Acids are complex substances whose molecules consist of hydrogen atoms (capable of being replaced by metal atoms) associated with an acidic residue.
general characteristics
Acids are classified into oxygen-free and oxygen-containing, as well as organic and inorganic.
Rice. 1. Classification of acids - oxygen-free and oxygen-containing.
Anoxic acids are solutions in water of binary compounds such as hydrogen halides or hydrogen sulfide. In solution, the polar covalent bond between hydrogen and an electronegative element is polarized by the action of dipole water molecules, and the molecules disintegrate into ions. the presence of hydrogen ions in the substance allows us to call aqueous solutions of these binary compounds acids.
Acids are named from the name of the binary compound by adding the ending -naya. for example, HF is hydrofluoric acid. An acid anion is named by the name of the element by adding the ending -ide, for example, Cl – chloride.
Oxygen-containing acids (oxoacids)– these are acid hydroxides that dissociate according to the acid type, that is, as protolytes. Their general formula is E(OH)mOn, where E is a non-metal or a metal with variable valency in the highest oxidation state. provided that when n is 0, then the acid is weak (H 2 BO 3 - boric), if n = 1, then the acid is either weak or of medium strength (H 3 PO 4 -orthophosphoric), if n is greater than or equal to 2, then the acid is considered strong (H 2 SO 4).
Rice. 2. Sulfuric acid.
Acidic hydroxides correspond to acidic oxides or anhydrides of acids, for example, sulfuric acid corresponds to sulfuric anhydride SO 3.
Chemical properties of acids
Acids are characterized by a number of properties that distinguish them from salts and other chemical elements:
- Action on indicators. How acid protolites dissociate to form H+ ions, which change the color of the indicators: a violet litmus solution becomes red, and an orange methyl orange solution becomes pink. Polybasic acids dissociate in stages, with each subsequent stage being more difficult than the previous one, since in the second and third stages increasingly weaker electrolytes dissociate:
H 2 SO 4 =H+ +HSO 4 –
The color of the indicator depends on whether the acid is concentrated or dilute. So, for example, when litmus is lowered into concentrated sulfuric acid, the indicator turns red, but in dilute sulfuric acid the color will not change.
- Neutralization reaction, that is, the interaction of acids with bases, resulting in the formation of salt and water, always occurs if at least one of the reagents is strong (base or acid). The reaction does not proceed if the acid is weak and the base is insoluble. For example, the reaction does not work:
H 2 SiO 3 (weak, water-insoluble acid) + Cu(OH) 2 – the reaction does not occur
But in other cases, the neutralization reaction with these reagents goes:
H 2 SiO 3 +2KOH (alkali) = K 2 SiO 3 +2H 2 O
- Interaction with basic and amphoteric oxides:
Fe 2 O 3 +3H 2 SO 4 =Fe 2 (SO 4) 3 +3H 2 O
- Interaction of acids with metals, standing in the voltage series to the left of hydrogen, leads to a process as a result of which a salt is formed and hydrogen is released. This reaction occurs easily if the acid is strong enough.
Nitric acid and concentrated sulfuric acid react with metals due to the reduction not of hydrogen, but of the central atom:
Mg+H 2 SO 4 +MgSO 4 +H 2
- Interaction of acids with salts occurs when a weak acid is formed as a result. If the salt reacting with the acid is soluble in water, then the reaction will also proceed if an insoluble salt is formed:
Na 2 SiO 3 (soluble salt of a weak acid) + 2HCl (strong acid) = H 2 SiO 3 (weak insoluble acid) + 2NaCl (soluble salt)
Many acids are used in industry, for example, acetic acid is necessary for preserving meat and fish products
Names of some inorganic acids and salts
| Acid formulas | Names of acids | Names of the corresponding salts |
| HClO4 | chlorine | perchlorates |
| HClO3 | hypochlorous | chlorates |
| HClO2 | chloride | chlorites |
| HClO | hypochlorous | hypochlorites |
| H5IO6 | iodine | periodates |
| HIO 3 | iodic | iodates |
| H2SO4 | sulfuric | sulfates |
| H2SO3 | sulfurous | sulfites |
| H2S2O3 | thiosulfur | thiosulfates |
| H2S4O6 | tetrathionic | tetrathionates |
| HNO3 | nitrogen | nitrates |
| HNO2 | nitrogenous | nitrites |
| H3PO4 | orthophosphoric | orthophosphates |
| HPO 3 | metaphosphoric | metaphosphates |
| H3PO3 | phosphorous | phosphites |
| H3PO2 | phosphorous | hypophosphites |
| H2CO3 | coal | carbonates |
| H2SiO3 | silicon | silicates |
| HMnO4 | manganese | permanganates |
| H2MnO4 | manganese | manganates |
| H2CrO4 | chrome | chromates |
| H2Cr2O7 | dichrome | dichromates |
| HF | hydrogen fluoride (fluoride) | fluorides |
| HCl | hydrochloric (hydrochloric) | chlorides |
| HBr | hydrobromic | bromides |
| HI | hydrogen iodide | iodides |
| H2S | hydrogen sulfide | sulfides |
| HCN | hydrogen cyanide | cyanides |
| HOCN | cyan | cyanates |
Let me briefly remind you, using specific examples, of how salts should be called correctly.
Example 1. The salt K 2 SO 4 is formed by a sulfuric acid residue (SO 4) and metal K. Salts of sulfuric acid are called sulfates. K 2 SO 4 - potassium sulfate.
Example 2. FeCl 3 - the salt contains iron and a hydrochloric acid residue (Cl). Name of salt: iron (III) chloride. Please note: in this case we must not only name the metal, but also indicate its valence (III). In the previous example, this was not necessary, since the valence of sodium is constant.
Important: the name of the salt should indicate the valence of the metal only if the metal has a variable valency!
Example 3. Ba(ClO) 2 - the salt contains barium and the remainder of hypochlorous acid (ClO). Salt name: barium hypochlorite. The valency of the metal Ba in all its compounds is two; it does not need to be indicated.
Example 4. (NH 4) 2 Cr 2 O 7. The NH 4 group is called ammonium, the valence of this group is constant. Name of salt: ammonium dichromate (dichromate).
In the above examples we only encountered the so-called. medium or normal salts. Acidic, basic, double and complex salts, salts of organic acids will not be discussed here.
7. Acids. Salt. Relationship between classes of inorganic substances
7.1. Acids
Acids are electrolytes, upon the dissociation of which only hydrogen cations H + are formed as positively charged ions (more precisely, hydronium ions H 3 O +).
Another definition: acids are complex substances consisting of a hydrogen atom and acid residues (Table 7.1).
Table 7.1
Formulas and names of some acids, acid residues and salts
| Acid formula | Acid name | Acid residue (anion) | Name of salts (average) |
|---|---|---|---|
| HF | Hydrofluoric (fluoric) | F − | Fluorides |
| HCl | Hydrochloric (hydrochloric) | Cl − | Chlorides |
| HBr | Hydrobromic | Br− | Bromides |
| HI | Hydroiodide | I − | Iodides |
| H2S | Hydrogen sulfide | S 2− | Sulfides |
| H2SO3 | Sulphurous | SO 3 2 − | Sulfites |
| H2SO4 | Sulfuric | SO 4 2 − | Sulfates |
| HNO2 | Nitrogenous | NO2− | Nitrites |
| HNO3 | Nitrogen | NO 3 − | Nitrates |
| H2SiO3 | Silicon | SiO 3 2 − | Silicates |
| HPO 3 | Metaphosphoric | PO 3 − | Metaphosphates |
| H3PO4 | Orthophosphoric | PO 4 3 − | Orthophosphates (phosphates) |
| H4P2O7 | Pyrophosphoric (biphosphoric) | P 2 O 7 4 − | Pyrophosphates (diphosphates) |
| HMnO4 | Manganese | MnO 4 − | Permanganates |
| H2CrO4 | Chrome | CrO 4 2 − | Chromates |
| H2Cr2O7 | Dichrome | Cr 2 O 7 2 − | Dichromates (bichromates) |
| H2SeO4 | Selenium | SeO 4 2 − | Selenates |
| H3BO3 | Bornaya | BO 3 3 − | Orthoborates |
| HClO | Hypochlorous | ClO – | Hypochlorites |
| HClO2 | Chloride | ClO2− | Chlorites |
| HClO3 | Chlorous | ClO3− | Chlorates |
| HClO4 | Chlorine | ClO 4 − | Perchlorates |
| H2CO3 | Coal | CO 3 3 − | Carbonates |
| CH3COOH | Vinegar | CH 3 COO − | Acetates |
| HCOOH | Ant | HCOO − | Formiates |
Under normal conditions, acids can be solids (H 3 PO 4, H 3 BO 3, H 2 SiO 3) and liquids (HNO 3, H 2 SO 4, CH 3 COOH). These acids can exist both individually (100% form) and in the form of diluted and concentrated solutions. For example, H 2 SO 4 , HNO 3 , H 3 PO 4 , CH 3 COOH are known both individually and in solutions.
A number of acids are known only in solutions. These are all hydrogen halides (HCl, HBr, HI), hydrogen sulfide H 2 S, hydrogen cyanide (hydrocyanic HCN), carbonic H 2 CO 3, sulfurous H 2 SO 3 acid, which are solutions of gases in water. For example, hydrochloric acid is a mixture of HCl and H 2 O, carbonic acid is a mixture of CO 2 and H 2 O. It is clear that using the expression “hydrochloric acid solution” is incorrect.
Most acids are soluble in water; silicic acid H 2 SiO 3 is insoluble. The overwhelming majority of acids have a molecular structure. Examples of structural formulas of acids:
In most oxygen-containing acid molecules, all hydrogen atoms are bonded to oxygen. But there are exceptions:
Acids are classified according to a number of characteristics (Table 7.2).
Table 7.2
Classification of acids
| Classification sign | Acid type | Examples |
|---|---|---|
| Number of hydrogen ions formed upon complete dissociation of an acid molecule | Monobase | HCl, HNO3, CH3COOH |
| Dibasic | H2SO4, H2S, H2CO3 | |
| Tribasic | H3PO4, H3AsO4 | |
| The presence or absence of an oxygen atom in a molecule | Oxygen-containing (acid hydroxides, oxoacids) | HNO2, H2SiO3, H2SO4 |
| Oxygen-free | HF, H2S, HCN | |
| Degree of dissociation (strength) | Strong (completely dissociate, strong electrolytes) | HCl, HBr, HI, H 2 SO 4 (diluted), HNO 3, HClO 3, HClO 4, HMnO 4, H 2 Cr 2 O 7 |
| Weak (partially dissociate, weak electrolytes) | HF, HNO 2, H 2 SO 3, HCOOH, CH 3 COOH, H 2 SiO 3, H 2 S, HCN, H 3 PO 4, H 3 PO 3, HClO, HClO 2, H 2 CO 3, H 3 BO 3, H 2 SO 4 (conc) | |
| Oxidative properties | Oxidizing agents due to H + ions (conditionally non-oxidizing acids) | HCl, HBr, HI, HF, H 2 SO 4 (dil), H 3 PO 4, CH 3 COOH |
| Oxidizing agents due to anion (oxidizing acids) | HNO 3, HMnO 4, H 2 SO 4 (conc), H 2 Cr 2 O 7 | |
| Anion reducing agents | HCl, HBr, HI, H 2 S (but not HF) | |
| Thermal stability | Exist only in solutions | H 2 CO 3, H 2 SO 3, HClO, HClO 2 |
| Easily decomposes when heated | H 2 SO 3 , HNO 3 , H 2 SiO 3 | |
| Thermally stable | H 2 SO 4 (conc), H 3 PO 4 |
All general chemical properties of acids are due to the presence in their aqueous solutions of excess hydrogen cations H + (H 3 O +).
1. Due to the excess of H + ions, aqueous solutions of acids change the color of litmus violet and methyl orange to red (phenolphthalein does not change color and remains colorless). In an aqueous solution of weak carbonic acid, litmus is not red, but pink; a solution over a precipitate of very weak silicic acid does not change the color of the indicators at all.
2. Acids interact with basic oxides, bases and amphoteric hydroxides, ammonia hydrate (see Chapter 6).
Example 7.1. To carry out the transformation BaO → BaSO 4 you can use: a) SO 2; b) H 2 SO 4; c) Na 2 SO 4; d) SO 3.
Solution. The transformation can be carried out using H 2 SO 4:
BaO + H 2 SO 4 = BaSO 4 ↓ + H 2 O
BaO + SO 3 = BaSO 4
Na 2 SO 4 does not react with BaO, and in the reaction of BaO with SO 2 barium sulfite is formed:
BaO + SO 2 = BaSO 3
Answer: 3).
3. Acids react with ammonia and its aqueous solutions to form ammonium salts:
HCl + NH 3 = NH 4 Cl - ammonium chloride;
H 2 SO 4 + 2NH 3 = (NH 4) 2 SO 4 - ammonium sulfate.
4. Non-oxidizing acids react with metals located in the activity series up to hydrogen to form a salt and release hydrogen:
H 2 SO 4 (diluted) + Fe = FeSO 4 + H 2
2HCl + Zn = ZnCl 2 = H 2
The interaction of oxidizing acids (HNO 3, H 2 SO 4 (conc)) with metals is very specific and is considered when studying the chemistry of elements and their compounds.
5. Acids interact with salts. The reaction has a number of features:
a) in most cases, when a stronger acid reacts with a salt of a weaker acid, a salt of a weak acid and a weak acid are formed, or, as they say, a stronger acid displaces a weaker one. The series of decreasing strength of acids looks like this:
Examples of reactions occurring:
2HCl + Na 2 CO 3 = 2NaCl + H 2 O + CO 2
H 2 CO 3 + Na 2 SiO 3 = Na 2 CO 3 + H 2 SiO 3 ↓
2CH 3 COOH + K 2 CO 3 = 2CH 3 COOK + H 2 O + CO 2
3H 2 SO 4 + 2K 3 PO 4 = 3K 2 SO 4 + 2H 3 PO 4
Do not interact with each other, for example, KCl and H 2 SO 4 (diluted), NaNO 3 and H 2 SO 4 (diluted), K 2 SO 4 and HCl (HNO 3, HBr, HI), K 3 PO 4 and H 2 CO 3, CH 3 COOK and H 2 CO 3;
b) in some cases, a weaker acid displaces a stronger one from a salt:
CuSO 4 + H 2 S = CuS↓ + H 2 SO 4
3AgNO 3 (dil) + H 3 PO 4 = Ag 3 PO 4 ↓ + 3HNO 3.
Such reactions are possible when the precipitates of the resulting salts do not dissolve in the resulting dilute strong acids (H 2 SO 4 and HNO 3);
c) in the case of the formation of precipitates that are insoluble in strong acids, a reaction may occur between a strong acid and a salt formed by another strong acid:
BaCl 2 + H 2 SO 4 = BaSO 4 ↓ + 2HCl
Ba(NO 3) 2 + H 2 SO 4 = BaSO 4 ↓ + 2HNO 3
AgNO 3 + HCl = AgCl↓ + HNO 3
Example 7.2. Indicate the row containing the formulas of substances that react with H 2 SO 4 (diluted).
1) Zn, Al 2 O 3, KCl (p-p); 3) NaNO 3 (p-p), Na 2 S, NaF; 2) Cu(OH) 2, K 2 CO 3, Ag; 4) Na 2 SO 3, Mg, Zn(OH) 2.
Solution. All substances of row 4 interact with H 2 SO 4 (dil):
Na 2 SO 3 + H 2 SO 4 = Na 2 SO 4 + H 2 O + SO 2
Mg + H 2 SO 4 = MgSO 4 + H 2
Zn(OH) 2 + H 2 SO 4 = ZnSO 4 + 2H 2 O
In row 1) the reaction with KCl (p-p) is not feasible, in row 2) - with Ag, in row 3) - with NaNO 3 (p-p).
Answer: 4).
6. Concentrated sulfuric acid behaves very specifically in reactions with salts. This is a non-volatile and thermally stable acid, therefore it displaces all strong acids from solid (!) salts, since they are more volatile than H2SO4 (conc):
KCl (tv) + H 2 SO 4 (conc.) KHSO 4 + HCl
2KCl (s) + H 2 SO 4 (conc) K 2 SO 4 + 2HCl
Salts formed by strong acids (HBr, HI, HCl, HNO 3, HClO 4) react only with concentrated sulfuric acid and only when in a solid state
Example 7.3. Concentrated sulfuric acid, unlike dilute one, reacts:
3) KNO 3 (tv);
Solution. Both acids react with KF, Na 2 CO 3 and Na 3 PO 4, and only H 2 SO 4 (conc.) react with KNO 3 (solid).
Answer: 3).
Methods for producing acids are very diverse.
Anoxic acids receive:
- by dissolving the corresponding gases in water:
HCl (g) + H 2 O (l) → HCl (p-p)
H 2 S (g) + H 2 O (l) → H 2 S (solution)
- from salts by displacement with stronger or less volatile acids:
FeS + 2HCl = FeCl 2 + H 2 S
KCl (tv) + H 2 SO 4 (conc) = KHSO 4 + HCl
Na 2 SO 3 + H 2 SO 4 Na 2 SO 4 + H 2 SO 3
Oxygen-containing acids receive:
- by dissolving the corresponding acidic oxides in water, while the degree of oxidation of the acid-forming element in the oxide and acid remains the same (with the exception of NO 2):
N2O5 + H2O = 2HNO3
SO 3 + H 2 O = H 2 SO 4
P 2 O 5 + 3H 2 O 2H 3 PO 4
- oxidation of non-metals with oxidizing acids:
S + 6HNO 3 (conc) = H 2 SO 4 + 6NO 2 + 2H 2 O
- by displacing a strong acid from a salt of another strong acid (if a precipitate insoluble in the resulting acids precipitates):
Ba(NO 3) 2 + H 2 SO 4 (diluted) = BaSO 4 ↓ + 2HNO 3
AgNO 3 + HCl = AgCl↓ + HNO 3
- by displacing a volatile acid from its salts with a less volatile acid.
For this purpose, non-volatile, thermally stable concentrated sulfuric acid is most often used:
NaNO 3 (tv) + H 2 SO 4 (conc.) NaHSO 4 + HNO 3
KClO 4 (tv) + H 2 SO 4 (conc.) KHSO 4 + HClO 4
- displacement of a weaker acid from its salts by a stronger acid:
Ca 3 (PO 4) 2 + 3H 2 SO 4 = 3CaSO 4 ↓ + 2H 3 PO 4
NaNO 2 + HCl = NaCl + HNO 2
K 2 SiO 3 + 2HBr = 2KBr + H 2 SiO 3 ↓
Acids are complex substances whose molecules include hydrogen atoms that can be replaced or exchanged for metal atoms and an acid residue.
Based on the presence or absence of oxygen in the molecule, acids are divided into oxygen-containing(H 2 SO 4 sulfuric acid, H 2 SO 3 sulfurous acid, HNO 3 nitric acid, H 3 PO 4 phosphoric acid, H 2 CO 3 carbonic acid, H 2 SiO 3 silicic acid) and oxygen-free(HF hydrofluoric acid, HCl hydrochloric acid (hydrochloric acid), HBr hydrobromic acid, HI hydroiodic acid, H 2 S hydrosulfide acid).
Depending on the number of hydrogen atoms in the acid molecule, acids are monobasic (with 1 H atom), dibasic (with 2 H atoms) and tribasic (with 3 H atoms). For example, nitric acid HNO 3 is monobasic, since its molecule contains one hydrogen atom, sulfuric acid H 2 SO 4 – dibasic, etc.
There are very few inorganic compounds containing four hydrogen atoms that can be replaced by a metal.
The part of an acid molecule without hydrogen is called an acid residue.
Acidic residues may consist of one atom (-Cl, -Br, -I) - these are simple acidic residues, or they may consist of a group of atoms (-SO 3, -PO 4, -SiO 3) - these are complex residues.
In aqueous solutions, during exchange and substitution reactions, acidic residues are not destroyed:
H 2 SO 4 + CuCl 2 → CuSO 4 + 2 HCl
The word anhydride means anhydrous, that is, an acid without water. For example,
H 2 SO 4 – H 2 O → SO 3. Anoxic acids do not have anhydrides.
Acids get their name from the name of the acid-forming element (acid-forming agent) with the addition of the endings “naya” and less often “vaya”: H 2 SO 4 - sulfuric; H 2 SO 3 – coal; H 2 SiO 3 – silicon, etc.
The element can form several oxygen acids. In this case, the indicated endings in the names of acids will be when the element exhibits a higher valence (the acid molecule contains a high content of oxygen atoms). If the element exhibits a lower valence, the ending in the name of the acid will be “empty”: HNO 3 - nitric, HNO 2 - nitrogenous.
Acids can be obtained by dissolving anhydrides in water. If the anhydrides are insoluble in water, the acid can be obtained by the action of another stronger acid on the salt of the required acid. This method is typical for both oxygen and oxygen-free acids. Oxygen-free acids are also obtained by direct synthesis from hydrogen and a non-metal, followed by dissolving the resulting compound in water:
H 2 + Cl 2 → 2 HCl;
H 2 + S → H 2 S.
Solutions of the resulting gaseous substances HCl and H 2 S are acids.
Under normal conditions, acids exist in both liquid and solid states.
Chemical properties of acids
Acid solutions act on indicators. All acids (except silicic) are highly soluble in water. Special substances - indicators allow you to determine the presence of acid.
Indicators are substances of complex structure. They change color depending on their interaction with different chemicals. In neutral solutions they have one color, in solutions of bases they have another color. When interacting with an acid, they change their color: the methyl orange indicator turns red, and the litmus indicator also turns red.
Interact with bases with the formation of water and salt, which contains an unchanged acid residue (neutralization reaction):
H 2 SO 4 + Ca(OH) 2 → CaSO 4 + 2 H 2 O.
Interact with base oxides with the formation of water and salt (neutralization reaction). The salt contains the acid residue of the acid that was used in the neutralization reaction:
H 3 PO 4 + Fe 2 O 3 → 2 FePO 4 + 3 H 2 O.
Interact with metals.
For acids to interact with metals, certain conditions must be met:
1. the metal must be sufficiently active with respect to acids (in the series of activity of metals it must be located before hydrogen). The further to the left a metal is in the activity series, the more intensely it interacts with acids;
2. the acid must be strong enough (that is, capable of donating hydrogen ions H +).
When chemical reactions of acid with metals occur, salt is formed and hydrogen is released (except for the interaction of metals with nitric and concentrated sulfuric acids):
Zn + 2HCl → ZnCl 2 + H 2 ;
Cu + 4HNO 3 → CuNO 3 + 2 NO 2 + 2 H 2 O.
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