Practical work No. 4. Chemistry 8th grade (to the textbook by Gabrielyan O.S.)

Signs of chemical reactions

Target: study the signs of chemical reactions, consolidate knowledge about the types of chemical reactions.
Equipment : test tubes, test tube rack, heating device, matches, test tube holder, 50 ml beaker, crucible tongs, copper wire, splinter, sheet of paper, spatula.
Reagents: solutions of sulfuric acid, iron (III) chloride, potassium thiocyanate, potassium carbonate, calcium chloride; marble, hydrochloric acid.

Experience 1.
Calcination of copper wire and the interaction of copper (II) oxide with sulfuric acid.

Work order:

1) Light the heater
Using crucible tongs, take the copper wire and bring it into the flame.
After some time, remove the wire from the flame and clean off any black deposits that have formed on it onto a sheet of paper.
We repeat the experiment several times.
Observed phenomena: During the heating process, the red copper wire becomes covered with a black coating, i.e. a new substance is formed.
Reaction equation:
2Cu + O 2 = 2CuO
This is a compound reaction.
Conclusion:

2) Place the resulting black coating in a test tube.
Add a solution of sulfuric acid to it and heat it carefully.
Observed phenomena: The black powder dissolves, the solution turns greenish-blue, i.e. new substances are formed.
Reaction equation:
2CuO + H 2 SO 4 = CuSO 4 + H 2 O
This is an exchange reaction.
Conclusion: a change in color is a sign of a chemical reaction.

Experience 2.
Interaction of marble with acid.

Place 1-2 pieces of marble into a glass.
Add hydrochloric acid to the glass so that the pieces are covered with it.
Observed phenomena: a violent release of colorless gas occurs, and the solution “boils.”
We light a torch and bring it into the glass.
Observed phenomena: the light goes out.
This means that the new substance formed is carbon dioxide.
Reaction equation:

This is an exchange reaction.
Conclusion: The release of gas is a sign of a chemical reaction.

Experience 3.

Pour 2 ml of iron (III) chloride solution FeCl 3 into a test tube, and then a few drops of potassium thiocyanate solution KSCN.
Observed phenomena: the solution turns blood red.
Reaction equation:

This is an exchange reaction.
Conclusion: a change in color is a sign of a chemical reaction.

Experience 4.
Reaction of sodium carbonate with calcium chloride.

Work order:

Pour 2 ml of sodium carbonate solution Na 2 CO 3 into a test tube.
Add a few drops of calcium chloride solution CaCl2.
Observed phenomena: a white precipitate forms.
Reaction equation:

This is an exchange reaction.
Conclusion: Precipitation is a sign of a chemical reaction.

General conclusion about the work: When performing practical work, the signs of chemical reactions were studied and knowledge about the types of chemical reactions was consolidated.

O.S.GABRIELYAN,
I.G. OSTROUMOV,
A.K.AKHLEBININ

START IN CHEMISTRY

7th grade

Continuation. For the beginning, see No. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10/2006

Chapter 3.
Phenomena occurring with substances

(ending)

§18. Chemical reactions.
Flow and termination conditions
chemical reactions

All previously discussed methods for separating mixtures are based on differences in the physical properties of the substances forming the mixtures and relate to physical phenomena. However, there are also chemical phenomena. Such phenomena are accompanied by the transformation of substances, they are called chemical reactions.

Let us compare the physical phenomena underlying the separation of mixtures and the chemical reactions leading to the production of new chemical compounds, using the example of a mixture of iron and sulfur powders.

Thoroughly mix iron filings and sulfur powder (7:4 ratio by weight). The result is a mixture of two simple substances, in which each retains its properties (suggest ways to separate the resulting mixture).
The mixture is transferred into a test tube and heated in the flame of an alcohol lamp. A chemical reaction of iron with sulfur begins, resulting in the formation of a new substance - iron sulfide. The reaction product is a complex substance whose properties differ from those of both iron and sulfur. For example, it is not attracted by a magnet, sinks in water, does not rust or burn (Fig. 78).

Let us describe the chemical reaction carried out in words:

iron + sulfur = iron sulfide

and chemical formulas:

In order for this chemical process to take place, two conditions were necessary: ​​contact of the reacting substances and the initial supply of heat (heating).

The first condition is mandatory for all chemical processes where two or more substances are involved.

The second is not always required. Demonstration experiment.

Place a small piece of marble in a test tube and add a solution of hydrochloric acid. Rapid gas evolution occurs (Fig. 79).

The test tube is closed with a stopper with a gas outlet tube and its tip is lowered into another test tube with lime water. The fact that a chemical reaction is taking place can be judged by the appearance of a white precipitate - clouding of lime water (Fig. 80).
No heating was required for both reactions.

You can describe the reactions taking place using the names of the substances:

marble + hydrochloric acid calcium chloride + carbon dioxide + water,

carbon dioxide + lime water calcium carbonate + water.

However, chemists use chemical formulas instead of words:

CaCO 3 + HCl CaCl 2 + CO 2 + H 2 O,

CO 2 + Ca(OH) 2 CaCO 3 + H 2 O.

For some reactions to occur, the contact of substances or their heating is not enough. If such reactions occur, they proceed very slowly. To speed up this process, special substances called catalysts are used.

Catalysts are substances that accelerate chemical reactions, but at the end of the reaction remain unchanged qualitatively and quantitatively.

Biological catalysts of protein nature are called enzymes, or enzymes.

Let us demonstrate the effect of catalysts using the following experiment.

The second is not always required. A small volume of hydrogen peroxide (more precisely, peroxide) solution is poured into a large test tube. Several grains of manganese dioxide powder are added to the solution, which acts as a catalyst. A rapid release of gas—oxygen—begins, as evidenced by the flashing of a smoldering splinter placed in the upper part of the test tube (Fig. 81).

Let's repeat a similar experiment, only instead of manganese dioxide, we place a little gruel of freshly chopped potatoes containing the enzyme into a test tube with hydrogen peroxide. We observe a rapid release of oxygen.

The chemical reaction that occurs can be represented using the names of the substances:

or their formulas:

Thus, a necessary condition for the occurrence of chemical reactions is the contact of the reacting substances. In some cases, heating or the use of catalysts is required.

Knowing the conditions for reactions to occur allows you to control them: speed up, slow down or stop altogether. The latter circumstance is very important, for example, for stopping combustion reactions when extinguishing fires.

As you know, combustion is the interaction of substances with oxygen in the air. Therefore, in order to extinguish a fire, it is necessary to stop the access of oxygen to burning objects. This is achieved by filling them with water, various foams, sand, throwing thick fabric or using special devices - fire extinguishers (Fig. 82).

1. What conditions are necessary for chemical reactions to occur?

2. Give examples of reactions from everyday life that do not require initial heating to occur.

3. What are catalysts? What are enzymes?

4. Name the methods of extinguishing fires that you know.

5. With the help of a teacher or special literature, review the design of a carbon dioxide fire extinguisher. What is the principle of its operation?

6. Read the instructions for using high-quality washing powders - synthetic detergents (SDCs) with added enzymes. What are the advantages of SMS containing enzymes over regular SMS?

7. Why do you extinguish fires or burning wooden buildings with water? What role does water play in this process?

8. Why can't you put out burning oil with water?

9. Why can’t you extinguish burning electrical appliances or electrical wiring with water?

§19. Signs of chemical reactions

You already know that the essence of chemical reactions is the transformation of one substance into another. Often such transformations are accompanied by external effects that are perceived by the senses. That's what they call.

signs of chemical reactions External signs of chemical reactions can be considered: the formation of a precipitate (Fig. 83, A
, cm. With. 10), gas release (Fig. 83, b ), odor, color change (Fig. 83, V

), release or absorption of heat.
In the previous paragraph you have already become acquainted with some signs of reactions. With. 10), gas release (Fig. 83, Thus, when iron filings interacted with sulfur powder, the color of the mixture changed and heat was released (see.

rice. 78,

The second is not always required. ). When marble interacted with hydrochloric acid, gas evolution was observed (see Fig. 79). When carbon dioxide reacted with lime water, a precipitate appeared (see Fig. 80). The flashing of a smoldering splinter in the presence of oxygen is also a sign of a reaction occurring (see Fig. 81). Let us illustrate these signs of chemical reactions using demonstration and student experiments.
A beaker contains a colorless alkali solution. It can be detected using special substances - indicators (from lat.
indico

- I indicate). An indicator for alkali is a colorless alcohol solution of phenolphthalein.

The second is not always required. In two beakers there are multi-colored solutions: violet-pink (potassium permanganate in an alkaline medium) and orange (acidified solution of potassium dichromate). A colorless sodium sulfite solution is added to both glasses. What indicates the occurrence of chemical reactions in glasses (Fig. 84)?

Student experiment. Dissolve a few crystals of potassium permanganate (literally two or three!) in a glass of water (wait until the substance is completely dissolved). Dip an ascorbic acid tablet into the resulting solution. What changes indicate a chemical reaction is taking place?

Student experiment. In a gas lighter with a transparent body you see a colorless liquid. This is a mixture of two gases, the names of which you could read at gas filling stations or household cylinders - propane and butane. What kind of gases are these if they have a liquid state of aggregation?
The fact is that there is increased pressure inside the tank. Press the valve without igniting the gas.

Do you hear hissing? Propane and butane burst out, taking on the gaseous state familiar to normal pressure.
Light your lighter. A chemical combustion reaction of propane and butane occurs (Fig. 85). Bring the flame briefly to the window glass. Explain the observed phenomenon.

Compare the color of a lighter's flame with the flame of a gas stove and candle. What kind of flame smokes?

The second is not always required. Trace the connection between the glow of the flame and its smoky properties.

The transition of propane and butane from a liquid state inside a lighter to a gaseous state outside it is a physical phenomenon. And the combustion of these gases is a chemical reaction.

The second is not always required. Some reactions are accompanied by the formation of sparingly soluble substances that precipitate.

A solution of ferric chloride is added to two beakers containing a colorless solution of sodium hydroxide and a yellowish solution of yellow blood salt (Fig. 86). What indicates chemical phenomena?

Stalactite columns take thousands of years to form. You can simulate a fragment of this process at home (task 9 at the end of this paragraph). It is clear that instead of a stalactite you will simply get a precipitate of calcium carbonate.

1. How do chemical phenomena differ from physical ones?

2. What phenomena would you classify the burning of a candle and the “burning” of an electric light bulb?

3. Give examples of reactions known from everyday life that are accompanied by a change in color, the release of gas, or the formation of a precipitate.

4. What process occurs when medications such as UPSA aspirin effervescent tablets or vitamin C are dissolved in water?

5. What qualitative reactions are used to distinguish between oxygen and carbon dioxide?

6. Marble sculptures are destroyed by so-called acid rain. What phenomenon occurs in this case?

7. Pour a pile of dry river sand into a deep plate. Soak the sand in alcohol. Make a small depression at the top of the cone and place in it a mixture of thoroughly mixed 2 g of baking soda and 13 g of powdered sugar. All that remains is to set fire to the mixture and observe the occurrence of several chemical reactions at once: the combustion of alcohol, the charring of sugar, the decomposition of soda when heated.

8. Pour half a glass of water into a quart glass jar and drop in a pea-sized portion of an effervescent aspirin tablet. What is observed in this case? To determine what gas is released as a result of a chemical reaction, lower a smoldering splinter into the jar (without touching the liquid).

9. Pour half a glass of boiled water and stir in half a teaspoon of slaked lime (available in hardware stores). All the powder will not dissolve, but this is not a problem. Let the mixture settle and pour the clear solution from the sediment into a clean glass.

Using a juice straw (be careful not to splash!), blow exhaled air through the solution. Soon it will become cloudy: a white precipitate will form. Make a conclusion about the occurrence of a chemical reaction in the glass.

PRACTICAL WORK No. 6.
Study of the corrosion process of iron
(home experiment)

You probably know the process of corrosion (rusting) of iron. Under the influence of external conditions, rust forms on metal. In this work you will find out how external conditions influence the rate of corrosion of iron.

To conduct the experiment you will need:

Three plastic bottles with caps of 250–500 ml;

Three large nails 5–10 cm long;

Sandpaper for stripping nails;

Boiled water;

Tap water;

Salt.

Nails should be washed with soap to remove the layer of oil that protects them from rusting.

When the nails are dry, sand their surface with sandpaper and rinse with boiled water.

Fill the first bottle completely with cold boiled water, put a nail in it and close the lid tightly.

Fill the second bottle halfway with cold tap water and place a nail in it. There is no need to close the bottle with a lid.

First add two tablespoons of table salt to the third bottle. Fill it halfway with cold tap water, close the lid and stir well. When all the salt has dissolved, place the third and final nail in the bottle.

There is no need to close the bottle with a lid.

To avoid confusion, use a felt-tip pen to number each bottle.

Place the bottles in a secluded place. If the water from the second and third bottles evaporates, simply add tap water to them.

After a week, rust will form on the nails.

Look where there is more and where there is less.

Record your observations by placing the bottle numbers next to the corresponding descriptions, for example:

Little or almost no rust has formed -...;

The rust is clearly visible, it adheres firmly to the nail -...;

I am reporting chemical “news”. My opponents, trying to create “devastating refutations” of my Drokino geological discoveries, stated in their opuses that concentrated sulfuric acid supposedly does not react with limestones and marbles, and I, therefore, am so “uneducated” and “generally crazy” that “ point blank” I don’t know this “well-known Truth”. As justification, they cite the idea that, supposedly, sulfuric acid forms gypsum, which, being an insoluble compound, covers limestone or marble with a film that protects it from further action of the acid and, therefore, “instantly blocks” this reaction. This “pearl” was first expressed by Dmitry Lvovich Bryzgalov (a kindergarten after-school teacher; on the Internet he writes “slop” to me anonymously); then the same idea was published by Boris Mikhailovich Lobastov (Krasnoyarsk geologist student), formulating it with special pathos: “in geology, studies for the presence of carbonates are carried out using hydrochloric acid, the concentration of which does not exceed 10%. Why not use sulfuric acid, especially in high concentration, since it is stronger? The thing is that the reaction of sulfuric acid and calcite (calcium carbonate) produces a very slightly soluble compound - calcium sulfate (aka gypsum), which instantly completely covers the surface of the carbonates and stops thereby the reaction." (word " stops" he highlighted in bold).

All the fuss is due to the fact that I used sulfuric acid of 93% concentration (“battery acid”) to test the Drokino rocks for the presence of carbonates (primarily calcite), although “according to the instructions” official geologists are “prescribed » Use 10% hydrochloric acid for this test. Seeing that I was doing tests with the wrong acid, my critics attacked me, trying to prove that the acid I used supposedly did not react with calcite, and that I was therefore ignorant, and all my geological results in the vicinity of Drokino - nonsense of a charlatan.

As a rule, I am simply too lazy to respond to this kind of “pearls”: after all, we are not talking about complex chemical exotics, but about banal basics from a school textbook. But since my unfortunate critics began to fervently multiply this “pearl” and passionately “repost” it with the goal of discrediting all my work in all areas, I found the time, CAPTURED this chemical reaction to VIDEO and posted this VIDEO on several servers; here, click on the choice (on the first link - DOWNLOAD ):

Duration: just over three minutes. First, a variant of this reaction is shown by pipetting acid onto a polished marble surface; then the same reaction is shown in a test tube (a piece of this marble is placed in a test tube with acid). For the marble - THANKS to Igor Yuryevich Tabakaev (this is a battle, that is, a fragment, from the Badalyk cemetery; do not be afraid: no one committed vandalism in the cemetery, this is exactly a battle). Marble (real, cemetery) is the most inert form of calcite (with chalk this reaction goes even faster). So - here's a video fact: this reaction is COMING (despite Bryzgalov and Lobastov)! It’s just that my supposedly “highly educated” opponents “point blank” don’t know that, firstly, gypsum is, although poorly, noticeably soluble in clean water; and secondly, it can react with sulfuric acid, first to form calcium HYDROSULPHATE Ca(HSO 4) 2 and then the associate CaSO 4 × 3H 2 SO 4, and both of these compounds are SOLUBLE (see, for example, “Analytical chemistry of calcium; p. 11"; or "Course of analytical chemistry. Volume one. Qualitative analysis; F.T. Goll, p. 292") and are formed the more readily the higher the acid concentration. Thus, in an excess of concentrated sulfuric acid, you will NOT see CaSO 4: you will get a transparent solution of Ca(HSO 4) 2 and CaSO 4 × 3H 2 SO 4.

P.S. The “vatness” of both such critics and the readers who agree with them is surprising. Well, is it really so difficult to take it and check it? After all, neither marble nor sulfuric acid 93% are either a shortage or prohibited products.

Marble (from Greek μάρμαρο - “shining stone”) is a common metamorphic rock, usually consisting of a single mineral, calcite. Marbles are the products of limestone metamorphosis - calcite marble; and products of dolomite metamorphization - dolomite marbles.

The structure is coarse-grained, medium-grained, fine-grained, fine-grained. Consists of calcite. It boils violently when exposed to dilute hydrochloric acid. Does not leave scratches on glass. The surfaces of the grains are smooth (perfect cleavage). Specific gravity 2.7 g/cm3. Hardness on the Mohs scale 3-4.

Marble has different colors. It is often colorfully colored and has an intricate pattern. The breed amazes with its unique patterns and colors. The black color of marble is due to the admixture of graphite, green – chlorite, red and yellow – iron oxides and hydroxides.

Features. Marble is characterized by a granular structure, calcite content, low hardness (does not leave scratches on glass), smooth grain surfaces (perfect cleavage), reaction under the action of dilute hydrochloric acid. Marble can be confused with harder rocks - quartzite and jasper. The difference is that quartzite and jasper do not react with dilute hydrochloric acid. In addition, marble does not scratch glass.

Composition and photo of marble

Mineralogical composition: calcite CaCO 3 up to 99%, admixtures of graphite and magnetite in the amount of up to 1%.

Chemical composition. Calcite marble has the composition: CaCO 3 95-99%, MgCO 3 up to 4%, traces of iron oxides Fe 2 O 3 and silica SiO 2. Dolomite marble is composed of 50% calcite CaCO 3, 35-40% dolomite MgCO 3, the SiO 2 content reaches up to 25%.

White marble. © Beatrice Murch Gray marble Black marble owes its color to graphite impurities The green color of marble is due to chlorite inclusions. The red color of marble is due to iron oxides.

Origin

The structure of limestones and dolomites undergoes changes under the influence of certain geological conditions (pressure, temperature), as a result of which marble is formed.

Application of marble

Marble is an excellent facing, decorative and sculptural material that was used in his works by the famous sculptor Michelangelo Buonarroti. Marble is used in the decoration of buildings, lobbies, underground metro halls, as a filler in colored concrete, and is used for the manufacture of slabs, bathtubs, washbasins and monuments. Marble of different shades is one of the main stones used to create the extremely beautiful Florentine mosaics.

David, Michelangelo Buonarroti. Photo Jörg Bittner Unna Aries sculpture made of white marble

Marble is used to make elegant cubes, lamps, and original tableware. Marble is used in ferrous metallurgy in the construction of open-hearth furnaces, in the electrical and glass industries. It is also used as a building material in road construction, and as a fertilizer in agriculture and for burning lime. Beautiful mosaic panels and tiles are made from marble chips.

Cast marble, from which bathrooms and countertops are made, only imitates the appearance, making objects look like natural marble and other natural decorative stones and minerals. And the price is much cheaper than natural stone, which to some extent makes it popular. The process of making cast marble involves mixing polyester resin and quartz sand.

Marble deposits

The largest marble deposit in Russia is Kibik-Kordonskoye (Krasnoyarsk Territory), where about twenty varieties of marble of different colors from white to greenish-gray are mined. There are large deposits of marble in the Urals - the Aydyrlinskoye and Koelginskoye deposits of white marble, located in the Orenburg and Chelyabinsk regions, respectively.

Black marble is mined at the Pershinsky deposit, yellow at the Oktyabrsky quarry, and lilac at the Gramatushinskoye deposit in the Sverdlovsk region.

Marble from Karelia (near the village of Tivdia), of a delicate fawn color with pink veins, was the first to be used for decorative finishing in Russia; it was used for the interior decoration of St. Isaac's and Kazan Cathedrals in St. Petersburg.

The stone is found on Lake Baikal (reddish-pink stone from Burovshchina), in Altai (Orokotoyskoye), and in the Far East (green marble). It is also mined in Armenia, Georgia (red marble from New Shroshi), Uzbekistan (Gazgan deposit of cream and black stone), Azerbaijan, Tajikistan, Kyrgyzstan and Greece (Paros Island).

Sculptural marble with a hardness of 3, which lends itself well to processing, is mined in Italy (Carrara). The world-famous sculptures of Michelangelo Buonarroti “David”, “Pieta”, “Moses” are made from Italian marble from the Carrara deposit.

Practical work includes four experiments.

Experience 1

Calcination of copper wire and interaction of copper (II) oxide with sulfuric acid

Light the alcohol lamp (gas burner). Take the copper wire with crucible tongs and bring it into the flame. After some time, remove the wire from the flame and clean off any black deposits that have formed on it onto a sheet of paper. Repeat the experiment several times. Place the resulting black deposit in a test tube and pour a solution of sulfuric acid into it. Warm the mixture. What are you observing?

Was a new substance formed when copper was heated? Write down the equation of the chemical reaction and determine its type based on the number and composition of the initial

substances and reaction products. What signs of a chemical reaction did you observe? Was a new substance formed when copper (II) oxide reacted with sulfuric acid? Determine the type of reaction based on the number and composition of the starting materials and reaction products and write down its equation.

1. When calcining copper wire, the copper will oxidize:

and black copper(II) oxide is formed. This is a compound reaction.

2. The resulting copper (II) oxide dissolves in sulfuric acid, the solution becomes blue, and copper (II) sulfate is formed:

This is an exchange reaction.

Interaction of marble with acid

Place 1-2 pieces of marble in a small glass. Pour enough hydrochloric acid into the glass to cover the pieces. Light a splinter and bring it into the glass.

Are new substances formed when marble reacts with acid? What signs of chemical reactions did you observe? Write down the equation of the chemical reaction and indicate its type based on the number and composition of the starting substances and reaction products.

1. Marble dissolved in hydrochloric acid, a chemical reaction occurred:

Experience 3

Reaction of iron (III) chloride with potassium thiocyanate

Pour 2 ml of a solution of iron (III) chloride into a test tube, and then a few drops of a solution of potassium thiocyanate KSCN - a salt of the acid HSCN, with an acid residue SCN -.

What signs accompany this reaction? Write down its equation and type of reaction based on the number and composition of the starting materials and reaction products.