Lesson in grade 8 on the topic: Hydrochloric acid and its salts.
purpose : study the chemical properties of hydrochloric acid, and consider the field of application of this acid.
Tasks :
Educational - in the process of research, to study the chemical properties of hydrochloric acid and get acquainted with the qualitative reaction to the chloride ion.
Developmental - to develop further skills in making equations chemical reactions; teach to compare, generalize, analyze and draw conclusions.
Educational - to develop cognitive activity through experiment.
Lesson type : a lesson in the assimilation of new knowledge.
Teaching method : explanatory-illustrative, problem-search, practical work, use of ICT.
Organizational forms : conversation, practical work, student messages.
ABOUT equipment and reagents: periodic system chemical elements, solubility table, test tube rack, hydrochloric acid, sodium hydroxide, silver nitrate, copper, magnesium, aluminum, blue litmus, methyl orange, phenolphthalein.
Techniques for enhancing the mental activity of students:
Analysis of educational information.
Disclosure of intersubject connections between chemistry, physics, biology.
Putting hypotheses.
Analysis and drawing up generalized conclusions.
During the classes.
opening speech teachers:
The topic of our lesson is "Hydrochloric acid and its properties." (slide 1)
Goethe's words will be the motto of our lesson:
“Just knowing is not everything, you need to be able to use knowledge”. (w. 2)
You will have to show how you can use your knowledge in different situations. First, let's remember what we know about acids. So first question:
What is acid? (a complex substance consisting of hydrogen atoms and an acidic residue).
How many hydrogen atoms can there be in acids? How are they classified according to this criterion? (one-, two-, three-basic). Give examples.
What can replace hydrogen? What is the result? (metals; salts).
Give the definition of salts. ( Complex substancesconsisting of metal atoms and an acid residue).
Concentrated hydrochloric acid (the mass fraction of hydrogen chloride is 37%) is a colorless solution, strongly fuming in humid air, with a pungent odor due to the release of hydrogen chloride. (slide 3 video of the experiment "Properties of fuming hydrochloric acid")
Getting hydrochloric acid:
1.In industry, it is obtained by burning hydrogen in chlorine and dissolving the reaction product in water.
2.In the laboratory H 2 SO 4 +2 NaCl →2 HCl + Na 2 SO 4
This gas dissolves easily in water: up to 450 volumes of hydrogen chloride - in one volume of water. In the test tube, hydrochloric acid is formed - a solution of hydrogen chloride in water.
1) Change in color of the indicator (litmus - red)
2) Interacts with metals (if the metal is in the row compiled by N.N. Beketov to hydrogen, then hydrogen is released and salt is formed.
An exception HNO 3 (other gases are released)
Mg + 2 HCl → MgCl 2 + H 2 2Al + 6HCl → 2AlCl 3 + 3H 2 Cu + HCl →
3) Interacts with basic and amphoteric oxides:
MgO + 2HCl → MgCl 2 + H 2 O ZnO + 2 HCl→ ZnCl 2 + H 2 O
4) Interacts with bases:
HCl + KOH → KCl + H 2 O 3HCl + Al (OH) 3 → AlCl 3 + 3H 2 O
5) Reacts with salts (in accordance with a number of acids, each previous acid can displace the next from the salt), another acid and another salt are formed.
HNO 3
H 2 SO 4 , HCl, H 2 SO 3 , H 2 CO 3 , H 2 S, H 2 SiO 3
────────────────────────
H 3 PO 4
CaCO 3 + 2HCl → CaCl 2 + H 2 O + CO 2
6) Reacts with silver nitrate, a white precipitate forms, which does not dissolve in water or acids.
HCl + AgNO 3 → AgCl ↓ + HNO 3
Silver nitrate is a reagent for hydrochloric acid and its salts, i.e. used as a qualitative reaction to recognize chloride ions.
Practical work
7) Interacts with oxidants (MnO 2 , KMO 4 , KClO 3 )
6HCl + KClO 3 \u003d KCl + 3H 2 O + 3Cl 2
Conclusion: in all the reactions we studied, chlorides - salts of hydrochloric acid were obtained.
We turn to the study of salts of hydrochloric acid, which are called chlorides.
Hydrochloric acid salts - chlorides .
Receiving:
1. Interaction of metals with chlorine.
2Fe + 3Cl 2 → 2FeCl 3
2. Interaction of hydrochloric acid with metals.
Mg + 2HCl → MgCl 2 + H 2
3. Interaction of hydrochloric acid with oxides
CaO + 2HCl → CaCl 2 + H 2 O
4. Interaction of hydrochloric acid with hydroxides
Ba (OH) 2 + 2HCl → BaCl 2 + 2H 2 O
5. Interaction of hydrochloric acid with some salts
Pb (NO 3) 2 + 2HCl → PbCl 2 ↓ + 2HNO 3
Most chlorides are water soluble (with the exception of silver, lead, and monovalent mercury chlorides).
Application of hydrochloric acid and its salts:
Hydrochloric acid is a part of gastric juice and promotes the digestion of protein foods in humans and animals.
Hydrogen chloride and hydrochloric acid are used for the production of drugs, dyes, solvents, and plastics.
Application of basic salts of hydrochloric acid:
KCl is a fertilizer, also used in the glass and chemical industries.
HgCl 2 - mercuric chloride - poison, used for disinfection in medicine, for dressing seeds in agriculture.
NaCl - table salt - raw material for the production of hydrochloric acid, sodium hydroxide, hydrogen, chlorine, bleach, soda. It is used in the leather and soap industry, in cooking and canning.
ZnCl 2 - for impregnation of wood against decay, in medicine, during brazing.
AgCl - used in black and white photography, since it has light sensitivity - it decomposes in the light to form free silver: 2AgCl \u003d\u003e 2Ag + Cl 2
Repetition and reinforcement assignments
№1. Carry out transformations according to the scheme:
HCl → Cl 2 → AlCl 3 → Al(OH) 3 → Al 2 O 3 → AlCl 3 → Cl 2
# 2. Substances are given:
Zn, Cu, Al, MgO, SiO 2 , Fe 2 O 3 , NaOH, Al(OH) 3 , Fe 2 (SO 4 ) 3 , CaCO 3 , Fe(NO 3 ) 3
Which of these substances will react with hydrochloric acid. Write chemical equations
Number 3. Solve the problem:
How much aluminum will react with an excess of hydrochloric acid to produce 5.6 L of hydrogen (NU)?
D / Z p. 49, task 4-5 p. 169.
Reflection
The teacher (together with the students evaluates the lesson, accepts their suggestions and wishes).
Guys, what have each of you learned in today's lesson?
Have you mastered the concepts: "chlorides", "inhibitor", " qualitative response»?
Were there moments of misunderstanding?
Were we able to resolve them during the conversation?
What are the most successful answers of your comrades?
What did you like or dislike in the lesson and why?
Students answer questions, assess the completeness of the knowledge gained, and self-assess their work. They identify the most interesting and complete answers, substantiate their point of view.
The degree of fulfillment of educational goals is revealed.
What is hydrochloric acid solution? It is a compound of water (H2O) and hydrogen chloride (HCl), which is a colorless thermal gas with a characteristic odor. Chlorides dissolve well and decompose into ions. Hydrochloric acid is the most famous compound that forms HCl, so you can tell about it and its features in detail.
Description
Hydrochloric acid solution belongs to the strong class. It is colorless, transparent and pungent. Although technical hydrochloric acid has a yellowish color, due to the presence of impurities and other elements. It "smokes" in the air.
It should be noted that this substance is also present in the body of every person. In the stomach, to be more precise, at a concentration of 0.5%. Interestingly, this amount is enough to completely destroy the razor blade. The substance will eat away at it in just a week.
Unlike the same sulfuric, by the way, the mass of hydrochloric acid in the solution does not exceed 38%. We can say that this indicator is a "critical" point. If you start to increase the concentration, the substance will simply evaporate, as a result of which the hydrogen chloride will simply evaporate along with the water. Plus, this concentration is only maintained at 20 ° C. The higher the temperature, the faster the evaporation takes place.
Interaction with metals
Hydrochloric acid solution can undergo many reactions. First of all, with metals that stand up to hydrogen in the series of electrochemical potentials. This is the sequence in which the elements move as they increase in such a characteristic measure as the electrochemical potential (φ 0). This indicator is extremely important in cation reduction half-reactions. In addition, it is this series that demonstrates the activity of metals shown by them in redox reactions.
So, interaction with them occurs with the release of hydrogen in the form of a gas and with the formation of salt. Here is an example of a reaction with sodium mild alkali metal: 2Na + 2HCl → 2NaCl + H 2.
Interaction with other substances proceeds according to similar formulas. This is how the reaction with aluminum, a light metal looks like: 2Al + 6HCl → 2AlCl 3 + 3H 2.
Reactions with oxides
The hydrochloric acid solution also interacts well with these substances. Oxides are binary compounds of an element with oxygen with an oxidation state of -2. All well-known examples are sand, water, rust, dyes, carbon dioxide.
Hydrochloric acid does not interact with all compounds, but only with metal oxides. The reaction also forms soluble salt and water. An example is the process that occurs between an acid and magnesium oxide, an alkaline earth metal: MgO + 2HCl → MgCl 2 + H 2 O.
Reactions with hydroxides
The so-called inorganic compounds, in the composition of which there is a hydroxyl group -OH, in which hydrogen and oxygen atoms are connected covalent bond... And, since the hydrochloric acid solution interacts only with metal hydroxides, it is worth mentioning that some of them are called alkalis.
So the resulting reaction is called neutralization. Its result is the formation of a weakly dissociating substance (that is, water) and salt.
As an example, we can cite the reaction of a small volume of a solution of hydrochloric acid and barium hydroxide, a soft alkaline earth malleable metal: Ba (OH) 2 + 2HCl \u003d BaCl 2 + 2H 2 O.
Interaction with other substances
In addition to the above, hydrochloric acid can react with other types of compounds. In particular with:
- Metal salts, which are formed by other, weaker acids. Here is an example of one of these reactions: Na 2 Co 3 + 2HCl → 2NaCl + H 2 O + CO 2. Shown here is the interaction with the carbonic acid salt (H 2 CO 3).
- Strong oxidizing agents. With manganese dioxide, for example. Or with potassium permanganate. Such reactions are accompanied by the release of chlorine. Here is one example: 2KMnO 4 + 16HCl → 5Cl 2 + 2MnCl 2 + 2KCl + 8H 2 O.
- Ammonia. It is hydrogen nitride with the formula NH 3, which is a colorless but pungent-smelling gas. The consequence of its reaction with a solution of hydrochloric acid is a mass of thick white smoke, consisting of small crystals of ammonium chloride. Which, by the way, is known to everyone as ammonia (NH 4 Cl). The interaction formula is as follows: NH 3 + HCl → NH 4 CL.
- Silver nitrate - inorganic compound (AgNO 3), which is a salt of nitric acid and silver metal. Due to the contact with a solution of hydrochloric acid, a qualitative reaction occurs - the formation of a curdled precipitate of silver chloride. Which does not dissolve in nitrogen. It looks like this: HCL + AgNO 3 → AgCl ↓ + HNO 3.
Getting the substance
Now we can talk about what is being done to form hydrochloric acid.
First, by burning hydrogen in chlorine, the main component is obtained - gaseous hydrogen chloride. Which is then dissolved in water. The result of this simple reaction is the formation of a synthetic acid.
This substance can also be obtained from offgas. These are chemical waste gases. They are formed in a wide variety of processes. For example, in the chlorination of hydrocarbons. The hydrogen chloride in their composition is called offgas. And the acid thus obtained, respectively.
It should be noted that in last years the share of off-gas substances in the total volume of its production is increasing. And the acid formed as a result of burning hydrogen in chlorine is displaced. However, in fairness, it should be noted that it contains fewer impurities.
Household use
Many cleaning products that household people use regularly contain a certain proportion of hydrochloric acid solution. 2-3 percent, and sometimes less, but it is there. That is why, when putting plumbing in order (washing out the tiles, for example), you need to wear gloves. Highly acidic products can damage your skin.
Another solution is used as a stain remover. It helps get rid of ink or rust on clothes. But in order for the effect to be noticeable, a more concentrated substance must be used. A 10% hydrochloric acid solution is suitable. By the way, it perfectly removes scale.
It is important to store the substance properly. Keep acid in glass containers and out of reach of animals and children. Even a weak solution on the skin or mucous membrane can cause chemical burns. If this happens, it is necessary to urgently rinse the areas with water.
In the field of construction
The use of hydrochloric acid and its solutions is a popular way to improve many building processes. For example, it is often added to concrete mix to increase frost resistance. In addition, it hardens faster this way, and the resistance of the masonry to moisture increases.
Hydrochloric acid is also used as a limestone remover. Its 10% solution is the best way to deal with dirt and marks on red bricks. It is not recommended to use it for cleaning others. The structure of other bricks is more sensitive to this substance.
In medicine
In this area, the substance in question is also actively used. Diluted hydrochloric acid has the following effects:
- Digests proteins in the stomach.
- Stops the development of malignant tumors.
- Helps in the treatment of cancer.
- Normalizes acid-base balance.
- Serves as an effective tool in the prevention of hepatitis, diabetes mellitus, psoriasis, eczema, rheumatoid arthritis, gallstone disease, rosacea, asthma, urticaria and many other ailments.
The idea came to mind to dilute the acid and use it inside in this form, and not in the composition of medicines? This is practiced, but it is strictly forbidden to do it without medical advice and receiving instructions. Incorrectly calculating the proportions, you can swallow an excess of hydrochloric acid solution, and simply burn your stomach.
By the way, you can still take medications that stimulate the production of this substance. And not only chemical ones. The same calamus, peppermint and wormwood contribute to this. Broths based on them can be made by yourself, and drunk for prevention.
Burns and poisoning
As effective as this remedy is, it is dangerous. Hydrochloric acid, depending on the concentration, can provoke four degrees of chemical burns:
- There is only redness and pain.
- Blisters with a clear liquid and edema appear.
- Necrosis of the upper layers of the skin is formed. Bubbles fill with blood or cloudy contents.
- The lesion reaches the tendons and muscles.
If the substance somehow gets into the eyes, rinse them with water and then with a soda solution. But in any case, the first thing to do is call an ambulance.
Acid ingestion is fraught with sharp pains in the chest and abdomen, laryngeal edema, and vomiting bloody masses. As a result, severe liver and kidney pathologies.
And the first signs of vapor poisoning include dry, frequent cough, choking, tooth damage, burning sensation in the mucous membranes and abdominal pain. First-aid measures include washing and rinsing the mouth with water and access to fresh air. Only a toxicologist can provide real help.
1.2679; G crnt 51.4 ° C, p crit 8.258 MPa, d crit 0.42 g / cm 3; -92.31 kJ / mol, DH pl 1.9924 kJ / mol (-114.22 ° C), DH eff 16.1421 kJ / mol (-8.05 ° C); 186.79 J / (mol TO); steam pressure (Pa): 133.32 · 10 -6 (-200.7 ° C), 2.775 · 10 3 (-130.15 ° C), 10.0 · 10 4 (-85.1 ° C), 74.0 * 10 4 (-40 ° C), 24.95 * 10 5 (O ° C), 76.9 * 10 5 (50 ° C); ur-nie temperature dependence steam pressure lgp (kPa) \u003d -905.53 / T + 1.75logT- -500.77 · 10 -5 T + 3.78229 (160-260 K); coeff. compressibility 0.00787; g 23 mN / cm (-155 ° C); r 0.29 · 10 7 Ohm · m (-85 ° C), 0.59 · 10 7 (-114.22 ° C). See also table. one.
Solubility of HCl in hydrocarbons at 25 ° C and 0.1 MPa (mol%): in pentane-0.47, hexane-1.12, heptane-1.47, octane-1.63. The p-conductivity of HC1 in alkyl and aryl halides is low, for example. 0.07 mol / mol for C 4 H 9 C1. The p-value in the range from -20 to 60 ° С decreases in the series dichloroethane-tri-chloroethane-tetrachloroethane-trichlorethylene. The p-stability at 10 ° C in a number of alcohols is approximately 1 mol / mol of alcohol, in ethers carbon fiber 0.6 mol / mol, in carboxylic acids 0.2 mol / mol. In ethers, stable adducts R 2 O · HCl are formed. The p-rimity of HC1 in chloride melts obeys Henry's law and is 2.51 · 10 -4 (800 ° С) for КСl, 1.75 · 10 -4 mol / mol (900 ° С), for NaCl 1.90 · 10 -4 mol / mol (900 ° C).
Salt to-that. Dissolution of HCl in water is highly exothermic. process, for infinitely decomp. water solution D H 0 dissolution of НСl -69.9 kJ / mol, ion Сl -- 167.080 kJ / mol; HC1 in water is fully ionized. The p-value of HC1 in water depends on the temperature (Table 2) and the partial pressure of HC1 in the gas mixture. Density of hydrochloric acid decomp. concentration and h at 20 ° C are presented in table. 3 and 4. With an increase in t-ry, h of hydrochloric acid decreases, for example: for 23.05% hydrochloric acid at 25 ° C h 1364 mPa s, at 35 ° C 1.170 mPa s hydrochloric acid containing h moles water per 1 mol of HC1 is [kJ / (kg · K)]: 3.136 (n \u003d 10), 3.580 (n \u003d 20), 3.902 (n \u003d 50), 4.036 (n \u003d 100), 4.061 (n \u003d 200 ).
HCl forms an azeotropic mixture with water (Table 5). In the HCl-water system, there are three eutectic. points: - 74.7 ° C (23.0% by mass of HCl); -73.0 ° C (26.5% HCl); -87.5 ° C (24.8% HC1, metastable phase). Known crystalline hydrates HCl nH 2 O, where n \u003d 8, 6 (mp -40 ° C), 4, 3 (mp -24.4 ° C), 2 (mp -17, 7 ° C) and 1 (mp -15.35 ° C). Ice crystallizes from 10% hydrochloric acid at -20, from 15% at -30, from 20% at -60 and from 24% at -80 ° C. The p-rimity of metal halides decreases with an increase in the concentration of HCl in hydrochloric acid, which is used for salting out them.
Chemical properties. Pure dry HCl begins to dissociate above 1500 ° C and is chemically passive. Mn. metals, C, S, P do not interact. even with liquid HCl. Reacts with nitrides, carbides, borides, sulfides above 650 ° C, with hydrides Si, Ge and B-in the presence. АlСl 3, with transition metal oxides - at 300 ° С and above. Oxidized O 2 and HNO 3 to Cl 2, with SO 3 gives C1SO 3 H. About p-tions with org. compounds, see Hydrohalogenation.
FROM hydrochloric acid is chemically very active. Dissolves with the release of H 2 all metals that have negative. normal potential,with me. oxides and hydroxides of metals forms chlorides, liberates free. to-you from salts such as phosphates, silicates, borates, etc.
Receiving.In the prom-sti HCl get a trace. methods-sulfate, synthetic. and from off-gases (side gases) of a number of processes. The first two methods are irrelevant. So, in the USA in 1965 the share of abgas hydrochloric acid was 77.6% in the total production volume, and in 1982-94%.
Production of hydrochloric acid (reactive, obtained by the sulfate method, synthetic, off-gas) consists in obtaining HCl followed by. its absorption in water. Depending on the method of removal of the heat of absorption (reaches 72.8 kJ / mol), the processes are divided into isothermal, adiabatic. and combined.
The sulfate method is based on interaction. NaCl with conc. H 2 SO 4 at 500-550 ° C. React. the gases contain from 50-65% HCl (muffle furnaces) to 5% HCl (fluidized bed reactor). It is proposed to replace H 2 SO 4 with a mixture of SO 2 and O 2 (process temperature approx. 540 ° C, cat.-Fe 2 O 3).
The direct synthesis of HCl is based on the chain p-tion of combustion: H 2 + Cl 2 2HCl + 184.7 kJ Equilibrium constant K p is calculated by ur-niy: logK p \u003d 9554 / T- 0.5331g T + 2.42.
P-tion is initiated by light, moisture, solid porous (charcoal, porous Pt) and some miner. in-you (quartz, clay). Synthesis is carried out with an excess of H 2 (5-10%) in combustion chambers made of steel, graphite, quartz, refractory bricks. Naib. modern material preventing HCl contamination, -phenol-formald impregnated graphite. resins. To prevent explosive combustion, the reagents are mixed directly in the torch of the burner flame. To the top. heat exchangers are installed in the zone of the combustion chambers for cooling the reaction. gases up to 150-160 ° C. Power of modern graphite furnaces reach 65 tons / day (in terms of 35% hydrochloric acid). In case of a deficiency of H 2, apply decomp. process modifications; for example, a mixture of Cl 2 with steam is passed through a layer of porous hot coal:
2Cl 2 + 2H 2 O + C: 4HCl + CO 2 + 288.9 kJ
The temperature of the process (1000-1600 ° C) depends on the type of coal and the presence in it of impurities that are catalysts (eg, Fe 2 O 3). The use of a mixture of CO with water vapor is promising:
CO + H 2 O + Cl 2: 2HCl + CO 2
More than 90% of hydrochloric acid in developed countries is obtained from off-gas HCl, formed during chlorination and dehydrochlorination of org. compounds, chlororg pyrolysis. waste, metal chlorides, obtaining potash non-chlorine. fertilizers, etc. Abgaz contain decomp. number of HC1, inert impurities (N 2, H 2, CH 4), slightly soluble in water org. substances (chlorobenzene, chloromethane), water-soluble substances (acetic acid, chloral), acidic impurities (Cl 2, HF, O 2) and water. Application of isothermal absorption is advisable at a low content of HC1 in offgas (but with a content of inert impurities less than 40%). Naib. Film absorbers are promising, which make it possible to extract from 65 to 85% HCl from the initial offgas.
Naib. adiabatic schemes are widely used. absorption. Abgas is injected into the bottom. part of the absorber, and water (or dilute hydrochloric acid) in a counterflow to the top. Hydrochloric acid is heated to boiling point due to the heat of dissolution of HCl. The change in the absorption temperature and concentration of HCl is given in Fig. 1. T-ra absorption is determined by the boiling point of the to-you of the corresponding concentration (max. T-ra-t. Boiling point of the azeotropic mixture is approx. 110 ° C).
In fig. 2 shows a typical adiabatic circuit. absorption of HCl from off-gases formed during chlorination (for example, obtaining chlorobenzene). НСl is absorbed in the absorber 1, and the remains of org, poorly soluble in water. in-in is separated from water after condensation in apparatus 2, purified in the tail column 4 and separators 3, 5 and get commercial hydrochloric acid.
Figure: 1. The distribution scheme of tp (curve 1) and
An aqueous solution of hydrogen chloride was called hydrochloric acid because for a long time it was obtained from table salt, acting on it with sulfuric acid. This so-called sulfate method for the production of hydrochloric acid was the only one for a long time. Then they began to obtain synthetic hydrogen chloride from chlorine and hydrogen. In addition, significant amounts of hydrogen chloride are obtained as a by-product of chlorination organic matter and other products.
Thus, in the industry, hydrochloric acid is obtained in the following ways:
- - sulfate;
- - synthetic;
- - from offgas (side gases) of a number of processes.
In all cases, the production of hydrochloric acid (reactive, obtained by the sulfate method, synthetic, off-gas) consists of two stages:
- 1) obtaining hydrogen chloride
- 2) absorption (absorption) of hydrogen chloride by water.
Depending on the method of removing the heat of absorption, which reaches 72.8 kJ / mol, the processes are divided into isothermal (at a constant temperature), adiabatic (without heat exchange with environment) and combined.
Sulfate method: based on the interaction of sodium chloride with sulfuric acid Н 2 SO 4 (92-93%) at 500-550 ° C.
2NaCl + H 2 SO 4\u003e Na 2 SO 4 + 2HCl
Less concentrated sulfuric acid is not used, since in this case the hydrogen chloride would be excessively diluted with water vapor, which would make it difficult to obtain concentrated hydrochloric acid. IN technological process is it preferable to use coarse-grained evaporated salt due to its porosity? it is easily saturated with acid to form a homogeneous mass. However, the evaporated salt contains a variable amount of moisture, which complicates the dosage of raw materials and the regulation of the temperature regime of the furnaces. Rock salt is characterized by constant moisture, but it is more polluted by impurities of CaSO 4, Fe 2 O 3 and others, passing into sodium sulfate. In addition, the use of rock salt is associated with the need for grinding and more intensive mixing with sulfuric acid.
Reaction gases from muffle furnaces contain 50-65% hydrogen chloride, and gases from fluidized bed reactors up to 5% HCl. At present, it is proposed to replace sulfuric acid with a mixture of SO 2 and O 2 using Fe 2 O 3 as a catalyst and carrying out the process at a temperature of 540 ° C.
The synthesis of hydrogen chloride from elements gives a concentrated hydrochloric gas (containing 80-90% and more HCl), which is easily liquefied, and its absorption in distilled water makes it possible to obtain a pure reactive acid, the concentration of which, if necessary, can reach 38%.
The direct synthesis of hydrochloric acid is based on a chain reaction of combustion:
H 2 + Cl 2 - 2HC1 + 184.7 kJ.
The reaction is initiated by light, moisture, solid porous substances (charcoal, spongy platinum) and some minerals (quartz, clay). Absolutely dry chlorine and hydrogen do not interact with each other. The presence of traces of moisture accelerates the reaction so intensely that it can explode. In production plants, a quiet, non-explosive combustion of hydrogen in a stream of chlorine is carried out. Hydrogen is supplied with an excess of 5-10%, which makes it possible to fully use the more valuable chlorine and obtain hydrochloric acid uncontaminated with chlorine.
The combustion of a mixture of chlorine and hydrogen is carried out in furnaces of various designs, which are small chambers made of refractory bricks, fused quartz, graphite or metal. The most modern material that prevents product contamination is graphite impregnated with phenol-formaldehyde resins. To prevent explosive combustion, the reagents are mixed directly in the torch of the burner flame. In the upper zone of the combustion chambers, heat exchangers are installed to cool the reaction gases to 150-160 ° C. The capacity of modern graphite furnaces reaches 65 tons / day. (in terms of hydrochloric acid containing 35% HCl).
Getting hydrochloric acid from chlorine and hydrogen? the main method of industrial production of this product.
In case of hydrogen deficiency, various process modifications are used. For example, a mixture of Cl 2 with steam is passed through a layer of porous incandescent coal:
2C1 2 + 2H 2 O + C\u003e 4HC1 + CO 2 + 288.9 kJ.
The process temperature is 1000-1600 ° C, depending on the type of coal and the presence of impurities in it, which are catalysts, for example Fe 2 O 3.
The use of a mixture of CO with water vapor is also promising:
CO + H 2 O + Cl 2\u003e 2HC1 + CO 2.
A significant amount of hydrochloric acid is currently obtained from off-gas hydrogen chloride HCl, which is formed during chlorination and dehydrochlorination. organic compounds, pyrolysis of organochlorine wastes, metal chlorides, production of non-chlorinated potash fertilizers, etc. Abgas gases contain various amounts of hydrogen chloride, inert impurities (N 2, H 2, CH 4), organic substances slightly soluble in water (chlorobenzene, chloromethanes), water-soluble substances ( acid, chloral), acidic impurities (Cl 2, HF, O 2) and water. When the content of inert impurities is less than 40%, it is advisable to use isothermal absorption of HCl in off-gas gases. The most promising are film absorbers, which make it possible to extract from 65% to 85% HCl from the original offgas.
In the Russian industry, adiabatic absorption schemes are most widely used to obtain hydrochloric acid. Off-gas gases are introduced into the lower part of the absorber, and water (or diluted hydrochloric acid) is injected in a countercurrent into the upper part. Hydrochloric acid is heated to boiling point due to the heat of dissolution of HCl. The dependence of the change in the absorption temperature and the HCl concentration is shown in Figure 1.
The absorption temperature is determined by the boiling point of the acid of the corresponding concentration; the maximum boiling point of the azeotropic mixture is about 110 ° C.
Figure: one.
A typical scheme for the adiabatic absorption of HCl from off-gases formed during chlorination (for example, when obtaining chlorobenzene) is shown in Figure 2. Hydrogen chloride is absorbed in absorber 1, and the residues of poorly water-soluble organic substances are separated from water after condensation in unit 2, and further purified in the tail column 4 and separators 3, 5 and get commercial hydrochloric acid.
Figure: 2 : Scheme of typical adiabatic absorption of hydrochloric acid from off-gases. one ? adiabatic absorber; 2? capacitor; 3, 5? separators; four ? tail string; 6? collection of organic phase; 7? collection of the aqueous phase; 8, 12? pumps; 9 ? stripping column; ten ? heat exchanger; eleven ? collection of commercial acid
The production of hydrochloric acid from off-gas gases using a combined absorption scheme is shown as a typical scheme in Figure 3. In the adiabatic absorption column, hydrochloric acid is obtained with a reduced concentration, but free from organic impurities. Acid with an increased concentration of HC1 is produced in an isothermal absorption column at low temperatures. The degree of extraction of HCl from off-gases when using dilute acids as absorbents is 95-99%. When pure water is used as an absorbent, the recovery is almost complete.
Figure: 3 : Scheme of a typical combined absorption of hydrochloric acid from off-gas gases 1 - adiabatic absorption column; 2 - capacitor; 3 - gas separator; 4 - separator; 5 - refrigerator; 6, 9 - acid collectors; 7 - pumps; 8 - isothermal absorber.
RATIO OF METALS TO ACIDS
Most often in chemical practice, strong acids such as sulfuricH 2 SO 4, hydrochloric HCl and nitrogen HNO 3 ... Next, consider the ratio of various metals to the listed acids.
Hydrochloric acid ( HCl)
Hydrochloric acid is the technical name for hydrochloric acid. It is obtained by dissolving gaseous hydrogen chloride in water -HCl ... Due to its low solubility in water, the concentration of hydrochloric acid under normal conditions does not exceed 38%. Therefore, regardless of the concentration of hydrochloric acid, the process of dissociation of its molecules in an aqueous solution is active:
HCl H + + Cl -
Hydrogen ions formed in this processH + act as an oxidizing agent, oxidizing metals located in the line of activity to the left of hydrogen ... The interaction proceeds according to the scheme:
Me + HCl salt +H 2
In this case, the salt is a metal chloride (NiCl 2, CaCl 2, AlCl 3 ), in which the number of chloride ions corresponds to the oxidation state of the metal.
Hydrochloric acid is a weak oxidizing agent; therefore, metals with variable valence are oxidized by it to lower positive oxidation states:
Fe 0 → Fe 2+
Co 0 → Co 2+
Ni 0 → Ni 2+
Cr 0 → Cr 2+
Mn 0 → Mn 2+ and dr .
Example:
2 Al + 6 HCl → 2 AlCl 3 + 3 H 2
2│ Al 0 - 3 e - → Al 3+ - oxidation
3│2 H + + 2 e - → H 2 - restoration
Hydrochloric acid passivates lead ( Pb ). Lead passivation is due to the formation on its surface of lead chloride, which is hardly soluble in water (II ), which protects the metal from further acid attack:
Pb + 2 HCl → PbCl 2 ↓ + H 2
Sulphuric acid (H 2 SO 4 )
The industry produces sulfuric acid of very high concentration (up to 98%). Consideration should be given to the difference in the oxidizing properties of a dilute solution and concentrated sulfuric acid in relation to metals.
Diluted sulfuric acid
In a dilute aqueous solution of sulfuric acid, most of its molecules dissociate:
H 2 SO 4 H + + HSO 4 -
HSO 4 - H + + SO 4 2-
Formed ions H + perform the function oxidizer .
Like hydrochloric acid, diluted sulfuric acid solution interacts only with active metals and average activity (located in the row of activity to hydrogen).
The chemical reaction proceeds according to the scheme:
Me + H 2 SO 4 ( smashed .) → salt + H 2
Example:
2 Al + 3 H 2 SO 4 (dil.) → Al 2 (SO 4) 3 + 3 H 2
1│2Al 0 - 6 e - → 2Al 3+ -oxidation
3│2 H + + 2 e - → H 2 - restoration
Metals with variable valence are oxidized with a dilute sulfuric acid solution to lower positive oxidation states:
Fe 0 → Fe 2+
Co 0 → Co 2+
Ni 0 → Ni 2+
Cr 0 → Cr 2+
Mn 0 → Mn 2+ and dr .
Lead ( Pb ) does not dissolve in sulfuric acid (if its concentration is below 80%) , since the resulting saltPbSO 4 insoluble and creates a protective film on the metal surface.
Concentrated sulfuric acid
In a concentrated solution of sulfuric acid (above 68%), most of the molecules are in undissociated condition, therefore sulfur acts as an oxidizing agent in the highest oxidation state (S +6 ). ConcentratedH 2 SO 4 oxidizes all metals, standard electrode potential which is less than the potential of the oxidizing agent - sulfate ionSO 4 2- (0.36V). In this regard, since concentrated react with sulfuric acid and some low-activity metals .
The process of interaction of metals with concentrated sulfuric acid in most cases proceeds according to the following scheme:
Me + H 2 SO 4 (conc.) salt + water + recovery product H 2 SO 4
Recovery products sulfuric acid can be the following sulfur compounds:
Practice has shown that when a metal interacts with concentrated sulfuric acid, a mixture of reduction products is released, consisting ofH 2 S, S and SO 2. However, one of these products is formed in the predominant amount. The nature of the main product is determined metal activity : the higher the activity, the deeper the process of sulfur recovery in sulfuric acid.
The interaction of metals of various activity with concentrated sulfuric acid can be represented by the following scheme:
Aluminum (Al ) and iron (Fe ) do not react with cold concentratedH 2 SO 4 , covered with dense oxide films, however, when heated, the reaction proceeds.
Ag , Au , Ru , Os , Rh , Ir , Pt do not react with sulfuric acid.
Concentrated sulfuric acid is strong oxidizing agent , therefore, when metals with variable valence interact with it, the latter are oxidized to higher oxidation states than in the case of a dilute acid solution:
Fe 0→ Fe 3+,
Cr 0→ Cr 3+,
Mn 0→ Mn 4+,
Sn 0→ Sn 4+
Lead ( Pb ) oxidizes to bivalent states with the formation of soluble lead hydrogen sulfatePb ( Hso 4 ) 2 .
Examples:
Active metal
8 A1 + 15 H 2 SO 4 (conc.) → 4A1 2 (SO 4) 3 + 12H 2 O + 3H 2 S
4│2 Al 0 - 6 e - → 2 Al 3+ - oxidation
3│ S 6+ + 8 e → S 2- - restoration
Medium activity metal
2 Cr + 4 H 2 SO 4 (conc.) → Cr 2 (SO 4) 3 + 4 H 2 O + S
1│ 2Cr 0 - 6e → 2Cr 3+ - oxidation
1│ S 6+ + 6 e → S 0 - restoration
Inactive metal
2Bi + 6H 2 SO 4 (conc.) → Bi 2 (SO 4) 3 + 6H 2 O + 3SO 2
1│ 2Bi 0 - 6e → 2Bi 3+ -oxidation
3│ S 6+ + 2 e → S 4+ - restoration
Nitric acid ( HNO 3 )
A feature of nitric acid is that the nitrogen contained inNO 3 - has the highest oxidation state +5 and therefore has strong oxidizing properties. The maximum value of the electrode potential for the nitrate ion is 0.96 V; therefore, nitric acid is a stronger oxidizing agent than sulfuric acid. The role of an oxidizing agent in the reactions of interaction of metals with nitric acid is played byN 5+ ... Consequently, hydrogen H 2 never stands out when metals interact with nitric acid ( regardless of concentration ). The process follows the scheme:
Me + HNO 3 salt + water + recovery product HNO 3
Recovery products HNO 3 :
Usually, when nitric acid reacts with metal, a mixture of reduction products is formed, but as a rule, one of them is predominant. Which of the products will be the main one depends on the concentration of the acid and the activity of the metal.
Concentrated nitric acid
A concentrated solution of an acid with a densityρ\u003e 1.25 kg / m 3, which corresponds
concentration\u003e 40%. Regardless of the activity of the metal, the reaction of interaction withHNO 3 (conc.) proceeds according to the scheme:
Me + HNO 3 (conc.)→ salt + water + NO 2
Noble metals do not react with concentrated nitric acid (Au , Ru , Os , Rh , Ir , Pt ), and a number of metals (Al , Ti , Cr , Fe , Co , Ni ) at low temperature passivated with concentrated nitric acid. The reaction is possible when the temperature rises, it proceeds according to the scheme presented above.
Examples of
Active metal
Al + 6 HNO 3 (conc.) → Al (NO 3) 3 + 3 H 2 O + 3 NO 2
1│ Al 0 - 3 e → Al 3+ - oxidation
3│ N 5+ + e → N 4+ - restoration
Medium activity metal
Fe + 6 HNO 3 (conc.) → Fe (NO 3) 3 + 3H 2 O + 3NO
1│ Fe 0 - 3e → Fe 3+ - oxidation
3│ N 5+ + e → N 4+ - restoration
Inactive metal
Ag + 2HNO 3 (conc.) → AgNO 3 + H 2 O + NO 2
1│ Ag 0 - e → Ag + - oxidation
1│ N 5+ + e → N 4+ - restoration
Diluted nitric acid
Recovery product nitric acid in a dilute solution depends on metal activity participating in the reaction:
Examples:
Active metal
8 Al + 30 HNO 3 (expanded) → 8Al (NO 3) 3 + 9H 2 O + 3NH 4 NO 3
8│ Al 0 - 3e → Al 3+ - oxidation
3│ N 5+ + 8 e → N 3- - restoration
Ammonia released during the reduction of nitric acid immediately interacts with an excess of nitric acid, forming a salt - ammonium nitrateNH 4 NO 3:
NH 3 + HNO 3 → NH 4 NO 3.
Medium activity metal
10Cr + 36HNO 3 (dil.) → 10Cr (NO 3) 3 + 18H 2 O + 3N 2
10│ Cr 0 - 3 e → Cr 3+ - oxidation
3│ 2 N 5+ + 10 e → N 2 0 - restoration
Besides molecular nitrogen (N 2 ) when metals of medium activity interact with dilute nitric acid, an equal amount is formed nitric oxide (I) - N 2 O ... In the reaction equation, you need to write one of these substances .
Inactive metal
3Ag + 4HNO 3 (dil.) → 3AgNO 3 + 2H 2 O + NO
3│ Ag 0 - e → Ag + - oxidation
1│ N 5+ + 3 e → N 2+ - restoration
"Aqua regia"
"Tsarskaya vodka" (previously called vodka acids) is a mixture of one volume of nitric acid and three to four volumes of concentrated hydrochloric acid, which has a very high oxidative activity. Such a mixture is capable of dissolving some low-activity metals that do not interact with nitric acid. Among them is the "king of metals" - gold. This effect of "aqua regia" is explained by the fact that nitric acid oxidizes hydrochloric acid with the release of free chlorine and the formation of nitrogen chloroxide (III ), or nitrosyl chloride -NOCl:
HNO 3 + 3 HCl → Cl 2 + 2 H 2 O + NOCl
2 NOCl → 2 NO + Cl 2
Chlorine at the time of release consists of atoms. Atomic chlorine is the strongest oxidizing agent, which allows "aqua regia" to affect even the most inert "noble metals".
Oxidation reactions of gold and platinum proceed according to the following equations:
Au + HNO 3 + 4 HCl → H + NO + 2H 2 O
3Pt + 4HNO 3 + 18HCl → 3H 2 + 4NO + 8H 2 O
On Ru, Os, Rh and Ir "Royal vodka" does not work.
E.A. Nudnova, M.V. Andryukhova
