Like acids. The education program requires students to memorize the names and formulas of six representatives of this group. And, looking through the table provided by the textbook, you notice in the list of acids the one that comes first and interested you in the first place - hydrochloric acid. Alas, neither properties nor any other information about it is studied in school classes. Therefore, those who are eager to gain knowledge outside school curriculum looking for additional information in all sorts of sources. But often many people do not find the information they need. And therefore, the topic of today’s article is devoted to this particular acid.
Definition
Hydrochloric acid is a strong monobasic acid. In some sources it may be called hydrochloric and hydrochloric acid, as well as hydrogen chloride.
Physical propertiesIt is a colorless, caustic liquid that fumes in air (photo on the right). However, industrial acid, due to the presence of iron, chlorine and other additives in it, has a yellowish color. Its highest concentration at a temperature of 20 o C is 38%. Density of hydrochloric acid with these parameters it is equal to 1.19 g/cm 3 . But this compound has completely different data in different degrees of saturation. As the concentration decreases, the numerical value of molarity, viscosity and melting point decreases, but the specific heat capacity and boiling point increase. Solidification of hydrochloric acid of any concentration gives various crystalline hydrates.
Chemical properties
All metals that stand before hydrogen in electrochemical series their voltages can interact with this compound, forming salts and releasing hydrogen gas. If they are replaced with metal oxides, the reaction products will be soluble salt and water. The same effect will occur when hydrochloric acid reacts with hydroxides. If you add to it any metal salt (for example, sodium carbonate), the remainder of which was taken from a weaker acid (carbonic acid), then the chloride of this metal (sodium), water and a gas corresponding to the acidic residue (in in this case- carbon dioxide).
Receipt
The compound now discussed is formed when hydrogen chloride gas, which can be produced by burning hydrogen in chlorine, is dissolved in water. Hydrochloric acid, which is obtained using this method, is called synthetic. Exhaust gases can also serve as a source for the extraction of this substance. And such hydrochloric acid will be called abgasic. IN Lately the level of production of hydrochloric acid using this method is much higher than its production by the synthetic method, although the latter produces a compound in more pure form. These are all the ways of its production in industry. However, in laboratories, hydrochloric acid is obtained in three ways (the first two differ only in temperature and reaction products) using various types interaction chemical substances, such as:
- The effect of saturated sulfuric acid on sodium chloride at a temperature of 150 o C.
- Interaction of the above substances under conditions with a temperature of 550 o C and above.
- Hydrolysis of aluminum or magnesium chlorides.
Hydrometallurgy and electroplating cannot do without the use of hydrochloric acid, where it is needed to clean the surface of metals during tinning and soldering and to obtain chlorides of manganese, iron, zinc and other metals. IN Food Industry this compound is known as food additive E507 - there it is an acidity regulator necessary to make seltzer (soda) water. Concentrated hydrochloric acid is also found in the gastric juice of any person and helps digest food. During this process, its degree of saturation decreases, because this composition is diluted with food. However, with prolonged fasting, the concentration of hydrochloric acid in the stomach gradually increases. And since this compound is very caustic, it can lead to stomach ulcers.
Conclusion
Hydrochloric acid can be both beneficial and harmful to humans. Contact with the skin results in severe chemical burns, and the vapors of this compound irritate the respiratory tract and eyes. But if you handle this substance carefully, it can come in handy more than once.
Hydrochloric acid is a solution of hydrogen chloride in water. Hydrogen chloride (HCl) under normal conditions is a colorless gas with a specific pungent odor. However, we are dealing with its aqueous solutions, so we will focus only on them.
Hydrochloric acid is a colorless transparent solution with a pungent odor of hydrogen chloride. In the presence of impurities of iron, chlorine or other substances, the acid has a yellowish-green color. The density of a hydrochloric acid solution depends on the concentration of hydrogen chloride in it; some data is given in table 6.9.
Table 6.9. Density of hydrochloric acid solutions of various concentrations at 20°C.
From this table it can be seen that the dependence of the density of a hydrochloric acid solution on its concentration can be described with accuracy satisfactory for technical calculations by the formula:
d = 1 + 0.5*(%) / 100
When dilute solutions boil, the HCl content in vapor is less than in solution, and when concentrated solutions boil, it is greater than in solution, which is reflected in the figure below. rice. 6.12 equilibrium diagram. The constantly boiling mixture (azeotrope) at atmospheric pressure has a composition of 20.22% wt. HCl, boiling point 108.6°C.
Finally, another important advantage of hydrochloric acid is the almost complete independence of the time of its acquisition from the time of year. As can be seen from rice. No. 6.13, acid of industrial concentration (32-36%) freezes at temperatures that are practically unattainable for the European part of Russia (from -35 to -45 ° C), in contrast to sulfuric acid, which freezes at positive temperatures, which requires the introduction of a tank heating operation.
Hydrochloric acid does not have the disadvantages of sulfuric acid.
Firstly, ferric chloride has increased solubility in hydrochloric acid solution (Fig. 6.14), which allows you to increase the concentration of ferric chloride in the solution to 140 g/l and even more; the danger of sediment formation on the surface disappears.
Working with hydrochloric acid can be carried out at any temperature inside the building (even at 10°C), and this does not cause noticeable changes in the composition of the solution.
Rice. 6.12. Liquid – vapor equilibrium diagram for the HCl – H 2 O system.
Rice. 6.13. State diagram (fusibility) of the HCl–H 2 O system.
Rice. 6.14. Equilibrium in the HCl – FeCl 2 system.
Finally, another very important advantage of hydrochloric acid is its full compatibility with flux, which uses chlorides.
Some disadvantage of hydrochloric acid as a reagent is its high volatility. The standards allow a concentration of 5 mg/m 3 of air volume in the workshop. The dependence of vapor pressure in an equilibrium state over an acid of various percentage concentrations is given in table 6.10. In general, when the acid concentration in the bath is less than 15% by weight, this condition is satisfied. However, when temperatures in the workshop rise (that is, in the summer), this indicator may be exceeded. Certain information about what acid concentration is permissible at a specific workshop temperature can be determined from rice. 6.15.
The dependence of the etching rate on concentration and temperature is shown in rice. 6.16.
Etching defects are usually caused by the following:
- using an acid with a higher or lower concentration compared to the optimal one;
- short etching duration (the expected etching duration at different acid and iron concentrations can be estimated from rice. 6.17;
- reduced temperature compared to optimal;
- lack of mixing;
- laminar movement of the etching solution.
These problems are usually solved using specific technological techniques.
Table 6.10. Dependence of the equilibrium concentration of hydrogen chloride on the acid concentration in the bath.
|
Acid concentration, % |
Acid concentration, % |
HCl concentration in air, mg/m3 |
|
Hydrogen chloride is a gas approximately 1.3 times heavier than air. It is colorless, but has a sharp, suffocating and characteristic odor. At a temperature of minus 84C, hydrogen chloride passes from a gaseous to a liquid state, and at minus 112C it solidifies. Hydrogen chloride dissolves in water. One liter of H2O can absorb up to 500 ml of gas. Its solution is called hydrochloric or hydrochloric acid. Concentrated hydrochloric acid at 20C is characterized by the maximum possible basic substance equal to 38%. The solution is a strong monobasic acid (it “smoke” in the air, and in the presence of moisture it forms an acid mist), it also has other names: hydrochloric acid, and according to the Ukrainian nomenclature - chloric acid. Chemical formula can be presented in this form: HCl. The molar mass is 36.5 g/mol. The density of concentrated hydrochloric acid at 20C is 1.19 g/cm³. This is a harmful substance that belongs to the second hazard class.
In its “dry” form, hydrogen chloride cannot react even with active metals, but in the presence of moisture the reaction proceeds quite vigorously. This strong hydrochloric acid is capable of reacting with all metals that are to the left of hydrogen in the voltage series. In addition, it interacts with basic and amphoteric oxides, bases, as well as with salts:
- Fe + 2HCl → FeCl2 + H2;
- 2HCl + CuO → CuCl2 + H2O;
- 3HCl + Fe(OH)3 → FeCl3 + 3H2O;
- 2HCl + Na2CO3 → 2NaCl + H2O + CO2;
- HCl + AgNO3 → AgCl↓ + HNO3.
In addition to the general properties characteristic of each strong acid, hydrochloric acid has reducing properties: in concentrated form it reacts with various oxidizing agents, releasing free chlorine. Salts of this acid are called chlorides. Almost all of them are highly soluble in water and completely dissociate into ions. Slightly soluble are: lead chloride PbCl2, silver chloride AgCl, monovalent mercury chloride Hg2Cl2 (calomel) and cuprous chloride CuCl. Hydrogen chloride is capable of reacting with a double or triple bond, resulting in the formation of chlorinated organic compounds.
In laboratory conditions, hydrogen chloride is produced by exposure to dry concentrated sulfuric acid. Reaction in different conditions may occur with the formation of sodium salts (acidic or moderate):
- H2SO4 + NaCl → NaHSO4 + HCl
- H2SO4 + 2NaCl → Na2SO4 + 2HCl.
The first reaction proceeds to completion at low heating, the second at higher temperatures. Therefore, in the laboratory, it is better to obtain hydrogen chloride using the first method, for which it is recommended to take the amount of sulfuric acid based on the production of the acid salt NaHSO4. Then, by dissolving hydrogen chloride in water, hydrochloric acid is obtained. In industry, it is obtained by burning hydrogen in an atmosphere of chlorine or by treating dry sodium chloride (only the second with concentrated sulfuric acid. Hydrogen chloride is also obtained as a by-product during the chlorination of saturated organic compounds. In industry, hydrogen chloride obtained by one of the above methods is dissolved in special towers in which liquid is passed from top to bottom, and gas is supplied from bottom to top, that is, according to the counterflow principle.
Hydrochloric acid is transported in special rubberized tanks or containers, as well as in polyethylene barrels with a capacity of 50 liters or glass bottles with a capacity of 20 liters. There is a risk of formation of explosive hydrogen-air mixtures. Therefore, contact of the hydrogen formed as a result of the reaction with air must be completely excluded, as well as (with the help of anti-corrosion coatings) contact of the acid with metals. Before removing the apparatus and pipelines where it was stored or transported for repairs, it is necessary to carry out nitrogen purging and monitor the state of the gas phase.
Hydrogen chloride is widely used in industrial production and laboratory practice. It is used to obtain salts and as a reagent in analytical studies. Technical hydrochloric acid is produced in accordance with GOST 857-95 (the text is identical international standard ISO 905-78), reagent - according to GOST 3118-77. The concentration of the technical product depends on the brand and variety and can be 31.5%, 33% or 35%, and externally the product is yellowish in color due to the content of impurities of iron, chlorine and other chemicals. The reactive acid should be a colorless and transparent liquid with a mass fraction of 35 to 38%.
Approximate solutions. In most cases, the laboratory has to use hydrochloric, sulfuric and nitric acids. Acids are commercially available in the form of concentrated solutions, the percentage of which is determined by their density.
Acids used in the laboratory are technical and pure. Technical acids contain impurities, and therefore are not used in analytical work.
Concentrated hydrochloric acid smokes in air, so you need to work with it in a fume hood. The most concentrated hydrochloric acid has a density of 1.2 g/cm3 and contains 39.11% hydrogen chloride.
The dilution of the acid is carried out according to the calculation described above.
Example. You need to prepare 1 liter of a 5% solution of hydrochloric acid, using a solution with a density of 1.19 g/cm3. From the reference book we find out that a 5% solution has a density of 1.024 g/cm3; therefore, 1 liter of it will weigh 1.024 * 1000 = 1024 g. This amount should contain pure hydrogen chloride:
An acid with a density of 1.19 g/cm3 contains 37.23% HCl (we also find it from the reference book). To find out how much of this acid should be taken, make up the proportion:
or 137.5/1.19 = 115.5 acid with a density of 1.19 g/cm3. Having measured out 116 ml of acid solution, bring its volume to 1 liter.
Sulfuric acid is also diluted. When diluting it, remember that you need to add acid to the water, and not vice versa. When diluting, strong heating occurs, and if you add water to the acid, it may splash, which is dangerous, since sulfuric acid causes severe burns. If acid gets on clothes or shoes, you should quickly wash the doused area with plenty of water, and then neutralize the acid with sodium carbonate or ammonia solution. In case of contact with the skin of your hands or face, immediately wash the area with plenty of water.
Particular care is required when handling oleum, which is a sulfuric acid monohydrate saturated with sulfuric anhydride SO3. According to the content of the latter, oleum comes in several concentrations.
It should be remembered that with slight cooling, oleum crystallizes and is in a liquid state only at room temperature. In air it smokes, releasing SO3, which forms sulfuric acid vapor when interacting with air moisture.
It is very difficult to transfer oleum from large to small containers. This operation should be carried out either under draft or in air, but where the resulting sulfuric acid and SO3 cannot have any harmful effect on people and surrounding objects.
If the oleum has hardened, it should first be heated by placing the container with it in a warm room. When the oleum melts and turns into an oily liquid, it must be taken out into the air and there poured into a smaller container, using the method of squeezing with air (dry) or an inert gas (nitrogen).
When mixed with water nitric acid heating also occurs (not as strong, however, as in the case of sulfuric acid), and therefore precautions must be taken when working with it.
Solid organic acids are used in laboratory practice. Handling them is much simpler and more convenient than liquid ones. In this case, care should only be taken to ensure that the acids are not contaminated with anything foreign. If necessary, solid organic acids are purified by recrystallization (see Chapter 15 “Crystallization”),
Precise solutions. Precise acid solutions They are prepared in the same way as approximate ones, with the only difference that at first they strive to obtain a solution of a slightly higher concentration, so that later it can be diluted precisely, according to calculations. For precise solutions, use only chemically pure preparations.
The required amount of concentrated acids is usually taken by volume calculated based on density.
Example. You need to prepare 0.1 and. H2SO4 solution. This means that 1 liter of solution should contain:
An acid with a density of 1.84 g/cmg contains 95.6% H2SO4 n to prepare 1 liter of 0.1 n. of the solution you need to take the following amount (x) of it (in g):
The corresponding volume of acid will be:
Having measured exactly 2.8 ml of acid from the burette, dilute it to 1 liter in a volumetric flask and then titrate with an alkali solution to establish the normality of the resulting solution. If the solution turns out to be more concentrated), the calculated amount of water is added to it from a burette. For example, during titration it was found that 1 ml of 6.1 N. H2SO4 solution contains not 0.0049 g of H2SO4, but 0.0051 g. To calculate the amount of water needed to prepare exactly 0.1 N. solution, make up the proportion:
Calculation shows that this volume is 1041 ml; the solution needs to be added 1041 - 1000 = 41 ml of water. You should also take into account the amount of solution taken for titration. Let 20 ml be taken, which is 20/1000 = 0.02 of the available volume. Therefore, you need to add not 41 ml of water, but less: 41 - (41*0.02) = = 41 -0.8 = 40.2 ml.
* To measure the acid, use a thoroughly dried burette with a ground stopcock. .
The corrected solution should be checked again for the content of the substance taken for dissolution. Accurate solutions of hydrochloric acid are also prepared using the ion exchange method, based on an accurately calculated sample of sodium chloride. The sample calculated and weighed on an analytical balance is dissolved in distilled or demineralized water, and the resulting solution is passed through a chromatographic column filled with a cation exchanger in the H-form. The solution flowing from the column will contain an equivalent amount of HCl.
As a rule, accurate (or titrated) solutions should be stored in tightly closed flasks. A calcium chloride tube must be inserted into the stopper of the vessel, filled with soda lime or ascarite in the case of an alkali solution, and with calcium chloride or simply cotton wool in the case of an acid.
To check the normality of acids, calcined sodium carbonate Na2COs is often used. However, it is hygroscopic and therefore does not fully satisfy the requirements of analysts. It is much more convenient to use acidic potassium carbonate KHCO3 for these purposes, dried in a desiccator over CaCl2.
When titrating, it is useful to use a “witness”, for the preparation of which one drop of acid (if an alkali is being titrated) or alkali (if an acid is being titrated) and as many drops of an indicator solution as added to the titrated solution are added to distilled or demineralized water.
The preparation of empirical, according to the substance being determined, and standard solutions of acids is carried out by calculation using the formulas given for these and the cases described above.
Hydrochloric acid is one of the most powerful and dangerous substances for humans on the list of hazardous substances. However, what is surprising is that it exists in the body of every person: hydrochloric acid is an integral part of gastric juice and plays an important role in the digestive process. In an amount of 0.2%, it promotes the transition of food masses from the stomach to the duodenum and neutralizes microbes entering the stomach from external environment. It also activates the enzyme pepsinogen, participates in the formation of secretin and some other hormones that stimulate the activity of the pancreas. For this purpose, it is used in medicine, prescribing its solution to patients to increase the acidity of gastric juice. In general, hydrochloric acid has wide applications in our lives. For example, in heavy industry - for the production of chlorides of various metals, in the textile industry - for the production of synthetic dyes; For the food industry, acetic acid is made from it, for the pharmaceutical industry - Activated carbon. It is also a component of various adhesives and hydrolytic alcohol. It is used for etching metals, cleaning various vessels, casing pipes of boreholes from carbonates, oxides and other sediments and contaminants. In metallurgy, hydrochloric acid is used to treat ores, and in the leather industry, leather is used before tanning and dyeing. Hydrochloric acid is transported in glass bottles or rubberized (rubber-coated) metal vessels, as well as in plastic containers.
What is it as a chemical?
Hydrochloric acid, or hydrochloric acid, is an aqueous solution of hydrogen chloride HCl, which is a clear, colorless liquid with a pungent odor of hydrogen chloride. The technical variety of acid has a yellowish-green color due to impurities of chlorine and iron salts. The maximum concentration of hydrochloric acid is about 36% HCl; such a solution has a density of 1.18 g/cm3. Concentrated acid “smoke” in air, since the released gaseous HCl forms tiny droplets of hydrochloric acid with water vapor.
Despite this characteristic, when in contact with air, hydrochloric acid is not flammable or explosive. But at the same time, it is one of the strongest acids and dissolves (with the release of hydrogen and the formation of salts - chlorides) all metals in the voltage series up to hydrogen. Chlorides are also formed when hydrochloric acid reacts with metal oxides and hydroxides. It behaves as a reducing agent with strong oxidizing agents.
Salts of hydrochloric acid are chlorides and, with the exception of AgCl, Hg2Cl2, are highly soluble in water. Materials such as glass, ceramics, porcelain, graphite, and fluoroplastic are resistant to hydrochloric acid.
Hydrochloric acid is obtained from hydrogen chloride in water, which, in turn, is either directly synthesized from hydrogen and chlorine, or obtained by the action of sulfuric acid on sodium chloride.
Industrially produced (technical) hydrochloric acid has a strength of at least 31% HCl (synthetic) and 27.5% HCl (from NaCI). A commercial acid is called concentrated if it contains 24% or more HCl; if the HCl content is less, then the acid is called dilute.