Iron hydroxide is a strong or weak base. Weak base and strong acid in the hydrolysis of salts. Anion hydrolysis

Salt hydrolysis" - To form an idea of chemistry as a productive force of society. Acetic acid CH3COOH is the oldest of organic acids. In acids - carboxyl groups, But all the acids here are weak.

All acids, their properties and bases are divided into strong and weak. For example, you cannot make a concentrated solution of a weak acid or a dilute solution of a strong base. Our water in this case plays the role of a base, as it receives a proton from of hydrochloric acid. Acids that dissociate completely into aqueous solutions are called strong.

For oxides hydrated with an indefinite number of water molecules, for example, Tl2O3 n H2O, it is unacceptable to write formulas like Tl(OH)3. Calling such compounds hydroxides is also not recommended.

For bases, one can quantify their strength, that is, the ability to split off a proton from an acid. All bases are solids with different colors. Attention! Alkalis are very caustic substances. If it comes into contact with the skin, alkali solutions cause severe long-healing burns, if they get into the eyes, they can cause blindness. When roasting cobalt minerals containing arsenic, volatile toxic arsenic oxide is released.

These properties of the water molecule are already known to you. II) and solution acetic acid. HNO2) - only one proton.

All bases are solids that have different colors. 1. They act on indicators. Indicators change their color depending on the interaction with different chemicals. When interacting with bases, they change their color: the methyl orange indicator turns into yellow, litmus indicator - in Blue colour, and phenolphthalein becomes fuchsia.

Cool the containers, for example by placing them in a vessel filled with ice. Three solutions will remain clear, and the fourth will quickly become cloudy, a white precipitate will begin to fall out. This is where the barium salt is located. Set this container aside. You can quickly determine barium carbonate in another way. This is fairly easy to make, all you need are porcelain evaporating cups and a spirit lamp. If it is a lithium salt, the color will be bright red. By the way, if barium salt were tested in the same way, the color of the flame should have been green.

An electrolyte is a substance that in the solid state is an insulator, that is, it does not conduct electric current, however, when dissolved or molten, becomes a conductor. Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on many factors: the nature of the electrolyte itself, the solvent, and the temperature. Therefore, this division itself is to a certain extent conditional. After all, the same substance can various conditions be both a strong electrolyte and a weak one.

Hydrolysis does not occur, no new compounds are formed, the acidity of the medium does not change. How does the acidity of the environment change? The reaction equations can not yet be written down. It remains for us to sequentially discuss 4 groups of salts and for each of them give a specific "scenario" of hydrolysis. In the next part, we will start with salts formed from a weak base and a strong acid.

After reading the article, you will be able to separate substances into salts, acids and bases. H solution, which common properties have acids and bases. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid.

The lower this value, the stronger the acid. Strong or weak - this is needed in the reference book of Ph.D. watch, but you need to know the classics. Strong acids are acids that can displace the anion of another acid from the salt.

The hydrolysis constant is equal to the ratio of the product of concentrations
hydrolysis products to the concentration of non-hydrolyzed salt.

Example 1 Calculate the degree of hydrolysis of NH 4 Cl.

Solution: From the table we find Kd (NH 4 OH) \u003d 1.8 ∙ 10 -3, from here

Kγ \u003d Kv / Kd k \u003d \u003d 10 -14 / 1.8 10 -3 \u003d 5.56 10 -10.

Example 2 Calculate the degree of hydrolysis of ZnCl 2 in 1 step in a 0.5 M solution.

Solution: Ionic hydrolysis equation Zn 2 + H 2 O ZnOH + + H +

Kd ZnOH +1=1.5∙10 -9; hγ=√(Kv/ [Kd basic ∙Cm]) = 10 -14 /1.5∙10 -9 ∙0.5=0.36∙10 -2 (0.36%).

Example 3 Compose ionic-molecular and molecular equations of hydrolysis of salts: a) KCN; b) Na 2 CO 3; c) ZnSO 4 . Determine the reaction of the medium solutions of these salts.

Solution: a) Potassium cyanide KCN is a salt of a weak monobasic acid (see Table I of the Appendix) HCN and a strong base KOH. When dissolved in water, KCN molecules completely dissociate into K + cations and CN - anions. K + cations cannot bind OH - water ions, since KOH is a strong electrolyte. Anions, on the other hand, CN - bind H + ions of water, forming molecules of a weak electrolyte HCN. The salt hydrolyzes at the anion. Ionic molecular equation hydrolysis

CN - + H 2 O HCN + OH -

or in molecular form

KCN + H 2 O HCN + KOH

As a result of hydrolysis, a certain excess of OH - ions appears in the solution; therefore, the KCN solution has an alkaline reaction (pH > 7).

b) Sodium carbonate Na 2 CO 3 is a salt of a weak polybasic acid and a strong base. In this case, the anions of the CO 3 2- salt, binding the hydrogen ions of water, form anions of the acidic salt of HCO - 3, and not H 2 CO 3 molecules, since HCO - 3 ions dissociate much more difficult than H 2 CO 3 molecules. Under normal conditions, hydrolysis proceeds in the first stage. The salt hydrolyzes at the anion. Ionic-molecular hydrolysis equation

CO2-3 + H 2 O HCO - 3 + OH -

or in molecular form

Na 2 CO 3 + H 2 O NaHCO 3 + NaOH

An excess of OH - ions appears in the solution, so the Na 2 CO 3 solution has an alkaline reaction (pH> 7).

c) Zinc sulfate ZnSO 4 - a salt of a weak polyacid base Zn (OH) 2 and a strong acid H 2 SO 4. In this case, Zn + cations bind hydroxide ions of water, forming cations of the basic salt ZnOH + . The formation of Zn(OH) 2 molecules does not occur, since ZnOH + ions dissociate much more difficult than Zn(OH) 2 molecules. Under normal conditions, hydrolysis proceeds in the first stage. The salt is hydrolyzed at the cation. Ionic-molecular hydrolysis equation

Zn 2+ + H 2 O ZnOH + + H +

or in molecular form

2ZnSO 4 + 2H 2 O (ZnOH) 2 SO 4 + H 2 SO 4

An excess of hydrogen ions appears in the solution, so the ZnSO 4 solution has an acidic reaction (pH< 7).

Example 4 What products are formed when solutions of A1(NO 3) 3 and K 2 CO 3 are mixed? Make an ion-molecular and molecular reaction equation.

Solution. Salt A1 (NO 3) 3 is hydrolyzed by the cation, and K 2 CO 3 - by the anion:

A1 3+ + H 2 O A1OH 2+ + H +

CO 2- 3 + H 2 O HSO - s + OH -

If the solutions of these salts are in the same vessel, then the hydrolysis of each of them is mutually enhanced, because the H + and OH - ions form a weak electrolyte molecule H 2 O. In this case, the hydrolytic equilibrium shifts to the right and the hydrolysis of each of the salts taken goes to the end with the formation A1 (OH) 3 and CO 2 (H 2 CO 3). Ionic-molecular equation:

2A1 3+ + ZSO 2- 3 + ZN 2 O \u003d 2A1 (OH) 3 + ZSO 2

molecular equation: ZSO 2 + 6KNO 3

2A1 (NO 3) 3 + ZK 2 CO 3 + ZN 2 O \u003d 2A1 (OH) 3

All acids, their properties and bases are divided into strong and weak. But do not dare to confuse such concepts as " strong acid” or “strong base” with their concentration. For example, you cannot make a concentrated solution of a weak acid or a dilute solution of a strong base. For example, hydrochloric acid, when dissolved in water, gives each of the two water molecules one of its protons.

When it happens chemical reaction in the hydronium ion, the hydrogen ion binds very strongly to the water molecule. The reaction itself will continue until its reagents are completely exhausted. Our water in this case plays the role of a base, as it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong acids.

When we know the very initial concentration of a strong acid, then in this case it is not difficult to calculate the concentration of hydronium ions and chloride ions in the solution. For example, if you take and dissolve 0.2 moles of gaseous hydrochloric acid in 1 liter of water, the concentration of ions after dissociation will be exactly the same.

Examples of strong acids:

1) HCl, hydrochloric acid;
2) HBr, hydrogen bromide;
3) HI, hydrogen iodine;
4) HNO3, nitric acid;
5) HClO4 - perchloric acid;
6) H2SO4 - sulphuric acid.

All known acids (with the exception of sulfuric acid) are listed above and are monoprotic, since their atoms donate one proton each; Sulfuric acid molecules can easily donate two of their protons, which is why sulfuric acid is diprotic.

Electrolytes are strong bases; they completely dissociate in aqueous solutions to form a hydroxide ion.

As with acids, calculating the concentration of hydroxide ion is very easy once you know the initial concentration of the solution. For example, a NaOH solution with a concentration of 2 mol/l dissociates into the same concentration of ions.

Weak acids. Foundations and properties

As for weak acids, they do not completely dissociate, that is, partially. It is very easy to distinguish between strong and weak acids: if the reference table shows its constant next to the name of an acid, then this acid is weak; if the constant is not given, then this acid is strong.

Weak bases also react well with water to form an equilibrium system. Weak acids are also characterized by a dissociation constant K.

		Foundations
medium strength
	Alkali metal hydroxides (KOH, NaOH, ZiOH), Ba(OH) 2, etc.		Na 4 OH and water-insoluble bases (Ca (OH) 2, Zi (OH) 2, AL (OH) 3, etc.

The hydrolysis constant is equal to the ratio of the product of the concentrations of hydrolysis products to the concentration of non-hydrolyzed salt.

Example 1 Calculate the degree of hydrolysis of NH 4 Cl.

Solution: From the table we find Kd (NH 4 OH) \u003d 1.8 ∙ 10 -3, from here

Kγ \u003d Kv / Kd k \u003d \u003d 10 -14 / 1.8 10 -3 \u003d 5.56 10 -10.

Example 2 Calculate the degree of hydrolysis of ZnCl 2 in 1 step in a 0.5 M solution.

Solution: Ionic equation for the hydrolysis of Zn 2 + H 2 OZnOH + + H +

Kd ZnOH +1=1.5∙10 -9; hγ=√(Kv/ [Kd basic ∙Cm]) = 10 -14 /1.5∙10 -9 ∙0.5=0.36∙10 -2 (0.36%).

Example 3 Compose ionic-molecular and molecular equations of hydrolysis of salts: a) KCN; b) Na 2 CO 3; c) ZnSO 4 . Determine the reaction of the medium solutions of these salts.

CN - + H 2 O HCN + OH -

or in molecular form

KCN + H 2 O HCN + KOH

As a result of hydrolysis, a certain excess of OH - ions appears in the solution; therefore, the KCN solution has an alkaline reaction (pH > 7).

CO2-3 + H 2 OHCO - 3 + OH -

or in molecular form

Na 2 CO 3 + H 2 O NaHCO 3 + NaOH

An excess of OH - ions appears in the solution, so the Na 2 CO 3 solution has an alkaline reaction (pH> 7).

Zn 2+ + H 2 OZnOH + + H +

or in molecular form

2ZnSO 4 + 2H 2 O (ZnOH) 2 SO 4 + H 2 SO 4

An excess of hydrogen ions appears in the solution, so the ZnSO 4 solution has an acidic reaction (pH< 7).

Example 4 What products are formed when solutions of A1(NO 3) 3 and K 2 CO 3 are mixed? Make an ion-molecular and molecular reaction equation.

Solution. Salt A1 (NO 3) 3 is hydrolyzed by the cation, and K 2 CO 3 - by the anion:

A1 3+ + H 2 O A1OH 2+ + H +

CO 2- 3 + H 2 O HCO - s + OH -

If the solutions of these salts are in the same vessel, then there is a mutual enhancement of the hydrolysis of each of them, because the H + and OH - ions form a weak electrolyte molecule H 2 O. In this case, the hydrolytic equilibrium shifts to the right and the hydrolysis of each of the salts taken goes to the end with the formation A1 (OH) 3 and CO 2 (H 2 CO 3). Ionic-molecular equation:

2A1 3+ + ZSO 2- 3 + ZN 2 O \u003d 2A1 (OH) 3 + ZSO 2

molecular equation: ZSO 2 + 6KNO 3

2A1 (NO 3) 3 + ZK 2 CO 3 + ZN 2 O \u003d 2A1 (OH) 3

Weak acids. Foundations and properties

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