An aqueous solution of which salt is acidic. Salt hydrolysis

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

Tasks with comments and solutions

In previous years, the assimilation of this element of content was tested by tasks with a choice of answers (basic level of complexity). Here are examples of such tasks.

Example 39. An aqueous solution has an acidic reaction of the medium

1) calcium nitrate

2) strontium chloride

3) aluminum chloride

4) cesium sulfate

Recall that medium salts formed by a weak base and a strong acid (hydrolysis by the cation) have an acid reaction of the medium. Among the proposed answers, there is such a salt - it is aluminum chloride. Therefore, the environment of its solution is acidic:

Example 40. Aqueous solutions of iron(III) sulfate and

1) calcium nitrate

2) strontium chloride

3) copper chloride

4) cesium sulfate

The aqueous medium of iron (III) sulfate is acidic, as for all salts formed by a weak base and a strong acid:

In the answer options there is only one such salt - copper chloride. Therefore, the environment of its solution is also acidic:

In the 2017 exam paper, knowledge of this content element will be tested with tasks of an increased level of complexity (tasks with a short answer). Here are examples of such tasks.

Example 41. Establish a correspondence between the name of the salt and the reaction of the environment of its aqueous solution.

The environment of an aqueous salt solution is determined by the type of its hydrolysis (if it is possible). Consider the relationship to the hydrolysis of each of the proposed salts.

A) Potassium nitrate KNO 3 is a salt of a strong acid and a strong base. Salts of this composition do not undergo hydrolysis. The medium of an aqueous solution of this salt is neutral (A-2).

B) Aluminum sulfate Al 2 (SO 4) 3 is a salt formed by a strong sulfuric acid and a weak base (aluminum hydroxide). Therefore, the salt will undergo hydrolysis at the cation:

As a result of the accumulation of H + ions, the environment of the salt solution will be acidic (B-1).

B) Potassium sulfide K 2 S is formed by a strong base and a very weak hydrosulphuric acid. Such salts undergo anion hydrolysis:

As a result of the accumulation of OH ions - the medium of the salt solution will be alkaline (B-3).

D) Sodium orthophosphate Na 3 PO 4 is formed by a strong base and a rather weak phosphoric acid. Therefore, the salt will undergo hydrolysis at the anion:

As a result of the accumulation of OH ions - the environment of the salt solution will be alkaline (G-3).

Summarize. The first solution is neutral, the second is acidic, the last two are alkaline.

To get the correct answer, we first establish the nature of the acids and bases that form these salts.

A) BeSO 4 is formed by a weak base and a strong sulfuric acid, such salts undergo hydrolysis at the cation.

B) KNO 2 is formed by a strong base and a weak nitrous acid, such salts undergo anion hydrolysis.

B) Pb (NO 3) 2 is formed by a weak base and a strong nitric acid, such salts undergo hydrolysis at the cation.

D) CuCl 2 is formed by a weak base and a strong hydrochloric acid, such salts undergo hydrolysis by the cation.

To get the correct answer, let's establish the nature of the acids and bases that form the proposed salts:

A) lithium sulfide Li 2 S - a salt formed by a strong base and a weak acid, undergoes anion hydrolysis;

B) potassium chlorate KClO 3 - a salt formed by a strong base and a strong acid, does not undergo hydrolysis;

B) ammonium nitrite NH 4 NO 2 - a salt formed by a weak base and a weak acid, hydrolysis occurs both in the cation and in the anion;

D) sodium propionate C 3 H 7 COONa - a salt formed by a strong base and a weak acid, hydrolysis occurs along the anion.

BUT	B	AT	G

In order to understand what hydrolysis of salts is, let us first recall how acids and alkalis dissociate.

What all acids have in common is that when they dissociate, hydrogen cations (H +) are necessarily formed, while when all alkalis dissociate, hydroxide ions (OH -) are always formed.

In this regard, if in a solution, for one reason or another, there are more H + ions, they say that the solution has an acid reaction of the environment, if OH − - an alkaline reaction of the environment.

If everything is clear with acids and alkalis, then what will be the reaction of the medium in salt solutions?

At first glance, it should always be neutral. And the truth is, where, for example, in a solution of sodium sulfide, an excess of hydrogen cations or hydroxide ions can come from. Sodium sulfide itself does not form ions of either type during dissociation:

Na 2 S \u003d 2Na + + S 2-

However, if you had, for example, aqueous solutions of sodium sulfide, sodium chloride, zinc nitrate and an electronic pH meter (a digital device for determining the acidity of a medium), you would find an unusual phenomenon. The instrument would show you that the pH of the sodium sulfide solution is greater than 7, i.e. it has a clear excess of hydroxide ions. The environment of the sodium chloride solution would be neutral (pH = 7), and the solution of Zn(NO 3) 2 would be acidic.

The only thing that meets our expectations is the sodium chloride solution medium. It turned out to be neutral, as expected.
But where did the excess of hydroxide ions in the sodium sulfide solution and hydrogen cations in the zinc nitrate solution come from?

Let's try to figure it out. To do this, we need to learn the following theoretical points.

Any salt can be thought of as the reaction product of an acid and a base. Acids and bases are divided into strong and weak. Recall that those acids and bases, the degree of dissociation of which is close to 100%, are called strong.

note: sulfurous (H 2 SO 3) and phosphoric (H 3 PO 4) are often referred to as medium strength acids, but when considering hydrolysis tasks, they should be classified as weak.

Acidic residues of weak acids are capable of reversibly interacting with water molecules, tearing off hydrogen cations H + from them. For example, a sulfide ion, being the acidic residue of a weak hydrosulphuric acid, interacts with it as follows:

S 2- + H 2 O ↔ HS - + OH -

HS - + H 2 O ↔ H 2 S + OH -

As can be seen, as a result of this interaction, an excess of hydroxide ions is formed, which is responsible for the alkaline reaction of the medium. That is, the acid residues of weak acids increase the alkalinity of the medium. In the case of salt solutions containing such acidic residues, it is said that for them anion hydrolysis.

Acid residues of strong acids, unlike weak ones, do not interact with water. That is, they do not affect the pH of the aqueous solution. For example, the chloride ion, being the acidic residue of strong hydrochloric acid, does not react with water:

That is, chloride ions do not affect the pH of the solution.

Of the metal cations, only those that correspond to weak bases are also able to interact with water. For example, the Zn 2+ cation, which corresponds to the weak base zinc hydroxide. In aqueous solutions of zinc salts, the following processes occur:

Zn 2+ + H 2 O ↔ Zn(OH) + + H +

Zn(OH) + + H 2 O ↔ Zn(OH) + + H +

As can be seen from the equations above, as a result of the interaction of zinc cations with water, hydrogen cations accumulate in the solution, which increase the acidity of the medium, that is, lower the pH. If the composition of the salt includes cations, which correspond to weak bases, in this case they say that the salt hydrolyzed at the cation.

Metal cations, which correspond to strong bases, do not interact with water. For example, the Na + cation corresponds to a strong base - sodium hydroxide. Therefore, sodium ions do not react with water and do not affect the pH of the solution in any way.

Thus, based on the foregoing, salts can be divided into 4 types, namely, formed:

1) strong base and strong acid,

Such salts contain neither acidic residues nor metal cations that interact with water, i.e. capable of affecting the pH of an aqueous solution. Solutions of such salts have a neutral reaction medium. Such salts are said to be do not undergo hydrolysis.

Examples: Ba(NO 3) 2 , KCl, Li 2 SO 4 etc.

2) strong base and weak acid

In solutions of such salts, only acid residues react with water. The environment of aqueous solutions of such salts is alkaline; in relation to salts of this type, they say that they hydrolyze at the anion

Examples: NaF, K 2 CO 3 , Li 2 S, etc.

3) weak base and strong acid

In such salts, cations react with water, and acidic residues do not react - salt hydrolysis at the cation, acidic environment.

Examples: Zn(NO 3) 2, Fe 2 (SO 4) 3, CuSO 4, etc.

4) weak base and weak acid.

Both cations and anions of acid residues react with water. The hydrolysis of salts of this kind is both cation and anion or. They also talk about such salts that they are exposed to irreversible hydrolysis.

What does it mean that they are irreversibly hydrolyzed?

Since in this case both metal cations (or NH 4 +) and anions of the acid residue react with water, both H + ions and OH − ions simultaneously appear in the solution, which form an extremely low dissociating substance - water (H 2 O).

This, in turn, leads to the fact that salts formed by acidic residues of weak bases and weak acids cannot be obtained by exchange reactions, but only by solid-phase synthesis, or cannot be obtained at all. For example, when mixing a solution of aluminum nitrate with a solution of sodium sulfide, instead of the expected reaction:

2Al(NO 3) 3 + 3Na 2 S \u003d Al 2 S 3 + 6NaNO 3 (- so the reaction does not proceed!)

The following reaction is observed:

2Al(NO 3) 3 + 3Na 2 S + 6H 2 O= 2Al(OH) 3 ↓+ 3H 2 S + 6NaNO 3

However, aluminum sulfide can be obtained without problems by fusing aluminum powder with sulfur:

2Al + 3S = Al 2 S 3

When aluminum sulfide is added to water, it, as well as when trying to obtain it in an aqueous solution, undergoes irreversible hydrolysis.

Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S

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.

A lesson conducted using a notebook for practical work by I.I. Novoshinsky, N.S. Novoshinskaya to the textbook Chemistry Grade 8 in the MOU “Secondary School No. 11”, Severodvinsk, Arkhangelsk Region, by a chemistry teacher O.A. Olkina in grade 8 (on the parallel ).

The purpose of the lesson: Formation, consolidation and control of students' abilities to determine the reaction of the environment of solutions using various indicators, including natural ones, using a notebook for practical work by I.I. Novoshinsky, N.S. Novoshinskaya to the textbook Chemistry Grade 8.

Lesson objectives:

1. Educational. To consolidate the following concepts: indicators, reaction of the medium (types), pH, filtrate, filtration based on the performance of practical work assignments. To check the knowledge of students, which reflect the relationship “solution of a substance (formula) - pH value (numerical value) - reaction of the environment”. Tell students about ways to reduce the acidity of soils in the Arkhangelsk region.
2. Developing. To promote the development of students' logical thinking based on the analysis of the results obtained in the course of practical work, their generalization, as well as the ability to draw a conclusion. Confirm the rule: practice proves the theory or refutes it. To continue the formation of the aesthetic qualities of the personality of students on the basis of a diverse range of solutions presented, as well as to support the interest of the children in the subject "Chemistry" being studied.
3. Nurturing. Continue to develop students' skills to perform practical work tasks, adhering to labor protection and safety rules, including correctly performing filtering and heating processes.

Practical work No. 6 “Determination of the pH of the medium”.

Purpose for students: Learn to determine the reaction of the environment of solutions of various objects (acids, alkalis, salts, soil solution, some solutions and juices), as well as to study plant objects as natural indicators.

Equipment and reagents: test tube rack, stopper, glass rod, ring rack, filter paper, scissors, chemical funnel, beakers, porcelain mortar and pestle, fine grater, clean sand, universal indicator paper, test solution, soil, boiled water, fruits, berries and other plant material, a solution of sodium hydroxide and sulfuric acid, sodium chloride.

During the classes

Guys! We have already got acquainted with such concepts as the reaction of the medium of aqueous solutions, as well as indicators.

What types of reactions in the environment of aqueous solutions do you know?

• neutral, alkaline and acid.

What are indicators?

• substances with which you can determine the reaction of the environment.

What indicators do you know?

• in solutions: phenolphthalein, litmus, methyl orange.

• dry: universal indicator paper, litmus paper, methyl orange paper

How can the reaction of an aqueous solution be determined?

• wet and dry.

What is the pH of the environment?

• pH value of hydrogen ions in solution (pH=– lg )

Let's remember which scientist introduced the concept of pH of the environment?

• Danish chemist Sorensen.

Well done!!! Now open the notebook for practical work on p.21 and read task number 1.

Task number 1. Determine the pH of the solution using a universal indicator.

Let's remember the rules when working with acids and alkalis!

Complete the experiment from task number 1.

Make a conclusion. Thus, if the solution has pH = 7, the medium is neutral, at pH< 7 среда кислотная, при pH >7 alkaline environment.

Task number 2. Get the soil solution and determine its pH using a universal indicator.

Read the task on p.21-p.22, complete the task according to the plan, put the results in the table.

Recall the safety rules when working with heating devices (alcohol).

What is filtering?

• the process of separating a mixture, which is based on the different throughput of the porous material - the filtrate in relation to the particles that make up the mixture.

What is a filtrate?

• it is a clear solution obtained after filtration.

Present the results in the form of a table.

What is the reaction of the soil solution medium?

• Sour

What needs to be done to improve soil quality in our region?

• CaCO 3 + H 2 O + CO 2 \u003d Ca (HCO 3) 2

Application of fertilizers that have an alkaline reaction of the environment: ground limestone and other carbonate minerals: chalk, dolomite. In the Pinezhsky district of the Arkhangelsk region there are deposits of such a mineral as limestone, near karst caves, so it is available.

Make a conclusion. The reaction of the environment of the resulting soil solution pH=4 is slightly acidic, therefore, liming is necessary to improve the quality of the soil.

Task number 3. Determine the pH of some solutions and juices using a universal indicator.

Read the task on p.22, complete the task according to the algorithm, put the results in the table.

juice source		juice source
Potato		silicate glue
fresh cabbage		table vinegar
Sauerkraut		Drinking soda solution
		Orange
		Fresh beets
		Boiled beets

Make a conclusion. Thus, different natural objects have different pH values: pH 1?7 – acidic environment (lemon, cranberry, orange, tomato, beet, kiwi, apple, banana, tea, potato, sauerkraut, coffee, silicate glue).

pH 7-14 alkaline environment (fresh cabbage, baking soda solution).

pH = 7 neutral medium (persimmon, cucumber, milk).

Task number 4. Study vegetable indicators.

What plant objects can act as indicators?

• berries: juices, flower petals: extracts, vegetable juices: root crops, leaves.
• substances that can change the color of the solution in different environments.

Read the task on p.23 and complete it according to the plan.

Record the results in a table.

Plant material (natural indicators)	Color of natural indicator solution
	Acid environment	Natural color of the solution (neutral medium)	Alkaline environment
Cranberry (juice)			Violet
Strawberries (juice)	orange	peach-pink
Blueberries (juice)		red-violet	blue - purple
Blackcurrant (juice)		red-violet	blue - purple

Make a conclusion. Thus, depending on the pH of the environment, natural indicators: cranberries (juice), strawberries (juice), blueberries (juice), black currants (juice) acquire the following colors: in an acidic environment - red and orange, in a neutral environment - red, peach - pink and violet colors, in an alkaline environment from pink through blue-violet to violet.

Consequently, the color intensity of the natural indicator can be judged by the reaction of the medium of a particular solution.

Tidy up your workspace when you're done.

Guys! Today was a very unusual lesson! Did you like?! Can the information learned in this lesson be used in everyday life?

Now complete the task that is given in your practice notebooks.

Task for control. Distribute the substances whose formulas are given below into groups depending on the pH of their solutions: HCl, H 2 O, H 2 SO 4, Ca (OH) 2, NaCl, NaOH, KNO 3, H 3 PO 4, KOH.

pH 17 - medium (acid), have solutions (HCl, H 3 PO 4, H 2 SO 4).

pH 714 medium (alkaline), have solutions (Ca (OH) 2, KOH, NaOH).

pH = 7 medium (neutral), have solutions (NaCl, H 2 O, KNO 3).

Evaluation for work _______________