Chemical reaction rate is the change in the amount of a reactant or reaction product per unit time per unit volume (for a homogeneous reaction) or per unit interface surface (for a heterogeneous reaction).
Law of mass action: dependence of the reaction rate on the concentration of reactants.
The higher the concentration, the greater the number of molecules contained in the volume. Consequently, the number of collisions increases, which leads to an increase in the speed of the process.
Kinetic equation– dependence of the reaction rate on concentration.
Solids equal 0
Molecularity of the reaction is the minimum number of molecules involved in an elementary chemical process. Based on molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.
General reaction order is the sum of the exponents of the degrees of concentration in the kinetic equation.
Reaction rate constant- coefficient of proportionality in the kinetic equation.
Van't Hoff's rule: For every 10-degree increase in temperature, the rate constant of a homogeneous elementary reaction increases two to four times
Active collision theory (TAC), there are three conditions necessary for a reaction to occur: Molecules must collide. This
important condition
, however, it is not enough, since a collision does not necessarily cause a reaction.
Molecules must have the necessary energy (activation energy). The molecules must be correctly oriented relative to each other.
Activation energy- the minimum amount of energy that is required to be supplied to the system for a reaction to occur.
Arrhenius equation
establishes the dependence of the rate constant of a chemical reaction on temperature
A - characterizes the frequency of collisions of reacting molecules
A catalyst is a substance that changes the rate of a chemical reaction, but is not consumed in the reaction and final products Excluded.
In this case, the change in the reaction rate occurs due to a change in the activation energy, and the catalyst with the reagents forms an activated complex.
Catalysis - chemical phenomenon, the essence of which is a change in speed chemical reactions under the action of certain substances (they are called catalysts).
Heterogeneous catalysis - The reactant and catalyst are in different phases - gaseous and solid.
Homogeneous catalysis - the reactants (reagents) and the catalyst are in the same phase - for example, both are gases or both are dissolved in some solvent.
Conditions chemical equilibrium
the state of chemical equilibrium is maintained as long as the reaction conditions remain unchanged: concentration, temperature and pressure.
Le Chatelier's principle: If any external influence is exerted on a system that is in equilibrium, then the equilibrium will shift towards the reaction that this action will weaken.
Equilibrium constant – This is a measure of the completeness of the reaction; the greater the value of the equilibrium constant, the higher the degree of conversion of starting substances into reaction products.
|
K r = C pr \ C out |
ΔG<0 К р >1 From pr > From out
ΔG>0 K p<1 С пр <С исх
I. Signs and conditions for chemical reactions
You already know many substances, have observed their transformations and the transformations accompanying these transformations. signs.
The most main feature A chemical reaction is the formation of new substances. But this can also be judged by some external signs of the course of reactions.
External signs of chemical reactions occurring:
- precipitation
- color change
- outgassing
- appearance of odor
- absorption and release of energy (heat, electricity, light)
It's obvious that For the occurrence and course of chemical reactions, certain conditions are necessary:
- contact starting materials(reagents)
- heating to a certain temperature
- the use of substances that accelerate chemical reactions (catalysts)
II. Thermal effect of a chemical reaction
DI. Mendeleev pointed out: the most important feature of all chemical reactions is the change in energy during their occurrence.
Each substance stores a certain amount of energy. We encounter this property of substances already at breakfast, lunch or dinner, since food allows our body to use the energy of a wide variety of chemical compounds contained in food. In the body, this energy is converted into movement, work, and is used to maintain a constant (and quite high!) body temperature.
The release or absorption of heat during chemical reactions is due to the fact that energy is spent on the process of destruction of some substances (destruction of bonds between atoms and molecules) and is released during the formation of other substances (formation of bonds between atoms and molecules).
Energy changes manifest themselves either in the release or absorption of heat.
Reactions that occur with the release of heat are called exothermic (from the Greek “exo” - out).
Reactions that occur with the absorption of energy are calledendothermic (from the Latin "endo" - inside).
Most often, energy is released or absorbed in the form of heat (less often in the form of light or mechanical energy). This heat can be measured. The measurement result is expressed in kilojoules (kJ) for one MOLE of reactant or (less commonly) for one mole of reaction product. The amount of heat released or absorbed during a chemical reaction is called thermal effect of reaction(Q).
Exothermic reaction:
Starting substances → reaction products + Q kJ
Endothermic reaction:
Starting substances → reaction products - Q kJ
The thermal effects of chemical reactions are needed for many technical calculations. Imagine yourself for a moment as a designer of a powerful rocket capable of launching spaceships and other payloads into orbit.
Let's say you know the work (in kJ) that will have to be spent to deliver a rocket with cargo from the surface of the Earth to orbit; you also know the work to overcome air resistance and other energy costs during the flight. How to calculate the required supply of hydrogen and oxygen, which (in a liquefied state) are used in this rocket as fuel and oxidizer?
Without the help of the thermal effect of the reaction of the formation of water from hydrogen and oxygen, this is difficult to do. After all, the thermal effect is the very energy that should launch the rocket into orbit. In the combustion chambers of a rocket, this heat is converted into the kinetic energy of molecules of hot gas (steam), which escapes from the nozzles and creates jet thrust.
In the chemical industry, thermal effects are needed to calculate the amount of heat to heat reactors in which endothermic reactions occur. In the energy sector, thermal energy production is calculated using the heat of combustion of fuel.
Dietitians use the thermal effects of food oxidation in the body to create proper diets not only for patients, but also for healthy people - athletes, workers in various professions. Traditionally, calculations here use not joules, but other energy units - calories (1 cal = 4.1868 J). The energy content of food is referred to any mass of food products: 1 g, 100 g, or even standard packaging of the product. For example, on the label of a jar of condensed milk you can read the following inscription: “calorie content 320 kcal/100 g.”
The area of chemistry that deals with the study of thermal effects and chemical reactions is called thermochemistry.
Equations of chemical reactions in which the thermal effect is indicated are called thermochemical.
In industry, conditions are selected so that the necessary reactions are carried out and harmful ones are slowed down.
TYPES OF CHEMICAL REACTIONS
Table 12 shows the main types of chemical reactions according to the number of particles involved in them. Drawings and equations of reactions often described in textbooks are given. decomposition, connections, substitution And exchange.
At the top of the table are presented decomposition reactions water and sodium bicarbonate. Shown is a device for passing direct electric current through water. The cathode and anode are metal plates immersed in water and connected to a source of electric current. Due to the fact that pure water practically does not conduct electric current, a small amount of soda (Na 2 CO 3) or sulfuric acid (H 2 SO 4) is added to it. When current passes through both electrodes, gas bubbles are released. In the tube where hydrogen is collected, the volume turns out to be twice as large as in the tube where oxygen is collected (its presence can be verified with the help of a smoldering splinter). The model diagram demonstrates the reaction of water decomposition. Chemical (covalent) bonds between atoms in water molecules are destroyed, and hydrogen and oxygen molecules are formed from the released atoms.
Model diagram connection reactions metallic iron and molecular sulfur S 8 shows that as a result of rearrangement of atoms during the reaction, iron sulfide is formed. In this case, the chemical bonds in the iron crystal (metallic bond) and the sulfur molecule (covalent bond) are destroyed, and the released atoms are combined to form ionic bonds to form a salt crystal.
Another reaction of the compound is the slaking of lime with CaO with water to form calcium hydroxide. At the same time, the burnt (quicklime) lime begins to heat up and loose slaked lime powder is formed.
TO substitution reactions refers to the interaction of a metal with an acid or salt. When a sufficiently active metal is immersed in a strong (but not nitric) acid, hydrogen bubbles are released. The more active metal displaces the less active metal from the solution of its salt.
Typical exchange reactions is a neutralization reaction and a reaction between solutions of two salts. The figure shows the preparation of barium sulfate precipitate. The progress of the neutralization reaction is monitored using the phenolphthalein indicator (the crimson color disappears).
Table 12
Types of chemical reactions
AIR. OXYGEN. COMBUSTION
Oxygen is the most abundant chemical element on Earth. Its content in the earth's crust and hydrosphere is presented in Table 2 "Occurrence of chemical elements." Oxygen accounts for approximately half (47%) of the mass of the lithosphere. It is the predominant chemical element of the hydrosphere. In the earth's crust, oxygen is present only in bound form (oxides, salts). The hydrosphere is also represented mainly by bound oxygen (part of the molecular oxygen is dissolved in water).
The atmosphere contains 20.9% free oxygen by volume. Air is a complex mixture of gases. Dry air consists of 99.9% nitrogen (78.1%), oxygen (20.9%) and argon (0.9%). The content of these gases in the air is almost constant. The composition of dry atmospheric air also includes carbon dioxide, neon, helium, methane, krypton, hydrogen, nitric oxide (I) (dianitrogen oxide, nitrogen hemioxide - N 2 O), ozone, sulfur dioxide, carbon monoxide, xenon, nitric oxide ( IV) (nitrogen dioxide – NO 2).
The composition of air was determined by the French chemist Antoine Laurent Lavoisier at the end of the 18th century (Table 13). He proved the oxygen content in the air and called it “life air.” To do this, he heated mercury on a stove in a glass retort, the thin part of which was placed under a glass cap placed in a water bath. The air under the hood turned out to be closed. When heated, mercury combined with oxygen, turning into red mercuric oxide. The “air” remaining in the glass bell after heating the mercury did not contain oxygen. The mouse, placed under the hood, was suffocating. Having calcined the mercury oxide, Lavoisier again isolated oxygen from it and again obtained pure mercury.
The oxygen content in the atmosphere began to increase noticeably about 2 billion years ago. As a result of the reaction photosynthesis a certain volume of carbon dioxide was absorbed and the same volume of oxygen was released. The figure in the table schematically shows the formation of oxygen during photosynthesis. During photosynthesis in the leaves of green plants containing chlorophyll, when solar energy is absorbed, water and carbon dioxide are converted into carbohydrates(sugar) and oxygen. The reaction of the formation of glucose and oxygen in green plants can be written as follows:
6H 2 O + 6CO 2 = C 6 H 12 O 6 + 6O 2.
The resulting glucose becomes insoluble in water starch, which accumulates in plants.
Table 13
Air. Oxygen. Combustion
Photosynthesis is a complex chemical process that includes several stages: the absorption and transport of solar energy, the use of sunlight energy to initiate photochemical redox reactions, the reduction of carbon dioxide and the formation of carbohydrates.
Sunlight is electromagnetic radiation of different wavelengths. In the chlorophyll molecule, when visible light (red and violet) is absorbed, electrons transition from one energy state to another. Only a small portion of solar energy (0.03%) reaching the Earth's surface is consumed for photosynthesis.
All carbon dioxide on Earth goes through the photosynthesis cycle on average in 300 years, oxygen in 2000 years, and ocean water in 2 million years. Currently, the atmosphere has a constant oxygen content. It is almost completely spent on respiration, combustion and decay of organic substances.
Oxygen is one of the most active substances. Processes involving oxygen are called oxidation reactions. These include combustion, breathing, rotting and many others. The table shows the combustion of oil, which occurs with the release of heat and light.
Combustion reactions can bring not only benefits, but also harm. Combustion can be stopped by cutting off the access of air (oxidizer) to the burning object using foam, sand or a blanket.
Foam fire extinguishers are filled with a concentrated solution of baking soda. When it comes into contact with concentrated sulfuric acid, located in a glass ampoule at the top of the fire extinguisher, a foam of carbon dioxide is formed. To activate the fire extinguisher, turn it over and hit the floor with a metal pin. In this case, the ampoule with sulfuric acid breaks and the carbon dioxide formed as a result of the reaction of the acid with sodium bicarbonate foams the liquid and throws it out of the fire extinguisher in a strong stream. Foamy liquid and carbon dioxide, enveloping a burning object, pushes away the air and extinguishes the flame.
Throughout our lives, we constantly encounter physical and chemical phenomena. Natural physical phenomena are so familiar to us that we have not attached much importance to them for a long time. Chemical reactions constantly occur in our body. The energy that is released during chemical reactions is constantly used in everyday life, in production, and when launching spaceships. Many of the materials from which the things around us are made are not taken from nature in a ready-made form, but are made using chemical reactions. In everyday life, it doesn’t make much sense for us to figure out what happened. But when studying physics and chemistry at a sufficient level, you cannot do without this knowledge. How to distinguish physical phenomena from chemical ones? Are there any signs that can help to do this?
During chemical reactions, new substances are formed from some substances, different from the original ones. By the disappearance of signs of the former and the appearance of signs of the latter, as well as by the release or absorption of energy, we conclude that a chemical reaction has occurred.
If you heat a copper plate, a black coating appears on its surface; When carbon dioxide is blown through lime water, a white precipitate forms; when wood burns, drops of water appear on the cold walls of the vessel; when magnesium burns, a white powder is obtained.
It turns out that signs of a chemical reaction are changes in color, smell, formation of sediment, and the appearance of gas.
When considering chemical reactions, it is necessary to pay attention not only to how they occur, but also to the conditions that must be met for the reaction to begin and proceed.
So, what conditions must be met for a chemical reaction to begin?
To do this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The more crushed the substances are, the larger the surface of their contact, the faster and more active the reaction between them occurs. For example, lump sugar is difficult to set on fire, but crushed and sprayed in the air it burns in a matter of seconds, forming a kind of explosion.
With the help of dissolution, we can crush a substance into tiny particles. Sometimes preliminary dissolution of the starting substances facilitates the chemical reaction between the substances.
In some cases, the contact of substances, for example, iron with moist air, is enough for a reaction to occur. But more often than not, the contact of substances alone is not enough for this: some other conditions must be met.
Thus, copper does not react with air oxygen at low temperatures of about 20˚-25˚С. To cause a reaction between copper and oxygen, it is necessary to use heat.
Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. When heating stops, the chemical reaction stops. For example, constant heat is required to decompose sugar.
In other cases, heating is required only for the reaction to occur, it gives an impetus, and then the reaction proceeds without heating. For example, we observe such heating during the combustion of magnesium, wood and other combustible substances.
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§ 1 Signs of chemical reactions
In chemical reactions, starting substances are transformed into other substances that have different properties. This can be judged by the external signs of chemical reactions: the formation of a gaseous or insoluble substance, the release or absorption of energy, a change in the color of the substance.
Heat a piece of copper wire in the flame of an alcohol lamp. We will see that the part of the wire that was in the flame has turned black.
Add 1-2 ml of acetic acid solution to baking soda powder. We observe the appearance of gas bubbles and the disappearance of soda.
Add 3-4 ml of copper chloride solution to the sodium hydroxide solution. In this case, the blue transparent solution will turn into a bright blue precipitate.
Add 1-2 drops of iodine solution to 2 ml of starch solution. And the translucent white liquid will become an opaque dark blue.
The most important sign of a chemical reaction is the formation of new substances.
But this can also be judged by some external signs of the reaction:
Precipitation;
Color change;
Gas release;
Odor appears;
The release or absorption of energy in the form of heat, electricity, or light.
For example, if you bring a lighted splinter to a mixture of hydrogen and oxygen or pass an electric discharge through this mixture, a deafening explosion will occur, and a new substance will form on the walls of the vessel - water. A reaction occurred in the formation of water molecules from hydrogen and oxygen atoms with the release of heat.
On the contrary, the decomposition of water into oxygen and hydrogen requires electrical energy.
§ 2 Conditions for the occurrence of a chemical reaction
However, certain conditions are necessary for a chemical reaction to occur.
Consider the combustion reaction of ethyl alcohol.
It occurs when alcohol interacts with oxygen in the air; for the reaction to begin, the molecules of alcohol and oxygen must come into contact. But if we open the cap of the alcohol lamp, then when the starting substances - alcohol and oxygen - come into contact, no reaction occurs. Let's bring a lit match. The alcohol on the wick of the alcohol lamp heats up and ignites, and a combustion reaction begins. The condition necessary for the reaction to occur here is initial heating.
Pour a 3% solution of hydrogen peroxide into a test tube. If we leave the test tube open, the hydrogen peroxide will slowly begin to decompose into water and oxygen. In this case, the reaction rate will be so low that we will not see any signs of gas evolution. Add some black manganese (IV) oxide powder. We observe rapid gas release. This is oxygen that was formed during the decomposition reaction of hydrogen peroxide.
A necessary condition for the start of this reaction was the addition of a substance that does not participate in the reaction, but accelerates it.
This substance is called a catalyst.
It is obvious that for the occurrence and course of chemical reactions certain conditions are necessary, namely:
Contact of starting substances (reagents),
Heating them to a certain temperature,
Application of catalysts.
§ 3 Features of chemical reactions
A characteristic feature of chemical reactions is that they are often accompanied by the absorption or release of energy.
Dmitry Ivanovich Mendeleev pointed out that the most important feature of all chemical reactions is the change in energy during their occurrence.
The release or absorption of heat during chemical reactions is due to the fact that energy is spent on the process of destruction of some substances (destruction of bonds between atoms and molecules) and is released during the formation of other substances (formation of bonds between atoms and molecules).
Energy changes manifest themselves either in the release or absorption of heat. Reactions that occur with the release of heat are called exothermic.
Reactions that occur with the absorption of heat are called endothermic.
The amount of heat released or absorbed is called the thermal effect of a reaction.
The thermal effect is usually denoted by the Latin letter Q and the corresponding sign: +Q for exothermic reactions and -Q for endothermic reactions.
The branch of chemistry that studies the thermal effects of chemical reactions is called thermochemistry. The first studies of thermochemical phenomena belonged to the scientist Nikolai Nikolaevich Beketov.
The value of the thermal effect is referred to 1 mole of the substance and expressed in kilojoules (kJ).
Most chemical processes occurring in nature, laboratory and industry are exothermic. These include all reactions of combustion, oxidation, combinations of metals with other elements, and others.
However, there are also endothermic processes, for example, the decomposition of water under the influence of electric current.
The thermal effects of chemical reactions vary widely from 4 to 500 kJ/mol. The thermal effect is most significant during combustion reactions.
Let's try to explain the essence of the ongoing transformations of substances and what happens to the atoms of the reacting substances. According to the atomic-molecular theory, all substances consist of atoms connected to each other into molecules or other particles. During the reaction process, the starting substances (reagents) are destroyed and new substances (reaction products) are formed. Thus, all reactions come down to the formation of new substances from the atoms that make up the original substances.
Therefore, the essence of a chemical reaction is the rearrangement of atoms, as a result of which new molecules (or other forms of matter) are obtained from molecules (or other particles).
List of used literature:
- NOT. Kuznetsova. Chemistry. 8th grade. Textbook for general education institutions. – M. Ventana-Graf, 2012.
