Sections: Chemistry

Lesson type: the acquisition of new knowledge.

Lesson type: conversation with demonstration of experiments.

Goals:

Educational- repeat the differences between chemical and physical phenomena. To form knowledge about the signs and conditions of the course chemical reactions.

Developing- to develop skills, based on knowledge of chemistry, to pose simple problems, to formulate hypotheses., to generalize.

Educational - continue the formation of the scientific outlook of students, foster a culture of communication through work in pairs "student-student", "student-teacher", as well as observation, attention, inquisitiveness, initiative.

Methods and methodological techniques: Conversation, demonstration of experiments; filling out the table, chemical dictation, independent work with cards.

Equipment and reagents... A laboratory rack with test tubes, an iron spoon for incineration of substances, a test tube with a gas outlet tube, an alcohol lamp, matches, solutions of iron chloride FeCL 3, potassium thiocyanate KNCS, copper sulfate (copper sulfate) CuSO 4, sodium hydroxide NaOH, sodium carbonate Na 2 CO 3, hydrochloric acid HCL, S.

During the classes

Teacher. We are studying the chapter "Changes in Substances" and we know that changes can be physical and chemical. What is the difference between a chemical phenomenon and a physical one?

Student. As a result of a chemical phenomenon, the composition of a substance changes, and as a result of a physical phenomenon, the composition of a substance remains unchanged, and only its state of aggregation or the shape and size of bodies changes.

Teacher. In the same experiment, chemical and physical phenomena can be observed simultaneously. If you flatten a copper wire with a hammer, you get a copper plate. The shape of the wire changes, but its composition remains the same. This is a physical phenomenon. If the copper plate is heated over high heat, the metallic sheen will disappear. The surface of the copper plate will be covered with a black coating that can be scraped off with a knife. This means that copper interacts with air and turns into a new substance. This is a chemical phenomenon. A chemical reaction takes place between the metal and the oxygen in the air.

Chemical dictation

Option 1

Exercise. Indicate what phenomena (physical or chemical) are we talking about. Explain your answer.

1. Combustion of gasoline in a car engine.

2. Preparation of powder from a piece of chalk.

3. Rotting plant residues.

4. Souring milk.

5. Rainfall

Option 2

1. Combustion of coal.

2. Snow melting.

3. Formation of rust.

4. Formation of frost on trees.

5. Glow of a tungsten filament in a light bulb.

Evaluation criteria

The maximum you can score is 10 points (1 point for a correctly indicated phenomenon and 1 point for justifying the answer).

Teacher. So, you know that all phenomena are subdivided into physical and chemical. Unlike physical phenomena, chemical phenomena, or chemical reactions, are the transformation of some substances into others. These transformations are accompanied by external signs. In order to introduce you to chemical reactions, I will conduct a series demonstration experiments... You need to identify the signs by which you can tell that a chemical reaction has occurred. Pay attention to what conditions are necessary for these chemical reactions to occur.

Demonstration experiment # 1

Teacher. In the first experiment, it is necessary to find out what happens to ferric chloride (111) when potassium thiocyanate solution KNCS is added to it.

FeCL 3 + KNCS = Fe (NCS) 3 +3 KCL

Student. The reaction is accompanied by a color change

Demonstration experiment # 2

Teacher. Pour 2 ml of copper sulfate into a test tube, add a little sodium hydroxide solution.

CuSO 4 + 2 NaOH = Cu (OH) 2 ↓ + Na 2 SO 4

Student... A blue precipitate forms Cu (OH) 2 ↓

Demonstration experiment # 3

Teacher. Add HCL acid solution to the obtained solution of Cu (OH) 2 ↓

Cu (OH) 2 ↓ + 2 HCL = CuCL 2 +2 HOH

Student... The precipitate dissolves.

Demonstration experiment # 4

Teacher. In a test tube with sodium carbonate solution, add a solution of hydrochloric acid HCL.

Na 2 CO 3 +2 HCL = 2 NaCL + H 2 O + CO 2

Student... Gas is evolved.

Demonstration experiment # 5

Teacher. Let's set fire to some sulfur in an iron spoon. Formed sulphurous gas-sulfur oxide (4) - SO 2.

S + O 2 = SO 2

Student. Sulfur ignites with a bluish flame, gives off plentiful pungent smoke, gives off heat and light.

Demonstration experiment # 6

Teacher. The decomposition reaction of potassium permangate is the reaction of obtaining and recognizing oxygen.

Student. Gas is evolved.

Teacher. This reaction proceeds with constant heating, as soon as it is stopped, the reaction also stops (the tip of the gas outlet tube of the device, where oxygen was obtained, is lowered into a test tube with water - while heating, oxygen is released, and it can be seen by the bubbles emerging from the end of the tube, if stop heating - the release of oxygen bubbles also stops).

Demonstration experiment # 7

Teacher. In a test tube with NH 4 CL ammonium chloride add a little alkali NaOH while heating. Ask one of the students to come up and smell the released ammonia. Warn the student about the pungent smell!

NH 4 CL + NaOH = NH 3 + HOH + NaCL

Student... Gas is emitted with a pungent odor.

Students write down signs of chemical reactions in a notebook.

Signs of chemical reactions

Generation (absorption) of heat or light

Color change

Gas evolution

Isolation (dissolution) of sediment

Odor change

Using the knowledge of students about chemical reactions, based on the performed demonstration experiments, we draw up a table of the conditions for the occurrence and course of chemical reactions

Teacher. You have studied the signs of chemical reactions and the conditions for their occurrence. Individual work by cards.

Which of the signs are characteristic of chemical reactions?

A) Sediment formation

B) Change of state of aggregation

C) Gas evolution

D) Grinding of substances

Final part

The teacher summarizes the lesson by analyzing the results. Gives marks.

Homework

Give examples of chemical phenomena that occur in labor activity your parents, in the household, in nature.

According to OS Gabrielyan's textbook "Chemistry -8th grade" § 26, exercise. 3.6 s 96

Throughout our lives, we are constantly confronted with physical and chemical phenomena... Natural physical phenomena are so familiar to us that we have not attached special importance to them for a long time. Chemical reactions are constantly taking place in our body. The energy that is released during chemical reactions is constantly used in everyday life, in production, when launching spaceships. Many of the materials from which the things around us are made are not taken from nature in a finished form, but are made using chemical reactions. In everyday life, it doesn't make much sense for us to understand what happened. But when studying physics and chemistry at a sufficient level, one cannot do without this knowledge. How to distinguish between physical and chemical phenomena? Are there any signs that can help you 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 a copper plate is calcined, a black coating appears on its surface; when blowing carbon dioxide a white precipitate forms through the water of lime; 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 the signs of chemical reactions are a change in color, odor, sediment formation, and the appearance of gas.

When considering chemical reactions, it is necessary to pay attention not only to how they proceed, but also to the conditions that must be met for the beginning and course of the reaction.

So, what conditions must be met in order for a chemical reaction to start?

For this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The more crushed the substances, the larger the surface of their contact, the faster and more actively the reaction between them proceeds. For example, lump sugar is difficult to ignite, but crushed and dispersed in the air, it burns out in a matter of fractions of a second, forming a kind of explosion.

By dissolving, we can shatter a substance into tiny particles. Sometimes the preliminary dissolution of the starting materials facilitates the chemical reaction between the substances.

In some cases, contact of substances, for example, iron with humid air, is enough for a reaction to occur. But more often one contact of substances is not enough for this: it is necessary to fulfill some other conditions.

Thus, copper does not react with atmospheric oxygen at a low temperature of about 20˚-25˚С. In order to cause the reaction of the copper compound with oxygen, it is necessary to resort to heating.

Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. If the heating stops, the chemical reaction stops. For example, constant heating is required to decompose sugar.

In other cases, heating is required only for the occurrence of a reaction, it gives an impetus, and then the reaction proceeds without heating. For example, we observe such heating when burning magnesium, wood and other combustible substances.

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The ability of various chemical reagents to interact is determined not only by their atomic-molecular structure, but also by the conditions for the occurrence of chemical reactions. In the practice of a chemical experiment, these conditions were intuitively realized and empirically taken into account, but theoretically they were not really investigated. 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 the 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 effect on the rate of occurrence of chemical reactions are the temperature and pressure in the reactor. Although any reaction takes time to complete, some reactions can be very fast and others extremely slow. So, the reaction of the formation of a precipitate of silver chloride when mixing solutions containing silver and chlorine ions takes several seconds. At the same time, a mixture of hydrogen and oxygen at room temperature and normal pressure can be stored for years without any reaction. But as soon as an electric spark is passed through the mixture, an explosion occurs. This example indicates 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 participating in the reaction.

In this case, the reactions proceeding 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 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 a differential equation:

where To - reaction rate constant;

R- the concentration of the substance.

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

If the 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 was found that this number is proportional to the concentration of molecules A and B. Then the second order reaction rate can be determined in differential form:

Speed ​​is highly temperature dependent. Empirical research it was found that for almost all chemical reactions the rate with an increase in temperature by 10 ° C approximately doubles. However, deviations from this empirical rule are observed, when the speed can increase only 1.5 times, and vice versa, the reaction speed is individual cases, for example, when denaturation of egg albumin (when boiling eggs), increases by 50 times. However, it should not be forgotten that these conditions can affect the nature and result of chemical reactions with a certain molecular structure. chemical compounds.

The most active in this respect are compounds of variable composition with weakened bonds between their components. It is on 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 the reactions. For comparison, you can cite the reaction of synthesis of ammonia from nitrogen and hydrogen. At the beginning, it could not be carried out either with the help of high 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 fraught with great technological difficulties, which were overcome after using the metal organic catalyst. 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 gene 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 more examples of the extremely high efficiency of catalysts in accelerating chemical reactions, special attention should be paid to the fact that the emergence and evolution of life on Earth would be impossible without the existence of enzymes, serving as essentially living catalysts.

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

Throughout our lives, we are constantly confronted with physical and chemical phenomena. Natural physical phenomena are so familiar to us that we have not attached special importance to them for a long time. Chemical reactions are constantly taking place in our body. The energy that is released during chemical reactions is constantly used in everyday life, in production, when launching spaceships. Many of the materials from which the things around us are made are not taken from nature in a finished form, but are made using chemical reactions. In everyday life, it doesn't make much sense for us to understand what happened. But when studying physics and chemistry at a sufficient level, one cannot do without this knowledge. How to distinguish between physical and chemical phenomena? Are there any signs that can help you 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 a copper plate is calcined, a black coating appears on its surface; when blowing carbon dioxide 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 the signs of chemical reactions are a change in color, odor, sediment formation, and the appearance of gas.

When considering chemical reactions, it is necessary to pay attention not only to how they proceed, but also to the conditions that must be met for the beginning and course of the reaction.

So, what conditions must be met in order for a chemical reaction to start?

For this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The more crushed the substances, the larger the surface of their contact, the faster and more actively the reaction between them proceeds. For example, lump sugar is difficult to ignite, but crushed and dispersed in the air, it burns out in a matter of fractions of a second, forming a kind of explosion.

By dissolving, we can shatter a substance into tiny particles. Sometimes the preliminary dissolution of the starting materials facilitates the chemical reaction between the substances.

In some cases, contact of substances, for example, iron with humid air, is enough for a reaction to occur. But more often one contact of substances is not enough for this: it is necessary to fulfill some other conditions.

Thus, copper does not react with atmospheric oxygen at a low temperature of about 20˚-25˚С. In order to cause the reaction of the copper compound with oxygen, it is necessary to resort to heating.

Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. If the heating stops, the chemical reaction stops. For example, constant heating is required to decompose sugar.

In other cases, heating is required only for the occurrence of a reaction, it gives an impetus, and then the reaction proceeds without heating. For example, we observe such heating when burning magnesium, wood and other combustible substances.

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I. Signs and conditions of chemical reactions

You already know many substances, you have observed their transformations and the accompanying transformations. signs.

The most the main feature chemical reaction is the formation of new substances. But this can be judged by some outward signs the course of reactions.

External signs of chemical reactions:

• precipitation
• color change
• gas evolution
• odor appearance
• absorption and release of energy (heat, electricity, light)

It's obvious that for the occurrence and course of chemical reactions, some conditions are necessary:

• contact of starting materials (reagents)
• heating to a certain temperature
• the use of substances that accelerate a chemical reaction (catalysts)

II. Heat effect of a chemical reaction

DI. Mendeleev pointed out: the most important feature of all chemical reactions is the change in energy in the course of their course.

Each substance has a certain amount of energy stored. We are faced with this property of substances already at breakfast, lunch or dinner, since food products allow 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, goes to maintain a constant (and quite high!) Body temperature.

The release or absorption of heat in the process of 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 are manifested either in the release or in the absorption of heat.

Reactions proceeding with the release of heat are called exothermic (from the Greek "exo" - outward).

Reactions proceeding with the absorption of energy are calledendothermic (from the Latin "endo" - inward).

Most often, energy is released or absorbed in the form of heat (less often - in the form of light or mechanical energy). This warmth can be measured. The measurement result is expressed in kilojoules (kJ) for one MOL of the reagent or (less commonly) for the mole of the reaction product. The amount of heat released or absorbed in a chemical reaction is called thermal effect of reaction(Q).

Exothermic reaction:

Initial substances → reaction products + Q kJ

Endothermic reaction:

Initial 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 into orbit spaceships and other payloads.

Suppose you know the work (in kJ) that will have to be spent to deliver a rocket with a load from the Earth's surface to orbit; you also know the work to overcome air resistance and other energy costs during flight. How to calculate necessary stock hydrogen and oxygen, which (in a liquefied state) are used in this rocket as a fuel and oxidizer?

It is difficult to do this without the help of the thermal effect of the reaction of water formation from hydrogen and oxygen. After all, the thermal effect is the very energy that should put the rocket into orbit. In the combustion chambers of the rocket, this heat is converted into kinetic energy of the molecules of the incandescent gas (vapor), which escapes from the nozzles and creates jet thrust.

In the chemical industry, thermal effects are needed to calculate the amount of heat for heating reactors in which endothermic reactions take place. In power engineering, heat production is calculated using the heats of combustion of fuel.

Nutritionists use the thermal effects of food oxidation in the body to formulate correct diets not only for sick people, but also for healthy people - athletes, workers of various professions. By tradition, for calculations, they use not joules, but other energy units - calories (1 cal = 4.1868 J). The energy content of food is related to any mass of food products: to 1 g, to 100 g, or even to the standard packaging of the product. For example, on the label of a can of condensed milk you can read the following inscription: "calorie content 320 kcal / 100 g".

The field of chemistry, which deals with the study of thermal effects, chemical reactions, is called thermochemistry.

The equations of chemical reactions in which the thermal effect is indicated are called thermochemical.