To completely remove volatile substances resulting from thermal decomposition, calcination is used, which can be carried out using a gas burner flame, in muffle or crucible furnaces. To calcinate the substance in a burner flame, it is placed in a metal or porcelain crucible. It is then inserted into the porcelain triangle so that it fits 2/3 of its height into the triangle. The porcelain triangle is placed on the tripod ring. Calcination is carried out in a fume hood.
Muffle furnaces are used for calcination of substances at elevated temperatures (up to 1600 °C). Reagents must not be spilled in the furnace working area. The hot crucibles are removed from muffle furnace long crucible tongs.
Filtration
This is the process of movement of a liquid or gas through a porous partition, which is accompanied by the deposition of solids suspended in them on the porous partition.
particles. The effectiveness of the filtration process is measured by the speed and completeness of separation of solid particles from liquid or gas. It is influenced by: viscosity (liquids with low viscosity are easier to filter), temperature (the higher the temperature, the easier the solution is filtered, since the viscosity of the liquid decreases when heated), pressure (the greater difference pressure on both sides of the filter, the higher the filtration speed), the size and nature of the solid particles (the larger the particle size compared to the pore size of the filter, the faster and easier the filtration).
Various organic and inorganic substances are used as filter materials. It must be remembered that for filtering you cannot use materials that interact in any way with the filtered liquid. For example, alkalis, especially concentrated ones, cannot be filtered through a filter made of pressed glass and other materials containing silicon dioxide, since SiO 2 dissolves in alkalis. Filter materials can be: fibrous (cotton wool, wool, various fabrics, synthetic fibers), granular (quartz sand), porous (paper, ceramics). The choice of filter material depends on the requirements for the purity of the solution, as well as on its properties.
Filtration can be carried out in various ways: under normal conditions, with heating, under vacuum. Under normal conditions, glass funnels are used for filtering. Some filter material, such as cotton wool or filter paper, is placed inside the funnel. Filter paper is used to make simple or pleated filters.
To prepare a simple filter, take a sheet of filter paper square shape. Fold first in half, then again, as shown in Figure a:
The result is a square reduced by 4 times. The corner of the folded square is cut along an arc with scissors. Separate one layer of paper from the other three with your finger and straighten it.
To prepare a pleated filter, first proceed in the same way as when making a simple one, then fold it in half and bend each half several times in one direction and the other like an accordion (Fig. b). The top edge of the filter should not reach the edge of the funnel by 5 mm. The filter, correctly placed in the funnel, is moistened with the filtered liquid or distilled water.
When filtering, the funnel is mounted on a ring stand. The tip of the funnel should touch the wall of the filtrate vessel.
The liquid is poured over a glass rod, pressing it against the wall of the funnel. If it is necessary to filter a hot solution, then use a special funnel for hot filtering with electric or water heating.
Filtration under reduced pressure (under vacuum) allows for more complete separation of solids from liquids. 
liquid and increase the speed of the process. To do this, assemble a device consisting of a filtering device - a Buchner funnel (1) connected to a Bunsen flask (2), the Buchner flask is connected to the pump via a rubber hose. The size of the Buchner funnel should correspond to the mass of the sediment, but not the liquid. Place two circles of filter paper on the mesh bottom of the Buchner funnel, moisten them with distilled water, connect the device to the pump, ensuring that the filter fits tightly to the funnel mesh. The filtering process begins. First, pour most of the liquid onto the filter, then shake the remaining liquid with the sediment and pour the mixture into a funnel. When filtering, the precipitate should not overfill the funnel, and the filtrate in the Bunsen flask should not reach the extension connecting the flask to the safety flask. At the end of filtration, first turn off the pump, then remove the funnel from the flask, and remove the precipitate onto a sheet of filter paper.
The condition of task C2 on the Unified State 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, non-paper chemistry. Not everyone understands the terms used and the processes involved. Let's try to figure it out.
Very often, concepts that seem completely clear to a chemist are perceived incorrectly by applicants. Here short dictionary such concepts.
Dictionary of obscure terms.
- Hitch- this is simply 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 :-)
- Ignite- heat the substance to a high temperature and heat until the end of the chemical reactions. This is not “mixing with potassium” or “piercing with a nail.”
- “A mixture of gases exploded”- this means that the substances reacted explosively. Usually an electric spark is used for this. The flask or vessel in this case don't explode!
- Filter- separate the precipitate from the solution.
- Filter- pass the solution through a filter to separate the precipitate.
- Filtrate- this is filtered solution.
- Dissolution of a substance- This is the transition of a substance into solution. It can occur without chemical reactions (for example, when dissolved in water table salt NaCl produces a solution of table salt NaCl, and not alkali and acid separately), or during the dissolution process the substance reacts with water and forms a solution of another substance (when barium oxide is dissolved, a solution of barium hydroxide is obtained). Substances can be dissolved not only in water, but also in acids, alkalis, etc.
- Evaporation- this is the removal of water and volatile substances from a solution without decomposing the solids contained in the solution.
- Evaporation- This is simply reducing the mass of water in a solution by boiling.
- Fusion- this is the joint heating of two or more solid substances to a temperature when their melting and interaction begins. It has nothing to do with river swimming :-)
- Sediment and residue.
These terms are very 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 (test tubes or flasks).
"Remainder"- is a substance that left, was not completely consumed 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 may be called the remainder. - Saturated a solution is a solution in which, at a given temperature, the concentration of a substance is the maximum 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 you can additionally dissolve some more amount of this substance until it becomes saturated.
Diluted And "very" diluted solution is a very conditional concept, more qualitative than quantitative. It is assumed that the concentration of the substance is low.
For acids and alkalis the term is also used "concentrated" solution. This is also a conditional characteristic. For example, concentrated hydrochloric acid is only about 40% concentrated. And concentrated sulfuric acid is an anhydrous, 100% acid.
In order to solve such problems, you need 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 various connections, electrolysis of solutions and melts 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 of the color and state of aggregation of most of the substances being studied - metals, non-metals, oxides, salts.
That is why we analyze this type of assignment at the very end of the study of general and inorganic chemistry.
Let's look at a few examples of such tasks.
Example 1: The product of the reaction of lithium with nitrogen was treated with water. The resulting gas was passed through a solution of sulfuric acid until the chemical reactions stopped. 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:Weighed aluminum was dissolved in dilute nitric acid, and a gaseous simple substance was released. Sodium carbonate was added to the resulting solution until gas evolution completely stopped. Dropped out the precipitate was filtered And calcined, filtrate evaporated, the resulting solid the rest was melted down with ammonium chloride. The released gas was mixed with ammonia and the resulting mixture was heated.
Solution:
Example 3: Aluminum oxide was fused with sodium carbonate, and the resulting solid was dissolved in water. Sulfur dioxide was passed through the resulting solution until the reaction completely stopped. The precipitate that 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 solution of hydrochloric acid, the resulting gas was passed through an excess of sodium hydroxide solution, then a solution of iron (II) chloride was added. The resulting precipitate was fired. 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. This released a gas that 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 Unified State Examination in Chemistry for independent work.
- Copper nitrate was calcined, and the resulting solid precipitate was dissolved in sulfuric acid. Hydrogen sulfide was passed through the solution, the resulting black precipitate was fired, and the solid residue was dissolved by heating in concentrated nitric acid.
- Calcium phosphate was fused with coal and sand, then the resulting simple substance was burned in excess oxygen, the combustion product was dissolved in excess caustic soda. A barium chloride solution was added to the resulting solution. The resulting precipitate was treated with excess phosphoric acid.
- 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.
- Dry sodium chloride was treated with concentrated sulfuric acid with low heating, and the resulting gas was passed into a solution of barium hydroxide. A solution of potassium sulfate was added to the resulting solution. The resulting sediment was fused with coal. The resulting substance was treated with hydrochloric acid.
- A sample of aluminum sulfide was treated with hydrochloric acid. At the same time, 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 lead nitrate solution. The resulting precipitate was treated with a solution of hydrogen peroxide.
- Aluminum powder was mixed with sulfur powder, the mixture was heated, the resulting substance was treated with water, a gas was released and a precipitate was formed, to which an excess of potassium hydroxide solution was added until completely dissolved. This solution was evaporated and calcined. An excess of hydrochloric acid solution was added to the resulting solid.
- The potassium iodide solution was treated with a chlorine solution. The resulting precipitate was treated with a solution of sodium sulfite. A solution of barium chloride was first added to the resulting solution, and after separation of the precipitate, a solution of silver nitrate was added.
- Gray-green powder of chromium (III) oxide was fused with an excess of alkali, the resulting substance was dissolved in water, resulting in a dark green solution. Hydrogen peroxide was added to the resulting alkaline solution. The result is a solution yellow color, which upon addition of sulfuric acid acquires Orange color. When hydrogen sulfide is passed through the resulting acidified orange solution, it becomes cloudy and turns green again.
- (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 and calcined. The resulting solid residue was fused with sodium carbonate.
- (MIOO 2011, training work) Silicon was dissolved in a concentrated solution of potassium hydroxide. Excess hydrochloric acid was added to the resulting solution. The cloudy solution was heated. The resulting precipitate was filtered and calcined with calcium carbonate. Write the equations for the reactions described.
Answers to tasks for independent solution:
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Calcination is the operation of heating solids to a high temperature (above 400 ° C) in order to: a) free them from volatile impurities; b) achieving constant mass; c) carrying out reactions occurring at high temperatures; d) ashing after preliminary combustion of organic substances. Heating to high temperatures is carried out in furnaces (muffle or crucible). Very often in laboratories it is necessary to calcinate substances such as CaCl2*bH2O, Na2SO4*10H2O, etc., for the purpose of dehydration. Calcination is usually carried out on gas stoves, the substance is placed on steel frying pans. If contamination of the preparation with iron cannot be allowed, then it should be calcined in fireclay plates or frying pans. You should never put a large amount of salt on the pan, as dehydration causes the salt to scatter, causing significant salt loss.
If you have to heat something in a porcelain or fireclay crucible, then the crucible is heated gradually: first on a small flame, then the flame is gradually increased. To avoid losses during ignition, crucibles are usually covered with lids. If you have to ashes something in such a crucible, then first, with low heating, burn the substance in an open crucible and only then close the crucible with a lid.
If the porcelain crucible is dirty inside after work, then to clean it, pour concentrated nitric acid or fuming hydrochloric acid into it and carefully heat it. If neither nitric nor hydrochloric acid removes contamination, then take a mixture of them in the following proportions: Nitric acid- 1 volume and hydrochloric acid - 3 volumes. Sometimes contaminated crucibles are treated either with a concentrated solution of KHSO4 when heated, or by melting this salt in a crucible and then washing it with water. There are, however, cases when all of the above techniques do not help; This type of crucible, which cannot be cleaned, is recommended to be used for some non-essential work.
In the practice of analytical work, when it is necessary to calcinate metal oxides, for example PerOz, care must be taken to ensure that the burner flame does not come into contact with the substance being calcined (to avoid reduction). In such cases, platinum plates are used with a hole in the center into which a crucible is inserted. These plates can be strengthened in asbestos cardboard. Instead of platinum, clay or fireclay plates that do not oxidize and do not collapse when ignited can be used. round hole in the center.
When calcining the precipitate in Gooch's crucible the latter is inserted into an ordinary one, several large sizes porcelain crucible so that the walls of both crucibles do not touch. To do this, the Gooch crucible is wrapped in a strip of moistened asbestos and, pressing, pressed into the safety crucible so that the distance between the bottom of both is equal to several millimeters. First, everything is dried together at 100 ° C, then the Gooch crucible is removed, and the safety crucible along with the asbestos ring is strongly calcined before the first use.
Platinum crucibles require very careful handling and are often burned through by inexperienced workers. To avoid this, heating platinum cookware on a bare flame must be carried out so that the internal cone of the burner flame does not touch the platinum. When this cone comes into contact with platinum, platinum carbide is formed. Severe destruction of platinum occurs at a temperature close to its melting point.
Minor surface damage is eliminated by heating in an oxidizing environment. A badly damaged crucible, together with the resulting platinum carbide powder (which must be collected), is handed over for remelting.
If the platinum crucible becomes dirty, it should be cleaned by heating pure nitric acid (without traces of hydrochloric acid) in it. If this does not help, KHSO4 or NaHS04 is melted in a crucible. When this does not achieve the goal, the walls of the crucible are wiped with the finest quartz (white) sand or fine emery (No. LLC).
Quartz crucibles are very convenient, having many valuable properties, such as: high thermal strength, chemical indifference to most substances, etc. However, it must be remembered that quartz is fused with alkalis or alkaline salts.
In some cases, calcination or heating must be carried out either in an oxidizing, or reducing, or neutral environment. Most often, tubular or special furnaces are used for these purposes, through which the corresponding gas is passed from a cylinder during calcination. To create an oxidizing environment, oxygen is passed through, and to create a reducing environment, hydrogen or carbon monoxide is passed through. A neutral atmosphere is created by passing argon
Rice. 231. Split oven for heating to high temperature.
and sometimes nitrogen. When deciding which gas should be used in each individual case, you need to know whether the selected gas will high temperature react with this substance. Even such a seemingly inert gas as nitrogen, under certain conditions, can form compounds such as nitrides.
For calcination using gas burners A split stove is very convenient (Fig. 231). It is made from two fireclay or diatomite bricks, hollowed out same size recesses so that when bricks are placed on top of each other, a chamber is formed inside. A hole with a diameter of 15 mm is drilled in the center of the top brick, and 25 mm in the center of the bottom brick. In the plane of contact of the bricks, grooves are made to strengthen the porcelain triangle in which the crucible is placed.
By heating this furnace with a Teklu or Mecker burner, you can reach a temperature of up to 1100 ° C. The temperature is adjusted by changing the distance of the furnace from the burner.
When calcination in a platinum crucible is not possible, so-called “soda” crucibles can be used. Finely crushed and pre-calcined sodium carbonate is poured into a porcelain crucible, for example No. 4, up to half its height. A smaller crucible is then pressed into the salt.
Rice. 232 Forming soda crucibles
Place overnight in a muffle furnace that is turned off after heating. By morning, the soda crucible is ready and alkaline melting can be carried out in it, for example, some ores or minerals. Na2CO3 melts at a temperature of 870° C; therefore, the “soda” crucible can be heated up to 600° C.
Task C2 of the Unified State Examination in Chemistry is a description chemical experiment, according to which you will need to create 4 reaction equations. According to statistics, this is one of the most difficult tasks; a very low percentage of those passing it cope with it. Below are recommendations for solving task C2.
Firstly, in order to correctly solve task C2 of the Unified State Exam in chemistry, you need to correctly imagine the actions to which substances are subjected (filtering, evaporation, roasting, calcination, sintering, fusion). It is necessary to understand where a physical phenomenon occurs with a substance, and where - chemical reaction. The most commonly used actions with substances are described below.
Filtration - a method of separating heterogeneous mixtures using filters - porous materials that allow liquid or gas to pass through, but retain solids. When separating mixtures containing a liquid phase, a solid substance 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 crystallizing from them a solid in the form of a crystalline hydrate, or until the solvent has completely evaporated in order to obtain the dissolved substance in its pure form.
Calcination – heating a substance to change it chemical composition. Calcination can be carried out in air or in an inert gas atmosphere. When calcined in air, crystalline hydrates lose water of crystallization, 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 2Sintering, fusion – This is the heating of two or more solid reagents, leading to their interaction. If the reagents are resistant to oxidizing agents, then sintering can be carried out in air:
Al 2 O 3 + Na 2 CO 3 → 2NaAlO 2 + CO 2If one of the reactants or the reaction product can be oxidized by air components, the process is carried out in an inert atmosphere, for example: Cu + CuO → Cu 2 O
Substances that are unstable to the action of air components oxidize when heated and react with air components:
2Сu + O 2 → 2CuO;
4Fe(OH) 2 + O 2 →2Fe 2 O 3 + 4H 2 OBurning – a heat treatment process leading to combustion of a substance.
Secondly, knowledge of the characteristic features of substances (color, smell, state of aggregation) It will serve as a hint or check for the correctness of the actions performed. Below are the most characteristic features gases, solutions, solids.
Signs of gas:
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 odor: HF, HCl, HBr, HI, SO 2 ( strong odors), NH 3 ( ammonia) – highly soluble in water and poisonous, PH 3 (garlic), H 2 S (rotten eggs) – 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 the concentration of yellow before brown), dichromates, for example, K 2 Cr 2 O 7
Green: Hydroxo complexes of chromium (III), for example, K 3, nickel salts (II), for example NiSO 4, manganates, for example, K 2 MnO 4
Blue: Copper (II) salts, for example CuSO 4
From pink to purple: Permanganates, e.g. KMnO 4
From green to blue: Chromium (III) salts, for example CrCl 3
Colored sediments:
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
Green: 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
Red: 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 (when sublimated - purple pairs), most metals.
Green: 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 the sequence of the resulting substances.
And finally, in order to solve such problems, you need to 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.
“Matter 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 over the summer. Oxygen. The old man didn’t like the way the Owl hooted and sighed. Grif. The woodpecker's knocking and bird voices will immediately fall silent. Make a food chain. Grass. Fats. Cold-blooded.
“Properties of living matter” - Reflection: Levels of organization of life: Criteria of life: Study new topic. Why are there many concepts of “LIFE”, but there is not one short and generally accepted one? How do the properties of living things manifest themselves at different levels of organization? Highlight the main features of the concept “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 contained in 1 mole? Epigraph. 1. B measuring cylinder measure out 12 tablespoons of water. It is measured in g/mol. Shows the mass of 1 mole of a substance. Lesson - research: “Amount of substance. Has a numerical value of 6.02 1023.
“Matter” - Currently, just over a hundred types of atoms are known. What if there are no clouds, but the Sun is shining? Draw appropriate conclusions. Carrying out. Use the dictionary to find the definition of the term “extraction.” Similarly (carefully!), evaporate 3–4 ml of sugar solution. On Earth you almost never encounter physical substances.
"Substance in Chemistry" - Gaseous substances. Chemical. Acetone. Carbon dioxide. Ability to react with other substances. Physical. Pick up the right word. Properties of substances. Simple substances. Liquid substances. Complex substances. Water. Oxygen. Today we begin to study one of the most ancient of 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 characteristics. The mass fractions of elements in the compound are equal: potassium - 43.1%, chlorine - 39.2%, oxygen - 17.7%. Simple substances - metals. Distribute the substances. Silver. Metals and non-metals. Carbon.
