All acids, their properties and bases are divided into strong and weak. But don't you dare confuse concepts like "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 a chemical reaction occurs 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 is sulfuric 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.

Like 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.

Decision: 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.

Decision: 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.

Decision: 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 hydrolysis equation

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 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).

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 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.

Decision. 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

Before discussing the chemical properties of bases and amphoteric hydroxides, let's clearly define what it is?

1) Bases or basic hydroxides include metal hydroxides in the oxidation state +1 or +2, i.e. the formulas of which are written either as MeOH or as Me(OH) 2 . However, there are exceptions. So, the hydroxides Zn (OH) 2, Be (OH) 2, Pb (OH) 2, Sn (OH) 2 do not belong to the bases.

2) Amphoteric hydroxides include metal hydroxides in the oxidation state +3, +4, and, as exceptions, hydroxides Zn (OH) 2, Be (OH) 2, Pb (OH) 2, Sn (OH) 2. Metal hydroxides in the oxidation state +4 are not found in the USE assignments, therefore they will not be considered.

Chemical properties of bases

All bases are divided into:

Recall that beryllium and magnesium are not alkaline earth metals.

In addition to being soluble in water, alkalis also dissociate very well in aqueous solutions, while insoluble bases have a low degree of dissociation.

This difference in solubility and ability to dissociate between alkalis and insoluble hydroxides leads, in turn, to noticeable differences in their chemical properties. So, in particular, alkalis are more chemically active compounds and are often capable of entering into those reactions that insoluble bases do not enter into.

Reaction of bases with acids

Alkalis react with absolutely all acids, even very weak and insoluble ones. For example:

Insoluble bases react with almost all soluble acids, do not react with insoluble silicic acid:

It should be noted that both strong and weak bases with the general formula of the form Me (OH) 2 can form basic salts with a lack of acid, for example:

Interaction with acid oxides

Alkalis react with all acidic oxides to form salts and often water:

Insoluble bases are able to react with all higher acid oxides corresponding to stable acids, for example, P 2 O 5, SO 3, N 2 O 5, with the formation of medium salts:

Insoluble bases of the form Me (OH) 2 react in the presence of water with carbon dioxide exclusively with the formation of basic salts. For example:

Cu(OH) 2 + CO 2 = (CuOH) 2 CO 3 + H 2 O

With silicon dioxide, due to its exceptional inertness, only the strongest bases, alkalis, react. In this case, normal salts are formed. The reaction does not proceed with insoluble bases. For example:

Interaction of bases with amphoteric oxides and hydroxides

All alkalis react with amphoteric oxides and hydroxides. If the reaction is carried out by fusing an amphoteric oxide or hydroxide with a solid alkali, such a reaction leads to the formation of hydrogen-free salts:

If aqueous solutions of alkalis are used, then hydroxo complex salts are formed:

In the case of aluminum, under the action of an excess of concentrated alkali, instead of the Na salt, the Na 3 salt is formed:

The interaction of bases with salts

Any base reacts with any salt only if two conditions are met simultaneously:

1) solubility of starting compounds;

2) the presence of a precipitate or gas among the reaction products

For example:

Thermal stability of bases

All alkalis, except Ca(OH) 2 , are resistant to heat and melt without decomposition.

All insoluble bases, as well as slightly soluble Ca (OH) 2, decompose when heated. The highest decomposition temperature for calcium hydroxide is about 1000 o C:

Insoluble hydroxides have much lower decomposition temperatures. So, for example, copper (II) hydroxide decomposes already at temperatures above 70 o C:

Chemical properties of amphoteric hydroxides

Interaction of amphoteric hydroxides with acids

Amphoteric hydroxides react with strong acids:

Amphoteric metal hydroxides in the +3 oxidation state, i.e. type Me (OH) 3, do not react with acids such as H 2 S, H 2 SO 3 and H 2 CO 3 due to the fact that salts that could be formed as a result of such reactions are subject to irreversible hydrolysis to the original amphoteric hydroxide and corresponding acid:

Interaction of amphoteric hydroxides with acid oxides

Amphoteric hydroxides react with higher oxides, which correspond to stable acids (SO 3, P 2 O 5, N 2 O 5):

Amphoteric metal hydroxides in the +3 oxidation state, i.e. type Me (OH) 3, do not react with acid oxides SO 2 and CO 2.

Interaction of amphoteric hydroxides with bases

Of the bases, amphoteric hydroxides react only with alkalis. In this case, if an aqueous solution of alkali is used, then hydroxo complex salts are formed:

And when amphoteric hydroxides are fused with solid alkalis, their anhydrous analogues are obtained:

Interaction of amphoteric hydroxides with basic oxides

Amphoteric hydroxides react when fused with oxides of alkali and alkaline earth metals:

Thermal decomposition of amphoteric hydroxides

All amphoteric hydroxides are insoluble in water and, like any insoluble hydroxides, decompose when heated to the corresponding oxide and water.

Salt hydrolysis" - To form an idea of ​​chemistry as a productive force of society. Acetic acid CH3COOH is the oldest of the 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 hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong acids.

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 a solution of 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 yellow, the litmus indicator turns blue, 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 a dielectric, that is, does not conduct electric current, however, in a dissolved or molten form it 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, under different 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, what are the general properties of 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.

We have defined hydrolysis remembered some facts about salts. Now we will discuss strong and weak acids and find out that the "scenario" of hydrolysis depends precisely on which acid and which base formed this salt.

← Hydrolysis of salts. Part I

Strong and weak electrolytes

Let me remind you that all acids and bases can be conditionally divided into strong and weak. Strong acids (and, in general, strong electrolytes) dissociate almost completely in aqueous solution. Weak electrolytes decompose into ions to a small extent.

Strong acids include:

• H 2 SO 4 (sulfuric acid),
• HClO 4 (perchloric acid),
• HClO 3 (chloric acid),
• HNO 3 (nitric acid),
• HCl (hydrochloric acid),
• HBr (hydrobromic acid),
• HI (hydroiodic acid).

The following is a list of weak acids:

• H 2 SO 3 (sulphurous acid),
• H 2 CO 3 (carbonic acid),
• H 2 SiO 3 (silicic acid),
• H 3 PO 3 (phosphorous acid),
• H 3 PO 4 (orthophosphoric acid),
• HClO 2 (chlorous acid),
• HClO (hypochlorous acid),
• HNO 2 (nitrous acid),
• HF (hydrofluoric acid),
• H 2 S (hydrosulfuric acid),
• most organic acids, e.g. acetic (CH 3 COOH).

Naturally, it is impossible to list all the acids that exist in nature. Only the most "popular" ones are listed. It should also be understood that the division of acids into strong and weak is rather arbitrary.

Things are much simpler with strong and weak bases. You can use the solubility table. All strong bases are soluble in base water, except for NH 4 OH. These substances are called alkalis (NaOH, KOH, Ca (OH) 2, etc.)

Weak bases are:

• all water-insoluble hydroxides (eg Fe(OH) 3 , Cu(OH) 2 etc.),
• NH 4 OH (ammonium hydroxide).

Salt hydrolysis. Key facts

It may seem to those reading this article that we have already forgotten about the main topic of the conversation, and have gone somewhere to the side. This is not true! Our conversation about acids and bases, about strong and weak electrolytes is directly related to the hydrolysis of salts. Now you will be convinced of it.

So let me give you the basic facts:

1. Not all salts undergo hydrolysis. Exist hydrolytically stable compounds such as sodium chloride.
2. Hydrolysis of salts can be complete (irreversible) and partial (reversible).
3. During the hydrolysis reaction, an acid or base is formed, the acidity of the medium changes.
4. The fundamental possibility of hydrolysis, the direction of the corresponding reaction, its reversibility or irreversibility are determined acid power and by force of foundation that form this salt.
5. Depending on the strength of the corresponding acid and resp. bases, all salts can be divided into 4 groups. Each of these groups has its own "scenario" of hydrolysis.

Example 4. Salt NaNO 3 is formed by a strong acid (HNO 3) and a strong base (NaOH). Hydrolysis does not occur, no new compounds are formed, the acidity of the medium does not change.

Example 5. Salt NiSO 4 is formed by a strong acid (H 2 SO 4) and a weak base (Ni (OH) 2). Hydrolysis occurs at the cation, during the reaction an acid and a basic salt are formed.

Example 6. Potassium carbonate is formed from a weak acid (H 2 CO 3) and a strong base (KOH). Anion hydrolysis, formation of alkali and acid salt. Alkaline solution.

Example 7. Aluminum sulfide is formed by a weak acid (H 2 S) and a weak base (Al (OH) 3). Hydrolysis occurs both at the cation and at the anion. irreversible reaction. During the process, H 2 S and aluminum hydroxide are formed. The acidity of the environment changes slightly.

Try it yourself:

Exercise 2. What type are the following salts: FeCl 3 , Na 3 PO 3 , KBr, NH 4 NO 2 ? Do these salts undergo hydrolysis? Cation or anion? What is formed during the reaction? 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 give a specific "scenario" of hydrolysis for each of them. In the next part, we will start with salts formed from a weak base and a strong acid.