We have given a definition 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 the given salt.

← Hydrolysis of salts. Part I

Strong and weak electrolytes

Let me remind you that all acids and bases can be divided into strong And weak. Strong acids (and, in general, strong electrolytes) in aqueous solution dissociate almost completely. Weak electrolytes disintegrate 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 (hydriodic acid).

Below is a list of weak acids:

• H 2 SO 3 (sulfurous 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 (hydrogen sulfide acid),
• most organic acids, eg acetic acid (CH 3 COOH).

Naturally, it is impossible to list all the acids existing in nature. Only the most “popular” ones are given. It should also be understood that the division of acids into strong and weak is quite arbitrary.

The situation is much simpler with strong and weak bases. You can use the solubility table. Strong reasons include all soluble in water bases other than NH 4 OH. These substances are called alkalis (NaOH, KOH, Ca(OH) 2, etc.)

Weak grounds are:

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

Hydrolysis of salts. Key facts

It may seem to those reading this article that we have already forgotten about the main topic of conversation and have gone somewhere aside. This is wrong! Our conversation about acids and bases, about strong and weak electrolytes is directly related to the hydrolysis of salts. Now you will see this.

So let me give you the basic facts:

1. Not all salts undergo hydrolysis. There are 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 and 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 strength And foundation force, which form this salt.
5. Depending on the strength of the respective acid and resp. bases, all salts can be divided into 4 groups. Each of these groups is characterized by its own “scenario” of hydrolysis.

Example 4. The 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, and the acidity of the medium does not change.

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

Example 6. Potassium carbonate is formed by a weak acid (H 2 CO 3) and a strong base (KOH). Hydrolysis by anion, 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 at both the cation and the anion. Irreversible reaction. During the process, H 2 S and aluminum hydroxide are formed. The acidity of the medium changes slightly.

Try it yourself:

Exercise 2. What type of salts are the following: FeCl 3, Na 3 PO 3, KBr, NH 4 NO 2? Are these salts subject to hydrolysis? By cation or by anion? What is formed during the reaction? How does the acidity of the environment change? You don’t have to write down the reaction equations for now.

All we have to do is discuss 4 groups of salts sequentially and give a specific “scenario” of hydrolysis for each of them. In the next part, we'll start with salts formed by a weak base and a strong acid.

Bases (hydroxides)– complex substances whose molecules contain one or more hydroxy OH groups. Most often, bases consist of a metal atom and an OH group. For example, NaOH is sodium hydroxide, Ca(OH) 2 is calcium hydroxide, etc.

There is a base - ammonium hydroxide, in which the hydroxy group is attached not to the metal, but to the NH 4 + ion (ammonium cation). Ammonium hydroxide is formed when ammonia is dissolved in water (the reaction of adding water to ammonia):

NH 3 + H 2 O = NH 4 OH (ammonium hydroxide).

The valency of the hydroxy group is 1. The number of hydroxyl groups in the base molecule depends on the valence of the metal and is equal to it. For example, NaOH, LiOH, Al (OH) 3, Ca(OH) 2, Fe(OH) 3, etc.

All reasons - solids that have different colors. Some bases are highly soluble in water (NaOH, KOH, etc.). However, most of them are not soluble in water.

Bases soluble in water are called alkalis. Alkali solutions are “soapy”, slippery to the touch and quite caustic. Alkalies include hydroxides of alkali and alkaline earth metals (KOH, LiOH, RbOH, NaOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2, etc.). The rest are insoluble.

Insoluble bases- these are amphoteric hydroxides, which act as bases when interacting with acids, and behave like acids with alkali.

Different bases have different abilities to remove hydroxy groups, so they are divided into strong and weak bases.

Strong bases in aqueous solutions easily give up their hydroxy groups, but weak bases do not.

Chemical properties of bases

The chemical properties of bases are characterized by their relationship to acids, acid anhydrides and salts.

1. Act on indicators. Indicators change color depending on interaction with different chemicals. In neutral solutions they have one color, in acid solutions they have another color. When interacting with bases, they change their color: the methyl orange indicator turns yellow, the litmus indicator turns blue, and phenolphthalein becomes fuchsia.

2. Interact with acid oxides With formation of salt and water:

2NaOH + SiO 2 → Na 2 SiO 3 + H 2 O.

3. React with acids, forming salt and water. The reaction of a base with an acid is called a neutralization reaction, since after its completion the medium becomes neutral:

2KOH + H 2 SO 4 → K 2 SO 4 + 2H 2 O.

4. Reacts with salts forming a new salt and base:

2NaOH + CuSO 4 → Cu(OH) 2 + Na 2 SO 4.

5. When heated, they can decompose into water and the main oxide:

Cu(OH) 2 = CuO + H 2 O.

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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) = 1.8∙10 -3, from here

Kγ=Kv/Kd k = =10 -14 /1.8∙10 -3 = 5.56∙10 -10 .

Example 2. Calculate the degree of hydrolysis of ZnCl 2 one step at a time in a 0.5 M solution.

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

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

Example 3. Make up ion-molecular and molecular equations for the hydrolysis of salts: a) KCN; b) Na 2 CO 3; c) ZnSO 4. Determine the reaction of the solution 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 - ions of water, since KOH is a strong electrolyte. The CN - anions bind the H + ions of water, forming molecules of the weak electrolyte HCN. The salt is hydrolyzed 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, so 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 the anions of the acid salt HCO - 3, and not H 2 CO 3 molecules, since HCO - 3 ions dissociate much more difficultly than H 2 CO 3 molecules. Under normal conditions, hydrolysis proceeds in the first stage. The salt is hydrolyzed at the anion. Ionic-molecular hydrolysis equation

CO 2- 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 is a salt of a weak polyacid base Zn(OH) 2 and a strong acid H 2 SO 4. In this case, Zn + cations bind hydroxyl ions of water, forming cations of the main salt ZnOH +. The formation of Zn(OH) 2 molecules does not occur, since ZnOH + ions dissociate much more difficultly than Zn(OH) 2 molecules. Under normal conditions, hydrolysis proceeds in the first stage. The salt hydrolyzes into the cation. Ionic-molecular hydrolysis equation

Zn 2+ + H 2 O ZnON + + 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 mixing solutions of A1(NO 3) 3 and K 2 CO 3? Write an ion-molecular and molecular equation for the reaction.

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 NSO - s + OH -

If 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 molecule of the weak electrolyte H 2 O. In this case, the hydrolytic equilibrium shifts to the right and the hydrolysis of each of the salts taken goes to completion with the formation A1(OH) 3 and CO 2 (H 2 CO 3). Ion-molecular equation:

2A1 3+ + ZSO 2- 3 + ZN 2 O = 2A1(OH) 3 + ZSO 2

molecular equation: 3SO 2 + 6KNO 3

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

Hydrolysis of salt" - To form an idea of ​​chemistry as a productive force of society. Acetic acid CH3COOH is the oldest of organic acids. In acids there are carboxyl groups, but all the acids here are not strong.

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, since it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong.

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

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

You already know such properties of the water molecule. II) and acetic acid solution. HNO2) - only one proton.

All bases are solid substances that have different colors. 1. Act on indicators. Indicators change color depending on 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 bowl of ice. Three solutions will remain clear, but the fourth will quickly become cloudy and a white precipitate will begin to form. This is where the barium salt is found. Set this container aside. You can quickly determine barium carbonate in another way. It's quite easy to do, all you need are porcelain steaming cups and a spirit lamp. If it is a lithium salt, the color will be bright red. By the way, if barium salt had been 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, it does not conduct electric current, however, when dissolved or molten 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 temperature. Therefore, this division itself is to a certain extent arbitrary. After all, one and the same substance can, under different conditions, be strong electrolyte, and weak.

Hydrolysis does not occur, no new compounds are formed, and the acidity of the medium does not change. How does the acidity of the environment change? You don’t have to write down the reaction equations for now. All we have to do is discuss 4 groups of salts sequentially and give a specific “scenario” of hydrolysis for each of them. In the next part, we'll start with salts formed by 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 general 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 indicator, the stronger the acid. Strong or weak - this is needed in the Ph.D. reference book. watch, but you need to know the classics. Strong acids are acids that can displace the anion of another acid from a salt.

12.4. Strength of acids and bases

The direction of displacement of the acid-base equilibrium is determined by the following rule:
Acid-base equilibria are biased towards the weaker acid and weaker base.

An acid is stronger the more easily it gives up a proton, and a base is stronger the more easily it accepts a proton and holds it more firmly. A molecule (or ion) of a weak acid is not inclined to donate a proton, and a molecule (or ion) of a weak base is not inclined to accept it, this explains the shift in equilibrium in their direction. The strength of acids as well as the strength of bases can only be compared in the same solvent
Since acids can react with different bases, the corresponding equilibria will be shifted in one direction or another to varying degrees. Therefore, to compare the strengths of different acids, we determine how easily these acids donate protons to solvent molecules. The strength of the grounds is determined similarly.

You already know that a water (solvent) molecule can both accept and donate a proton, that is, it exhibits both the properties of an acid and the properties of a base. Therefore, both acids and bases can be compared with each other in strength in aqueous solutions. In the same solvent, the strength of the acid depends largely on the energy of tearing connections A-N, and the strength of the base depends on the energy of the formed B-H bond.
For quantitative characteristics acid strength in aqueous solutions, you can use the acid-base equilibrium constant for the reversible reaction of a given acid with water:
HA + H 2 O A + H 3 O.

To characterize the strength of an acid in dilute solutions in which the water concentration is almost constant, use acidity constant:

,

Where K to(HA) = K c·.

In a completely similar way, to quantitatively characterize the strength of a base, you can use the acid-base equilibrium constant of the reversible reaction of a given base with water:

A + H 2 O HA + OH,

and in dilute solutions - basicity constant

, Where K o (HA) = K c ·.

In practice, to assess the strength of a base, the acidity constant of the acid obtained from a given base is used (the so-called " conjugate" acid), since these constants are related by the simple relation

K o (A) = TO(H 2 O)/ K k(NA).

In other words, The weaker the conjugate acid, the stronger the base. And vice versa, the stronger the acid, the weaker the conjugate base .

Acidity and basicity constants are usually determined experimentally. The values ​​of the acidity constants of various acids are given in Appendix 13, and the values ​​of the basicity constants of bases are given in Appendix 14.
To estimate what fraction of the molecules of an acid or base in a state of equilibrium has undergone a reaction with water, a value similar (and homogeneous) to the mole fraction is used and is called degree of protolysis(). For acid NA

.

Here, the value with the subscript “pr” (in the numerator) characterizes the reacted part of the acid molecules NA, and the value with the subscript “out” (in the denominator) characterizes the initial portion of the acid.
According to the reaction equation

n pr (HA) = n(H3O) = n(A) c pr(HA) = c(H3O) = c(A);
==a · With ref(NA);
= (1 – a) · With ref(NA).

Substituting these expressions into the acidity constant equation, we obtain

Thus, knowing the acidity constant and the total concentration of the acid, it is possible to determine the degree of protolysis of this acid in a given solution. Similarly, the base basicity constant can be expressed through the degree of protolysis, therefore, in general form

This equation is a mathematical expression Ostwald's dilution law. If the solutions are diluted, that is, the initial concentration does not exceed 0.01 mol/l, then the approximate ratio can be used

K= 2 · c ref.

To roughly estimate the degree of protolysis, this equation can also be used at concentrations up to 0.1 mol/l.
Acid-base reactions are reversible processes, but not always. Let us consider the behavior of hydrogen chloride and hydrogen fluoride molecules in water:

A hydrogen chloride molecule gives up a proton to a water molecule and becomes a chloride ion. Therefore, in water, hydrogen chloride exhibits properties of an acid, and water itself - properties of a base. The same thing happens with the hydrogen fluoride molecule, and, therefore, hydrogen fluoride also exhibits the properties of an acid. Therefore, an aqueous solution of hydrogen chloride is called hydrochloric (or hydrochloric) acid, and an aqueous solution of hydrogen fluoride is called hydrofluoric (or hydrofluoric) acid. But there is a significant difference between these acids: hydrochloric acid reacts with excess water irreversibly (completely), and hydrofluoric acid reacts reversibly and slightly. Therefore, a hydrogen chloride molecule easily donates a proton to a water molecule, but a hydrogen fluoride molecule does this with difficulty. Therefore, hydrochloric acid is classified as strong acids, and fluorescent – ​​to weak.

Strong acids: HCl, HBr, HI, HClO 4, HClO 3, H 2 SO 4, H 2 SeO 4, HNO 3 and some others.
Now let's turn our attention to the right-hand sides of the equations for the reactions of hydrogen chloride and hydrogen fluoride with water. The fluoride ion can accept a proton (by removing it from the oxonium ion) and turn into a hydrogen fluoride molecule, but the chloride ion cannot. Consequently, the fluoride ion exhibits the properties of a base, while the chloride ion does not exhibit such properties (but only in dilute solutions).
Like acids, there are strong And weak grounds.

Strong base substances include all highly soluble ionic hydroxides (they are also called " alkalis"), since when they are dissolved in water, hydroxide ions completely go into solution.

Weak bases include NH 3 ( K O= 1.74·10 –5) and some other substances. These also include practically insoluble hydroxides of elements that form metals ("metal hydroxides") because when these substances interact with water, only an insignificant amount of hydroxide ions passes into solution.
Weak base particles (they are also called " anionic bases"): F, NO 2, SO 3 2, S 2, CO 3 2, PO 4 3 and other anions formed from weak acids.
The anions Cl, Br, I, HSO 4, NO 3 and other anions formed from strong acids do not have basic properties
The cations Li, Na, K, Ca 2, Ba 2 and other cations that are part of strong bases do not have acidic properties.

In addition to acid and base particles, there are also particles that exhibit both acidic and basic properties. You already know such properties of the water molecule. In addition to water, these are hydrosulfite ion, hydrosulfide ion and other similar ions. For example, HSO 3 exhibits the properties of an acid
HSO 3 + H 2 O SO 3 + H 3 O and base properties
HSO 3 + H 2 O H 2 SO 3 + OH.

Such particles are called ampholytes.

Most ampholyte particles are molecules of weak acids that have lost some protons (HS, HSO 3, HCO 3, H 2 PO 4, HPO 4 2 and some others). The HSO 4 anion does not exhibit basic properties and is a rather strong acid ( TO K = 1.12. 10–2), and therefore does not belong to ampholytes. Salts containing such anions are called acid salts.

Examples of acid salts and their names:

As you've probably noticed, acid-base and redox reactions have a lot in common. Follow common features and the diagram shown in Figure 12.3 will help you find the differences between these types of reactions.

ACID STRENGTH, BASE STRENGTH, ACIDITY CONSTANT, BASICITY CONSTANT, CONJUGATED ACID, CONJUGATED BASE, DEGREE OF PROTOLYSIS, OSTWALD'S LAW OF DILUTION, STRONG ACID, WEAK ACID, STRONG BASE, WEAK BASE, ALKALI, A NIONE BASE, AMPHOLYTES, ACID SALTS
1.Which acid is more inclined to donate a proton in an aqueous solution: a) nitric or nitrogenous, b) sulfuric or sulfurous, c) sulfuric or hydrochloric, d) hydrogen sulfide or sulfurous? Write down reaction equations. In the case of reversible reactions, write down the expression for the acidity constants.
2.Compare the atomization energy of HF and HCl molecules. Are these data consistent with the strength of hydrofluoric and hydrochloric acids?
3.Which particle is a stronger acid: a) a carbonic acid molecule or a bicarbonate ion, b) a phosphoric acid molecule, a dihydrogen phosphate ion or a hydrogen phosphate ion, c) a hydrogen sulfide molecule or a hydrosulfide ion?
4. Why don’t you find acidity constants for sulfuric, hydrochloric, nitric and some other acids in Appendix 13?
5.Prove the validity of the relationship connecting the basicity constant and the acidity constant of conjugate acids and bases.
6. Write down the equations for the reactions with water: a) hydrogen bromide and nitrous acid, b) sulfuric and sulfurous acids, c) nitric acid and hydrogen sulfide. What are the differences between these processes?
7. For the following ampholytes: HS, HSO 3, HCO 3, H 2 PO 4, HPO 4 2, H 2 O - draw up equations for the reactions of these particles with water, write down expressions for the acidity and basicity constants, write down the values ​​of these constants from Appendix 13 and 14. Determine which properties, acidic or basic, predominate in these particles?
8.What processes can occur when phosphoric acid is dissolved in water?
Comparison of the reactivity of strong and weak acids.

12.5. Acid-base reactions of oxonium ions

Both acids and bases differ in strength, solubility, stability, and some other characteristics. The most important of these characteristics is strength. The most characteristic properties of acids are manifested in strong acids. In solutions of strong acids, the acid particles are oxonium ions. Therefore, in this section we will consider reactions in solutions that occur during the interaction of oxonium ions with various substances, containing base particles. Let's start with the strongest foundations.

a) Reactions of oxonium ions with oxide ions

Among the very strong bases, the most important is the oxide ion, which is part of the basic oxides, which, as you remember, are ionic substances. This ion is one of the strongest bases. Therefore, basic oxides (for example, composition MO), even those that do not react with water, easily react with acids. Reaction mechanism:

In these reactions, the oxide ion does not have time to go into solution, but immediately reacts with the oxonium ion. Consequently, the reaction occurs on the surface of the oxide. Such reactions go to completion, since a very weak ampholyte (water) is formed from a strong acid and a strong base.

Example. Reaction of nitric acid with magnesium oxide:

MgO + 2HNO 3p = Mg(NO 3) 2p + H 2 O.

All basic and amphoteric oxides react this way with strong acids, but if an insoluble salt is formed, the reaction in some cases slows down very much, since a layer of insoluble salt prevents the penetration of the acid to the surface of the oxide (for example, the reaction of barium oxide with sulfuric acid).

b) Reactions of oxonium ions with hydroxide ions

Of all the base species that exist in aqueous solutions, the hydroxide ion is the strongest base. Its basicity constant (55.5) is many times higher than the basicity constants of other base particles. Hydroxide ions are part of alkalis and, when dissolved, go into solution. The mechanism of reaction of oxonium ions with hydroxide ions:

.

Example 1. Reaction of hydrochloric acid with sodium hydroxide solution:

HCl p + NaOH p = NaCl p + H 2 O.

Like reactions with basic oxides, such reactions go to completion (irreversible) because as a result of the transfer of a proton by an oxonium ion (a strong acid, K K = 55.5) hydroxide ion (strong base, K O = 55.5) water molecules (a very weak ampholyte, K K= K O = 1.8·10 -16).
Recall that reactions of acids with bases (including alkalis) are called neutralization reactions.
You already know that pure water contains oxonium and hydroxide ions (due to autoprotolysis of water), but their concentrations are equal and extremely insignificant: With(H 3 O) = With(OH) = 10 -7 mol/l. Therefore, their presence in water is practically invisible.
The same is observed in solutions of substances that are neither acids nor bases. Such solutions are called neutral.

But if you add an acid or base substance to water, an excess of one of these ions will appear in the solution. The solution will become sour or alkaline.

Hydroxide ions are part of not only alkalis, but also practically insoluble bases, as well as amphoteric hydroxides(amphoteric hydroxides in this regard can be considered as ionic compounds). Oxonium ions also react with all these substances, and, as in the case of basic oxides, the reaction occurs on the surface of the solid. Reaction mechanism for hydroxide composition M(OH) 2:

.

Example 2. Reaction of a solution of sulfuric acid with copper hydroxide. Since the hydrogen sulfate ion is a rather strong acid ( K K 0.01), the reversibility of its protolysis can be neglected and the equations of this reaction can be written as follows:

Cu(OH) 2 + 2H 3 O = Cu 2 + 4H 2 O
Cu(OH) 2 + H 2 SO 4р = CuSO 4 + 2H 2 O.

c) Reactions of oxonium ions with weak bases

As in solutions of alkalis, solutions of weak bases also contain hydroxide ions, but their concentration is many times lower than the concentration of the base particles themselves (this ratio is equal to the degree of protolysis of the base). Therefore, the rate of the neutralization reaction of hydroxide ions is many times less than the rate of the neutralization reaction of the base particles themselves. Consequently, the reaction between oxonium ions and base particles will be predominant.

Example 1. Reaction of neutralization of hydrochloric acid with ammonia solution:

.

The reaction produces ammonium ions (a weak acid, K K = 6·10 -10) and water molecules, but since one of the initial reagents (ammonia) the base is weak ( K O = 2·10 -5), then the reaction is reversible

But the equilibrium in it is very strongly shifted to the right (towards the reaction products), so much so that reversibility is often neglected by writing the molecular equation of this reaction with an equal sign:

HCl p + NH 3p = NH 4 Cl p + H 2 O.

Example 2. Reaction of hydrobromic acid with a solution of sodium bicarbonate. Being an ampholyte, the bicarbonate ion behaves like a weak base in the presence of oxonium ions:

The resulting carbonic acid can be contained in aqueous solutions only in very small concentrations. As the concentration increases, it decomposes. The decomposition mechanism can be imagined as follows:

Summary chemical equations:

H 3 O + HCO 3 = CO 2 + 2H 2 O
HBr р + NaHCO 3р = NaBr р + CO 2 + H 2 O.

Example 3. Reactions that occur when merging solutions of perchloric acid and potassium carbonate. The carbonate ion is also a weak base, although stronger than the bicarbonate ion. The reactions between these ions and the oxonium ion are completely analogous. Depending on the conditions, the reaction may stop at the stage of formation of a bicarbonate ion, or may lead to the formation of carbon dioxide:

a) H 3 O + CO 3 = HCO 3 + H 2 O
HClO 4p + K 2 CO 3p = KClO 4p + KHCO 3p;
b) 2H 3 O + CO 3 = CO 2 + 3H 2 O
2HClO 4p + K 2 CO 3p = 2KClO 4p + CO 2 + H 2 O.

Similar reactions occur even when salts containing base particles are insoluble in water. As in the case of basic oxides or insoluble bases, in this case the reaction also occurs on the surface of the insoluble salt.

Example 4. Reaction between hydrochloric acid and calcium carbonate:
CaCO 3 + 2H 3 O = Ca 2 + CO 2 + 3H 2 O
CaCO 3p + 2HCl p = CaCl 2p + CO 2 + H 2 O.

An obstacle to such reactions may be the formation of an insoluble salt, a layer of which will impede the penetration of oxonium ions to the surface of the reagent (for example, in the case of the interaction of calcium carbonate with sulfuric acid).

NEUTRAL SOLUTION, ACIDIC SOLUTION, ALKALINE SOLUTION, NEUTRALIZATION REACTION.
1.Draw up diagrams of the mechanisms of reactions of oxonium ions with the following substances and particles: FeO, Ag 2 O, Fe(OH) 3, HSO 3, PO 4 3 and Cu 2 (OH) 2 CO 3. Using the diagrams, create ionic reaction equations.
2.Which of the following oxides will oxonium ions react with: CaO, CO, ZnO, SO 2, B 2 O 3, La 2 O 3? Write ionic equations for these reactions.
3.Which of the following hydroxides will oxonium ions react with: Mg(OH)2, B(OH)3, Te(OH)6, Al(OH)3? Write ionic equations for these reactions.
4. Make up ionic and molecular equations for the reactions of hydrobromic acid with solutions of the following substances: Na 2 CO 3, K 2 SO 3, Na 2 SiO 3, KHCO 3.
5. Make up ionic and molecular equations for the reactions of a solution of nitric acid with the following substances: Cr(OH) 3, MgCO 3, PbO.
Reactions of solutions of strong acids with bases, basic oxides and salts.

12.6. Acid-base reactions of weak acids

Unlike solutions of strong acids, solutions of weak acids contain not only oxonium ions as acid particles, but also molecules of the acid itself, and there are many times more acid molecules than oxonium ions. Therefore, in these solutions, the predominant reaction will be the reaction of the acid particles themselves with the base particles, and not the reactions of oxonium ions. The rate of reactions involving weak acids is always lower than the rate of similar reactions involving strong acids. Some of these reactions are reversible, and the more, the weaker the acid involved in the reaction.

a) Reactions of weak acids with oxide ions

This is the only group of reactions of weak acids that proceed irreversibly. The speed of the reaction depends on the strength of the acid. Some weak acids (hydrogen sulfide, carbon, etc.) do not react with low-active basic and amphoteric oxides (CuO, FeO, Fe 2 O 3, Al 3 O 3, ZnO, Cr 2 O 3, etc.).

Example. The reaction that occurs between manganese(II) oxide and a solution of acetic acid. The mechanism of this reaction:

Reaction equations:
MnO + 2CH 3 COOH = Mn 2 + 2CH 3 COO + H 2 O
MnO + 2CH 3 COOH p = Mn(CH 3 COO) 2p + H 2 O. (Salts of acetic acid are called acetates)

b) Reactions of weak acids with hydroxide ions

As an example, consider how phosphoric (orthophosphoric) acid molecules react with hydroxide ions:

As a result of the reaction, water molecules and dihydrogen phosphate ions are obtained.
If after completion of this reaction hydroxide ions remain in the solution, then dihydrogen phosphate ions, being ampholytes, will react with them:

Hydrophosphate ions are formed, which, also being ampholytes, can react with an excess of hydroxide ions:

.

Ionic equations for these reactions

H 3 PO 4 + OH H 2 PO 4 + H 2 O;
H 2 PO 4 + OH HPO 4 2 + H 2 O;
HPO 4 + OH PO 4 3 + H 2 O.

The equilibria of these reversible reactions are shifted to the right. In an excess of alkali solution (for example, NaOH), all these reactions proceed almost irreversibly, so their molecular equations are usually written as follows:

H 3 PO 4р + NaOH р = NaH 2 PO 4р + H 2 O;
NaH 2 PO 4р + NaOH р = Na 2 HPO 4р;
Na 2 HPO 4р + NaOH р = Na 3 PO 4р + H 2 O.

If the target product of these reactions is sodium phosphate, then the overall equation can be written:
H 3 PO 4 + 3NaOH = Na 3 PO 4 + 3H 2 O.

Thus, a molecule of phosphoric acid, entering into acid-base interactions, can sequentially donate one, two or three protons. In a similar process, a molecule of hydrosulfide acid (H 2 S) can donate one or two protons, and a molecule of nitrous acid (HNO 2) can donate only one proton. Accordingly, these acids are classified as tribasic, dibasic and monobasic.

The corresponding characteristic of the base is called acidity.

Examples of one-acid bases are NaOH, KOH; examples of diacid bases are Ca(OH) 2, Ba(OH) 2.
The strongest of weak acids can also react with hydroxide ions included in the composition insoluble bases and even amphoteric hydroxides.

c) Reactions of weak acids with weak bases

Almost all of these reactions are reversible. According to general rule Equilibria in such reversible reactions are shifted towards weaker acids and weaker bases.

BASICITY OF ACID, ACIDITY OF BASE.
1.Draw up diagrams of the mechanisms of reactions occurring in an aqueous solution between formic acid and the following substances: Fe 2 O 3, KOH and Fe(OH) 3. Using the diagrams, create ionic and molecular equations for these reactions. (tetraaquazinc ion) and 3aq aq+ H 3 O .
4. In what direction will the equilibrium in this solution shift a) when it is diluted with water, b) when a solution of a strong acid is added to it?