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 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, 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, you 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 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, which 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 acquaint you with chemical reactions, I will conduct a series of 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 resulting Cu (OH) 2 solution
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 coming out of 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 on the 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 the work of 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, at startup 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 physical from 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 falls out; 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. Heating must be used to induce a reaction between copper and oxygen.
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 practice 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. Thus, 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 dependent on temperature. 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 of 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 first, it was not possible to carry out it 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 an ordinary 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 will 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, one should turn Special attention that the emergence and evolution of life on Earth would be impossible without the existence enzymes, serving as essentially living catalysts.
Despite the fact that enzymes have general properties, inherent in all catalysts, however, they are 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 run into serious limitations. We can only talk about modeling some functions of enzymes and using these models for theoretical analysis the activity of living systems, and also partially for practical application isolated enzymes to speed up certain chemical reactions.
Chemical reaction rate Is a change in the amount of a reactant or a reaction product per unit of time per unit volume (for a homogeneous reaction) or per unit of interface (for a heterogeneous reaction).
Mass action law: dependence of the reaction rate on the concentration of reactants. The higher the concentration, the more molecules are contained in the volume. Consequently, the number of collisions increases, which leads to an increase in the speed of the process.
Kinetic equation- the dependence of the reaction rate on concentration.
Solid bodies equal to 0
Molecularity of the reaction Is the minimum number of molecules participating in an elementary chemical process. In terms of 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 indicators of the degrees of concentration in the kinetic equation.
Reaction rate constant- coefficient of proportionality in the kinetic equation.
Van't Hoff's rule: With an increase in temperature for every 10 degrees, the rate constant of a homogeneous elementary reaction increases two to four times
Active collision theory(TAS), there are three conditions required for a reaction to occur:
The molecules must collide. it important condition however, it is not enough, since a collision does not necessarily result in a reaction.
Molecules must have the required energy (activation energy).
The molecules must be correctly oriented relative to each other.
Activation energy- the minimum amount of energy that needs to be reported to the system in order for a reaction to occur.
Arrhenius equation sets 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 itself is not consumed in the reaction and is not included in the final products.
In this case, a 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 a change in the rates of chemical reactions under the action of certain substances (they are called catalysts).
Heterogeneous catalysis - the reagent and the catalyst are in different phases - gaseous and solid.
Homogeneous catalysis - the reagents (reagents) and the catalyst are in the same phase - for example, both are gases or both are dissolved in some kind of solvent.
Chemical equilibrium conditions
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 the system in equilibrium, then the equilibrium will shift towards the reaction, which this action will weaken.
Equilibrium constant - This is a measure of the completeness of the course of the reaction, the greater the value of the equilibrium constant, the higher the degree of conversion of the starting materials into reaction products.
|
K p = C pr \ C ref |
ΔG<0 К р >1 C pr> C ref
ΔG> 0 K p<1 С пр <С исх
1. Chemical reactions. Signs and conditions of their course. Chemical equations. The law of conservation of mass of substances. Types of chemical reactions.
2. What volume of gas can be obtained by the interaction of 60g, 12% potassium carbonate solution with sulfuric acid.
Chemical reaction
- the transformation of one or more substances into another.
Types of chemical reactions:
1) Compound reaction- these are reactions as a result of which one more complex is formed from two substances.
2) Decomposition reaction is a reaction as a result of which several simpler ones are formed from one complex substance.
3) Substitution reaction- these are reactions between simple and complex substances, as a result of which a new simple and new complex substance is formed.
4) Exchange reaction Are reactions between two complex substances, as a result of which they exchange their constituent parts.
Reaction conditions:
1) Close contact of substances.
2) Heating
3) Grinding (reactions are fastest in solutions)
Any chemical reaction can be depicted using a chemical equation.
Chemical equation Is a conditional record of a chemical reaction using chemical formulas and coefficients.
The chemical equations are based on mass conservation law
: the mass of the substances that entered into the reaction is equal to the mass of the substances obtained as a result of the reaction.
Signs of chemical reactions:
· Color change
· Gas evolution
· Precipitation
· Generation of heat and light
· Odor release
2.
Ticket number 7
1. Basic provisions of TEJ. - theory of electrical dissociation.
2. How many grams of magnesium containing 8% impurities can react with 40 g of hydrochloric acid.
Substances soluble in water can dissociate, i.e. decay into oppositely charged ions.
Electrical dissociation –
decomposition of the electrolyte into ions during dissolution or melting.
Electrolytes –
substances, solutions or melts of which conduct electric current (acids, salts, alkalis).
They are formed by ionic bonds (salts, alkalis), or covalent, strongly polar (acids).
Not electrolytes –
substances, solutions of which do not conduct electric current (solution of sugar, alcohol, glucose)
When dissociated, electrolytes break down into cations (+) and anions (-)
Jonah -
charged particle, into which atoms turn, as a result of giving and taking ē
The chemical properties of electrolyte solutions are determined by the properties of those ions that are formed during dissociation.
Acid
- an electrolyte that dissociates into hydrogen cations and an acid residue anion.
Sulfuric acid dissociates into 2 H cations with a charge (+) and
anion SO 4 with charge (-)
Foundations
- an electrolyte that dissociates into metal cations and hydroxide anions.
Salt - electrolyte, which in aqueous solution dissociates into metal cations and acid residue anions.
2.
1. Reactions of ion exchange.