Methodological development of the lesson

"Environment of aqueous solutions"

Target: formation of research competence of students in the study of the environment of aqueous solutions of electrolytes and methods of its qualitative analysis.

Tasks:

1. To form an idea of ​​students about the types of environment of aqueous solutions (acidic, neutral, alkaline);
2. Consider the concept of "indicators" and the main types of indicators (litmus, phenolphthalein, methyl orange);
3. To study the color change of indicators in different environments;
4. To reveal in the course of a chemical experiment the most optimal indicator for determining the acidic and alkaline environment of the solution;
5. Analyze the relationship between the solution medium and the pH value;
6. To form the skills of students with a universal indicator;
7. To reveal the dependence of the color of the juices of some plants (in particular, red cabbage) on the solution medium.

Form: lesson - research. This form allows you to simulate all stages of chemical research in the study of a particular topic.

This lesson harmoniously combines the problematic method and a chemical experiment, which serves as a means of proving or refuting the hypotheses put forward.

The leading form of activity in the lesson is the independent work of students in pairs or groups, performing the same or different tasks (according to options), aimed at obtaining a wider range of information for the whole class.

Methodological comments are written in italics.

Organizational moment. Stage I - motivational

Good afternoon! The world around us is full of substances of various structures and properties. Knowing them will enable us to know ourselves.

The most optimal and capacious way of knowledge is research. Today I invite us to imagine ourselves not as students and a teacher, but as employees of a serious laboratory, venerable chemistry researchers. (Game Technology) Slide #1

To begin, let me ask you a question that was addressed to me by one of my colleagues: "What do ancient Carthage and modern Holland have in common?" ( problem learning) (Discussion of answer options)

In fact, environmental problems are common, which are characteristic of both one and the other state.

History reference:At one time, Carthage was a very powerful state that defended its dominance in the Mediterranean. As a result of the third Punic War, the half-million city of Carthage was completely destroyed, and the surviving inhabitants were sold into slavery. The Romans chanted "Carthago delendam esse!" ("Carthage must be destroyed!").Slide #2

The place where the city was located was covered with salt. No one sprinkles modern Holland with salt, but this state is actively fighting global environmental problems, including those caused by floods. (interdisciplinary connections)

Problem question:

Do you think there are environmental problems in Yegorievsk? Which?

(Clogging of the soil, pollution of water bodies, atmosphere, a lot of garbage on the streets, etc.)

One of the most important problems iswater purity problem. Water enters the water supply from pumping stations that raise it from great depths, from artesian wells. But once the source of water in the village of Vysokoye (on the site of which Yegorievsk arose) was the Guslitsa River. Slide #3

Consider a modern sample of water from the Guslitsa River. Assess the color, transparency, smell, presence of suspended particles.

All of these methods of analysis areorganoleptic.Explain the name of the concept. (Ie, carried out with the help of human senses).

Question for thought: Based only on the results of organoleptic methods, can we conclude that the water samples are ecologically clean?

(It is impossible. The water may contain particles that we cannot see - outwardly invisible).

We approached to the problem : How to determine the presence of invisible particles in a solution? (problem learning)

Stage II - Problem Solving

Target our today's research: to study some ways of qualitative analysis of aqueous solutions (i.e. the content of different particles in them). What methods can be used?

(You can carry out chemical reactions -qualitative reactionsproving the presence of certain particles in the solution.)

And you can use special substances - indicators.

Question for thought:You are familiar with indicators from the course of biology, physics and other academic disciplines. What do you think is the meaning of the term "indicator" in chemistry?

Fixing a definition on a slide: slide number 4

Indicator is a substance that changes its color depending on the medium of the solution.

Question for thought:Do you understand everything in this definition?

(What is a “solution medium”? What is it like?) This subject of today's lesson, write it down in your notebook:

« Aqueous solution medium ».

Great science - logic!... and knowledge of classes of inorganic compounds will help you to identify the types of media of aqueous solutions.

I propose to build the first logical chain by answering the relevant questions:

1. What class do substances with the formulas belong to: HCl, H 2 SO 4 , HNO 3 , H 2 S? (acids) Slide #5
2. What cations are formed in solution during the dissociation of this class of compounds? (hydrogen cations)

Write on the blackboard the equation for the dissociation of nitric acid

HNO 3 → H + + NO 3 -

Hint: The name of the solution medium in this case comes from the name of the corresponding class of compounds ( acid environment).

1. Build the following logical chain for compounds expressed by the formulas: NaOH, Ca(OH) 2 , KOH, Ba(OH) 2 . (bases, alkalis) Slide #6

Write on the board the equation for the complete dissociation of barium hydroxide

Ba(OH) 2 → Ba 2+ + 2OH -

Hint: Remember the classification of bases! Do all bases in aqueous solution decompose into ions? The name of the medium comes from the name of the soluble bases. (alkaline)

1. What class do the following substances belong to: potassium sulfate, barium chloride, calcium nitrate? (salts). Slide number 7 K 2 SO 4 , BaCl 2 , Ca(NO 3) 2
2. When these compounds are dissolved in water, particles are formed that characterize the acidic or alkaline nature of the solution? (not formed)

Write an equation for the dissociation of potassium sulfate on the board

K 2 SO 4 → 2K + + SO 4 2-

Hint: The name of the medium comes from the absence of hydrogen cations and hydroxo anions. (neutral)

Let's make a scheme for classifying environments Diagram on the board(pedagogy of cooperation)

ENVIRONMENT OF AQUEOUS SOLUTIONS

_______________ ________________

___________________

(physical education for the eyes)

So, we found out that there are three types of aqueous solutions (acidic, neutral and alkaline).

Indicators, which we already talked about at the beginning of the lesson, will help us measure the level of acidity of the aquatic environment.

Indicators - These are substances that change their color depending on the medium of the solution.

The indicators are different. Today we will introduce you to the three main ones:blue litmus, methyl orange and phenolphthalein.

Each of them changes color differently depending on the solution medium, so our task is to choose the most optimal indicator for each solution medium.

To work, let's make a table: Slide #9

		Methyl orange	Phenolphthalein
acid solution
alkali solution
Salt solution

Pour 2-3 ml of hydrochloric acid solution into three test tubes. Add 1 drop of indicators to each of them (methyl orange in test tube No. 1, phenolphthalein in test tube No. 2, blue litmus in test tube No. 3).

Record the observed changes in your notebook.

Exercise: Note the name of the indicator that is most convenient to use to determine the acidic environment of an aqueous solution!

Pour 2-3 ml of sodium hydroxide solution into three test tubes. Add 1 drop of indicators to each of them (methyl orange in test tube No. 1, phenolphthalein in test tube No. 2, blue litmus in test tube No. 3).

Watch for color change. Record observed changes in a notebook

Exercise: Mark the name of the indicator that is most convenient to use to determine the alkaline environment of an aqueous solution!

Discussion of the results of the experiment. Filling in the table in the notebook (students) and on the slide (teacher).(pedagogy of cooperation)

Formulation of conclusions:In an acidic environment, the color of methyl orange becomes red, litmus - red, phenolphthalein does not change its color. Therefore, the most optimal indicator for determining the acidic environment of a solution ismethyl orange.

In an alkaline environment, the color of methyl orange becomes yellow, litmus - blue, phenolphthalein - raspberry. Therefore, the most optimal indicator for determining the alkaline environment isphenolphthalein.

You are armed with new knowledge. Can you now study the environment of the water sample?

Try to determine the environment of the water sample using optimal indicators, only for this, pour a small amount of test water from the beaker into three clean test tubes and add the appropriate indicator (phenolphthalein, methyl orange) to each.

Do you observe significant color changes of indicators in solutions? (Not).

What hypotheses can you put forward?

1. The medium of the solution is not strongly acidic, or not strongly alkaline, so indicators cannot tell the difference.
2. The environment is neutral, so the color of the indicators does not change.

Indeed, the range of characteristics of the solution medium is very wide: from strongly acidic to strongly alkaline.

It is expressed in units from 0 to 14, which is called the pH value (p-ash) -pH indicator.(advanced learning)

Hydrogen indicatoris the value characterizing the content of hydrogen cations in the solution. There are accurate universal indicators.Slide #10

Advance learning. Scientifically speaking, pH is the negative decimal logarithm of the concentration of hydrogen ions in a solution. So far, there are a lot of incomprehensible words for you, but in the 11th grade we will return to the study of this value and consider it in more detail from the standpoint of the knowledge that you will have by that time.

Assignment in a notebook:

Using the information obtained, identify the relationship between the pH value and the medium of the solution. Write your conclusions in your notebook.

Findings:

At pH > 7, the solution medium alkaline

At pH = 7, the solution medium neutral

At pH< 7 среда раствора sour

To determine the pH value and more accurately determine the medium of the solution, there are different methods: acid-base titration, measuring the electromotive force (EMF), or using universal indicator paper.

Dip the universal indicator paper into the water sample in the beaker.

Compare the color obtained on it with the colored pH scale.

Question for thought: What is the medium of your sample solution?

Be sure to specify the type of medium by strength (weak, strong).

problem question: Well, now can you draw a conclusion about the ecological state of the water sample issued to you?

(No. Because we do not know environmental standards, we do not know what to compare our samples with).

You can compare the acidity level of the issued samples with the conditional scale of pH values ​​of some solutions.

A pH scale is drawn up on the slide Slide #11

Problematic issues:

1. What liquids do you think are not recommended for people with stomach ulcers? Why?

(All weakly and strongly acidic solutions (coffee, lemon, apple, tomato juice, Coca-Cola) can cause an exacerbation of peptic ulcer due to excessive acidity).

1. What do you think is common between ammonia, which housewives add to water for washing windows, and soap, with which we wash our hands?

(Both soap solution and ammonia are alkaline to help remove dirt.)Slide #12

problem question:Sometimes we need to determine the environment of the solution at home. And there is no universal indicator paper at hand. What to do? (problem learning)

Information: It turns out that some vegetables and fruits have an indicator ability. They contain a pH-sensitive pigment (anthocyanin).

These are fruits of dark blue, purple color: beets, blackberries, black currants, cherries, dark grapes and, including red cabbage.

Information : At home, you can make indicator papers.

Take the juice of red cabbage and saturate sheets of filter paper with it. Leaves should be allowed to dry. Then cut the filter paper into thin strips.Indicator papers are ready!Good luck with your experiments! (humane-personal)

III stage. The final stage of the study:

We are coming to the end of our research. Earlier you said that in order to draw a conclusion about the acidity of water samples, we must have useful information about the sanitary and hygienic standards in force in the world and in our country.

Helpful information: In accordance with the Hygienic requirements for water quality of centralized drinking water supply systems (SanPiN 2.1.4.559-96), drinking water must be harmless in chemical composition and have favorable organoleptic properties.

The pH index for drinking water should correspond to the norm of 6-9 units, for reservoirs 6.5 - 8.5. Researchers have found that an acidic environment is especially detrimental to aquatic inhabitants than an alkaline one. In aquatic plants, an increase in the acidity of water, first of all, affects the violation of calcium metabolism and the formation of cell membranes, their division, as well as the course of the photosynthesis reaction.

For water bodies and drinking water, the content of nitrates should not exceed 45 mg/l, phosphates - 3.5 mg/l. Nitrate and phosphate - ions contribute to the overgrowth of water bodies with vegetation, causing the growth of plankton. That, in turn, dies off and absorbs a large amount of oxygen, depriving water of the ability to self-purify. Nitrates can be toxic to humans and aquatic life.

The increased content of iron in water causes the deposition of iron in the liver and significantly outpaces alcoholism in terms of harmfulness. The maximum permissible concentration of iron in water is 0.3 mg/l. (health-saving technologies)

III. Reflection Issues for discussion:

1. Is the pH value of the test water correct?
2. In which preparations is the solution acidic?
3. In which preparations is the solution environment alkaline?
4. How do indicators change color in such an environment?

Key question:

Do you think that the information received so far about the quality of water samples is enough to make a final conclusion about its environmental suitability and purity? (Not enough. It is necessary to conduct a complete qualitative analysis for the content of various particles - ions in it).

Conclusion: you need to study the subject for a long time and painstakingly in order to draw complete and correct conclusions from the research.

D.Z. paragraph 28, ex. №2,3 page 46

Lecture: Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline

Salt hydrolysis

We continue to study the patterns of chemical reactions. When studying the topic, you learned that during electrolytic dissociation in an aqueous solution, the particles involved in the reaction of substances dissolve in water. This is hydrolysis. Various inorganic and organic substances, in particular salts, are exposed to it. Without understanding the process of hydrolysis of salts, you will not be able to explain the phenomena that occur in living organisms.

The essence of salt hydrolysis is reduced to the exchange process of interaction of ions (cations and anions) of the salt with water molecules. As a result, a weak electrolyte is formed - a low-dissociating compound. An excess of free H + or OH - ions appears in an aqueous solution. Remember, the dissociation of which electrolytes forms H + ions, and which OH -. As you guessed, in the first case we are dealing with an acid, which means that the aqueous medium with H + ions will be acidic. In the second case, alkaline. In the water itself, the medium is neutral, since it slightly dissociates into H + and OH - ions of the same concentration.

The nature of the environment can be determined using indicators. Phenolphthalein detects an alkaline environment and colors the solution crimson. Litmus turns red with acid and blue with alkali. Methyl orange - orange, in an alkaline environment it becomes yellow, in an acidic environment - pink. The type of hydrolysis depends on the type of salt.

Salt types

So, any salt is an interaction of an acid and a base, which, as you understand, are strong and weak. Strong are those whose degree of dissociation α is close to 100%. It should be remembered that sulfurous (H 2 SO 3) and phosphoric (H 3 PO 4) acid are often referred to as medium strength acids. When solving hydrolysis problems, these acids must be classified as weak.

Acids:

Strong: HCl; HBr; Hl; HNO3; HClO 4 ; H2SO4. Their acid residues do not interact with water.

Weak: HF; H2CO3; H 2 SiO 3 ; H2S; HNO2; H2SO3; H3PO4; organic acids. And their acidic residues interact with water, taking hydrogen cations H + from its molecules.

Reasons:

Strong: soluble metal hydroxides; Ca(OH) 2 ; Sr(OH) 2 . Their metal cations do not interact with water.

Weak: insoluble metal hydroxides; ammonium hydroxide (NH 4 OH). And metal cations here interact with water.

Based on this material, considersalt types :

Salts with a strong base and a strong acid. For example: Ba (NO 3) 2, KCl, Li 2 SO 4. Features: do not interact with water, which means they do not undergo hydrolysis. Solutions of such salts have a neutral reaction medium.

Salts with a strong base and a weak acid. For example: NaF, K 2 CO 3 , Li 2 S. Features: acid residues of these salts interact with water, anion hydrolysis occurs. The medium of aqueous solutions is alkaline.

Salts with weak bases and strong acids. For example: Zn (NO 3) 2, Fe 2 (SO 4) 3, CuSO 4. Features: only metal cations interact with water, cation hydrolysis occurs. Wednesday is sour.

Salts with a weak base and a weak acid. For example: CH 3 COONН 4, (NH 4) 2 CO 3 , HCOONН 4. Features: both cations and anions of acid residues interact with water, hydrolysis occurs by cation and anion.

An example of hydrolysis at the cation and the formation of an acidic environment:

Hydrolysis of ferric chloride FeCl 2

FeCl 2 + H 2 O ↔ Fe(OH)Cl + HCl(molecular equation)

Fe 2+ + 2Cl - + H + + OH - ↔ FeOH + + 2Cl - + H+ (full ionic equation)

Fe 2+ + H 2 O ↔ FeOH + + H + (abbreviated ionic equation)

An example of anion hydrolysis and the formation of an alkaline environment:

Hydrolysis of sodium acetate CH 3 COONa

CH 3 COONa + H 2 O ↔ CH 3 COOH + NaOH(molecular equation)

Na + + CH 3 COO - + H 2 O ↔ Na + + CH 3 COOH + OH- (full ionic equation)

CH 3 COO - + H 2 O ↔ CH 3 COOH + OH -(abbreviated ionic equation)

An example of co-hydrolysis:

• Hydrolysis of aluminum sulfide Al 2 S 3

Al 2 S 3 + 6H2O ↔ 2Al(OH) 3 ↓+ 3H 2 S

In this case, we see complete hydrolysis, which occurs if the salt is formed by a weak insoluble or volatile base and a weak insoluble or volatile acid. In the solubility table there are dashes on such salts. If during the ion exchange reaction a salt is formed that does not exist in an aqueous solution, then it is necessary to write the reaction of this salt with water.

For example:

2FeCl 3 + 3Na 2 CO 3 ↔ Fe 2 (CO 3) 3+ 6NaCl

Fe 2 (CO 3) 3+ 6H 2 O ↔ 2Fe(OH) 3 + 3H 2 O + 3CO 2

We add these two equations, then what is repeated in the left and right parts, we reduce:

2FeCl 3 + 3Na 2 CO 3 + 3H 2 O ↔ 6NaCl + 2Fe(OH) 3 ↓ + 3CO 2

The reaction of a solution of substances in a solvent can be of three types: neutral, acidic and alkaline. The reaction depends on the concentration of hydrogen ions H + in solution.

Pure water dissociates to a very small extent into H + ions and hydroxyl ions OH - .

pH value

The pH is a convenient and common way of expressing the concentration of hydrogen ions. For pure water, the H + concentration is equal to the OH - concentration, and the product of the H + and OH - concentrations, expressed in gram-ions per liter, is a constant value equal to 1.10 -14

From this product, you can calculate the concentration of hydrogen ions: =√1.10 -14 =10 -7 /g-ion/l/.

This equilibrium /"neutral"/ state is usually denoted by pH 7/p - the negative logarithm of the concentration, H - hydrogen ions, 7 - the exponent with the opposite sign/.

A solution with a pH greater than 7 is alkaline, it contains fewer H + ions than OH - ; a solution with a pH less than 7 is acidic, there are more H + ions in it than OH - .

Liquids used in practice have a concentration of hydrogen ions that usually varies within the pH range from 0 to 1

Indicators

Indicators are substances that change color depending on the concentration of hydrogen ions in a solution. With the help of indicators determine the reaction of the environment. The most famous indicators are bromobenzene, bromothymol, phenolphthalein, methyl orange, etc. Each of the indicators operates within certain pH ranges. For example, bromthymol changes from yellow at pH 6.2 to blue at pH 7.6; neutral red indicator - from red at pH 6.8 to yellow at pH 8; bromobenzene - from yellow jari pH 4.0 to blue at pH 5.6; phenolphthalein - from colorless at pH 8.2 to purple at pH 10.0, etc.

None of the indicators work throughout the entire pH scale from 0 to 14. However, in restoration practice, it is not necessary to determine high concentrations of acids or alkalis. Most often there are deviations of 1 - 1.5 pH units from neutral in both directions.

To determine the reaction of the environment in restoration practice, a mixture of various indicators is used, selected in such a way that it marks the slightest deviations from neutrality. This mixture is called a "universal indicator".

The universal indicator is a clear orange liquid. With a slight change in the medium towards alkalinity, the indicator solution acquires a greenish tint, with an increase in alkalinity - blue. The greater the alkalinity of the test liquid, the more intense the blue color becomes.

With a slight change in the environment towards acidity, the solution of the universal indicator becomes pink, with an increase in acidity - red /carmine or mottled hue/.

Changes in the reaction of the environment in the paintings occur as a result of their damage by mold; often there are changes in areas where labels are pasted with alkaline glue /casein, office, etc./.

For analysis, you need to have, in addition to the universal indicator, distilled water, clean white filter paper and a glass rod.

Analysis progress

A drop of distilled water is applied to the filter paper and allowed to soak. A second drop is applied next to this drop and applied to the test area. For better contact, the paper with the second drop on top is rubbed with a glass shelf. Then, a drop of universal indicator is applied to the filter paper in the areas of water droplets. The first drop of water serves as a control, with the color of which the drop soaked in the solution from the test area is compared. The discrepancy in color with the control drop indicates a change - a deviation of the medium from neutral.

NEUTRALIZATION OF ALKALINE ENVIRONMENT

The treated area is moistened with a 2% aqueous solution of acetic or citric acid. To do this, wind a small amount of cotton wool around the tweezers, moisten it in an acid solution, wring it out and apply it to the indicated area.

reaction be sure to check universal indicator!

The process is continued until the entire area is completely neutralized.

After a week, check the environment should be repeated.

ACID NEUTRALIZATION

The area to be treated is moistened with a 2% aqueous solution of ammonium hydroxide /ammonia/. The procedure for carrying out neutralization is the same as in the case of an alkaline medium.

The media check should be repeated after one week.

WARNING: The neutralization process requires great care, as over-treatment can lead to over-acidification or over-alkalination of the treated area. In addition, water in solutions can cause shrinkage of the canvas.

Hydrolysis is the interaction of substances with water, as a result of which the medium of the solution changes.

Cations and anions of weak electrolytes are able to interact with water to form stable low-dissociation compounds or ions, as a result of which the solution medium changes. Water formulas in hydrolysis equations are usually written as H-OH. When reacting with water, the cations of weak bases take away the hydroxyl ion from the water, and an excess of H + is formed in the solution. The solution becomes acidic. Anions of weak acids attract H + from water, and the reaction of the medium becomes alkaline.

In inorganic chemistry, most often one has to deal with the hydrolysis of salts, i.e. with the exchange interaction of salt ions with water molecules in the process of their dissolution. There are 4 variants of hydrolysis.

1. Salt is formed by a strong base and a strong acid.

Such a salt is practically not subjected to hydrolysis. At the same time, the equilibrium of water dissociation in the presence of salt ions is almost not disturbed, therefore pH = 7, the medium is neutral.

Na + + H 2 O Cl - + H 2 O

2. If the salt is formed by a cation of a strong base and an anion of a weak acid, then hydrolysis occurs at the anion.

Na 2 CO 3 + HOH \(\leftrightarrow\) NaHCO 3 + NaOH

Since OH - ions accumulate in the solution, the medium is alkaline, pH> 7.

3. If the salt is formed by a cation of a weak base and an anion of a strong acid, then hydrolysis proceeds along the cation.

Cu 2+ + HOH \(\leftrightarrow\) CuOH + + H +

СuCl 2 + HOH \(\leftrightarrow\) CuOHCl + HCl

Since H + ions accumulate in the solution, the medium is acidic, pH<7.

4. A salt formed by a cation of a weak base and an anion of a weak acid undergoes hydrolysis both at the cation and at the anion.

CH 3 COONH 4 + HOH \(\leftrightarrow\) NH 4 OH + CH 3 COOH

CH 3 COO - + + HOH \(\leftrightarrow\) NH 4 OH + CH 3 COOH

Solutions of such salts have either a slightly acidic or slightly alkaline environment, i.e. the pH value is close to 7. The reaction of the medium depends on the ratio of the acid and base dissociation constants. The hydrolysis of salts formed by very weak acids and bases is practically irreversible. These are mainly sulfides and carbonates of aluminum, chromium, and iron.

Al 2 S 3 + 3HOH \(\leftrightarrow\) 2Al(OH) 3 + 3H 2 S

When determining the medium of a salt solution, it must be taken into account that the medium of the solution is determined by the strong component. If the salt is formed by an acid that is a strong electrolyte, then the medium of the solution is acidic. If the base is a strong electrolyte, then it is alkaline.

Example. Solution has an alkaline environment

1) Pb(NO 3) 2 ; 2) Na 2 CO 3 ; 3) NaCl; 4) NaNO 3

1) Pb (NO 3) 2 lead (II) nitrate. Salt is made up of a weak base and strong acid, means the solution medium sour.

2) Na 2 CO 3 sodium carbonate. Salt formed strong base and a weak acid, then the solution medium alkaline.

3) NaCl; 4) NaNO 3 Salts are formed by the strong base NaOH and the strong acids HCl and HNO 3 . The medium of the solution is neutral.

Correct answer 2) Na2CO3

An indicator paper was dipped into the salt solutions. In NaCl and NaNO 3 solutions, it did not change color, which means the solution medium neutral. In a solution of Pb (NO 3) 2 turned red, the solution medium sour. In a solution of Na 2 CO 3 turned blue, the solution medium alkaline.

Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline

According to the theory of electrolytic dissociation, in an aqueous solution, solute particles interact with water molecules. Such an interaction can lead to a hydrolysis reaction (from the Greek. hydro- water, lysis disintegration, decay).

Hydrolysis is a reaction of the metabolic decomposition of a substance by water.

Various substances undergo hydrolysis: inorganic - salts, carbides and hydrides of metals, non-metal halides; organic - haloalkanes, esters and fats, carbohydrates, proteins, polynucleotides.

Aqueous solutions of salts have different pH values ​​and different types of media - acidic ($pH 7$), neutral ($pH = 7$). This is due to the fact that salts in aqueous solutions can undergo hydrolysis.

The essence of hydrolysis is reduced to the exchange chemical interaction of salt cations or anions with water molecules. As a result of this interaction, a low-dissociating compound (weak electrolyte) is formed. And in an aqueous salt solution, an excess of free $H^(+)$ or $OH^(-)$ ions appears, and the salt solution becomes acidic or alkaline, respectively.

Salt classification

Any salt can be thought of as the product of the interaction of a base with an acid. For example, the salt $KClO$ is formed by the strong base $KOH$ and the weak acid $HClO$.

Depending on the strength of the base and acid, four types of salts can be distinguished.

Consider the behavior of salts of various types in solution.

1. Salts formed by a strong base and a weak acid.

For example, the potassium cyanide salt $KCN$ is formed by the strong base $KOH$ and the weak acid $HCN$:

$(KOH)↙(\text"strong monoacid base")←KCN→(HCN)↙(\text"weak monoacid acid")$

1) a slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be written in a simplified way using the equation

$H_2O(⇄)↖(←)H^(+)+OH^(-);$

$KCN=K^(+)+CN^(-)$

The $H^(+)$ and $CN^(-)$ ions formed during these processes interact with each other, binding into weak electrolyte molecules - hydrocyanic acid $HCN$, while the hydroxide - the $OH^(-)$ ion remains in solution, thus making it alkaline. Hydrolysis occurs at the $CN^(-)$ anion.

We write the full ionic equation of the ongoing process (hydrolysis):

$K^(+)+CN^(-)+H_2O(⇄)↖(←)HCN+K^(+)+OH^(-).$

This process is reversible, and the chemical equilibrium is shifted to the left (in the direction of the formation of the starting substances), because water is a much weaker electrolyte than hydrocyanic acid $HCN$.

$CN^(-)+H_2O⇄HCN+OH^(-).$

The equation shows that:

a) there are free hydroxide ions $OH^(-)$ in the solution, and their concentration is greater than in pure water, so the salt solution $KCN$ has alkaline environment($pH > 7$);

b) $CN^(-)$ ions participate in the reaction with water, in which case they say that there is anion hydrolysis. Other examples of anions that react with water are:

Consider the hydrolysis of sodium carbonate $Na_2CO_3$.

$(NaOH)↙(\text"strong monoacid base")←Na_2CO_3→(H_2CO_3)↙(\text"weak dibasic acid")$

The salt is hydrolyzed at the $CO_3^(2-)$ anion.

$2Na^(+)+CO_3^(2-)+H_2O(⇄)↖(←)HCO_3^(-)+2Na^(+)+OH^(-).$

$CO_2^(2-)+H_2O⇄HCO_3^(-)+OH^(-).$

Hydrolysis products - acid salt$NaHCO_3$ and sodium hydroxide $NaOH$.

The environment of an aqueous solution of sodium carbonate is alkaline ($pH > 7$), because the concentration of $OH^(-)$ ions increases in the solution. The acid salt $NaHCO_3$ can also undergo hydrolysis, which proceeds to a very small extent, and it can be neglected.

To summarize what you have learned about anion hydrolysis:

a) at the anion of the salt, as a rule, they hydrolyze reversibly;

b) the chemical equilibrium in such reactions is strongly shifted to the left;

c) the reaction of the medium in solutions of similar salts is alkaline ($рН > 7$);

d) during the hydrolysis of salts formed by weak polybasic acids, acidic salts are obtained.

2. Salts formed from a strong acid and a weak base.

Consider the hydrolysis of ammonium chloride $NH_4Cl$.

$(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4Cl→(HCl)↙(\text"strong monobasic acid")$

Two processes take place in an aqueous solution of salt:

1) a slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be written in a simplified way using the equation:

$H_2O(⇄)↖(←)H^(+)+OH^(-)$

2) complete dissociation of salt (strong electrolyte):

$NH_4Cl=NH_4^(+)+Cl^(-)$

The resulting $OH^(-)$ and $NH_4^(+)$ ions interact with each other to form $NH_3 H_2O$ (weak electrolyte), while the $H^(+)$ ions remain in the solution, causing the most of its acidic environment.

Full ionic hydrolysis equation:

$NH_4^(+)+Cl^(-)+H_2O(⇄)↖(←)H^(+)+Cl^(-)NH_3 H_2O$

The process is reversible, the chemical equilibrium is shifted towards the formation of the starting substances, because water $Н_2О$ is a much weaker electrolyte than ammonia hydrate $NH_3·H_2O$.

Abbreviated ionic hydrolysis equation:

$NH_4^(+)+H_2O⇄H^(+)+NH_3 H_2O.$

The equation shows that:

a) there are free hydrogen ions $H^(+)$ in the solution, and their concentration is greater than in pure water, so the salt solution has acid environment($pH

b) ammonium cations $NH_4^(+)$ participate in the reaction with water; in that case they say it's coming cation hydrolysis.

Multicharged cations can also participate in the reaction with water: two-shot$M^(2+)$ (for example, $Ni^(2+), Cu^(2+), Zn^(2+)…$), except for alkaline earth metal cations, three-shot$M^(3+)$ (for example, $Fe^(3+), Al^(3+), Cr^(3+)…$).

Let us consider the hydrolysis of nickel nitrate $Ni(NO_3)_2$.

$(Ni(OH)_2)↙(\text"weak diacid base")←Ni(NO_3)_2→(HNO_3)↙(\text"strong monobasic acid")$

The salt is hydrolyzed at the $Ni^(2+)$ cation.

Full ionic hydrolysis equation:

$Ni^(2+)+2NO_3^(-)+H_2O(⇄)↖(←)NiOH^(+)+2NO_3^(-)+H^(+)$

Abbreviated ionic hydrolysis equation:

$Ni^(2+)+H_2O⇄NiOH^(+)+H^(+).$

Hydrolysis products - basic salt$NiOHNO_3$ and nitric acid $HNO_3$.

The medium of an aqueous solution of nickel nitrate is acidic ($ pH

The hydrolysis of the $NiOHNO_3$ salt proceeds to a much lesser degree and can be neglected.

To summarize what you have learned about cation hydrolysis:

a) by the cation of the salt, as a rule, they are hydrolyzed reversibly;

b) the chemical equilibrium of reactions is strongly shifted to the left;

c) the reaction of the medium in solutions of such salts is acidic ($ pH

d) during the hydrolysis of salts formed by weak polyacid bases, basic salts are obtained.

3. Salts formed from a weak base and a weak acid.

It is obviously already clear to you that such salts undergo hydrolysis both at the cation and at the anion.

A weak base cation binds $OH^(-)$ ions from water molecules, forming weak base; anion of a weak acid binds $H^(+)$ ions from water molecules, forming weak acid. The reaction of solutions of these salts can be neutral, slightly acidic or slightly alkaline. It depends on the dissociation constants of two weak electrolytes - an acid and a base, which are formed as a result of hydrolysis.

For example, consider the hydrolysis of two salts: ammonium acetate $NH_4(CH_3COO)$ and ammonium formate $NH_4(HCOO)$:

1) $(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4(CH_3COO)→(CH_3COOH)↙(\text"strong monobasic acid");$

2) $(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4(HCOO)→(HCOOH)↙(\text"weak monobasic acid").$

In aqueous solutions of these salts, weak base cations $NH_4^(+)$ interact with hydroxide ions $OH^(-)$ (recall that water dissociates $H_2O⇄H^(+)+OH^(-)$), and anions weak acids $CH_3COO^(-)$ and $HCOO^(-)$ interact with $Н^(+)$ cations to form molecules of weak acids — acetic $CH_3COOH$ and formic $HCOOH$.

Let us write the ionic equations of hydrolysis:

1) $CH_3COO^(-)+NH_4^(+)+H_2O⇄CH_3COOH+NH_3 H_2O;$

2) $HCOO^(-)+NH_4^(+)+H_2O⇄NH_3 H_2O+HCOOH.$

In these cases, hydrolysis is also reversible, but the equilibrium is shifted towards the formation of hydrolysis products—two weak electrolytes.

In the first case, the solution medium is neutral ($рН = 7$), because $K_D(CH_3COOH)=K+D(NH_3 H_2O)=1.8 10^(-5)$. In the second case, the medium of the solution is weakly acidic ($pH

As you have already noticed, the hydrolysis of most salts is a reversible process. In a state of chemical equilibrium, only part of the salt is hydrolyzed. However, some salts are completely decomposed by water, i.e. their hydrolysis is an irreversible process.

In the table "Solubility of acids, bases and salts in water" you will find a note: "decompose in the aquatic environment" - this means that such salts undergo irreversible hydrolysis. For example, aluminum sulfide $Al_2S_3$ in water undergoes irreversible hydrolysis, since the $H^(+)$ ions that appear during hydrolysis at the cation are bound by the $OH^(-)$ ions formed during hydrolysis at the anion. This enhances hydrolysis and leads to the formation of insoluble aluminum hydroxide and hydrogen sulfide gas:

$Al_2S_3+6H_2O=2Al(OH)_3↓+3H_2S$

Therefore, aluminum sulfide $Al_2S_3$ cannot be obtained by an exchange reaction between aqueous solutions of two salts, for example aluminum chloride $AlCl_3$ and sodium sulfide $Na_2S$.

Other cases of irreversible hydrolysis are also possible, they are not difficult to predict, because for the irreversibility of the process it is necessary that at least one of the hydrolysis products leave the reaction sphere.

To summarize what you have learned about both cation and anion hydrolysis:

a) if salts are hydrolyzed both by cation and anion reversibly, then the chemical equilibrium in hydrolysis reactions is shifted to the right;

b) the reaction of the medium is either neutral, or slightly acidic, or slightly alkaline, which depends on the ratio of the dissociation constants of the formed base and acid;

c) salts can be hydrolyzed by both the cation and the anion irreversibly if at least one of the hydrolysis products leaves the reaction sphere.

4. Salts formed by a strong base and a strong acid do not undergo hydrolysis.

You obviously came to this conclusion yourself.

Consider the behavior of $KCl$ in potassium chloride solution.

$(KOH)↙(\text"strong monoacid base")←KCl→(HCl)↙(\text"strong monobasic acid").$

Salt in an aqueous solution dissociates into ions ($KCl=K^(+)+Cl^(-)$), but when interacting with water, a weak electrolyte cannot be formed. The solution medium is neutral ($рН=7$), because the concentrations of $H^(+)$ and $OH^(-)$ ions in the solution are equal, as in pure water.

Other examples of such salts may be alkali metal halides, nitrates, perchlorates, sulfates, chromates and dichromates, alkaline earth metal halides (other than fluorides), nitrates and perchlorates.

It should also be noted that the reversible hydrolysis reaction is completely subject to Le Chatelier's principle. So salt hydrolysis can be enhanced(and even make it irreversible) in the following ways:

a) add water (reduce concentration);

b) heat the solution, thus increasing the endothermic dissociation of water:

$H_2O⇄H^(+)+OH^(-)-57$ kJ,

which means that the amount of $H^(+)$ and $OH^(-)$, which are necessary for salt hydrolysis, increases;

c) bind one of the hydrolysis products into a sparingly soluble compound or remove one of the products into the gas phase; for example, the hydrolysis of ammonium cyanide $NH_4CN$ will be greatly enhanced by the decomposition of ammonia hydrate to form ammonia $NH_3$ and water $H_2O$:

$NH_4^(+)+CN^(-)+H_2O⇄NH_3 H_2O+HCN.$

$NH_3()↖(⇄)H_2$

Salt hydrolysis

Legend:

Hydrolysis can be suppressed (significantly reduced the amount of salt undergoing hydrolysis) by proceeding as follows:

a) increase the concentration of the solute;

b) cool the solution (to weaken hydrolysis, salt solutions should be stored concentrated and at low temperatures);

c) introduce one of the hydrolysis products into the solution; for example, acidify the solution if its medium is acidic as a result of hydrolysis, or alkalinize if it is alkaline.

Significance of hydrolysis

Salt hydrolysis has both practical and biological significance. Since ancient times, ash has been used as a detergent. The ash contains potassium carbonate $K_2CO_3$, which is hydrolyzed as an anion in water, the aqueous solution becomes soapy due to the $OH^(-)$ ions formed during hydrolysis.

At present, we use soap, washing powders and other detergents in everyday life. The main component of soap is sodium and potassium salts of higher fatty carboxylic acids: stearates, palmitates, which are hydrolyzed.

The hydrolysis of sodium stearate $C_(17)H_(35)COONa$ is expressed by the following ionic equation:

$C_(17)H_(35)COO^(-)+H_2O⇄C_(17)H_(35)COOH+OH^(-)$,

those. the solution is slightly alkaline.

In the composition of washing powders and other detergents, salts of inorganic acids (phosphates, carbonates) are specially introduced, which enhance the washing effect by increasing the pH of the medium.

Salts that create the necessary alkaline environment of the solution are contained in the photographic developer. These are sodium carbonate $Na_2CO_3$, potassium carbonate $K_2CO_3$, borax $Na_2B_4O_7$ and other salts hydrolyzed by the anion.

If the acidity of the soil is insufficient, the plants develop a disease - chlorosis. Its signs are yellowing or whitening of the leaves, lag in growth and development. If $pH_(soil) > 7.5$, then ammonium sulfate $(NH_4)_2SO_4$ fertilizer is added to it, which increases acidity due to hydrolysis by the cation passing in the soil:

$NH_4^(+)+H_2O⇄NH_3 H_2O$

The biological role of the hydrolysis of some salts that make up our body is invaluable. For example, the composition of the blood includes bicarbonate and sodium hydrogen phosphate salts. Their role is to maintain a certain reaction of the environment. This occurs due to a shift in the equilibrium of hydrolysis processes:

$HCO_3^(-)+H_2O⇄H_2CO_3+OH^(-)$

$HPO_4^(2-)+H_2O⇄H_2PO_4^(-)+OH^(-)$

If there is an excess of $H^(+)$ ions in the blood, they bind to the hydroxide ions $OH^(-)$, and the equilibrium shifts to the right. With an excess of $OH^(-)$ hydroxide ions, the equilibrium shifts to the left. Due to this, the acidity of the blood of a healthy person fluctuates slightly.

Another example: human saliva contains $HPO_4^(2-)$ ions. Thanks to them, a certain environment is maintained in the oral cavity ($рН=7-7.5$).