts less vigorously with water at ordinary temperatures:
Ca + 2H 2 O = Ca(OH) 2 + H 2 + Q;
Aluminum reacts with water already at elevated temperatures:
2Al + 6H 2 O = 2Al(OH)z + ZH 2 - Q;
And gold is one of the inactive metals; it does not react with water either at normal or at elevated temperatures.
The rate of a chemical reaction is directly dependent on concentrations of reactants . So, for the reaction:
C 2 H 4 + 3O 2 = 2CO 2 + 2H 2 O;
The expression for the reaction rate is:
v = k**[O 2 ] 3 ;
Where k is the rate constant of a chemical reaction, numerically equal to the rate of this reaction, provided that the concentrations of the reacting components are equal to 1 g/mol; the values of [C 2 H 4 ] and [O 2 ] 3 correspond to the concentrations of the reacting substances raised to the power of their stoichiometric coefficients. The greater the concentration of [C 2 H 4 ] or [O 2 ], the more collisions of molecules of these substances per unit time, and therefore the greater the rate of chemical reaction.
The rates of chemical reactions, as a rule, are also directly dependent on reaction temperature . Naturally, with increasing temperature, the kinetic energy of molecules increases, which also leads to greater collisions of molecules per unit time. Numerous experiments have shown that with every 10 degree change in temperature, the reaction rate changes by 2-4 times (van't Hoff rule):
where V T 2 is the rate of chemical reaction at T 2; V ti is the rate of chemical reaction at T 1 ; g is the temperature coefficient of the reaction rate.
Influence degree of grinding of substances
the speed of reaction is also directly dependent. The finer the particles of reacting substances are, the more to a greater extent They come into contact with each other per unit time, the greater the speed of the chemical reaction. Therefore, as a rule, reactions between gaseous substances or solutions proceed faster than in the solid state.
Changes in pressure affect the rate of reaction between substances in a gaseous state. Being in a closed volume at a constant temperature, the reaction proceeds at a rate of V 1. If in this system we increase the pressure (hence, reduce the volume), the concentrations of the reacting substances will increase, the collision of their molecules per unit time will increase, the reaction rate will increase to V 2 (v 2 > v 1).
Catalysts are substances that change the rate of a chemical reaction, but remain unchanged after the chemical reaction ends. The influence of catalysts on the rate of a reaction is called catalysis. Catalysts can both accelerate a chemical dynamic process and slow it down. When the interacting substances and the catalyst are in the same state of aggregation, we speak of homogeneous catalysis, and with heterogeneous catalysis, the reactants and the catalyst are in different states of aggregation. The catalyst and reagents form an intermediate complex. For example, for a reaction:
The catalyst (K) forms a complex with A or B - AK, VK, which releases K upon interaction with a free particle A or B:
AK + B = AB + K
VK + A = VA + K;
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The rate of a chemical reaction depends on many factors, including the nature of the reactants, the concentration of the reactants, temperature, and the presence of catalysts. Let's consider these factors.
1). Nature of reactants. If there is an interaction between substances with an ionic bond, then the reaction proceeds faster than between substances with a covalent bond.
2.) Concentration of reactants. For a chemical reaction to take place, the molecules of the reacting substances must collide. That is, the molecules must come so close to each other that the atoms of one particle experience the action of the electric fields of the other. Only in this case will electron transitions and corresponding rearrangements of atoms be possible, as a result of which molecules of new substances are formed. Thus, the rate of chemical reactions is proportional to the number of collisions that occur between molecules, and the number of collisions, in turn, is proportional to the concentration of the reactants. Based on experimental material, the Norwegian scientists Guldberg and Waage and, independently of them, the Russian scientist Beketov in 1867 formulated the basic law of chemical kinetics - law of mass action(ZDM): at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances to the power of their stoichiometric coefficients. For the general case:
the law of mass action has the form:
The recording of the law of mass action for a given reaction is called basic kinetic equation of the reaction. In the basic kinetic equation, k is the reaction rate constant, which depends on the nature of the reactants and temperature.
Most chemical reactions are reversible. During such reactions, their products, as they accumulate, react with each other to form the starting substances:
Forward reaction rate:
Feedback speed:
At the moment of equilibrium:
Hence the law of mass action in a state of equilibrium takes the form:
,
where K is the reaction equilibrium constant.
3) Effect of temperature on reaction rate. The rate of chemical reactions, as a rule, increases when the temperature is exceeded. Let's consider this using the example of the interaction of hydrogen with oxygen.
2H 2 + O 2 = 2H 2 O
At 20 0 C, the reaction rate is practically zero and it would take 54 billion years for the interaction to progress by 15%. At 500 0 C, it will take 50 minutes to form water, and at 700 0 C the reaction occurs instantly.
The dependence of the reaction rate on temperature is expressed van't Hoff's rule: with an increase in temperature by 10 o, the reaction rate increases by 2–4 times. Van't Hoff's rule is written:
4) Effect of catalysts. The rate of chemical reactions can be controlled using catalysts– substances that change the rate of a reaction and remain after the reaction in unchanged quantities. Changing the rate of a reaction in the presence of a catalyst is called catalysis. Distinguish positive(reaction speed increases) and negative(reaction rate decreases) catalysis. Sometimes a catalyst is formed during a reaction; such processes are called autocatalytic. There are homogeneous and heterogeneous catalysis.
At homogeneous In catalysis, the catalyst and reactants are in the same phase. For example:
At heterogeneous In catalysis, the catalyst and reactants are in different phases. For example:
Heterogeneous catalysis is associated with enzymatic processes. All chemical processes processes occurring in living organisms are catalyzed by enzymes, which are proteins with certain specialized functions. In solutions in which enzymatic processes take place, there is no typical heterogeneous environment, due to the absence of a clearly defined phase interface. Such processes are referred to as microheterogeneous catalysis.
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, 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.