DEFINITION
Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.
Most often, chemical reactions are understood as the process of transformation of initial substances (reagents) into final substances (products).
Chemical reactions are written using chemical equations containing the formulas of the starting materials and reaction products. According to the law of conservation of mass, the number of atoms of each element in the left and right sides chemical equation equally. Usually, the formulas of the starting substances are written on the left side of the equation, and the formulas of the products are written on the right. The equality of the number of atoms of each element in the left and right parts of the equation is achieved by placing integer stoichiometric coefficients in front of the formulas of substances.
Chemical equations may contain additional information about the features of the reaction: temperature, pressure, radiation, etc., which is indicated by the corresponding symbol above (or “under”) the equals sign.
All chemical reactions can be grouped into several classes, which have certain characteristics.
Classification of chemical reactions according to the number and composition of the initial and resulting substances
According to this classification, chemical reactions are divided into reactions of combination, decomposition, substitution, exchange.
As a result compound reactions from two or more (complex or simple) substances, one new substance is formed. AT general view The equation for such a chemical reaction will look like this:
For example:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2
SO 3 + H 2 O \u003d H 2 SO 4
2Mg + O 2 \u003d 2MgO.
2FeCl 2 + Cl 2 = 2FeCl 3
Combination reactions are in most cases exothermic, i.e. flow with the release of heat. If the reaction involves simple substances, then such reactions are most often redox (ORD), i.e. occur with a change in the oxidation states of the elements. It is unambiguous to say whether the reaction of the connection between complex substances can't be treated as an OR.
Reactions in which several other new substances (complex or simple) are formed from one complex substance are classified as decomposition reactions. In general, the equation for a chemical decomposition reaction will look like this:
For example:
CaCO 3 CaO + CO 2 (1)
2H 2 O \u003d 2H 2 + O 2 (2)
CuSO 4 × 5H 2 O \u003d CuSO 4 + 5H 2 O (3)
Cu (OH) 2 \u003d CuO + H 2 O (4)
H 2 SiO 3 \u003d SiO 2 + H 2 O (5)
2SO 3 \u003d 2SO 2 + O 2 (6)
(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O (7)
Most decomposition reactions proceed with heating (1,4,5). May be decomposed by electric current(2). The decomposition of crystalline hydrates, acids, bases and salts of oxygen-containing acids (1, 3, 4, 5, 7) proceeds without changing the oxidation states of the elements, i.e. these reactions do not apply to OVR. OVR decomposition reactions include the decomposition of oxides, acids and salts, formed by elements in higher degrees oxidation (6).
Decomposition reactions are also found in organic chemistry, but under other names - cracking (8), dehydrogenation (9):
C 18 H 38 \u003d C 9 H 18 + C 9 H 20 (8)
C 4 H 10 \u003d C 4 H 6 + 2H 2 (9)
At substitution reactions a simple substance interacts with a complex one, forming a new simple and a new complex substance. In general, the equation for a chemical substitution reaction will look like this:
For example:
2Al + Fe 2 O 3 \u003d 2Fe + Al 2 O 3 (1)
Zn + 2HCl = ZnCl 2 + H 2 (2)
2KBr + Cl 2 \u003d 2KCl + Br 2 (3)
2KSlO 3 + l 2 = 2KlO 3 + Cl 2 (4)
CaCO 3 + SiO 2 \u003d CaSiO 3 + CO 2 (5)
Ca 3 (RO 4) 2 + ZSiO 2 = ZCaSiO 3 + P 2 O 5 (6)
CH 4 + Cl 2 = CH 3 Cl + Hcl (7)
Substitution reactions are mostly redox reactions (1 - 4, 7). Examples of decomposition reactions in which there is no change in oxidation states are few (5, 6).
Exchange reactions called the reactions that occur between complex substances, in which they exchange their constituent parts. Usually this term is used for reactions involving ions located in aqueous solution. In general, the equation for a chemical exchange reaction will look like this:
AB + CD = AD + CB
For example:
CuO + 2HCl \u003d CuCl 2 + H 2 O (1)
NaOH + HCl \u003d NaCl + H 2 O (2)
NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 (3)
AgNO 3 + KBr = AgBr ↓ + KNO 3 (4)
CrCl 3 + ZNaOH = Cr(OH) 3 ↓+ ZNaCl (5)
Exchange reactions are not redox. special case these exchange reactions are neutralization reactions (reactions of interaction of acids with alkalis) (2). Exchange reactions proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous substance (3), a precipitate (4, 5) or a poorly dissociating compound, most often water (1, 2).
Classification of chemical reactions according to changes in oxidation states
Depending on the change in the oxidation states of the elements that make up the reactants and reaction products, all chemical reactions are divided into redox (1, 2) and those occurring without changing the oxidation state (3, 4).
2Mg + CO 2 \u003d 2MgO + C (1)
Mg 0 - 2e \u003d Mg 2+ (reductant)
C 4+ + 4e \u003d C 0 (oxidizing agent)
FeS 2 + 8HNO 3 (conc) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O (2)
Fe 2+ -e \u003d Fe 3+ (reductant)
N 5+ + 3e \u003d N 2+ (oxidizing agent)
AgNO 3 + HCl \u003d AgCl ↓ + HNO 3 (3)
Ca(OH) 2 + H 2 SO 4 = CaSO 4 ↓ + H 2 O (4)
Classification of chemical reactions by thermal effect
Depending on whether heat (energy) is released or absorbed during the reaction, all chemical reactions are conditionally divided into exo - (1, 2) and endothermic (3), respectively. The amount of heat (energy) released or absorbed during a reaction is called the heat of the reaction. If the equation indicates the amount of released or absorbed heat, then such equations are called thermochemical.
N 2 + 3H 2 = 2NH 3 +46.2 kJ (1)
2Mg + O 2 \u003d 2MgO + 602.5 kJ (2)
N 2 + O 2 \u003d 2NO - 90.4 kJ (3)
Classification of chemical reactions according to the direction of the reaction
According to the direction of the reaction, there are reversible (chemical processes, the products of which are able to react with each other under the same conditions in which they are obtained, with the formation of starting substances) and irreversible (chemical processes, the products of which are not able to react with each other with the formation of starting substances ).
For reversible reactions, the equation in general form is usually written as follows:
A + B ↔ AB
For example:
CH 3 COOH + C 2 H 5 OH ↔ H 3 COOS 2 H 5 + H 2 O
Examples irreversible reactions the following reactions can serve:
2KSlO 3 → 2KSl + ZO 2
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Evidence of the irreversibility of the reaction can serve as the reaction products of a gaseous substance, a precipitate or a low-dissociating compound, most often water.
Classification of chemical reactions by the presence of a catalyst
From this point of view, catalytic and non-catalytic reactions are distinguished.
A catalyst is a substance that speeds up a chemical reaction. Reactions involving catalysts are called catalytic. Some reactions are generally impossible without the presence of a catalyst:
2H 2 O 2 \u003d 2H 2 O + O 2 (MnO 2 catalyst)
Often, one of the reaction products serves as a catalyst that accelerates this reaction (autocatalytic reactions):
MeO + 2HF \u003d MeF 2 + H 2 O, where Me is a metal.
Examples of problem solving
EXAMPLE 1
“Chemical reactions. Signs and conditions of their occurrence "Baryshova I.V. GOU SOSH №1980. Moscow.
Learning objectives. To form knowledge about the signs and conditions of the flow chemical reactions, on this basis, to improve the ability to distinguish physical processes from chemical ones.
Development tasks. To improve the ability to explain the dependence of the course of chemical reactions on external conditions.
Experiment. Melting of paraffin, charring of sugar, burning of a torch, interaction of copper sulfate with ammonia, interaction of copper (II) sulfate and sodium hydroxide, interaction of solutions of sodium carbonate and hydrochloric acid, interaction of sodium thiosulfate with sulfuric acid. Making models of molecules.
Planned learning outcomes. Students should be able to use examples of specific chemical reactions to indicate the conditions for their occurrence and further course, as well as signs of reactions.
Planned results of development. Students should be able to explain the relationship between conditions and the possibility of chemical reactions.
During the classes.
All changes that occur with substances in nature are called phenomena. In nature, biological, chemical and physical phenomena. But today we will compare chemical and physical phenomena
In the process of demonstrating experiments (crushing a piece of sugar and carbonizing sugar), we find out the essence of the occurring phenomena and draw up a table.
Give your examples of physical and chemical phenomena.
Chemical phenomena are called chemical reactions. Let's simulate at the atomic-molecular level the chemical reaction of water decomposition.
Making water molecules and demonstrating a chemical phenomenon (working with models).
To consolidate knowledge, we conduct a conversation with students and answer questions.
Golden foliage swirled
In the pinkish water of the pond.
Like butterflies, a light flock with
Fading flies to the star ...
(S. Yesenin).
Teacher questions:
1. What phenomenon in the life of plants is mentioned in the poems of S. Yesenin?
2. Is leaf fall related to physical or chemical phenomena?
3. What is the reason for the change in the color of tree leaves in autumn, what physical or chemical phenomena occur in this case?
4. What pigment causes the green color of plant leaves?
To develop students' skills in self-control of knowledge, we conduct a test control.
1. Chemical phenomena (as opposed to physical ones) include:
Combustion of gasoline in a car engine
Souring milk
Snow melting
Frost formation on trees.
Rainfall
Volcanic eruption
Rotting plant debris
Ice drift on the river.
Precipitation
Change in the state of aggregation
Gas evolution
Substance reduction.
burning coal
Preparation of powder from a piece of chalk
Rust formation
The glow of a tungsten filament in a light bulb.
Why do we need to know the conditions for the occurrence and conditions for the occurrence of chemical reactions?
In order to control the course of chemical reactions, sometimes a chemical reaction must be stopped, for example, in a fire, we strive to stop the combustion reaction.
We will talk about the combustion reaction in the next lesson.
The lesson ends with a reflection-evaluation stage.
Demonstration of the entertaining experience "Volcano"
During this lesson, we learned how to work with chemical glassware, create models of molecules, distinguish between chemical and physical phenomena, know the conditions for the occurrence and course of reactions, and draw conclusions.
Chemical reactions are detected by various accompanying phenomena.
Heating of a mixture of substances, glow, light flashes, explosions. It is easy to understand that all this is the result of the release of energy. Chemical reactions should be carried out with great care, primarily protecting the eyes, keeping vessels with substances at a maximum distance from the face. If the result of the reaction is not known in advance, then experiments are carried out with very small amounts of substances. Works with volatile, poisonous, strong-smelling substances must not be carried out indoors without good exhaust ventilation (draught).
EXPERIENCE 2.1. A little bromine Br 2 (a heavy red-brown liquid with a pungent odor) and aluminum shavings are placed in a thick-walled test tube (Fig. 2.1). The tube is closed with a stopper with a long tube. The reaction is slow at first, and changes are not noticeable. Gradually it accelerates and ends with a bright flash. The mixture is heated, the vapors of the unreacted excess of bromine rise high up the tube. Reaction equation:
Color change. It is quite natural that the initial substances and reaction products are characterized by different properties, including that they can have completely different colors. Color change can be demonstrated by many interesting experiments.
EXPERIENCE 2.2. Add 3-5 drops of copper sulfate solution CuSO 4 to a test tube with a dilute ammonia solution. An intense purple color appears due to the formation of a new substance:
Gas release. Gaseous substances as reaction products can be isolated from solutions, as well as from molten and solid mixtures. A lot of gas bubbles rise to the surface of the liquid. Splashing of liquid occurs, which should be avoided. Sometimes foam is formed. When performing such experiments, the vessels must not be tightly closed.
EXPERIENCE 2.3. Crystalline sodium nitrate NaNOg is placed in a test tube with a layer of about 5 mm. Carefully heated in the flame of a gas burner until melting (308 ° C). The release of gas bubbles begins, in which a smoldering splinter lights up. This proves that the escaping gas is oxygen. More than 60 cm 3 of oxygen is released from 0.5 g of NaN0 3:
Rice. 2.1. The device for carrying out the reaction of aluminum with bromine
Rice. 2.2. Device for obtaining gas
To collect gas in the form of an individual substance, various devices are used, one of which is shown in Fig. 2.2. The appearance and disappearance of odor is associated with the formation or absorption of gaseous substances. It is also a practically important sign of the occurrence of chemical reactions.
experience 2 .4. In a porcelain mortar, calcium hydroxide Ca(OH) 2 powder is ground with ammonium chloride NH 4 C1. Ammonia odor appears:
Precipitation formation. Chemical reactions in solutions often lead to the formation of substances that are insoluble in water or other liquids. Reactions in aqueous solutions are usually considered.
experience 2.5. To a solution of lead nitrate Pb (N0 3) 2 add a solution of potassium chromate K 2 Cr0 4 yellow color. An insoluble bright yellow substance lead chromate PbCrO 4 appears, settling on the bottom of the vessel. This substance is used as a pigment - yellow crown.
task 2 .1. What are the conditions for chemical reactions to take place? Use the given examples, as well as your observations and assumptions, to answer.
task 2.2. Potassium chromate solution was added to two test tubes with a solution of lead nitrate without measuring the volume. After the lead chromate had settled to the bottom, the solution in one of the tubes turned out to be yellow, and in the other it was colorless. What will be observed when the potassium chromate solution is added to the test tubes again?
Classification of chemical reactions
Consider the classification of chemical reactions according to the change in the number and composition of the starting materials and reaction products.
Connection reactions. Two starting materials can form one product. This type of transformation is called connection reaction.
A mixture of gray zinc powder and yellow sulfur powder, when sufficiently heated on an iron plate, itself begins to heat up to a light red glow. Part of the sulfur evaporates. After the reaction is completed, the product cools down and turns into a white mass of zinc sulfide ZnS:
Reactions of this type are possible both between simple and complex substances. White powder of calcium oxide, or quicklime CaO, when mixed with water, heats up, turning into a loose white mass - calcium hydroxide Ca (OH) 2, or slaked lime:
decomposition reactions. A substance under changing conditions can turn into two or more new substances. The corresponding reactions are called decomposition reactions.
The blue powder of copper hydroxide Cu (OH) 2 or the precipitate of this substance in a test tube with a solution with slight heating (70-90 ° C) turns black, turning into copper oxide CuO:
There are also very unstable substances that decompose at the slightest heating, and also exist only at low temperatures.
Lead (IV) chloride РbС1 4 is a yellow liquid at room temperature. With weak heating, it decomposes with an explosion, turning into lead (P) chloride:
substitution reactions. The atoms or groups of atoms that make up one of the reactants may replace some of the atoms in the other reactant. This interaction of substances is called substitution reaction.
experience 2.6. Iron in the form of sawdust or a small product (carnation, paper clip), lowered into hydrochloric acid(solution of hydrogen chloride HC1 in water), replaces hydrogen, forming a pale green solution of iron (I) chloride:
The liberated hydrogen can be collected using the apparatus shown in Fig. 2.2, but replacing the large flask with a test tube. When a lit match approaches the test tube, hydrogen instantly burns out - a characteristic whistling sound is heard.
With strong heating of a mixture of soda Na 2 C0 3 with white quartz sand Si0 2, the C0 2 group is replaced, which is released in the form carbon dioxide, to the group Si0 2:
After calcination, white sodium silicate remains.
Exchange reactions. Reagents can exchange atoms or groups of atoms, and this interaction is called exchange reaction.
The isolation of a precipitate from a solution very often occurs as a result of an exchange. When colorless solutions of barium chloride BaCl 2 and magnesium sulfate MgSO 4 are mixed, a white suspension (suspension) of water-insoluble barium sulfate BaSO 4 is formed, which
gradually settles to the bottom of the tube. Above the precipitate is a colorless solution of magnesium chloride:
The reaction of formation of barium sulfate is often used to analyze (test) solutions for the presence of compounds chemical element barium.
exercise 2 .3. Determine what type the reaction between lead nitrate and potassium chromate is (p. 45).
exercise 2.4. Find other examples of exchange reactions in the studied material.
transfer reactions. There are chemical reactions characterized by the fact that an atom or a group of atoms passes from the structural unit of one substance to structural unit another substance. They're called transfer reactions.
EXPERIENCE 2.7. A colorless solution of tin(II) chloride SnCl 2 is added to the water-insoluble white powder of silver chloride AgCl. The mixture turns black due to the formation of small grains of silver. Chlorine atoms move from silver chloride to tin chloride:
The transfer reaction can proceed as a real transport of particles from one substance to another. If in a special vessel desiccator(Fig. 2.3) place blue crystals of copper sulfate CuS0 4 5H 2 0 and white powder of phosphorus oxide P 2 0 5 in open cups, then after a few days the crystals turn white, losing water, and phosphorus oxide
Rice. 2.3. Desiccator with substances involved in the transfer of water
pa reacts with it, turning into metaphosphoric acid:
Water is transported as vapor through air space in a desiccator.
QUESTIONS AND EXERCISES
1. Give your own examples of reactions accompanied by characteristic phenomena.
2. What types of reactions are considered in Section 2.1?
3. Ammonium carbonate (NH 4) 2 C0 3, powder white color, has a faint smell of ammonia. On the outdoors the substance gradually disappears, decomposing into gaseous substances. Write the reaction equation.
4. What type are the following reactions:
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7th grade
The purpose of the lesson:
- study and primary awareness of chemical phenomena; clarification of signs and conditions of chemical reactions;
- improving the ability to distinguish physical processes from chemical ones; the formation of practical skills when working with chemical reagents and laboratory equipment.
Lesson objectives:
educational:
- consolidate students' knowledge of physical and chemical phenomena;
- to promote the formation of students' knowledge about the signs and conditions of chemical reactions;
developing:
- develop the ability to pose problems and solve them, establish interdisciplinary connections;
- develop practical skills to work with laboratory equipment and reagents in accordance with the rules on safety;
- continue the formation of skills to formalize the results of the educational experiment;
- develop the ability to self- and mutual control.
educational:
- to cultivate a culture of communication through work in pairs;
- to continue the formation of ideas about the positive role of chemistry to explain the ongoing processes in nature and in the human body.
Lesson type: lesson of studying and primary consolidation of knowledge.
Teaching methods: conversation, teacher's story, demonstration experiment, demonstration of video clips , use of EFU.
Equipment: PC, media projector, kits for laboratory work, workbook.
Means of education: multimedia presentation.
Lesson stages
- Teacher activity
- Student activities
- Note
During the classes
I. Organizational moment - 1 min
During the lesson you will work with electronic applications. I hope that the lesson will be interesting and useful for you.
Target: creating a positive emotional mood for learning educational material, formulation of the goals and objectives of the lesson, expected results.
II. Checking homework - 5 min
- Guys, what phenomena can occur in our lives?
- What do you think, which of these phenomena should be studied by physics, and which - by chemistry?
– Formulate definitions of physical and chemical phenomena.
Chemistry is one of the sciences that helps to understand the secrets of nature. But this must be learned. One of the necessary skills is the ability to observe phenomena, to distinguish, to determine whether a phenomenon belongs to a certain group. Pictures of physical and chemical phenomena will be presented on the screen.
physical phenomenon- this is a phenomenon in which the state of aggregation of a substance or the shape of the body formed by it changes, while the substance itself remains unchanged.
chemical phenomenon It is a phenomenon in which one substance is transformed into another.
III. Motivation to discover new knowledge - 2 min
Tell me, now you can distinguish a physical phenomenon from a chemical reaction?
- Is your knowledge enough for this?
What is the main distinguishing feature of a chemical reaction?
“There are millions of chemical reactions going on around the world every minute—in our stomachs when we eat, in a cake being baked, or in a car engine. But how to determine whether a chemical reaction is occurring or not?
In chemical processes, some substances are transformed into others, which have new properties that the original substances did not possess. These transformations are accompanied by bright and visual changes. These visible changes are called signs.
– What else do you need to know about chemical reactions in order to accurately distinguish them from physical phenomena?
So what topic are we going to study today in the lesson?
- There must be something very noticeable
– Know the signs of chemical phenomena (reactions)
– Signs of chemical reactions
Target : preparation for conscious perception of the material, stimulation of cognitive interest.
IV. Actualization of the subjective experience of students - 3 min
“Chemistry cannot be learned in any way without seeing the practice itself and without taking up chemical operations.”
M.V. Lomonosov
How do you understand these words?
- What would you like to learn and what to learn in the lesson:
- identify - (signs of chemical reactions)
- define - (differences between chemical and physical phenomena)
- get to know - (with the conditions for the course of reactions)
- conduct - (chemical experiment)
- mark - (meaning of chemical reactions)
The main sign of a chemical reaction is the formation of new substances with new properties. You know many substances, but your knowledge will be incomplete if we do not study the phenomena that occur with them. That's why, today in the lesson you should conduct various reactions and find out their main features.
- In order to conduct experiments, you need to know what substances you need to take for the reaction.
(students form the objectives of the lesson)
Target: activation personal experience each student and its use in teaching, preparation for the perception of new material, assessment by the teacher of the level of preparedness of students.
V. Discovery of new knowledge, study of new material (laboratory experiment) - 12 min
- During the study, you will act as young chemists - experimenters. We have to work with reagents, including acids and alkalis. Remember the safety rules that you need to know when working with these substances.
When analyzing a laboratory experiment, do not forget to write down your observations in a workbook and draw conclusions. You can use your eyes, hands, memory, erudition, safety rules and reagents.
- What experiments were carried out and what signs were observed?
What conditions do you think must be met for a chemical reaction to occur?
(the guys list the rules of TB)
Conditions for the occurrence of chemical reactions: contact of substances, heating
Target: formation of new knowledge about chemical reactions, signs, conditions of the reaction and classification of reactions according to the thermal effect.
VI. Consolidation of the studied - 5 min
Using electronic applications, the guys consolidate the studied material.
– Read the description of the phenomena and indicate which part of the text talks about a physical phenomenon, and which part about a chemical reaction . List the signs of a chemical reaction.
1) When a candle burns, the wax first melts and wets the wick, and then burns out, forming carbon dioxide and water vapor. Combustion is accompanied by the formation of a flame.
2) In the process of smelting iron, metal is formed from ore. Hot iron is poured into molds in which it gradually cools and hardens.
3) To prepare lemonade, citric acid crystals are poured into the water. They gradually dissolve. Then soda is added to the water with acid. As a result, gas bubbles appear.
4) When water is heated, it evaporates, and when an electric current is applied to it, water decomposes into two simple gaseous substances: oxygen and hydrogen.
5) With prolonged chewing of black bread, it is crushed. Then you will feel a sweetish taste, as the bread begins to be digested by the action of saliva.
6) To prepare the dough, first the yeast is dissolved in water. Then they are added to the rest of the dough components. As a result, gas bubbles appear, which make the dough airy.
- So, let's write down in the workbook the signs and conditions for the reactions
Wax melting- a physical phenomenon, because its state of aggregation changes, and combustion - chemical, the text refers to the formation of two new substances.
Turning ore into pig iron- the process is chemical, and the cooling and solidification of cast iron is physical.
Dissolution of citric acid crystals- a physical phenomenon, because particle size changes. After adding soda, a chemical reaction took place - as bubbles of some new substance appear.
Water evaporation- a physical phenomenon, since the aggregate state of a substance changes, and decomposition under the action of a current is chemical, since new substances have formed.
chopping bread is a physical phenomenon. Digestion by saliva is chemical.
1-2 minutes children get acquainted with the text and discuss it. The teacher guides the rest of the children to discuss the answers of each group.
Turn on the screen.
VII. Applying what you learned - 3 min
- Well done boys! You learned in practice about some of the signs of chemical reactions. But ... has all the information now sounded? What do you think?
– Quite right! We didn't say anything about where man in his Everyday life can meet with chemical reactions and observe their signs, what kind chemical reactions are happening in nature and what signs they are accompanied. Can you give examples?
- Probably not all!
- Volcanic eruptions. Great amount(release of energy in the form of heat and light)
- In summer the leaves on the trees are green, and in autumn they are yellow and red
– Cave formation, as do minerals such as limestone interact with water and carbon dioxide, causing them to dissolve?
VIII. Control and self-control - 5 min
1. Match
Phenomena: one). Dissolution of salt in water; 2). Lead melting; 3). Protein decay; 4). Blackening of a silver spoon.
Name of the event: a). physical phenomenon; b) Chemical phenomenon.
(1a; 2a; 3b; 4b).
2. Specify the signs of the following chemical reactions
The name of a chemical reaction.
Sign of chem. reactions.
- Souring of milk.
- Burning gasoline.
- The breakdown of sugar.
- Scale formation in the kettle.
- Opening a bottle of soda water
- Your time is up. Check your answers and rate yourself. .
Target: reproductive reproduction of the studied material, systematization and generalization, performance of tasks on the studied topic, primary verification of assimilation and understanding of the material .
IX. Homework - 1 min
Give proverbs that talk about phenomena. Define a group of phenomena: physical or chemical
Target: clarification, homework .
X. Summing up. Reflection - 2 min
- During the lesson, you tried to master the most important thing in the process of cognition - the ability to find the truth with the help of evidence, that is, to conduct research. Many scientists of past centuries relied in their research only on intuition and, as a result, were often mistaken. You have found your truth with the help of experiments.
Without chemical reactions, life and all the variety of substances is impossible. And the task of a person, having studied this world, is to try to use the knowledge gained for good.
Continue the sentence:
- Now I know that...
- Now I know how...
- Now I can...
- I was surprised...
- Express your opinion about the lesson by choosing an emoticon.
What else would you like to know about chemical reactions?
- In conclusion, I want to say that chemistry is an amazing science, you are just beginning to learn it. Thank you for your cooperation.
Throughout life, we are constantly confronted with 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 are constantly taking place in our body. The energy that is released during chemical reactions is constantly used in everyday life, at work, at start-up spaceships. Many of the materials from which the things around us are made are not taken in nature in finished form, but are made using chemical reactions. In everyday life, it does not make much sense for us to understand what happened. But when studying physics and chemistry at a sufficient level, this knowledge is indispensable. How to distinguish physical phenomena from chemical ones? Are there any signs that can help to do this?
In chemical reactions, new substances are formed from some substances, which are different from the original ones. By the disappearance of the signs of the first and the appearance of signs of the second, 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; blowing carbon dioxide through lime water produces a white precipitate; when wood burns, drops of water appear on the cold walls of the vessel; when magnesium is burned, a white powder is obtained.
It turns out that the signs of chemical reactions are a change in color, smell, the formation of a precipitate, the appearance of a 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 reaction to start and proceed.
So, what conditions must be met in order for a chemical reaction to begin?
For this, first of all, it is necessary to bring the reacting substances into contact (combine, mix them). The finer the substances more surface their contact, the faster and more actively the reaction between them proceeds. For example, lump sugar is difficult to ignite, but crushed and sprayed in the air, it burns out in a matter of fractions of a second, forming a kind of explosion.
With the help of dissolution, we can break the substance into tiny particles. Sometimes the preliminary dissolution of the starting substances facilitates the chemical reaction between the substances.
In some cases, the contact of substances, such as iron with moist air, is enough for a reaction to occur. But more often than not, one contact of substances is not enough for this: some other conditions must be met.
So, copper does not react with atmospheric oxygen at a low temperature of about 20˚-25˚С. To cause the reaction of the combination of copper with oxygen, it is necessary to resort to heating.
Heating affects the occurrence of chemical reactions in different ways. Some reactions require continuous heating. Heating stops - the chemical reaction stops. For example, constant heating is necessary 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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