Signs and conditions for the flow of chemical reactions. Signs and conditions for chemical reactions Select the conditions for chemical reactions

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 external signs chemical reactions: formation of a gaseous or insoluble substance, release or absorption of energy, change in 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 solution acetic acid 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 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 All chemical reactions involve changes in energy as they occur.

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:

1. NOT. Kuznetsova. Chemistry. 8th grade. Tutorial for educational institutions. – M. Ventana-Graf, 2012.

1. Indicate whether the phenomena depicted in the pictures are physical or chemical.

2. Match.

Examples of chemical reactions:
I. interaction of marble with hydrochloric acid;
II. interaction of iron with sulfur;
III. hydrogen peroxide decomposition;
IV. interaction of carbon dioxide with lime water.

Conditions for chemical reactions to occur:
a) contact of substances;
b) heating;
c) use of a catalyst.

Answer: I - a; II - a, b; III - in; IV - a.

3. Fill out diagram 2.

4. "Crossword - in reverse." All the words in the crossword have already been entered. Define each word as precisely as possible.

"Key word" is the first chemical reaction with which man became acquainted.

1. One of the four states of matter.
2. Formation of a solid in solution during a chemical reaction.
3. The position of two or more bodies, objects, substances.
4. A portable or mobile device for extinguishing fires.
5. The process is characterized by an increase in temperature.
6. A chemical substance that accelerates a reaction, but is not part of the reaction products.
7. The impact of objects on each other.

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 reactions occurring.

External signs of chemical reactions occurring:

• precipitation
• color change
• gas evolution
• 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 of starting substances (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 put 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.

The ability of various chemical reagents to interact is determined not only by their atomic-molecular structure, but also by the conditions under which chemical reactions occur. In the practice of chemical experimentation, these conditions were intuitively recognized and empirically taken into account, but were not truly studied theoretically. Meanwhile, the yield of the resulting reaction product largely depends on them.

These conditions include, first of all, thermodynamic conditions that characterize the dependence of reactions on temperature, pressure and some other factors. To an even greater extent, the nature and especially the rate of reactions depend on kinetic conditions, which are determined by the presence of catalysts and other additives to the reagents, as well as the influence of solvents, reactor walls and other conditions.

Thermodynamic factors that have a significant impact on the rate of chemical reactions are temperature and pressure in the reactor. Although any reaction requires certain time, but some reactions can occur very quickly and others extremely slowly. Thus, the reaction of the formation of a silver chloride precipitate when mixing solutions containing silver and chlorine ions takes several seconds. At the same time, a mixture of hydrogen and oxygen can be stored at room temperature and normal pressure for years, and no reaction will occur. But as soon as an electric spark is passed through the mixture, an explosion occurs. This example demonstrates that the rate of chemical reactions is influenced by many different conditions: exposure to electricity, ultraviolet and x-rays, the concentration of reagents, their stirring, and even the presence of other substances not involved in the reaction.

In this case, reactions occurring in a homogeneous system consisting of one phase proceed, as a rule, faster than in a heterogeneous system consisting of several phases. A typical example of a homogeneous reaction is the reaction of the natural decay of a radioactive substance, the rate of which is proportional to the concentration of the substance R. This speed can be expressed by the differential equation:

Where To - reaction rate constant;

R- concentration of the substance.

Such a reaction is called a first-order reaction, and the time required for the original amount of substance to be reduced by half is called half-life.

If a reaction occurs as a result of the interaction of two molecules Aw B, then its speed will be proportional to the number of their collisions. It has been established that this number is proportional to the concentration of molecules A and B. Then we can determine the rate of a second-order reaction in differential form:

The speed depends significantly on temperature. Empirical studies have established that for almost all chemical reactions, the rate approximately doubles with an increase in temperature by 10 °C. However, deviations from this empirical rule are also observed, when the speed can increase only 1.5 times, and conversely, the reaction rate in some cases, for example, during the denaturation of egg albumin (when boiling eggs), increases 50 times. It should not be forgotten, however, that these conditions can affect the nature and result of chemical reactions with a certain structure of the molecules of chemical compounds.

The most active in this regard are compounds of variable composition with weakened bonds between their components. It is precisely at them that the action of various catalysts is primarily directed, which significantly accelerate the course of chemical reactions. Thermodynamic factors such as temperature and pressure have less influence on reactions. For comparison, we can cite the reaction of the synthesis of ammonia from nitrogen and hydrogen. At first it could not be accomplished either with the help of great pressure or high temperature, and only the use of specially treated iron as a catalyst led to success for the first time. However, this reaction is associated with great technological difficulties, which were overcome when a metal-organic catalyst was used. In its presence, ammonia synthesis occurs at a normal temperature of 18 ° C and normal atmospheric pressure, which opens up great prospects not only for the production of fertilizers, but in the future such a change in the genetic structure of cereals (rye and wheat) when they do not need nitrogen fertilizers. Even greater opportunities and prospects arise with the use of catalysts in other branches of the chemical industry, especially in “fine” and “heavy” organic synthesis.

Without giving any more examples about the extremely high efficiency catalysts in accelerating chemical reactions should be reversed special attention that the emergence and evolution of life on Earth would have been impossible without the existence enzymes, serving essentially as living catalysts.

Despite the fact that enzymes have common properties inherent in all catalysts, they are nevertheless not identical to the latter, since they function within living systems. Therefore, all attempts to use the experience of living nature to accelerate chemical processes in the inorganic world encounter serious limitations. We can only talk about modeling some enzyme functions and using these models for theoretical analysis activities of living systems, and also partially for the practical use of isolated enzymes to accelerate certain chemical reactions.