h3po4 h2so4 exhibits the strongest acidic properties. Acid formulas

7. Acids. Salt. Relationship between classes of inorganic substances

7.1. acids

Acids are electrolytes, during 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

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 in individual (100% form) and in the form of dilute 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 hydrohalic (HCl, HBr, HI), hydrogen sulfide H 2 S, hydrocyanic (hydrocyanic HCN), coal 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, coal is a mixture of CO 2 and H 2 O. It is clear that using the expression "hydrochloric acid solution" is wrong.

Most acids are soluble in water, silicic acid H 2 SiO 3 is insoluble. The vast 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 features (Table 7.2).

Table 7.2

Acid classification

All the general chemical properties of acids are due to the presence in their aqueous solutions of an excess of hydrogen cations H + (H 3 O +).

1. Due to an excess of H + ions, aqueous solutions of acids change the color of violet and methyl orange litmus to red (phenolphthalein does not change color, remains colorless). In an aqueous solution of weak carbonic acid, the 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 Ch. 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) SO3.

Decision. The transformation can be carried out using H 2 SO 4:

BaO + H 2 SO 4 \u003d 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 \u003d NH 4 Cl - ammonium chloride;

H 2 SO 4 + 2NH 3 = (NH 4) 2 SO 4 - ammonium sulfate.

4. Non-oxidizing acids with the formation of a salt and the release of hydrogen react with metals located in the row of activity to hydrogen:

H 2 SO 4 (diff) + Fe = FeSO 4 + H 2

2HCl + Zn \u003d ZnCl 2 \u003d H 2

The interaction of oxidizing acids (HNO 3 , H 2 SO 4 (conc)) with metals is very specific and is considered in the study of 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 is formed and a weak acid, or, as they say, a stronger acid displaces a weaker one. The series of decreasing strength of acids looks like this:

Examples of ongoing reactions:

2HCl + Na 2 CO 3 \u003d 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 \u003d 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 (diff), NaNO 3 and H 2 SO 4 (diff), 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 the salt:

CuSO 4 + H 2 S \u003d CuS ↓ + H 2 SO 4

3AgNO 3 (razb) + 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 between a strong acid and a salt formed by another strong acid is possible:

BaCl 2 + H 2 SO 4 \u003d 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 series in which the formulas of substances that react with H 2 SO 4 are given (diff).

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.

Decision. All substances of series 4 interact with H 2 SO 4 (razb):

Na 2 SO 3 + H 2 SO 4 \u003d Na 2 SO 4 + H 2 O + SO 2

Mg + H 2 SO 4 \u003d 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. It is a non-volatile and thermally stable acid, therefore it displaces all strong acids from solid (!) Salts, since they are more volatile than H 2 SO 4 (conc):

KCl (tv) + H 2 SO 4 (conc) KHSO 4 + HCl

2KCl (tv) + 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 in the solid state

Example 7.3. Concentrated sulfuric acid, unlike dilute sulfuric acid, reacts:

3) KNO 3 (TV);

Decision. 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 (tv).

Answer: 3).

Methods for obtaining 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 (g) → H 2 S (solution)

• from salts by displacement by stronger or less volatile acids:

FeS + 2HCl \u003d 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

oxygenated acids receive:

• by dissolving the corresponding acid oxides in water, while the oxidation state of the acid-forming element in the oxide and acid remains the same (NO 2 is an exception):

N 2 O 5 + H 2 O \u003d 2HNO 3

SO 3 + H 2 O \u003d 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 forms that is insoluble in the resulting acids):

Ba (NO 3) 2 + H 2 SO 4 (razb) \u003d BaSO 4 ↓ + 2HNO 3

AgNO 3 + HCl = AgCl↓ + HNO 3

• displacement of a volatile acid from its salts by 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

• by displacing a weaker acid from its salts with 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- electrolytes, during the dissociation of which only H + ions are formed from positive ions:

HNO 3 ↔ H + + NO 3 -;

CH 3 COOH ↔ H + +CH 3 COO -.

All acids are classified into inorganic and organic (carboxylic), which also have their own (internal) classifications.

Under normal conditions, a significant amount of inorganic acids exist in a liquid state, some in a solid state (H 3 PO 4, H 3 BO 3).

Organic acids with up to 3 carbon atoms are easily mobile, colorless liquids with a characteristic pungent odor; acids with 4-9 carbon atoms are oily liquids with an unpleasant odor, and acids with a large number of carbon atoms are solids that are insoluble in water.

Chemical formulas of acids

Consider the chemical formulas of acids using the example of several representatives (both inorganic and organic): hydrochloric acid -HCl, sulfuric acid - H 2 SO 4, phosphoric acid - H 3 PO 4, acetic acid - CH 3 COOH and benzoic acid - C 6 H5COOH. The chemical formula shows the qualitative and quantitative composition of the molecule (how many and which atoms are included in a particular compound) Using the chemical formula, you can calculate the molecular weight of acids (Ar (H) \u003d 1 amu, Ar (Cl) \u003d 35.5 a.m.). m.u., Ar(P) = 31 a.m.u., Ar(O) = 16 a.m.u., Ar(S) = 32 a.m.u., Ar(C) = 12 a.u.m.):

Mr(HCl) = Ar(H) + Ar(Cl);

Mr(HCl) = 1 + 35.5 = 36.5.

Mr(H 2 SO 4) = 2×Ar(H) + Ar(S) + 4×Ar(O);

Mr(H 2 SO 4) \u003d 2 × 1 + 32 + 4 × 16 \u003d 2 + 32 + 64 \u003d 98.

Mr(H 3 PO 4) = 3×Ar(H) + Ar(P) + 4×Ar(O);

Mr(H 3 PO 4) \u003d 3 × 1 + 31 + 4 × 16 \u003d 3 + 31 + 64 \u003d 98.

Mr(CH 3 COOH) = 3×Ar(C) + 4×Ar(H) + 2×Ar(O);

Mr(CH 3 COOH) = 3x12 + 4x1 + 2x16 = 36 + 4 + 32 = 72.

Mr(C 6 H 5 COOH) = 7×Ar(C) + 6×Ar(H) + 2×Ar(O);

Mr(C 6 H 5 COOH) = 7x12 + 6x1 + 2x16 = 84 + 6 + 32 = 122.

Structural (graphic) formulas of acids

The structural (graphic) formula of a substance is more visual. It shows how atoms are connected to each other within a molecule. Let us indicate the structural formulas of each of the above compounds:

Rice. 1. Structural formula of hydrochloric acid.

Rice. 2. Structural formula of sulfuric acid.

Rice. 3. Structural formula of phosphoric acid.

Rice. 4. Structural formula of acetic acid.

Rice. 5. Structural formula of benzoic acid.

Ionic formulas

All inorganic acids are electrolytes, i.e. capable of dissociating in an aqueous solution into ions:

HCl ↔ H + + Cl - ;

H 2 SO 4 ↔ 2H + + SO 4 2-;

H 3 PO 4 ↔ 3H + + PO 4 3-.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Consider the most common formulas of acids in the educational literature:

It is easy to see what unites all the formulas of acids is the presence of hydrogen atoms (H), which comes first in the formula.

Determination of the valency of the acid residue

From the above list, it can be seen that the number of these atoms may differ. Acids, which contain only one hydrogen atom, are called monobasic (nitric, hydrochloric, and others). Sulfuric, carbonic, silicic acids are dibasic, since their formulas contain two H atoms each. A tribasic phosphoric acid molecule contains three hydrogen atoms.

Thus, the amount of H in the formula characterizes the basicity of the acid.

That atom, or group of atoms, which are written after hydrogen, is called acid residues. For example, in hydrosulfide acid, the residue consists of one atom - S, and in phosphoric, sulfuric and many others - of two, and one of them is necessarily oxygen (O). On this basis, all acids are divided into oxygen-containing and anoxic.

Each acid residue has a certain valence. It is equal to the number of H atoms in the molecule of this acid. The valency of the HCl residue is equal to one, since it is a monobasic acid. The residues of nitric, perchloric, and nitrous acids have the same valency. The valency of the sulfuric acid residue (SO 4) is two, since there are two hydrogen atoms in its formula. A trivalent phosphoric acid residue.

Acid residues - anions

In addition to valency, acid residues have charges and are anions. Their charges are listed in the solubility table: CO 3 2− , S 2− , Cl − and so on. Please note: the charge of the acid residue numerically coincides with its valency. For example, in silicic acid, the formula of which is H 2 SiO 3, the acid residue SiO 3 has a valence equal to II and a charge of 2-. Thus, knowing the charge of the acid residue, it is easy to determine its valency and vice versa.

Summarize. Acids are compounds formed by hydrogen atoms and acid residues. From the point of view of the theory of electrolytic dissociation, another definition can be given: acids are electrolytes, in solutions and melts of which there are hydrogen cations and anions of acid residues.

Hints

The chemical formulas of acids, as a rule, are memorized, as are their names. If you have forgotten how many hydrogen atoms are in a particular formula, but you know what its acidic residue looks like, a solubility table will come to your aid. The charge of the residue coincides in modulus with the valence, and that with the amount of H. For example, you remember that the residue of carbonic acid is CO 3. According to the solubility table, you determine that its charge is 2-, which means that it is divalent, that is, carbonic acid has the formula H 2 CO 3.

Often there is confusion with the formulas of sulfuric and sulphurous, as well as nitric and nitrous acids. Here, too, there is one point that makes it easier to remember: the name of the acid from the pair in which there are more oxygen atoms ends in -naya (sulfuric, nitric). An acid with fewer oxygen atoms in the formula has a name ending in -ista (sulphurous, nitrogenous).

However, these tips will only help if you are familiar with the acid formulas. Let's repeat them again.

Names of some inorganic acids and salts

Let me briefly remind you with specific examples of how salts should be properly named.

Example 1. Salt K 2 SO 4 is formed by the rest of sulfuric acid (SO 4) and metal K. Salts of sulfuric acid are called sulfates. K 2 SO 4 - potassium sulfate.

Example 2. FeCl 3 - the composition of the salt includes iron and the rest of hydrochloric acid (Cl). Name of the salt: iron(III) chloride. Please note: in this case, we not only have to name the metal, but also indicate its valency (III). In the previous example, this was not necessary, since the valency of sodium is constant.

Important: in the name of the salt, the valency of the metal should be indicated only if this metal has a variable valency!

Example 3. Ba (ClO) 2 - the composition of the salt includes barium and the remainder of hypochlorous acid (ClO). Name of salt: barium hypochlorite. The valency of the Ba metal in all its compounds is two, it is not necessary to indicate it.

Example 4. (NH 4) 2 Cr 2 O 7. The NH 4 group is called ammonium, the valency of this group is constant. Salt name: ammonium dichromate (bichromate).

In the above examples, we met only the so-called. medium or normal salts. Acid, basic, double and complex salts, salts of organic acids will not be discussed here.