Calcination of substances. How to Solve C2 in Chemistry - Hints and Tips

"Substance and energy" - Carbohydrates. Let's save our nature. Why do animals eat? Make a food web. Signs of a living organism. A family of tits eats 35 thousand caterpillars during the summer. Oxygen. The old man did not like how the Owl hooted and sighed. Vulture. The woodpecker's knock And bird voices will immediately fall silent. Make up a food chain. Grass. Fats. Cold-blooded.

"Properties of living matter" - Reflection: Levels of organization of life: Criteria of life: Study of a new topic. Why there are many concepts of "LIFE", but there is not a single short and generally accepted one? How are the properties of living things manifested at various levels of organization? Highlight the main features of the concept of "Biological system". Organizing time.

"Amount of substance" - Molar mass is numerically equal to the relative molecular mass of the substance. How many structural units are there in 1 mole? Epigraph. 1. Measure 12 tablespoons of water into a graduated cylinder. g\mol is measured. Shows the mass in 1 mole of a substance. Lesson - research: “Amount of substance. It has a numerical value of 6.02 1023.

"Substance" - Currently, a little more than a hundred types of atoms are known. And if there are no clouds, but the sun is shining? Draw the appropriate conclusions. Holding. Look up the definition of "extraction" in the dictionary. Similarly (be careful!) Evaporate 3-4 ml of sugar solution. On Earth, you almost never encounter physical matter.

"Substance in chemistry" - Gaseous substances. Chemical. Acetone. Carbon dioxide. The ability to react with other substances. Physical. Choose the right word. Properties of substances. simple substances. liquid substances. Complex substances. Water. Oxygen. Today we begin to study one of the oldest of the important sciences - chemistry.

"Classification of substances" - Classification of substances. Acidic. It is not a hydroxide: Eliminate the substance that is superfluous according to the classification characteristic. The mass fractions of elements in the compound are equal: potassium - 43.1%, chlorine - 39.2%, oxygen - 17.7%. Simple substances are metals. Distribute substances. Silver. Metals and non-metals. Carbon.

The condition of task C2 for the exam in chemistry is a text describing the sequence of experimental actions. This text needs to be converted into reaction equations.

The difficulty of such a task is that schoolchildren have little idea of ​​experimental, not "paper" chemistry. Not everyone understands the terms used and the ongoing processes. Let's try to figure it out.

Very often, concepts that seem completely clear to a chemist are misunderstood by applicants. Here is a short glossary of such terms.

Dictionary of obscure terms.

1. Hinge- it's just a certain portion of a substance of a certain mass (it was weighed on the scales). It has nothing to do with the canopy over the porch :-)
2. Ignite- heat the substance to a high temperature and heat until the end of chemical reactions. This is not "potassium mixing" or "piercing with a nail."
3. "Blow up a mixture of gases"- this means that the substances reacted with an explosion. Usually an electric spark is used for this. The flask or vessel at the same time do not explode!
4. Filter- separate the precipitate from the solution.
5. Filter- pass the solution through a filter to separate the precipitate.
6. Filtrate- it's filtered solution.
7. Dissolution of a substance is the transition of a substance into a solution. It can occur without chemical reactions (for example, when sodium chloride NaCl is dissolved in water, a solution of sodium chloride NaCl is obtained, and not alkali and acid separately), or in the process of dissolution, the substance reacts with water and forms a solution of another substance (when barium oxide is dissolved, it will turn out barium hydroxide solution). Substances can be dissolved not only in water, but also in acids, alkalis, etc.
8. Evaporation- this is the removal of water and volatile substances from a solution without decomposition of the solids contained in the solution.
9. Evaporation- this is simply a decrease in the mass of water in a solution by boiling.
10. fusion- this is the joint heating of two or more solids to a temperature when they begin to melt and interact. It has nothing to do with swimming on the river :-)
11. Sediment and residue.
    These terms are often confused. Although these are completely different concepts.
    "The reaction proceeds with the release of a precipitate"- this means that one of the substances obtained in the reaction is slightly soluble. Such substances fall to the bottom of the reaction vessel (tubes or flasks).
    "Remainder" is a substance that left, was not spent completely or did not react at all. For example, if a mixture of several metals was treated with acid, and one of the metals did not react, it can be called remainder.
12. Saturated A solution is a solution in which, at a given temperature, the concentration of a substance is the highest possible and no longer dissolves.

    unsaturated a solution is a solution in which the concentration of a substance is not the maximum possible; in such a solution, some more amount of this substance can be additionally dissolved until it becomes saturated.

    Diluted and "very" diluted solution - these are very conditional concepts, rather qualitative than quantitative. It is assumed that the concentration of the substance is low.

    The term is also used for acids and bases. "concentrated" solution. This is also conditional. For example, concentrated hydrochloric acid has a concentration of only about 40%. And concentrated sulfuric is an anhydrous, 100% acid.

In order to solve such problems, it is necessary to clearly know the properties of most metals, non-metals and their compounds: oxides, hydroxides, salts. It is necessary to repeat the properties of nitric and sulfuric acids, potassium permanganate and dichromate, redox properties of various compounds, electrolysis of solutions and melts of various substances, decomposition reactions of compounds of different classes, amphotericity, hydrolysis of salts and other compounds, mutual hydrolysis of two salts.

In addition, it is necessary to have an idea about the color and state of aggregation of most of the studied substances - metals, non-metals, oxides, salts.

That is why we analyze this type of tasks at the very end of the study of general and inorganic chemistry.
Let's look at some examples of such tasks.

Example 1: The reaction product of lithium with nitrogen was treated with water. The resulting gas was passed through a solution of sulfuric acid until the chemical reactions ceased. The resulting solution was treated with barium chloride. The solution was filtered and the filtrate was mixed with sodium nitrite solution and heated.

Solution:

Example 2:Hinge aluminum was dissolved in dilute nitric acid, and a gaseous simple substance was released. Sodium carbonate was added to the resulting solution until the gas evolution ceased completely. dropped out the precipitate was filtered and calcined, filtrate evaporated, the resulting solid the rest was fused with ammonium chloride. The evolved gas was mixed with ammonia and the resulting mixture was heated.

Solution:

Example 3: Aluminum oxide was fused with sodium carbonate, the resulting solid was dissolved in water. Sulfur dioxide was passed through the resulting solution until the complete cessation of interaction. The precipitate formed was filtered off, and bromine water was added to the filtered solution. The resulting solution was neutralized with sodium hydroxide.

Solution:

Example 4: Zinc sulfide was treated with a hydrochloric acid solution, the resulting gas was passed through an excess of sodium hydroxide solution, then a solution of iron (II) chloride was added. The precipitate obtained was calcined. The resulting gas was mixed with oxygen and passed over the catalyst.

Solution:

Example 5: Silicon oxide was calcined with a large excess of magnesium. The resulting mixture of substances was treated with water. At the same time, a gas was released, which was burned in oxygen. The solid combustion product was dissolved in a concentrated solution of cesium hydroxide. Hydrochloric acid was added to the resulting solution.

Solution:

Tasks C2 from the USE options in chemistry for independent work.

1. Copper nitrate was calcined, the resulting solid precipitate was dissolved in sulfuric acid. Hydrogen sulfide was passed through the solution, the resulting black precipitate was calcined, and the solid residue was dissolved by heating in concentrated nitric acid.
2. Calcium phosphate was fused with coal and sand, then the resulting simple substance was burned in an excess of oxygen, the combustion product was dissolved in an excess of sodium hydroxide. A solution of barium chloride was added to the resulting solution. The resulting precipitate was treated with an excess of phosphoric acid.
3. Copper was dissolved in concentrated nitric acid, the resulting gas was mixed with oxygen and dissolved in water. Zinc oxide was dissolved in the resulting solution, then a large excess of sodium hydroxide solution was added to the solution.
4. Dry sodium chloride was treated with concentrated sulfuric acid at low heating, the resulting gas was passed into a barium hydroxide solution. A solution of potassium sulfate was added to the resulting solution. The resulting precipitate was fused with coal. The resulting substance was treated with hydrochloric acid.
5. A weighed portion of aluminum sulfide was treated with hydrochloric acid. In this case, gas was released and a colorless solution was formed. An ammonia solution was added to the resulting solution, and the gas was passed through a solution of lead nitrate. The precipitate thus obtained was treated with a solution of hydrogen peroxide.
6. Aluminum powder was mixed with sulfur powder, the mixture was heated, the resulting substance was treated with water, gas was released and a precipitate formed, to which an excess of potassium hydroxide solution was added until complete dissolution. This solution was evaporated and calcined. An excess of hydrochloric acid solution was added to the resulting solid.
7. The potassium iodide solution was treated with a chlorine solution. The resulting precipitate was treated with sodium sulfite solution. First, a solution of barium chloride was added to the resulting solution, and after separating the precipitate, a solution of silver nitrate was added.
8. Gray-green powder of chromium (III) oxide was fused with an excess of alkali, the resulting substance was dissolved in water, and a dark green solution was obtained. Hydrogen peroxide was added to the resulting alkaline solution. A yellow solution was obtained, which turns orange when sulfuric acid is added. When hydrogen sulfide is passed through the resulting acidified orange solution, it becomes cloudy and turns green again.
9. (MIOO 2011, training work) Aluminum was dissolved in a concentrated solution of potassium hydroxide. Carbon dioxide was passed through the resulting solution until the precipitation ceased. The precipitate was filtered off and calcined. The resulting solid residue was fused with sodium carbonate.
10. (MIOO 2011, training work) Silicon was dissolved in a concentrated solution of potassium hydroxide. An excess of hydrochloric acid was added to the resulting solution. The cloudy solution was heated. The separated precipitate was filtered off and calcined with calcium carbonate. Write the equations of the described reactions.

Answers to tasks for independent solution:

1. or

2. Task C2 of the Unified State Exam in chemistry is a description of a chemical experiment, according to which 4 reaction equations will need to be compiled. According to statistics, this is one of the most difficult tasks, a very low percentage of those who pass it cope with it. Below are recommendations for solving task C2.

   Firstly, in order to correctly solve the task C2 of the Unified State Examination in chemistry, you need to correctly imagine the actions that substances undergo (filtration, evaporation, roasting, calcination, sintering, fusion). It is necessary to understand where a physical phenomenon occurs with a substance, and where a chemical reaction occurs. The most commonly used actions with substances are described below.

   Filtration - a method for separating heterogeneous mixtures using filters - porous materials that pass liquid or gas, but retain solids. When separating mixtures containing a liquid phase, a solid remains on the filter, filtrate .

   Evaporation - the process of concentrating solutions by evaporating the solvent. Sometimes evaporation is carried out until saturated solutions are obtained, with the aim of further crystallization of a solid substance in the form of a crystalline hydrate, or until the solvent is completely evaporated in order to obtain a pure solute.

   Ignition - heating a substance to change its chemical composition. The calcination can be carried out in air and in an inert gas atmosphere. When calcined in air, crystalline hydrates lose crystallization water, for example, CuSO 4 ∙ 5H 2 O → CuSO 4 + 5H 2 O
   Thermally unstable substances decompose:
   Cu(OH) 2 →CuO + H 2 O; CaCO 3 → CaO + CO 2

   Sintering, fusion - This is the heating of two or more solid reactants, leading to their interaction. If the reagents are resistant to the action of oxidizing agents, then sintering can be carried out in air:
   Al 2 O 3 + Na 2 CO 3 → 2NaAlO 2 + CO 2

   If one of the reactants or the reaction product can be oxidized by air components, the process is carried out with an inert atmosphere, for example: Сu + CuO → Cu 2 O

   Substances that are unstable to the action of air components, when ignited, oxidize, react with air components:
   2Сu + O 2 → 2CuO;
   4Fe(OH) 2 + O 2 →2Fe 2 O 3 + 4H 2 O

   Burning - a heat treatment process that leads to the combustion of a substance.

   Secondly, knowledge of the characteristic features of substances (color, smell, state of aggregation) will serve you as a hint or verification of the correctness of the actions performed. Below are the most characteristic features of gases, solutions, solids.

   Signs of gases:

   Painted: Cl 2 - yellow-green; NO 2 - brown; O 3 - blue (all have smells). All are poisonous, dissolve in water, Cl 2 and NO 2 react with her.

   Colorless, odorless: H 2 , N 2 , O 2 , CO 2 , CO (poison), NO (poison), inert gases. All are poorly soluble in water.

   Colorless with odour: HF, HCl, HBr, HI, SO 2 (pungent odors), NH 3 (ammonia) are highly soluble in water and poisonous, PH 3 (garlic), H 2 S (rotten eggs) are slightly soluble in water, poisonous.

   Colored solutions:

   Yellow: Chromates, for example K 2 CrO 4, solutions of iron (III) salts, for example, FeCl 3.

   Orange: Bromine water, alcohol and alcohol-water solutions of iodine (depending on concentration from yellow before brown), dichromates, for example, K 2 Cr 2 O 7

   Greens: Hydroxocomplexes of chromium (III), for example, K 3, nickel (II) salts, for example NiSO 4, manganates, for example, K 2 MnO 4

   Blue: Copper (II) salts, such as CuSO 4

   Pink to purple: Permanganates, e.g. KMnO 4

   From green to blue: Salts of chromium (III), for example, CrCl 3

   Colored precipitation:

   Yellow: AgBr, AgI, Ag 3 PO 4 , BaCrO 4 , PbI 2 , CdS

   Brown: Fe(OH) 3 , MnO 2

   Black, black-brown: Sulfides of copper, silver, iron, lead

   Blue: Cu(OH) 2 , KFe

   Greens: Cr (OH) 3 - gray-green, Fe (OH) 2 - dirty green, turns brown in air

   Other colored substances:

   yellow : sulfur, gold, chromates

   Orange: copper oxide (I) - Cu 2 O, dichromates

   Reds: bromine (liquid), copper (amorphous), red phosphorus, Fe 2 O 3, CrO 3

   Black: СuO, FeO, CrO

   Gray with a metallic sheen: Graphite, crystalline silicon, crystalline iodine (during sublimation - purple vapors), most metals.

   Greens: Cr 2 O 3, malachite (CuOH) 2 CO 3, Mn 2 O 7 (liquid)

   Thirdly, when solving C2 tasks in chemistry, for greater clarity, it can be recommended to draw up transformation schemes or a sequence of substances obtained.

   And finally, in order to solve such problems, one must clearly know the properties of metals, non-metals and their compounds: oxides, hydroxides, salts. It is necessary to repeat the properties of nitric and sulfuric acids, potassium permanganate and dichromate, redox properties of various compounds, electrolysis of solutions and melts of various substances, decomposition reactions of compounds of different classes, amphotericity, hydrolysis of salts.

   
   
   
   
   
   
   

   The filtered and washed precipitate still contains moisture; usually it is dried and calcined. These operations make it possible to obtain a substance with a strictly defined chemical composition.

   Drying of the precipitate. The precipitate is dried along with the filter. Cover the funnel with the precipitate with a sheet of damp filter paper. Its edges are pressed tightly against the outer surface of the funnel, excess paper is removed. It turns out a paper lid that sits tightly on the funnel and protects the sediment from dust.

   After that, the funnel with sediment should be placed for 20-30 minutes in an oven with shelves with round holes. A funnel is inserted into one of them. The temperature in the cabinet is maintained no higher than 90-105 ° C - with stronger heating, the filter chars and disintegrates.

   The precipitates are calcined in porcelain crucibles of various sizes. Before starting the calcination, it is necessary to know the mass of the empty crucible. To do this, the crucible is preliminarily calcined to a constant mass, i.e., until its mass ceases to change. The crucibles are calcined in an electric muffle furnace, in a crucible furnace or on a gas burner, but always under the same temperature conditions under which the precipitate is supposed to be calcined. The calcination temperature is roughly judged by the color of the heat of the muffle (crucible) furnace:

   The crucible intended for calcination is taken with crucible tongs by the edge and placed in a muffle furnace. After 25-30 minutes of calcination, it is removed from the furnace, allowed to cool on a sheet of asbestos (or on a granite tile) and transferred to a desiccator. The latter is closed with a lid not immediately, but after 1-2 minutes; otherwise, when cooling, a vacuum is created in the desiccator and the lid will be difficult to open. Then the desiccator is taken to the weighing room and left for 15-20 minutes so that the crucible takes the temperature of the scales.

   After weighing the crucible on an analytical balance, it is again calcined for 15–20 min, cooled in a desiccator, and the weighing is repeated. If the result of the last weighing differs from the previous one by no more than ±0.0002 g, it is considered that the crucible has been adjusted to constant weight, i.e., it has been prepared for calcining the precipitate. Otherwise, the crucible is ignited, cooled and weighed again. The results of all weighings must be recorded in the laboratory journal.

   
   
   

   Ignition of the precipitate.Water of crystallization or constitutional water, which even dried sediment may contain, must be completely removed by calcination. In addition, during calcination, chemical decomposition of the substance often occurs. For example, calcium oxalate CaC 2 O 4 H 2 O, obtained by precipitation of Ca 2+ ions with ammonium oxalate, loses water of crystallization already upon drying:

   CaC 2 O 4 H 2 O → CaC 2 O 4 + H 2 O

   When ignited weakly, it releases carbon monoxide and turns into calcium carbonate:

   CaC 2 O 4 → CO 2 + CaCO 3

   Finally, upon strong calcination, calcium carbonate decomposes to form carbon dioxide and calcium oxide:

   CaCO 3 → CaO + CO 2

   The mass of calcium oxide and calculate the result of the determination. The temperature and duration of calcination of precipitation can be different.

   In the calcination technique itself, two cases are distinguished.

   1. Ignition of the precipitate without separation of the filter. This method is used when the calcined precipitate does not interact with the carbonized filter carbon. So, without removing the filter, precipitates of oxides of Al 2 O 3 , CaO and some others are calcined.

   The porcelain crucible, brought to constant weight, is placed on glossy (preferably black) paper. Carefully remove the dried filter with sediment from the funnel and, holding it over the crucible, roll it up. After that, carefully place it in the crucible. If, upon careful examination, traces of sediment are found on the funnel, then carefully wipe its inner surface with a piece of an ashless filter, which is placed in the same crucible. Finally, grains of sediment that spilled onto the paper when the filter was rolled up are also shaken off into the crucible. Then put the crucible on an electric stove and carefully ash (burn) the filter. Sometimes, instead, the crucible is inserted into a porcelain triangle on a tripod ring and heated on a small burner flame. It is desirable that the filter slowly char and decay without flashing, since combustion leads to the loss of the smallest sediment particles. If it does catch fire, then the flame is by no means blown out, but only stops heating and waits for the burning to stop.

   Having finished the ashing of the filter, the crucible is transferred to the muffle furnace and calcined for 25-30 minutes. The crucible is cooled in a desiccator, weighed and the value of its mass is recorded in a laboratory journal. Repeat calcination (15-20 min), cooling and weighing until a constant weight of the crucible with sediment is reached.

   2. Ignition of the precipitate with separation of the filter. This method is used when the filter carbonization precipitate can chemically interact with carbon (recover). For example, a precipitate of silver chloride AgCl is reduced by carbon to free silver; it is impossible to ignite it together with the filter.

   The well-dried precipitate is poured out of the filter as completely as possible onto glossy paper and covered with a beaker (or inverted funnel) to prevent loss. The filter with sediment particles remaining on it is placed in a crucible (brought to constant weight), burned and calcined. The previously separated precipitate is added to the calcined residue in the same crucible. After that, as usual, the contents of the crucible are calcined to constant weight.

   If the precipitate is filtered off with a glass crucible, then instead of calcination, drying to constant weight is resorted to. Of course, the filter crucible must first be brought to constant mass at the same temperature.

   If an irreparable error is made during the analysis (for example, part of the sediment is lost, part of the solution with sediment is spilled, etc.), then the determination should be started again without wasting time on obtaining a deliberately incorrect result.

   This lesson is a practical exercise during which various experiments are carried out, representing both physical and chemical processes. Conducted chemical reactions are given characteristics indicating the conditions for the onset and course of reactions, as well as their signs.

   Topic: Initial chemical ideas

   Lesson: Practice 3. Chemical reactions

   EXPERIENCE 1.

   A piece of paraffin is placed on a metal plate and heated. As a result, we observe a change in the state of aggregation of paraffin (transition to a liquid state). Despite the fact that the molten paraffin has become colorless (the color has changed), this phenomenon belongs to the physical ones, because. the composition of the substance remained the same, only its state of aggregation changed.

   

   Rice. 1. Melting of paraffin

   EXPERIENCE 2.

   Light a candle and let it burn a little. In the process of burning a candle, the wick and paraffin burn out, part of the paraffin melts, heating up from the heat released during the combustion process. The burning of the wick and paraffin is a chemical process, because starting materials are converted into new reaction products. These products are gaseous, because the candle is reduced in size. Combustion is accompanied by the release of heat and light.

   The melting of paraffin, as mentioned above, refers to physical phenomena. Describe the process of burning a candle. The conditions for the start of the reaction are arson and contact of the wick with air. The condition for the course of the reaction is the influx of fresh air (if it is stopped, the candle will go out). Signs of a reaction are the release of heat and light.

   

   2. Electronic version of the journal "Chemistry and Life" ().

   Homework

   With .14-15 №№ 9, 10 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.