Determination of the nature of the solution environment of acids and alkalis. Determining the nature of the solution environment of acids and alkalis using indicators

Chemical properties of oxides: basic, amphoteric, acidic

Oxides are complex substances consisting of two chemical elements, one of which is oxygen with an oxidation state ($-2$).

The general formula of oxides is: $E_(m)O_n$, where $m$ is the number of atoms of the element $E$, and $n$ is the number of oxygen atoms. Oxides can be hard(sand $SiO_2$, varieties of quartz), liquid(hydrogen oxide $H_2O$), gaseous(carbon oxides: carbon dioxide $CO_2$ and carbon dioxide $CO$ gases). Based on their chemical properties, oxides are divided into salt-forming and non-salt-forming.

Non-salt-forming These are oxides that do not react with alkalis or acids and do not form salts. There are few of them, they contain non-metals.

Salt-forming These are oxides that react with acids or bases to form salt and water.

Among the salt-forming oxides there are oxides basic, acidic, amphoteric.

Basic oxides- these are oxides that correspond to bases. For example: $CaO$ corresponds to $Ca(OH)_2, Na_2O to NaOH$.

Typical reactions of basic oxides:

1. Basic oxide + acid → salt + water (exchange reaction):

$CaO+2HNO_3=Ca(NO_3)_2+H_2O$.

2. Basic oxide + acidic oxide → salt (compound reaction):

$MgO+SiO_2(→)↖(t)MgSiO_3$.

3. Basic oxide + water → alkali (compound reaction):

$K_2O+H_2O=2KOH$.

Acidic oxides- these are oxides that correspond to acids. These are non-metal oxides:

N2O5 corresponds to $HNO_3, SO_3 - H_2SO_4, CO_2 - H_2CO_3, P_2O_5 - H_3PO_4$, as well as metal oxides with high oxidation states: $(Cr)↖(+6)O_3$ corresponds to $H_2CrO_4, (Mn_2)↖(+7 )O_7 — HMnO_4$.

Typical acid oxide reactions:

1. Acid oxide + base → salt + water (exchange reaction):

$SO_2+2NaOH=Na_2SO_3+H_2O$.

2. Acidic oxide + basic oxide → salt (compound reaction):

$CaO+CO_2=CaCO_3$.

3. Acid oxide + water → acid (compound reaction):

$N_2O_5+H_2O=2HNO_3$.

This reaction is possible only if the acid oxide is soluble in water.

Amphoteric are called oxides, which, depending on conditions, exhibit basic or acidic properties. These are $ZnO, Al_2O_3, Cr_2O_3, V_2O_5$. Amphoteric oxides do not directly combine with water.

Typical reactions of amphoteric oxides:

1. Amphoteric oxide + acid → salt + water (exchange reaction):

$ZnO+2HCl=ZnCl_2+H_2O$.

2. Amphoteric oxide + base → salt + water or complex compound:

$Al_2O_3+2NaOH+3H_2O(=2Na,)↙(\text"sodium tetrahydroxoaluminate")$

$Al_2O_3+2NaOH=(2NaAlO_2)↙(\text"sodium aluminate")+H_2O$.

Lesson topic: Creative tasks in GIA variants

Lesson location: general lesson in 9th grade (in preparation for the State Examination in Chemistry).

Lesson duration: (60 min.).

Lesson content:

The lesson is structurally divided into 3 parts, corresponding to the questions in the GIA options.

Obtaining gaseous substances. Qualitative reactions to gaseous substances (oxygen, hydrogen, carbon dioxide, ammonia) (A 14).

Determining the nature of the solution environment of acids and alkalis using indicators. Qualitative reactions to ions in solution (chloride, sulfate, carbonate ions, ammonium ion) (A 14).

Chemical properties of simple substances. Chemical properties of complex substances. Qualitative reactions to ions in solution (chloride, sulfate, carbonate ions, ammonium ion). Obtaining gaseous substances. Qualitative reactions to gaseous substances (oxygen, hydrogen, carbon dioxide) (C 3).

During the lesson, the teacher uses a multimedia presentation: “Creative tasks in GIA variants”, “Safety precautions in chemistry lessons”, “Creative tasks in GIA variants” for the 3rd part of the lesson.

The purpose of the lesson: Prepare 9th grade students for the State Examination in Chemistry on specific issues.Purpose of the work: to consolidate knowledge about the properties of inorganic compounds of different classes, about qualitative reactions to ions.Deepen students' knowledge of chemistry and develop interest in the subject.

Lesson Objectives :

- Deepen, systematize and consolidate,students' knowledge of methods of production, collection and properties of various gases;

Develop the ability to analyze, compare, generalize, and establish cause and effect relationships;

Introduce you to the methodology for completing tasks of GIA variants on this topic;

Develop skills and abilities to work with chemical reagents and chemical equipment;

Promote the development of skills to apply knowledge in specific situations;

Broaden the horizons of students, increase motivation for learning, socialization of students through independent activities;

Help students gain real experience in solving non-standard tasks;

Develop educational and communication skills;

To promote the development of children’s skills in self-assessment and control of their activities;

Help students prepare for entry into secondary education.

Objectives for students:

Get acquainted with performing creative tasks in GIA variants (A-14, C3);

Learn to solve non-standard creative problems;

Exercise control and self-control of your activities.

(Students read out).

Lesson type:

Lesson on improving knowledge, skills and abilities (lesson on the formation of skills and abilities, targeted application of those learned in the GIA variants)

lesson of generalization and systematization of knowledge;

combined.

Forms of work:

Frontal, group, individual, collective.

Methods and means of teaching:independent work of students, which they did at home, in class,individual work, group work, laboratory experience, work at the board, using ICT, handouts and abstract world objects.

Lesson effectiveness:

During the lesson, the teacher created conditions for active student activity, including creative activity.

Equipment: balloons, soap bubbles, individual cards, task cards, practical work assignments, homework cards, reflection sheet, “How did I learn the material?” test,computer, projector, screen,presentations. Tables: solubility, color of indicators, determination of ions. Tables at the blackboard.

Reagents: sodium carbonate, sodium chloride and sodium sulfate, hydrochloric acid, silver nitrate, barium chloride, calcium carbonate, water, ammonium chloride. Indicators: methyl orange, phenolphthalein, litmus).

Test "Our mood"

( Before the lesson, students are invited to take squares of any color that the children want to take):

Red – energetic (ready to work).

Yellow is the color of joy and good mood.

Blue is the color of calm and balance.

Green is bored, but I hope that this mood will change.

Brown – isolation.

Black is gloomy.

Lesson motto: Words of Goethe: “It is not enough to know, you must also apply.

It’s not enough to want, you have to do it.”

During the classes:

Warm-up:

Founder of the theory of Electrolytic dissociation (Arrhenius).

The process of disintegration of an electrolyte into ions is called? (ED).

What substances are called electrolytes? (Substances whose aqueous solutions or melts conduct electric current).

Positively charged ions are called (cations).

Negatively charged ions are called (anions).

When alkalis dissociate, ions are formed (hydroxide ions).

List the conditions for the occurrence of ion exchange reactions (ion exchange reactions go to completion in three cases: 1. As a result of the reaction, a precipitate is formed; 2. a slightly dissociating substance or water; 3. a gaseous substance is formed) (student answers).

When acids dissociate, ions (hydrogen ions) are formed.

First part of the lesson.

Obtaining gaseous substances. Qualitative reactions to gaseous substances (oxygen, hydrogen, carbon dioxide, ammonia)

Need to know:

Physical and chemical properties of gases (hydrogen, oxygen, carbon dioxide, ammonia).

Gas collection methods.

Name and operation of devices for producing gases

The main methods of obtaining gases in industry and laboratories

Gas identification ( qualitative reactions) .

1. Variety of gases. Distribute the gases you know into groups (individual work - students complete the task on separate pieces of paper, the answers are recorded on the screen, mutual testing is organized, and students are graded).

Formulas of gaseous substances are printed on sheets of paper and placed on the board in advance:

O 2 ,CO,H 2 ,NO 2 , CO 2 , N 2 , N.H. 3 , H 2 S, CI 2 , HCI.

1) gases – simple substances;

2) gases - oxides;

3) colored gases;

4) gases with a characteristic odor;

Answer: 1) Simple substances: N 2 , O 2 , H 2 , Cl 2 .

2) Oxides: CO, CO 2 ,NO 2 .

3) Colored gases: Cl 2 ,NO 2 .

4) Gases with a characteristic odor: Cl 2 ,NO 2 , N.H. 3 , H 2 S, HCl.

2. Determine what gas fills the ball. To do this: Calculate the air density of the gases given to you.

Balloons of different colors are suspended on the board, located at different heights. Within 5 minutes, students must determine which gas, from those whose formulas are listed below, fills each ball: NH 3 , CO 2 , N 2 , ABOUT 2 .

We create groups. Each group receives its own gas (a ball of a different color, corresponding to the color of the cylinders in which liquefied gas is transported. For example, oxygen: the ball is blue), the properties of which the group will determine. Group 1 - H 2 , 2nd group - O 2 , 3rd group - CO 2 , 4th group - NH 3 . Students also give the answer: why are the balls located at different heights?

3. Experience : Why do air bubbles fly down? (Water gun). The children give the answer.

Work in groups:

4. Name the physical properties of the gases given to you. Briefly. (Work in groups).

Oxygen-

Hydrogen –

Ammonia –

Carbon dioxide -

5.Answer the question: What methods of collecting gases do you know? Let's look at the slide:

Devices for collecting gases.

2) What gases can be collecteddevice in Figure 1 and 2?

Which are lighter than air 1, heavier - 2.

3) What gases can be collected with the device in Figure 3?

Gases that are insoluble in water.

4) What device number will you use to collect?

Group 1 - hydrogen? 2- oxygen?

ABOUT We are working on this issue according to the State Inspectorate’s instructions:

A) ammonia B) oxygen

C) carbon dioxide D) hydrogen sulfide

The production of which gas is shown in the figure?

A) ammonia B) oxygen

C) carbon dioxide D) hydrogen

6 . We will work out laboratory and industrial methods for producing gases on GIA issues: (according to the handout, table 1.)

TO what kind of gas do they get?

A) ammonia B) oxygen

C) carbon dioxide D) hydrogen

What gas do you get?

A) ammonia

B) oxygen

B) carbon dioxide

D) hydrogen

What gas do you get?

A) ammonia

B) oxygen

B) carbon dioxide

D) hydrogen

What gas do you get?

A) ammonia B) oxygen C) carbon dioxide D) hydrogen

What gas do you get?

A) ammonia B) oxygen

B) carbon dioxide D) hydrogen

7 .How to distinguish gases from each other?

What gas is being determined?

A) ammonia B) oxygen

C) carbon dioxide D) hydrogen

What gas are the balloons filled with?

A) hydrogen sulfide B) oxygen

C) carbon dioxide D) hydrogen

What gas is being transferred?

A) ammonia B) oxygen

C) carbon dioxide D) hydrogen

A distinctive feature of tasks A14 2012 were questions on drawings.

Thus, in the GIA assignments there are the following questions about drawings:

What gas is being collected? (Collecting methods)

What gas do you get? (Methods of obtaining)

What gas is being determined? (Identification)

Presentation

2.Second part of the lesson.

Determining the nature of the solution environment of acids and alkalis using indicators.

Qualitative reactions to ions in solution (chloride, sulfate, carbonate ions, ammonium ion.

Safety Rules (presentation).

1. Laboratory experience.

Safety precautions in chemistry lessons (Multimedia presentation)

In groups, identify the substances given to you.

Group 1

HCI), alkalis (NaOH) and water (H 2 O). Using the given substances (methyl orange), determine which test tube contains each substance.

Group 2

Group 3

Appendix 1.2 (for group 1-3)

Practical work No. 1

Lesson objectives:

Equipment: board, chalk, table “Determining the nature of the solution of acids and alkalis using indicators”, “Table of solubility of acids, bases, salts in water”, a stand with test tubes, an alcohol lamp, matches, a holder for test tubes.

Reagents: solutions: sodium hydroxide, hydrochloric acid, water, indicator - methyl orange.

Group 1

Three test tubes under numbers contain solutions: acids (HCI), alkalis (NaOH) and water (H 2 O). Using the given indicator substances (methyl orange, phenolphthalein, litmus), determine which test tube contains each substance.

Instructions for use.

Assignment: three numbered test tubes (1, 2, 3) contain the following substances: acids (HCI), alkalis (NaOH) and water (H 2 O).

Using characteristic reactions, recognize which test tube contains these substances.

Perform experiments 1, 2, 3.

Pour 2 - 3 ml of solution into test tube No. 1 and add 1 - 2 drops of a solution of methyl orange indicator, litmus, phenolphthalein, how did the color of the solution change?

Pour a solution of methyl orange indicator, litmus, phenolphthalein into test tube No. 2.

What are you observing?

Pour a solution of methyl orange indicator, litmus, phenolphthalein into test tube No. 3.

What are you observing?

3. Fill out the table.

Make the necessary notes in your notebook, voice the conclusion (one student from the group speaks). See handouts appendix 1.2.

Color change

in an acidic environment

Color change in alkaline environment

Practical work No. 1

Topic: Qualitative reactions to ions.

Purpose of the work: using characteristic reactions to recognize inorganic substances.

Improve skills in conducting chemical experiments;

In a practical way, confirm the conditions for carrying out ion exchange reactions.

Lesson objectives:

Educational: with the help of a chemical experiment, consolidate the knowledge, skills and abilities of students in the section “Theory of electrolytic dissociation” (characteristic reactions to inorganic substances).

Developmental: promote the development of thinking (analyze, compare, highlight the main thing, establish cause-and-effect relationships), the development of cognitive interests.

Educational: promote the formation of personality qualities (responsibility, collectivism, initiative).

Type of lesson: application of knowledge, skills and abilities in practice.

Type of lesson: practical work.

Teaching methods: analytical, comparative, generalizing, classification.

Group 2

Three test tubes with numbers contain solutions: sodium carbonate, sodium chloride and sodium sulfate. Using the given substances (hydrochloric acid, silver nitrate, barium chloride), determine which test tube contains each substance.

Instructions for use.

To perform this experiment, divide the contents of each numbered test tube into three samples.

Progress:

1. Table for recording work completion in the form:

2. Perform experiments 1, 2, 3.

What are you observing?

What are you observing?

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

3. Fill out the table.

4. Draw a general conclusion. Write down the results of the experimental part of the work in the report table. When preparing a report, use §§ 2,3,4.

Make the necessary notes

Practical work No. 1

Topic: Qualitative reactions to ions.

Purpose of the work: using characteristic reactions to recognize inorganic substances.

Improve skills in conducting chemical experiments;

In a practical way, confirm the conditions for carrying out ion exchange reactions.

Lesson objectives:

Educational: with the help of a chemical experiment, consolidate the knowledge, skills and abilities of students in the section: “Theory of electrolytic dissociation” (characteristic reactions to inorganic substances).

Developmental: promote the development of thinking (analyze, compare, highlight the main thing, establish cause-and-effect relationships), the development of cognitive interests.

Educational: promote the formation of personality qualities (responsibility, collectivism, initiative).

Type of lesson: application of knowledge, skills and abilities in practice.

Type of lesson: practical work.

Teaching methods: analytical, comparative, generalizing, classification.

Equipment: board, chalk, table of solubility of acids, bases, salts in water, rack with test tubes, alcohol lamp, matches, test tube holder.

Reagents: solutions: hydrochloric acid, water, indicator - silver nitrate, calcium carbonate, sodium carbonate and sodium chloride, hydrochloric acid, ammonium chloride.

Group 3

Three numbered test tubes contain solids: calcium carbonate, ammonium chloride and sodium chloride. Using the given substances (hydrochloric acid, silver nitrate, sodium hydroxide), determine which test tube contains each substance.

Instructions for use.

To perform this experiment, divide the contents of each numbered test tube into three samples.

Progress:

1. A table for recording the completion of work in the form:

2. Perform experiments 1, 2, 3.

Pour the silver nitrate solution into test tube No. 1.

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

Pour a solution of hydrochloric acid into test tube No. 2.

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

Pour sodium hydroxide solution into test tube No. 3.

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

3. Fill out the table.

4. Draw a general conclusion. Write down the results of the experimental part of the work in the report table. When preparing a report, use §§ 2,3,4.

Make the necessary notesin the notebook, the conclusion is voiced. (One student from the group speaks). See handouts appendix 1.2.

3. Third part of the lesson

Chemical properties of simple substances. Chemical properties of complex substances.

Qualitative reactions to ions in solution (chloride, sulfate, carbonate ions, ammonium ion).

Obtaining gaseous substances.

Qualitative reactions to gaseous substances (oxygen, hydrogen, carbon dioxide, ammonia)

Creative tasks, task C 3, the tasks are difficult.

3. Each group is asked to solve one combined problem. The group decides together. The solution is written down on the board.

3.1 : Pavel Bazhov’s fairy tale “The Mistress of the Copper Mountain” mentions a beautiful ornamental stone - malachite, from which vases, boxes, and jewelry are made. Chemical formula of malachite (CuOH) 2 CO 3 . During the thermal decomposition of malachite, three complex substances are formed: one black solid and two gaseous. When one of the resulting gaseous substances is passed through lime water, it becomes cloudy due to the formation of sediment.

Write down the chemical formula and name of the resulting precipitate. Write two molecular equations for the reactions that were carried out.

Answer:t 0

(CuOH) 2 CO 3 → 2CuO + CO 2 +H 2 O

Malachite

CO 2 +Ca(OH) 2 →CaCO 3 ↓+H 2 O

This reaction is used to detect carbon monoxide (IV).

3.2: Substance X 1 obtained by reacting aluminum with yellow powder. When water acts on X 1 a poisonous gas with the smell of rotten eggs is released. This gas burns to form substance X 2 with a pungent odor. Define X 1 their 2. Write the equations for the reactions occurring. Indicate the molar mass of substance X 2.

Answer:

2Al +3S → Al 2 S 3

Al 2 S 3 + 6 N 2 ABOUT→ 3H 2 S + 2Al(OH) 3

2 H 2 S + 3 O 2 → 2 SO 2 + 2H 2 ABOUT

Al 2 S 3 - X 1 , SO 2 - X 2 M (SO 2 ) = 64 g/mol

All experiments with hydrogen sulfide are carried out in a fume hood!

3.3: To determine the qualitative composition of a substance, students were given a metal salt, 1 kg of which in 1854 cost 270 times more than silver, and in the middleXXcentury has already been widely used for the manufacture of light metal structures. After dissolving the crystals of the given salt in water, the students poured the resulting clear solution into two test tubes.

A few drops of sodium hydroxide solution were added to one of them, and a gel-like white precipitate formed. A few drops of barium chloride solution were added to another test tube containing a salt solution, which formed a white, milk-like precipitate.

Write down the chemical formula and name of the salt given. Create two equations for the reactions that were carried out in the process of recognizing it.

Answer:

Al 2 (SO 4 ) 3 +6NaOH→3Na 2 SO 4 +2Al(OH) 3 ↓ gel-likewhitesediment

Al 2 ( SO 4 ) 3 + 3 BaCl 2 → 3 BaSO 4 ↓+ 2 AlCl 3

white, milky sediment

Summing up the lesson. Reflection. Grading.

Evaluative-reflective block

Let's analyze your work in groups. The floor is given to the leader of each group.

What questions did we cover in class today?

Which of these questions did you find most difficult?

Test Appendix 3

How did I learn the material?

Homework (the assignment is printed for each student)

Task No. 1.

To carry out the experiments, the researchers were given a substance that was yellow crystals insoluble in water. This substance is known to be used in making matches and vulcanizing rubber. As a result of the interaction of the released substance with concentrated sulfuric acid when heated, gaseous oxide and water are formed. And when the resulting oxide is passed through a solution of barium hydroxide, a white precipitate forms, which dissolves with further passage of the gas.

Write down the chemical formula and name the salt obtained as a result of the second experiment. Write two molecular reaction equations that correspond to the experiments the students conducted while exploring salt.

t 0

S+2H 2 SO 4 = 3SO 2 + 2H 2 O

SO 2 +Ba(OH) 2 = BaSO 4 ↓ + 2H 2 O

Barium sulfate

(At the end of the lesson, students who want to change the square to a square of a different color are asked to do so. Test "Our mood").

Annex 1.

Table. Ion determination

Reaction result

H +

Indicators

Color change

Ag+

Cl -

White sediment

WITHu 2+

OH -

S 2-

Blue sediment

Black sediment

Coloring the flame blue-green

Fe 2+

OH -

Greenish sediment that turns brown over time

Fe 3+

OH -

Brown sediment

Zn 2+

OH -

S 2-

White precipitate, if in excessOH - dissolves

White sediment

Al 3+

OH -

A white, jelly-like precipitate that, when in excessOH - dissolves

N.H. 4 +

OH -

Ammonia smell

Ba 2+

SO 4 2-

White sediment

Coloring the flame yellow-green

Ca 2+

CO 3 2-

White sediment

Coloring the flame brick red

Na +

Flame color yellow

K +

Flame color is purple (through cobalt glass)

Cl -

Ag +

White sediment

H 2 SO 4 *

Release of colorless gas with a pungent odor (HCl)

Br -

Ag +

H 2 SO 4 *

Yellowish sediment

SelectionSO 2 AndBr 2 (brown color)

I -

Ag +

H 2 SO 4 +

Yellow precipitate

SelectionH 2 SAndI 2 (purple)

SO 3 2-

H +

SelectionSO 2 - a gas with a pungent odor that bleaches a solution of fuchsin and violet ink

CO 3 2-

H +

Emission of odorless gas causing cloudiness in lime water

CH 3 COO -

H 2 SO 4

Smell of acetic acid

NO 3 -

H 2 SO 4 (conc.) andCu

Brown gas release

SO 4 2-

Ba 2+

White sediment

P.O. 4 3-

Ag +

Yellow precipitate

OH -

Indicators

Changing the color of indicators

Appendix 2.

Practical work No. 1

Topic: Qualitative reactions to ions.

Purpose of the work: using characteristic reactions to recognize inorganic substances.

Improve skills in conducting chemical experiments;

In a practical way, confirm the conditions for carrying out ion exchange reactions.

Lesson objectives:

Educational: with the help of a chemical experiment, consolidate the knowledge, skills and abilities of students in the section “Theory of electrolytic dissociation” (characteristic reactions to inorganic substances).

Developmental: promote the development of thinking (analyze, compare, highlight the main thing, establish cause-and-effect relationships), the development of cognitive interests.

Educational: promote the formation of personality qualities (responsibility, collectivism, initiative).

Type of lesson: application of knowledge, skills and abilities in practice.

Type of lesson: practical work.

Teaching methods: analytical, comparative, generalizing, classification.

Equipment: board, chalk, table of solubility of acids, bases, salts in water, rack with test tubes, alcohol lamp, matches, test tube holder.

Reagents: sodium carbonate, sodium chloride and sodium sulfate, hydrochloric acid, silver nitrate, barium chloride.

Group 2

Three test tubes with numbers contain solutions: sodium carbonate, sodium chloride and sodium sulfate. Using the given substances (hydrochloric acid, silver nitrate, barium chloride), determine which test tube contains each substance.

Instructions for use.

To perform this experiment, divide the contents of each numbered test tube into three samples.

Progress:

1. Table for recording work completion in the form:

2. Perform experiments 1, 2, 3.

Pour barium chloride solution into test tube No. 1.

What are you observing?

Write the reaction equation in molecular, full ionic, and abbreviated ionic forms.

Pour a solution of hydrochloric acid into test tube No. 2.

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

Pour the silver nitrate solution into test tube No. 3.

What are you observing?

Write the reaction equation in molecular, full ionic, and reduced ionic forms.

3. Fill out the table.

4. Draw a general conclusion. Write down the results of the experimental part of the work in the report table. When preparing a report, use §§ 2,3,4.

Make the necessary notes in the notebook, the conclusion is voiced. (One student from the group speaks).

Appendix 3

Test

How did I learn the material?

1. Received solid knowledge, mastered all the material 9-10 points

2. Partially mastered the material 7-8 points

3. I didn’t understand much, I still need to work 4-6 points

4. What grade would you give yourself for participating in groups? (Give yourself this rating here).

Chemically, the pH of a solution can be determined using acid-base indicators.

Acid-base indicators are organic substances whose color depends on the acidity of the medium.

The most common indicators are litmus, methyl orange, and phenolphthalein. Litmus turns red in an acidic environment and blue in an alkaline environment. Phenolphthalein is colorless in an acidic environment, but turns crimson in an alkaline environment. Methyl orange turns red in an acidic environment, and yellow in an alkaline environment.

In laboratory practice, a number of indicators are often mixed, selected so that the color of the mixture changes over a wide range of pH values. With their help, you can determine the pH of a solution with an accuracy of one. These mixtures are called universal indicators.

There are special devices - pH meters, with which you can determine the pH of solutions in the range from 0 to 14 with an accuracy of 0.01 pH units.

Hydrolysis of salts

When some salts are dissolved in water, the equilibrium of the water dissociation process is disrupted and, accordingly, the pH of the environment changes. This is because salts react with water.

Hydrolysis of salts – chemical exchange interaction of dissolved salt ions with water, leading to the formation of weakly dissociating products (molecules of weak acids or bases, anions of acid salts or cations of basic salts) and accompanied by a change in the pH of the medium.

Let's consider the process of hydrolysis depending on the nature of the bases and acids that form the salt.

Salts formed by strong acids and strong bases (NaCl, kno3, Na2so4, etc.).

Let's say that when sodium chloride reacts with water, a hydrolysis reaction occurs to form an acid and a base:

NaCl + H 2 O ↔ NaOH + HCl

To get a correct idea of ​​the nature of this interaction, let us write the reaction equation in ionic form, taking into account that the only weakly dissociating compound in this system is water:

Na + + Cl - + HOH ↔ Na + + OH - + H + + Cl -

When canceling identical ions on the left and right sides of the equation, the water dissociation equation remains:

H 2 O ↔ H + + OH -

As you can see, there are no excess H + or OH - ions in the solution compared to their content in water. In addition, no other weakly dissociating or sparingly soluble compounds are formed. From this we conclude that salts formed by strong acids and bases do not undergo hydrolysis, and the reaction of solutions of these salts is the same as in water, neutral (pH = 7).

When composing ion-molecular equations for hydrolysis reactions, it is necessary:

1) write down the salt dissociation equation;

2) determine the nature of the cation and anion (find the cation of a weak base or the anion of a weak acid);

3) write down the ionic-molecular equation of the reaction, taking into account that water is a weak electrolyte and that the sum of charges should be the same on both sides of the equation.

Salts formed by a weak acid and a strong base

(Na 2 CO 3 , K 2 S, CH 3 COONa And etc. .)

Consider the hydrolysis reaction of sodium acetate. This salt in solution breaks down into ions: CH 3 COONa ↔ CH 3 COO - + Na + ;

Na + is the cation of a strong base, CH 3 COO - is the anion of a weak acid.

Na + cations cannot bind water ions, since NaOH, a strong base, completely disintegrates into ions. Anions of weak acetic acid CH 3 COO - bind hydrogen ions to form slightly dissociated acetic acid:

CH 3 COO - + HON ↔ CH 3 COOH + OH -

It can be seen that as a result of the hydrolysis of CH 3 COONa, an excess of hydroxide ions was formed in the solution, and the reaction of the medium became alkaline (pH > 7).

Thus we can conclude that salts formed by a weak acid and a strong base hydrolyze at the anion ( An n - ). In this case, the salt anions bind H ions + , and OH ions accumulate in the solution - , which causes an alkaline environment (pH>7):

An n - + HOH ↔ Han (n -1)- + OH - , (at n=1 HAn is formed - a weak acid).

Hydrolysis of salts formed by di- and tribasic weak acids and strong bases proceeds stepwise

Let's consider the hydrolysis of potassium sulfide. K 2 S dissociates in solution:

K 2 S ↔ 2K + + S 2- ;

K + is the cation of a strong base, S 2 is the anion of a weak acid.

Potassium cations do not take part in the hydrolysis reaction; only weak hydrosulfide anions interact with water. In this reaction, the first step is the formation of weakly dissociating HS - ions, and the second step is the formation of a weak acid H 2 S:

1st stage: S 2- + HOH ↔ HS - + OH - ;

2nd stage: HS - + HOH ↔ H 2 S + OH - .

The OH ions formed in the first stage of hydrolysis significantly reduce the likelihood of hydrolysis in the next stage. As a result, a process that occurs only in the first stage is usually of practical importance, which, as a rule, is limited to when assessing the hydrolysis of salts under normal conditions.

Depending on which H + or OH - ions are in excess in an aqueous solution, the following types (characters) of solution media are distinguished:

1) sour

2) alkaline

3) neutral

At acidic nature of the environment the solution contains an excess of hydrogen cations H +, and the concentration of hydroxide ions is close to zero.

At alkaline environment there is an excess of hydroxide ions OH - in the solution, and the concentration of H + cations is close to zero.

At neutral environment solution, the concentrations of H + and OH - ions are equal to each other and practically equal to zero (0.0000001 mol/l).

There are some organic substances whose color changes depending on the nature of the environment. This phenomenon is widely used in chemistry. Some of the most common indicators are litmus, phenolphthalein and methyl orange (methyl orange). The color of these substances depending on the nature of the environment is presented in the following table:

	indicator color
indicator	in a neutral environment	in an acidic environment	in an alkaline environment
litmus	violet	red	blue
phenolphthalein	colorless	colorless	crimson
methyl orange (methyl orange)	orange	pink	yellow

As you can see, a specific property of phenolphthalein is that this indicator does not allow one to distinguish between neutral and acidic environments - in both environments it is not colored in any way. This property is undoubtedly a disadvantage, however, phenolphthalein is widely used due to its exceptional sensitivity to even a slight excess of OH - ions.

Obviously, using indicators you can distinguish acids, alkalis and distilled water from each other. However, it should be remembered that acidic, alkaline and neutral environments can be observed not only in solutions of acids, alkalis and distilled water. The solution environment can also be different in salt solutions depending on their relationship to hydrolysis.

For example, a solution of sodium sulfite can be distinguished from a solution of sodium sulfate using phenolphthalein. Sodium sulfite is a salt formed by a strong base and a weak acid, so its solutions will have an alkaline reaction. Phenolphthalein will turn crimson in its solution. Sodium sulfate is formed by a strong base and a strong acid, i.e. does not undergo hydrolysis, and its aqueous solutions will have a neutral reaction. In the case of sodium sulfate solution, phenolphthalein will remain colorless.

In task 18 of the OGE in chemistry, we demonstrate knowledge of indicators and pH, as well as qualitative reactions to ions in solution.

Theory for task No. 18 OGE in chemistry

Indicators

An indicator is a chemical substance that changes color depending on the pH of the environment.

The most well-known indicators are phenolphthalein, methyl orange, litmus and the universal indicator. Their colors depending on the environment in the picture below:

And here are the colors of the indicators in more detail with real-life examples:

We've dealt with indicators; let's move on to qualitative reactions to ions.

Qualitative reactions to ions

Qualitative reactions to cations and anions are presented in the table below.

How to correctly cope with task 18 in the OGE test in chemistry?

To do this, you need to select a qualitative reaction to one of the options provided and make sure that this reagent does not react with the second substance.

Analysis of typical options for task No. 18 OGE in chemistry

First version of the task

Establish a correspondence between two substances and a reagent that can be used to distinguish between these substances.

Substances:

A) Na2CO3 and Na2SiO3

B) K2CO3 and Li2CO3

B) Na2SO4 and NaOH

Reagent:

1) CuCl2

4) K3PO4

Let's consider each case.

Na2CO3 and Na2SiO3

1. the reaction with copper chloride does not occur in both cases, since copper carbonate and silicate decompose in an aqueous solution
2. with hydrochloric acid, in the case of sodium carbonate, gas is released, and in the case of silicate, a precipitate forms - this is qualitative reaction to silicates
3. with phosphate there are also no qualitative reactions to sodium

K2CO3 and Li2CO3

1. These substances do not react with copper chloride (in fact, a precipitate of copper hydroxide precipitates, but this reaction cannot distinguish the two reagents)
2. Both react with hydrochloric acid to release carbon dioxide.
3. These substances do not react with magnesium oxide, and magnesium oxide does not enter into ion exchange reactions
4. with phosphate lithium precipitates as phosphate , but no potassium

We have one last option left - copper chloride. Indeed, copper hydroxide precipitates with sodium hydroxide, but the reaction does not occur with sulfate.