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Law of conservation of mass of substances. Chemical equations

Lesson #14. Law of conservation of mass of matter. Chemical equations

Law of conservation of mass of substances

Problematic question: Will the mass of the reactants change compared to the mass of the reaction products?

To answer this question, observe the following experiment.

Video experiment: .

Description of the experiment: Place 2 grams of crushed copper in a conical flask. Stopper the flask tightly and weigh. Remember the mass of the flask. Gently heat the flask for 5 minutes and observe the changes that occur. Stop heating and when the flask has cooled, weigh it. Compare the mass of the flask before heating with the mass of the flask after heating.

Conclusion: The mass of the flask did not change after heating.

Let's watch other video experiments:

Conclusion: The mass of substances before and after the reaction did not change.

Formulation law of conservation of mass: The mass of substances that entered into the reaction is equal to the mass of the formed substances.

From the point of view of atomic-molecular science, this law is explained by the fact that in chemical reactions total atoms do not change, but only their rearrangement occurs.

The law of conservation of mass was theoretically discovered in 1748 and experimentally confirmed in 1756 by the Russian scientist M.V. Lomonosov.

The French scientist Antoine Lavoisier in 1789 finally convinced the scientific world of the universality of this law. Both Lomonosov and Lavoisier used very precise scales in their experiments. They heated metals (lead, tin, and mercury) in sealed vessels and weighed the starting materials and reaction products.

Chemical equations

The law of conservation of mass of substances is used when drawing up equations of chemical reactions.

Chemical equation - this is a conditional record chemical reaction through chemical formulas and coefficients.

Let's watch the video - experiment: .

As a result of the chemical interaction of sulfur and iron, a substance was obtained - iron sulfide (II) – it differs from the original mixture. Neither iron nor sulfur can be visually detected in it. It is also impossible to separate them using a magnet. A chemical change has occurred.

The starting materials that take part in chemical reactions are called reagents.

New substances formed as a result of a chemical reaction are called products.

Let us write the ongoing reaction in the form of a chemical reaction equation:

Fe + S = FeS

Algorithm for composing a chemical reaction equation

Let's create an equation for the chemical reaction between phosphorus and oxygen

1. On the left side of the equation we write down the chemical formulas of the reagents (substances that react). Remember! Molecules of most simple gaseous substancesdiatomic – H 2 ; N 2 ; O 2 ; F 2 ; Cl 2 ; Br 2 ; I 2 . Between the reagents we put a “+” sign, and then an arrow:

P + O 2 →

2. On the right side (after the arrow) we write the chemical formula of the product (the substance formed during the interaction). Remember! Chemical formulas must be made using the valences of atoms chemical elements:

P+O 2 → P 2 O 5

First, the number of atoms that are contained more in the reacting substances (products) is equalized.

IN in this case these are oxygen atoms.

Find the least common multiple of the numbers of oxygen atoms in the left and right parts equations The smallest multiple for sodium atoms is –10:

We find the coefficients by dividing the smallest multiple by the number of atoms of a given type, and put the resulting numbers into the reaction equation:

The law of conservation of mass of a substance is not satisfied, since the number of phosphorus atoms in the reactants and reaction products is not equal, we act similarly to the situation with oxygen:

We obtain the final form of the chemical reaction equation. We replace the arrow with an equal sign. The law of conservation of mass of matter is satisfied:

4 P+5O 2 = 2P 2 O 5

ASSIGNMENT TASKS

№1.

Convert following diagrams into the equations of chemical reactions by placing the necessary coefficients and replacing the arrows with an equal sign:

Zn+O 2 → ZnO

Fe+Cl 2 →FeCl 3

Mg + HCl → MgCl 2 +H 2

Al(OH) 3 → Al 2 O 3 +H 2 O

HNO 3 → H 2 O+NO 2 +O 2

CaO+H 2 O→Ca(OH) 2

H 2 +Cl 2 →HCl

KClO 3 → KClO 4 +KCl

Fe(OH) 2 +H 2 O+O 2 →Fe(OH) 3

KBr+ Cl 2 → KCl+ Br 2

№2.

Using the algorithm for composing equations of chemical reactions, compose equations for the reactions of interaction between the following pairs of substances:
1) Na and O 2
2) Na and Cl 2
3) Al and S

Lesson on the topic

“LAW OF CONSERVATION OF MASS OF SUBSTANCES.

EQUATIONS OF CHEMICAL REACTIONS"

I propose developing a lesson in the 8th grade according to the program of O. S. Gabrielyan.

Lesson objectives: to form ideas about the law of conservation of mass of substances, to develop the ability to apply it, to explain the essence of chemical reactions and the process of drawing up equations of chemical reactions, to develop the ability to identify what is essential, to draw conclusions, to establish interdisciplinary connections, to develop experimental skills, to form ideological concepts about the knowability of nature.

Epigraph for the lesson:

Experience!

Tell me, what are you proud of?

What are you?

You are the fruit of mistakes and tears,

The energy spent is counted.

Everywhere: “What’s new?” - you hear.

Yes, think about the old things first!

You will find a lot of new things for yourself in it!

A. Maikov

We start the lesson with repetition homework, updating knowledge about physical and chemical phenomena with the help of creative homework and excerpts from works of fiction.

As homework for this lesson, students were asked to draw physical and chemical phenomena: photosynthesis, boiling a kettle, rusting a nail, burning a fire, melting ice cream, burning a light bulb, bending a nail, dissolving sugar, moving a clock pendulum, cooking scrambled eggs, calling from class and etc. Based on the drawings of their classmates, students determine what kind of phenomenon it is.

I love the storm in early May,

When the first thunder of spring

As if frolicking and playing,

Rumbling in the blue sky.

F. I. Tyutchev. Spring thunderstorm

The last cloud of the scattered storm!

Alone you rush across the clear azure,

You alone cast a dull shadow,

You alone sadden the jubilant day.

A. S. Pushkin. Cloud

My fire shines in the fog:

The sparks go out as they fly...

Ya. P. Polonsky. Songs of the Gypsy

The naughty guy has already frozen his finger,

It's both painful and funny to him,

And his mother threatens him through the window...

A. S. Pushkin. Eugene Onegin

It's already evening.

Dew Glistens on the nettles.

I'm standing by the road

Leaning against the willow tree.

There is great light from the moon

Right on our roof.

Somewhere the song of a nightingale

I hear it in the distance.

S. A. Yesenin. It's already evening. Dew...

Updating knowledge key terms, concepts are carried out in the form of an oral survey or dictation. List of tested concepts: chemical phenomenon, physical phenomenon, index, coefficient, equation of a chemical reaction, chemical formula, signs and conditions of chemical reactions, exchange reactions, substitution, combination, decomposition.

Then we move on to studying new material. Behind the equations of chemical reactions lies an amazing and not yet fully understood world. In order to advance along the path of understanding it, it is necessary to conduct an experiment. We provide instructions on safety rules when working with glass and heating.

Exercise: Carry out the indicated reactions and tell us about your observations.

Students are preliminarily divided into four groups according to their level of training™ (with the help of a psychologist). Participants in each group receive instruction cards.

1. Combustion of phosphorus in a closed vessel

Put a little red phosphorus (about the size of a pea) into a round-bottomed flask, close the flask with a stopper, and weigh. Then heat the flask (in the place where the phosphorus is located). After the chemical reaction has occurred, cool the flask and reweigh it.

Has the mass of the flask changed? Write the equation for the oxidation of phosphorus to phosphorus oxide (V). Indicate the type of reaction, name the conditions and signs of the reaction.

2. Decomposition of basic copper carbonate (H)

Put some salt (СuОН) into the test tube. 2 CO 3 . Pour 30-40 ml of lime water into the flask. Weigh the device consisting of a test tube with salt, a stopper with a gas outlet tube and a flask with lime water. Heat a test tube containing basic copper(II) carbonate, vent pipe should be immersed in lime water. After the tube has cooled, weigh the device again.

Has the mass of the device changed? Write the equation for the decomposition reaction of salt (СuОН) 2 CO 3 to carbon monoxide (IV), copper oxide (II) and water. Indicate the type of reaction, name the conditions and signs of the reaction.

3. Reaction between solutions of sodium sulfate and barium chloride

On the scales, balance a Landolt vessel, in one elbow of which there is a solution of sodium sulfate, and in the other - barium chloride. Drain the solutions. A chemical reaction has occurred.

Did the mass of the substances change before and after the reaction? Write an equation, indicate the type of reaction, name the conditions and signs of the reaction.

4. Reaction between solutions of alkali and copper (II) sulfate

On a scale, equilibrate two beakers with solutions of copper(II) sulfate and sodium hydroxide. Drain the solutions.

Is the balance of the scales out of balance? Write the reaction equation, indicate the type of reaction, name the conditions and characteristics of the reaction.

Students perform the experiment according to the instructions and make appropriate notes in their notebooks.

We inform you that the experiment performed by the first group is an analogue of the historical experiment conducted by M.V. Lomonosov. We show a portrait of a scientist, listen to a student’s report on the life and work of M.V. Lomonosov.

Please note Special attention students that M.V. Lomonosov, for the first time in the history of science, formulated one of the basic laws of nature - the law of conservation of matter. He wrote: “All changes that occur in nature are such states that, as much as is taken from one body, so much will be added to another... This universal natural law extends to the very rules of movement...” Emphasizing the outstanding merits of Lomonosov, we say that the best monument to the great scientist is our knowledge.

Students write down in their notebooks the modern formulation of the law of conservation of mass of substances in chemical reactions.

To consolidate knowledge, we suggest completing several tasks, then we organize a self-assessment - we project the answers onto the board through an overhead projector.

We invite students to write a mini-essay at home on the topic “Chemical phenomena outside the window.”

12.02.2015 5575 688 Khairulina Liliya Evgenievna

Objective of the lesson: to formulate the concept of the law of conservation of mass, to teach how to compose reaction equations
Lesson objectives:
Educational: experimentally prove and formulate the law of conservation of mass of substances.
Developmental: give the concept of a chemical equation as a conditional recording of a chemical reaction using chemical formulas; start developing skills in writing chemical equations
Educational: instill an interest in chemistry, broaden your horizons

During the classes
I. Organizational moment
II. Frontal survey:
- What are physical phenomena?
- What are chemical phenomena?
- Examples of physical and chemical phenomena
- Conditions for chemical reactions to occur
III. Learning new material

Formulation of the law of conservation of mass: the mass of substances that entered into a reaction is equal to the mass of the formed substances.
From the point of view of atomic-molecular science, this law is explained by the fact that during chemical reactions the total number of atoms does not change, but only their rearrangement occurs.

The law of conservation of mass of substances is the basic law of chemistry; all calculations for chemical reactions are made on its basis. It is with the discovery of this law that the emergence of modern chemistry as an exact science is associated.
The law of conservation of mass was theoretically discovered in 1748 and experimentally confirmed in 1756 by the Russian scientist M.V. Lomonosov.
The French scientist Antoine Lavoisier in 1789 finally convinced the scientific world of the universality of this law. Both Lomonosov and Lavoisier used very precise scales in their experiments. They heated metals (lead, tin, and mercury) in sealed vessels and weighed the starting materials and reaction products.

Chemical equations
The law of conservation of mass of substances is used when drawing up equations of chemical reactions.
A chemical equation is a conventional representation of a chemical reaction using chemical formulas and coefficients.
Let's watch a video - experiment: Heating a mixture of iron and sulfur.
As a result of the chemical interaction of sulfur and iron, a substance is obtained - iron (II) sulfide - it differs from the original mixture. Neither iron nor sulfur can be visually detected in it. It is also impossible to separate them using a magnet. A chemical change has occurred.
The starting materials that take part in chemical reactions are called reagents.
New substances formed as a result of a chemical reaction are called products.
Let us write the ongoing reaction in the form of a chemical reaction equation:
Fe + S = FeS
Algorithm for composing a chemical reaction equation
Let's create an equation for the chemical reaction between phosphorus and oxygen
1. On the left side of the equation we write down the chemical formulas of the reagents (substances that react). Remember! The molecules of most simple gaseous substances are diatomic - H2; N2; O2; F2; Cl2; Br2; I2. Between the reagents we put a “+” sign, and then an arrow:
P + O2 →
2. On the right side (after the arrow) we write the chemical formula of the product (the substance formed during the interaction). Remember! Chemical formulas must be compiled using the valences of atoms of chemical elements:

P + O2 → P2O5

3. According to the law of conservation of mass of substances, the number of atoms before and after the reaction must be the same. This is achieved by placing coefficients in front of the chemical formulas of the reagents and products of a chemical reaction.
First, the number of atoms that are contained more in the reacting substances (products) is equalized.
In this case, these are oxygen atoms.
Find the least common multiple of the numbers of oxygen atoms on the left and right sides of the equation. The smallest multiple for sodium atoms is –10:
We find the coefficients by dividing the smallest multiple by the number of atoms of a given type, and put the resulting numbers into the reaction equation:
The law of conservation of mass of a substance is not satisfied, since the number of phosphorus atoms in the reactants and reaction products is not equal, we act similarly to the situation with oxygen:
We obtain the final form of the chemical reaction equation. We replace the arrow with an equal sign. The law of conservation of mass of matter is satisfied:
4P + 5O2 = 2P2O5

IV. Consolidation
V. D/z

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Slide 2

The only path leading to knowledge is action.

Lesson objectives: Educational - experimentally prove the law of conservation of mass of substances. Based on this law, form the concept of the material balance of a chemical reaction. To form a concept of the equation of a chemical reaction as a conventional notation reflecting the transformations of substances. Developmental - develop the ability to pose simple problems, formulate hypotheses and test them experimentally; improve skills in working with laboratory equipment and reagents; develop the ability for logical thinking. Educational - to continue the formation of the scientific worldview of students; cultivate communicative competence, as well as observation, attention, initiative. Using the example of the life and work of M.V. Lomonosov, cultivate interest in the study of chemistry.

Slide 3

Discovery of the law of conservation of mass of substances

1789 Robert Boyle 1673 1748 M. V. Lomonosov Antoine Lavoisier

Slide 4

Boyle carried out many experiments on calcining metals in sealed retorts, and each time the mass of scale turned out to be greater than the mass of the metal being calcined.

Slide 5

Slide 6

Russian scientist M.V. Lomonosov suggested that sensory experience deceives us. On July 5, 1748, he wrote in a letter to Leonhard Euler:

Slide 7

“All changes in nature that occur are of such a state that whatever is taken away from one body, the same amount is added to another. So, if there is a decrease in matter somewhere, it will increase in another place; no matter how many hours someone puts into vigil, the same amount of sleep will be taken away..."

Slide 8

“The mass of substances that entered into a reaction is equal to the mass of substances formed as a result of the reaction” is a modern formulation of the law of conservation of mass of substances.

Slide 9

Slide 10

Only in 1756 did Lomonosov manage to experimentally test the theoretically discovered law of conservation of mass of substances. Like Boyle, the Russian scientist experimented in sealed retorts. But, unlike Boyle, Lomonosov weighed the vessels both before and after calcination without opening them.

Slide 11

Slide 12

Much later, this law, regardless of M.V. Lomonosov, was discovered by the French scientist A. Lavoisier.

Slide 13

Slide 14

A chemical formula is a conventional recording of the composition of a substance using chemical symbols and indices. The index shows the number of atoms in the formula unit of a substance. The coefficient shows the number of 5H2O particles not connected to each other. Coefficient Chemical formula Index Based on this law, chemical reaction equations are compiled using chemical formulas, coefficients and mathematical signs.