The expert spoke about typical mistakes in the exam in chemistry. Chemical reactions - Knowledge hypermarket Key words and phrases

Unlike physical phenomena, in chemical phenomena, or chemical reactions, as you know, there is a transformation of some substances into others. These transformations are accompanied outward signs: the formation of a precipitate or gas, a change in color, the release or absorption of heat, the appearance of a smell, etc.

Remember the experiments that the teacher showed you in the first lessons. The interaction of marble - calcium carbonate CaCO 3 - with hydrochloric acid Hcl was accompanied by the release of carbon dioxide - carbon monoxide (IV) CO 2 . And passing it through transparent lime water - a solution of calcium hydroxide Ca (OH) 2 - occurred with the formation of a precipitate - calcium carbonate CaCO 3.

We will observe the course of some chemical reactions and establish signs that confirm the formation of new substances and the appearance of new properties in them - insolubility or low solubility in water, smell, color, etc.

Pour 2 ml of iron (III) chloride solution FeClg into a test tube, and then add a few drops of potassium thiocyanate solution KSCN. We will see the appearance of a blood-red solution of a new substance - iron (III) thiocyanate Fe(SCN) 3 .

We mix iron and sulfur powders, no new substances appear. Iron from this mixture will be attracted by a magnet, and when the mixture is lowered into water, sulfur floats to the surface, that is, the mixture can be very easily separated. However, if this mixture of iron and sulfur powders is heated, then a chemical reaction will begin, which will continue further without heating with the release of heat - we will see how the mixture heats up. After the end of the reaction, a new substance will be obtained - iron (II) sulfide FeS. It is gray in color, sinks in water and is not attracted by a magnet (Fig. 92).

Rice. 92.
Separation of a mixture of iron and sulfur powders (left). Interaction of iron with sulfur (right)

Let's set fire to a little sulfur in an iron spoon - it will light up with a bluish flame and give a plentiful acrid smoke of sulfur dioxide - sulfur oxide (IV) SO 2. The course of the reaction can be judged by a change in color, the appearance of a gas with a pungent odor, and the release of heat and light.

The instantaneous burning reaction of magnesium powder was used as a "flash" in photography until the electric lamp appeared. Combustion reactions are a special case of a large group of chemical reactions that proceed with the release of heat (Fig. 93).

Rice. 93.
Combustion reactions:
a - candles; b - magnesium tape

The latter, for example, include the decomposition reaction of mercury oxide (II), which you can see in the photographs (Fig. 94), since due to the toxicity of mercury and its compounds, this reaction is prohibited in school conditions.

Rice. 94.
Decomposition of mercury oxide (II):
a - the initial substance mercury oxide (II) (red); b - one of the reaction products - mercury (silver-white)

Schematically, exothermic and endothermic reactions are shown in Figure 95.

Rice. 95.
Reactions:
a - exothermic; b - endothermic

In conclusion, consider what conditions must be met for a chemical reaction to occur.

  1. It is necessary that the reacting substances come into contact, and the larger the area of ​​\u200b\u200btheir contact, the faster the chemical reaction proceeds. Therefore, solids are crushed and mixed, and highly soluble substances are dissolved and the solutions are drained.
  2. The second important condition is heating. Some reactions (usually exothermic ones) go without heating, and only for some exothermic reactions it is necessary only for the reaction to begin, but for endothermic reactions, heating is necessary throughout the entire reaction.
  3. Some reactions proceed under the influence of electric current, light, etc.

Keywords and phrases

  1. Signs of chemical reactions.
  2. Conditions for the flow of chemical reactions.
  3. Reactions are exo- and endothermic.
  4. combustion reaction.

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  1. Refer to the electronic application. Study the material of the lesson and complete the suggested tasks.
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Questions and tasks

  1. Place a slice of lemon or a few crystals of citric acid in a glass of strongly brewed fresh tea. What are you watching?
  2. Prepare a homemade fizzy drink with citric acid. Dissolve a little acid at the tip of a teaspoon in water, and then add the same amount of baking (drinking) soda to the resulting solution. What are you watching?
  3. Old copper coins and bronze items are often covered with a greenish coating, while silver ones are black. What does the appearance of these raids say? How to clean products from them?
  4. What chemical error did journalists make when they wrote, for example, the following phrase in their reports: “The scene of the incident was illuminated by continuous flashes of magnesium”?
  5. Tell us about the design and operation of a fire extinguisher.
  6. Why can't fired oil products or flared electrical wires be extinguished with water? How to pay them off?

In chemical phenomena or chemical reactions, as you know, there is a transformation of one substance into another. These transformations are accompanied by external signs: the appearance of a precipitate or gas, a change in color, the release or absorption of heat, and the appearance of an odor.

Remember the experiments that the teacher showed you in the first lessons. The interaction of marble - calcium carbonate CaCO3 - with hydrochloric acid HCl was accompanied by an abundant release of carbon dioxide - carbon monoxide (IV) CO2. And passing it through transparent lime water - calcium hydroxide Ca (OH) 2 - occurred with the formation of a precipitate - calcium carbonate CaCO3.

We will observe the course of certain chemical reactions and establish signs that confirm the formation of new substances and the appearance of new properties in them - insolubility or low solubility in water, smell, color, etc.

Pour 2 ml of a solution of iron (III) chloride FeCl3 into a test tube, and then add a few drops of a solution of potassium thiocyanate KNCS. We will see the appearance of a blood-red solution of a new substance - iron (III) thiocyanate Fe(NCS)3.

We mix iron and sulfur powders, no new substances appear. Iron from this mixture will be attracted by a magnet, and when the mixture is lowered into water, sulfur floats to the surface, that is, the mixture can be very easily separated. However, if this mixture of iron and sulfur powders is heated, then a chemical reaction will begin, which will continue further without heating with the release of heat - we will see how the mixture heats up. After the end of the reaction, a new substance will be obtained - iron (II) sulfide FeS. It is gray in color, sinks in water and is not attracted by a magnet.

We set fire to a little sulfur in an iron spoon - the eye will light up with a bluish flame and give abundant acrid smoke of sulfur dioxide - sulfur oxide (IV) 802. The course of the reaction can be judged by a change in color, the appearance of a gas with a pungent odor, the release of heat and light.

Reactions that release heat and light are called combustion reactions.

The instantaneous burning reaction of magnesium powder was used as a "flash" in photography until the electric lamp appeared. Combustion reactions are a special case of a large group of chemical reactions that proceed with the release of heat.

Reactions that proceed with the release of heat are called exothermic (exothermic - “outward”), and those that proceed with the absorption of heat are called gndothermic (endo - “inside”).

The latter, for example, include the reaction of water decomposition at high temperatures, with the formation of hydrogen and oxygen molecules.

In conclusion, consider what conditions must be met for a chemical reaction to occur.

1. It is necessary that the reacting substances come into contact, and the larger the area of ​​\u200b\u200btheir contact, the faster the chemical reaction proceeds. Therefore, solids are crushed and mixed or dissolved, and solutions of substances that dissolve well in water are drained.

2. The second important condition is heating. Exothermic reactions require oHO only to start the reaction, while endothermic reactions require heating throughout the reaction.

2. Conditions for the flow of chemical reactions.

3. Reactions and endothermic.

4. Combustion reaction.

Dip a piece of lemon or a few crystals of citric acid into a glass of strongly brewed fresh tea. What are you watching?

Make a homemade fizzy drink with citric acid. Dissolve a little acid on the tip of a teaspoon in water, and then add the same amount of baking (drinking) soda to the resulting solution. What are you watching?

Old copper coins and bronze items are often covered with a greenish coating, and silver ones with black. What does the appearance of these raids indicate? How to clean products from them?

What chemical mistake do journalists make when they write, for example, such a phrase in their reports: "The scene of the incident was illuminated by continuous flashes of magnesium"?

Tell us about the design and operation of a fire extinguisher.

Why can't fired oil products or electric wires be extinguished with water? How to pay them off?

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The checking expert told "PU" about the most common mistakes in the exam in chemistry last year. So, what you need to pay attention to when preparing:

1. Drawing up equations for the reactions of oxides with water. Remember that neither amphoteric nor basic oxides react with water. Basic oxides react with water only if strong bases are obtained - alkalis.

2. Drawing up equations for the reactions of nitric acid with metals. Important: nitric acid interacts with many metals, oxidizing them not at the expense of H +, but at the expense of nitrogen in the highest oxidation state (+5). As a result of such reactions, almost no H2 is released. The reaction produces: metal nitrate, nitrogen reduction product and water.

3. Determination of nitrate decomposition products. Repeat the schemes according to which alkali and alkaline earth metals, nitrates of metals located in the electrochemical series of metal voltages from Mg to Cu inclusive, and Li nitrate, nitrates of metals located in the electrochemical series of metal voltages to the right of Cu are decomposed. Remember there is a special case– decomposition of NH4NO3.

4. Determination of reaction products of metals and non-metals with alkalis. Please note that copper and iron do not react with alkalis, while zinc and aluminum will dissolve in alkalis. In addition, chlorine reacts with alkalis, and during the reaction, disproportionation of halogens occurs.

5. Determination of the environment in the preparation of equations of redox reactions. It is important to remember that if the reaction takes place in an alkaline medium, there cannot be an acid among its products, and vice versa, if the medium in which the reaction proceeds was acidic, there cannot be alkali in the reaction products.

6. Determination of oxidation products of benzene homologues. Do not forget that during the oxidation of benzene homologues with potassium permanganate, if the hydrocarbon radical connected to the benzene ring contains two or more carbon atoms, the carbon chain of the hydrocarbon radical breaks, resulting in the formation of aromatic carboxylic acids.

7. Determining the possibilities of disproportionation and counterproportionation. Solve the equations again and remember that when nitrogen dioxide interacts with water and with alkali, nitrogen is disproportionated: nitric and nitrous acid are formed, respectively (in the case of water) and a mixture of nitrate and nitrite (in the case of reaction with alkali).

8. Ignoring Markovnikov's rule and Zaitsev's rule. Reread the rule again and practice solving problems where you need to apply it.

9. Inattentive reading of the conditions of the problem, which does not make it possible to solve it correctly. The wording of assignments for test exams and for the main one sometimes differs. Read the terms to the end.

10. General errors in the preparation of reaction equations. Please note that when compiling reaction equations, it is very important to correctly arrange all the coefficients.

... Gold rusts and steel decays,

The marble crumbles. Everything is ready for death.

Sadness is the strongest thing on earth

And more durable - the royal word ...

(A. Akhmatova. From the collection “The Run of Time”)

Answer: Of course, gold does not rust, does not corrode. This is a mistake, but how accurately the difference in the dynamics of the destruction of steel and marble is noticed!

And what inaccuracy can be seen in L. Lavrenev's poem "Nobuzh"?

... The air turns into ozone ...

Answer: it is not air that turns into ozone, but oxygen.

Science fiction writers often use chemical terminology, while often making inaccuracies and errors. For example, in A. Belyaev's novel "The Air Seller" we read:

“Mr. Bailey opened the sixth door and I saw an amazing sight. Before us was a huge underground grotto. Dozens of lamps illuminated a large lake, its water was distinguished by an unusually beautiful blue color ...

Liquid air, Bailey said

I was amazed. Until now, I have seen liquid air only in small vessels in our laboratory.

Answer: I would like to note that the temperature at which air becomes liquid must be below minus 196 degrees. At this temperature, it is impossible for a person to be near this “lake”, he will die from hypothermia.

Do you think sulfuric acid smells? Let us turn to the popular story by A.N. Rybakov "The Adventures of Krosh":

"... The smell of sulfuric acid standing in the electrical shop reminded me of the trousers burned in Lipki ...".

What did the author of the story want to say, and how should the phrase be modified so that it sounds chemically competent? Of course, in the electric arc welding shop it could not smell of sulfuric acid - it simply has nowhere to come from. Then what did the author mean? In electric arc welding, materials, most often metals, are fused with the heat of an electric arc, the temperature of which reaches 5000 degrees. With such high temperature may form: ozone, nitric oxide (II).

Therefore, the phrase in the text should be changed as follows:

“... The smell of ozone, nitrogen oxides and nitric acid in the electrical shop reminded me of trousers burned in Lipki ...”

The catchphrase "without phosphorus there is no thought" appeared after Gensing established the presence of phosphorus ions in the brain tissue. The mechanistic materialists of the last century were especially fond of using it.

Although all the most important physiological processes in the human body, starting with muscle contraction and ending with thinking, are associated with chemical transformations of phosphorus, but this does not give reason to believe that the presence of phosphorus alone ensures the ability of the brain to think.

Calling phosphorus "white", it should be borne in mind that we are making a mistake. IN pure form white phosphorus is a colorless transparent substance.

White phosphorus crystals are very similar to diamonds, they are completely colorless, perfectly transparent and refract light so strongly that they play with all the colors of the rainbow in the sun.

Often statements require appropriate explanations. For example, in I. Silvinsky we read:

... You see - marble:

It's just calcium...

Marble is not calcium, but salt, calcium carbonate. This is mistake.

And of course, one cannot fail to recall an excerpt from the famous "The Hound of the Baskervilles" by A.K. Doyle, in which a significant chemical error was made: "…Yes! It was a dog, huge, pitch black. But none of us mortals have ever seen such a dog. Flames shot out of her open mouth, sparks flew from her eyes, iridescent fire flickered across her muzzle and nape. In no one's inflamed brain could a vision more terrible, more disgusting than this hellish creature that jumped out of the fog at us ... A terrible dog, the size of a young lioness. Its huge maw was still glowing with bluish flames, its deep-seated wild eyes were circled with fiery circles. I touched this luminous head and, taking my hand away, saw that my fingers also glowed in the darkness.

“Phosphorus,” I said.

White phosphorus glows in the dark, and, firstly, it is poisonous, and secondly, it ignites spontaneously in air, so it cannot be used for application to the body of an animal.