Conditions for chemical reactions. Chemical processes and conditions for their occurrence. Which of the signs are characteristic of chemical reactions
Sections: Chemistry
Lesson type: acquisition of new knowledge.
Type of lesson: a conversation with a demonstration of experiments.
Goals:
Educational- to repeat the differences between chemical phenomena and physical ones. To form knowledge about the signs and conditions of chemical reactions.
Educational- develop skills based on knowledge of chemistry, pose simple problems, formulate hypotheses., generalize.
Educational - continue the formation of the scientific outlook of students, cultivate 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 in the table, chemical dictation, independent work with cards.
Equipment and reagents. Laboratory stand with test tubes, an iron spoon for burning 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, powder S.
During the classes
Teacher. We study the chapter "Changes that occur with 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, one can simultaneously observe chemical and physical phenomena. 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 a copper plate is heated over high heat, the metallic luster 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 oxygen in the air.
Chemical dictation
Option 1
Exercise. Indicate what phenomena (physical or chemical) in question. Explain your answer.
1. Combustion of gasoline in a car engine.
2. Preparation of powder from a piece of chalk.
3. Decay of plant residues.
4. Souring of milk.
5. Rainfall
Option 2
1. Burning coal.
2. Melting snow.
3. Rust formation.
4. Formation of frost on trees.
5. The glow of a tungsten filament in a light bulb.
Evaluation criteria
You can score a maximum of 10 points (1 point for a correctly indicated phenomenon and 1 point for justifying the answer).
Teacher. So, you know that all phenomena are divided into physical and chemical. Unlike physical phenomena, in chemical phenomena, or chemical reactions, one substance is transformed into another. These transformations are accompanied by external signs. In order to introduce you to chemical reactions, I will conduct a series of demonstration experiments. You need to identify 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.
Demo Experience #1
Teacher. In the first experiment, you need to find out what happens to iron chloride (111) when a solution of potassium thiocyanate KNCS is added to it.
FeCL 3 + KNCS = Fe(NCS) 3 +3 KCL
Student. The reaction is accompanied by a color change
Demo Experience #2
Teacher. Pour 2 ml of copper sulfate into a test tube, add a little sodium hydroxide solution.
CuSO 4 + 2 NaOH \u003d Cu (OH) 2 ↓ + Na 2 SO 4
Student. A blue precipitate Cu (OH) 2↓
Demo Experience #3
Teacher. To the resulting solution of Cu (OH) 2↓ add a solution of acid HCL
Cu (OH) 2↓ + 2 HCL \u003d CuCL 2 +2 HOH
Student. The precipitate dissolves.
Demo Experience #4
Teacher. Pour a solution of hydrochloric acid HCL into a test tube with a solution of sodium carbonate.
Na 2 CO 3 +2 HCL \u003d 2 NaCL + H 2 O + CO 2
Student. Gas is released.
Demo Experience #5
Teacher. Let's set fire to a little sulfur in an iron spoon. Sulfur dioxide is formed - sulfur oxide (4) - SO 2.
S + O 2 \u003d SO 2
Student. Sulfur ignites with a bluish flame, gives off abundant acrid smoke, heat and light are released.
Demonstration experience No. 6
Teacher. The decomposition reaction of potassium permangate is the reaction of obtaining and recognizing oxygen.
Student. Gas is released.
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 received, is lowered into a test tube with water - while heating, oxygen is released, and it can be seen by the bubbles emerging from the tip of the tube, if stop heating - the release of oxygen bubbles also stops).
Demonstration experience No. 7
Teacher. In a test tube with NH 4 CL ammonium chloride, add a little NaOH while heating. Ask one of the students to come up and smell the ammonia that is released. Warn the student about the strong smell!
NH 4 CL + NaOH \u003d NH 3 + HOH + NaCL
Student. A gas with a pungent odor is released.
Students write down signs of chemical reactions in a notebook.
Signs of chemical reactions
Emission (absorption) of heat or light
Color change
Gas evolution
Isolation (dissolution) of the precipitate
Smell change
Using the knowledge of students about chemical reactions, on the basis of the demonstration experiments done, we compile a table of the conditions for the occurrence and occurrence of chemical reactions
Teacher. You have studied the signs of chemical reactions and the conditions for their occurrence. Individual work on cards.
Which of the signs are characteristic of chemical reactions?
A) Precipitation
B) Change in the state of aggregation
B) Gas evolution
D) Grinding of substances
Final part
The teacher sums up the lesson by analyzing the results. Gives grades.
Homework
Give examples of chemical phenomena that occur in the work of your parents, in the household, in nature.
According to O.S. Gabrielyan's textbook "Chemistry - Grade 8" § 26, exercise. 3.6 p.96
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, in production, and when launching spacecraft. 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 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 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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In industry, such 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 decomposition reactions water and sodium bicarbonate. A device for passing a direct electric current through water is shown. The cathode and anode are metal plates immersed in water and connected to an electric current source. Due to the fact that pure water practically does not conduct electricity, 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 is twice as large as in the tube where oxygen is collected (you can verify its presence with the help of a smoldering splinter). The model scheme 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 scheme compound reactions metallic iron and molecular sulfur S 8 shows that as a result of the 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 combine to form ionic bonds into a salt crystal.
Another reaction of the compound is the slaking of lime CaO with water to form calcium hydroxide. At the same time, burnt (quicklime) lime begins to warm up and a loose powder of slaked lime 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 its salt solution.
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 course of the neutralization reaction is monitored using the phenolphthalein indicator (crimson color disappears).
Table 12
Types of chemical reactions
AIR. OXYGEN. COMBUSTION
Oxygen is the most common chemical element on Earth. Its content in the earth's crust and hydrosphere is presented in Table 2 "The prevalence of chemical elements". Oxygen accounts for approximately half (47%) of the mass of the lithosphere. It is the predominant chemical element in 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 of free oxygen contains 20.9% by volume. Air is a complex mixture of gases. Dry air is 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) (diazot oxide, nitrogen hemioxide - N 2 O), ozone, sulfur dioxide, carbon monoxide, xenon, nitrogen oxide ( IV) (nitrogen dioxide - NO 2).
The composition of the air was determined by the French chemist Antoine Laurent Lavoisier at the end of the 18th century (Table 13). He proved the content of oxygen in the air, and called it "vital air". To do this, he heated mercury on a furnace in a glass retort, the thin part of which was placed under a glass cap, lowered into a water bath. The air under the cap turned out to be closed. When heated, mercury combined with oxygen, turning into red mercury oxide. The "air" remaining in the glass cap after heating the mercury contained no oxygen. The mouse, placed under the cap, suffocated. Having calcined mercury oxide, Lavoisier again isolated oxygen from it and again received pure mercury.
The oxygen content in the atmosphere began to noticeably increase 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 \u003d C 6 H 12 O 6 + 6O 2.
The resulting glucose becomes insoluble in water. starch that 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. Photosynthesis consumes only a small part of the solar energy (0.03%) reaching the Earth's surface.
All carbon dioxide available on Earth goes through the cycle of photosynthesis in an average of 300 years, oxygen - in 2000 years, ocean water - in 2 million years. At present, a constant oxygen content has been established in the atmosphere. It is almost completely spent on respiration, combustion and decay of organic matter.
Oxygen is one of the most active substances. Processes involving oxygen are called oxidation reactions. These include combustion, breathing, decay and many others. The table shows the combustion of oil, which goes with the release of heat and light.
Combustion reactions can bring not only benefits, but also harm. Combustion can be stopped by stopping the air (oxidizer) from reaching the burning object with 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, which is in a glass ampoule at the top of the fire extinguisher, carbon dioxide foam is formed. To activate the fire extinguisher, turn over and hit the floor with a metal pin. At the same time, 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 with a strong jet. Foamy liquid and carbon dioxide, enveloping the burning object, push the air and extinguish the flame.
1. Indicate whether the phenomena shown in the figures are physical or chemical.
2. Set the match.
Examples of chemical reactions:
I. interaction of marble with hydrochloric acid;
II. interaction of iron with sulfur;
III. decomposition of hydrogen peroxide;
IV. interaction of carbon dioxide with lime water.
Conditions for the occurrence of chemical reactions:
a) contact of substances;
b) heating;
c) the use of a catalyst.
Answer: I - a; II - a, b; III - in; IV - a.
3. Fill in chart 2.
4. "Crossword - vice versa." All the words in the crossword have already been entered. Give each of the words as precise a definition as possible.
"Keyword" - the first chemical reaction that man met.
1. One of the four states of matter.
2. The formation of a solid in solution during a chemical reaction.
3. The position of two or more bodies, objects, substances.
4. Portable or mobile device for extinguishing fires.
5. The process is characterized by an increase in temperature.
6. A chemical that speeds up a reaction, but is not part of the reaction products.
7. The impact of objects on each other.
The rate of a chemical reaction is the change in the amount of the reactant or reaction product per unit time per unit volume (for a homogeneous reaction) or per unit interface (for a heterogeneous reaction).
Law of acting masses: 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 reaction rate on concentration.
Rigid bodies are 0
Reaction molecularity is the minimum number of molecules involved in an elementary chemical process. By 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 degrees increase in temperature, the rate constant of a homogeneous elementary reaction increases two to four times.
Theory of active collisions(TAC), there are three conditions necessary for a reaction to occur:
The molecules must collide. This is an important condition, but it is not sufficient, since a reaction will not necessarily occur during a collision.
Molecules must have the necessary energy (activation energy).
The molecules must be correctly oriented relative to each other.
Activation energy is the minimum amount of energy that must 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
R is the universal gas constant.
Influence of catalysts on the reaction rate.
A catalyst is a substance that changes the rate of a chemical reaction, but is not itself consumed in the reaction and is not included in the final products.
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 - a chemical phenomenon, the essence of which is to change the rates of chemical reactions under the action of certain substances (they are called catalysts).
Heterogeneous catalysis - the reactant and the 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 for 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 in equilibrium, then the equilibrium will shift in the direction of 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 the starting materials into reaction products.
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K p \u003d C pr \ C ref |
ΔG<0 К р >1 C pr > C ref
ΔG>0 K p<1 С пр <С исх
