Theory of electrolytic dissociation. Solutions

Electrolytes as chemical substances have been known since ancient times. However, they have conquered most areas of their application relatively recently. We will discuss the industry's highest priority areas for using these substances and figure out what the latter are and how they differ from each other. But let's start with an excursion into history.

Story

The oldest known electrolytes are salts and acids, discovered in the Ancient world. However, ideas about the structure and properties of electrolytes have evolved over time. Theories of these processes have evolved since the 1880s, when a number of discoveries were made related to theories of the properties of electrolytes. Several qualitative leaps were observed in theories describing the mechanisms of interaction of electrolytes with water (after all, only in solution do they acquire the properties due to which they are used in industry).

Now we will examine in detail several theories that had the greatest influence on the development of ideas about electrolytes and their properties. And let's start with the most common and simple theory, which each of us went through in school.

Arrhenius theory of electrolytic dissociation

In 1887, the Swedish chemist and Wilhelm Ostwald created the theory of electrolytic dissociation. However, it’s not that simple here either. Arrhenius himself was a proponent of the so-called physical theory of solutions, which did not take into account the interaction of the constituents of a substance with water and argued that free charged particles (ions) exist in the solution. By the way, it is from this position that electrolytic dissociation is considered in school today.

Let's talk about what this theory provides and how it explains to us the mechanism of interaction of substances with water. Like any other, she has several postulates that she uses:

1. When interacting with water, the substance breaks down into ions (positive - cation and negative - anion). These particles undergo hydration: they attract water molecules, which, by the way, are charged positively on one side and negatively on the other (forming a dipole), as a result they are formed into aqua complexes (solvates).

2. The dissociation process is reversible - that is, if a substance has broken up into ions, then under the influence of any factors it can again turn into its original form.

3. If you connect electrodes to the solution and turn on the current, the cations will begin to move to the negative electrode - the cathode, and the anions to the positively charged one - the anode. That is why substances that are highly soluble in water conduct electric current better than water itself. For the same reason they were called electrolytes.

4. electrolyte characterizes the percentage of a substance that has undergone dissolution. This indicator depends on the properties of the solvent and the dissolved substance itself, on the concentration of the latter and on the external temperature.

Here, in fact, are all the main postulates of this simple theory. We will use them in this article to describe what happens in an electrolyte solution. We will look at examples of these connections a little later, but now let’s look at another theory.

Lewis theory of acids and bases

According to the theory of electrolytic dissociation, an acid is a substance in the solution of which a hydrogen cation is present, and a base is a compound that disintegrates in solution into a hydroxide anion. There is another theory, named after the famous chemist Gilbert Lewis. It allows us to somewhat expand the concept of acid and base. According to Lewis's theory, acids are molecules of a substance that have free electron orbitals and are capable of accepting an electron from another molecule. It is easy to guess that the bases will be particles that are capable of donating one or more of their electrons to the “use” of the acid. What is very interesting here is that not only an electrolyte, but also any substance, even insoluble in water, can be an acid or base.

Brendsted-Lowry protolytic theory

In 1923, independently of each other, two scientists - J. Brønsted and T. Lowry - proposed a theory that is now actively used by scientists to describe chemical processes. The essence of this theory is that the meaning of dissociation comes down to the transfer of a proton from an acid to a base. Thus, the latter is understood here as a proton acceptor. Then the acid is their donor. The theory also explains well the existence of substances that exhibit the properties of both acids and bases. Such compounds are called amphoteric. In the Bronsted-Lowry theory, the term ampholytes is also used for them, while acids or bases are usually called protolytes.

We come to the next part of the article. Here we will tell you how strong and weak electrolytes differ from each other and discuss the influence of external factors on their properties. And then we will begin to describe their practical application.

Strong and weak electrolytes

Each substance interacts with water individually. Some dissolve well in it (for example, table salt), while others do not dissolve at all (for example, chalk). Thus, all substances are divided into strong and weak electrolytes. The latter are substances that interact poorly with water and settle at the bottom of the solution. This means that they have a very low degree of dissociation and high bond energy, which does not allow the molecule to disintegrate into its constituent ions under normal conditions. Dissociation of weak electrolytes occurs either very slowly or with increasing temperature and concentration of this substance in solution.

Let's talk about strong electrolytes. These include all soluble salts, as well as strong acids and alkalis. They easily disintegrate into ions and are very difficult to collect into precipitation. Current in electrolytes, by the way, is carried out precisely thanks to the ions contained in the solution. Therefore, strong electrolytes conduct current best. Examples of the latter: strong acids, alkalis, soluble salts.

Factors influencing the behavior of electrolytes

Now let's figure out how changes in the external environment affect Concentration directly affects the degree of dissociation of the electrolyte. Moreover, this relationship can be expressed mathematically. The law describing this relationship is called Ostwald's dilution law and is written as follows: a = (K / c) 1/2. Here a is the degree of dissociation (taken in fractions), K is the dissociation constant, different for each substance, and c is the concentration of the electrolyte in the solution. Using this formula, you can learn a lot about a substance and its behavior in solution.

But we have deviated from the topic. In addition to concentration, the degree of dissociation is also affected by the temperature of the electrolyte. For most substances, increasing it increases solubility and chemical activity. This is precisely what can explain the occurrence of some reactions only at elevated temperatures. Under normal conditions, they go either very slowly or in both directions (this process is called reversible).

We have analyzed the factors that determine the behavior of a system such as an electrolyte solution. Now let's move on to the practical application of these, without a doubt, very important chemicals.

Industrial use

Of course, everyone has heard the word “electrolyte” in relation to batteries. The car uses lead-acid batteries, the electrolyte in which is 40% sulfuric acid. To understand why this substance is needed there at all, it is worth understanding the operating features of batteries.

So what is the principle of operation of any battery? They undergo a reversible reaction of converting one substance into another, as a result of which electrons are released. When charging a battery, an interaction of substances occurs that does not occur under normal conditions. This can be thought of as the accumulation of electricity in a substance as a result of a chemical reaction. During the discharge, the reverse transformation begins, leading the system to the initial state. These two processes together constitute one charge-discharge cycle.

Let's look at the above process using a specific example - a lead-acid battery. As you might guess, this current source consists of an element containing lead (as well as lead dioxide PbO 2) and acid. Any battery consists of electrodes and the space between them filled with electrolyte. As the latter, as we have already found out, in our example we use sulfuric acid with a concentration of 40 percent. The cathode of such a battery is made of lead dioxide, and the anode consists of pure lead. All this is because different reversible reactions take place at these two electrodes with the participation of ions into which the acid has dissociated:

PbO 2 + SO 4 2- + 4H + + 2e - = PbSO 4 + 2H 2 O (reaction occurring at the negative electrode - cathode).
Pb + SO 4 2- - 2e - = PbSO 4 (Reaction occurring at the positive electrode - anode).

If we read the reactions from left to right, we get processes that occur when the battery is discharged, and if from right to left, we get processes that occur when the battery is charged. In each of these reactions, these reactions are different, but the mechanism of their occurrence is generally described in the same way: two processes occur, in one of which electrons are “absorbed”, and in the other, on the contrary, they “leave out”. The most important thing is that the number of electrons absorbed is equal to the number of electrons released.

Actually, besides batteries, there are many applications for these substances. In general, the electrolytes, examples of which we have given, are only a grain of the variety of substances that are united under this term. They surround us everywhere, everywhere. Here, for example, is the human body. Do you think these substances are not there? You are very mistaken. They are found everywhere in us, and the largest amount is made up of blood electrolytes. These include, for example, iron ions, which are part of hemoglobin and help transport oxygen to the tissues of our body. Blood electrolytes also play a key role in regulating water-salt balance and heart function. This function is performed by potassium and sodium ions (there is even a process that occurs in cells called the potassium-sodium pump).

Any substances that you can dissolve even a little are electrolytes. And there is no branch of industry or our life where they are not used. It's not just car batteries and batteries. These are any chemical and food production, military factories, clothing factories, and so on.

The composition of the electrolyte, by the way, varies. Thus, acidic and alkaline electrolytes can be distinguished. They are fundamentally different in their properties: as we have already said, acids are proton donors, and alkalis are acceptors. But over time, the composition of the electrolyte changes due to the loss of part of the substance; the concentration either decreases or increases (it all depends on what is lost, water or electrolyte).

We come across them every day, but few people know exactly the definition of such a term as electrolytes. We've looked at examples of specific substances, so let's move on to slightly more complex concepts.

Physical properties of electrolytes

Now about physics. The most important thing to understand when studying this topic is how current is transmitted in electrolytes. Ions play a decisive role in this. These charged particles can transfer charge from one part of the solution to another. Thus, anions always tend to the positive electrode, and cations - to the negative. Thus, by acting on the solution with electric current, we separate the charges on different sides of the system.

A very interesting physical characteristic is density. Many properties of the compounds we are discussing depend on it. And the question often comes up: “How to increase the density of the electrolyte?” In fact, the answer is simple: it is necessary to reduce the water content in the solution. Since the density of the electrolyte is largely determined, it largely depends on the concentration of the latter. There are two ways to achieve your plan. The first is quite simple: boil the electrolyte contained in the battery. To do this, you need to charge it so that the temperature inside rises to just above one hundred degrees Celsius. If this method does not help, do not worry, there is another one: simply replace the old electrolyte with a new one. To do this, you need to drain the old solution, clean the insides from residual sulfuric acid with distilled water, and then fill in a new portion. As a rule, high-quality electrolyte solutions immediately have the desired concentration. After replacement, you can forget for a long time about how to increase the density of the electrolyte.

The composition of the electrolyte largely determines its properties. Characteristics such as electrical conductivity and density, for example, strongly depend on the nature of the solute and its concentration. There is a separate question about how much electrolyte a battery can contain. In fact, its volume is directly related to the declared power of the product. The more sulfuric acid inside the battery, the more powerful it is, i.e., the more voltage it can produce.

Where will this be useful?

If you are a car enthusiast or just interested in cars, then you yourself understand everything. Surely you even know how to determine how much electrolyte is in the battery now. And if you are far from cars, then knowledge of the properties of these substances, their use and how they interact with each other will not be superfluous. Knowing this, you will not be confused if you are asked to tell what electrolyte is in the battery. Although, even if you are not a car enthusiast, but you have a car, then knowledge of the battery structure will not be superfluous and will help you with repairs. It will be much easier and cheaper to do everything yourself than to go to an auto center.

And in order to better study this topic, we recommend reading a chemistry textbook for school and universities. If you know this science well and have read enough textbooks, the best option would be “Chemical Current Sources” by Varypaev. The entire theory of operation of batteries, various batteries and hydrogen cells is outlined there in detail.

Conclusion

We've come to the end. Let's summarize. Above we discussed everything related to such a concept as electrolytes: examples, theory of structure and properties, functions and applications. Once again, it is worth saying that these compounds form part of our life, without which our bodies and all areas of industry could not exist. Do you remember about blood electrolytes? Thanks to them we live. What about our cars? With this knowledge, we can fix any problem related to the battery, since we now understand how to increase the density of the electrolyte in it.

It’s impossible to tell everything, and we didn’t set such a goal. After all, this is not all that can be told about these amazing substances.

All substances can be divided into electrolytes and non-electrolytes. Electrolytes include substances whose solutions or melts conduct electric current (for example, aqueous solutions or melts of KCl, H 3 PO 4, Na 2 CO 3). Non-electrolyte substances do not conduct electric current when melted or dissolved (sugar, alcohol, acetone, etc.).

Electrolytes are divided into strong and weak. Strong electrolytes in solutions or melts completely dissociate into ions. When writing chemical reaction equations, this is emphasized by an arrow in one direction, for example:

HCl→ H + + Cl -

Ca(OH) 2 → Ca 2+ + 2OH -

Strong electrolytes include substances with a heteropolar or ionic crystal structure (Table 1.1).

Table 1.1 Strong electrolytes

Weak electrolytes only partially disintegrate into ions. Along with ions, melts or solutions of these substances contain overwhelmingly undissociated molecules. In solutions of weak electrolytes, in parallel with dissociation, the reverse process occurs - association, that is, the combination of ions into molecules. When writing the reaction equation, this is emphasized by two oppositely directed arrows.

CH 3 COOH D CH 3 COO - + H +

Weak electrolytes include substances with a homeopolar type of crystal lattice (Table 1.2).

Table 1.2 Weak electrolytes

The equilibrium state of a weak electrolyte in an aqueous solution is quantitatively characterized by the degree of electrolytic dissociation and the electrolytic dissociation constant.

The degree of electrolytic dissociation α is the ratio of the number of molecules dissociated into ions to the total number of molecules of the dissolved electrolyte:

The degree of dissociation shows what part of the total amount of dissolved electrolyte disintegrates into ions and depends on the nature of the electrolyte and solvent, as well as on the concentration of the substance in the solution, has a dimensionless value, although it is usually expressed as a percentage. With infinite dilution of the electrolyte solution, the degree of dissociation approaches unity, which corresponds to complete, 100%, dissociation of the molecules of the dissolved substance into ions. For solutions of weak electrolytes α<<1. Сильные электролиты в растворах диссоциируют полностью (α =1). Если известно, что в 0,1 М растворе уксусной кислоты степень электрической диссоциации α =0,0132, это означает, что 0,0132 (или 1,32%) общего количества растворённой уксусной кислоты продиссоциировало на ионы, а 0,9868 (или 98,68%) находится в виде недиссоциированных молекул. Диссоциация слабых электролитов в растворе подчиняется закону действия масс.

In general, a reversible chemical reaction can be represented as:

a A+ b B D d D+ e E

The reaction rate is directly proportional to the product of the concentration of reacting particles in powers of their stoichiometric coefficients. Then for the direct reaction

V 1 = k 1 [A] a[B] b,

and the speed of the reverse reaction

V 2 = k 2 [D] d[E] e.

At some point in time, the rates of the forward and reverse reactions will equalize, i.e.

This state is called chemical equilibrium. From here

k 1 [A] a[B] b=k 2 [D] d[E] e

Grouping constants on one side and variables on the other, we get:

Thus, for a reversible chemical reaction in a state of equilibrium, the product of the equilibrium concentrations of the reaction products in powers of their stoichiometric coefficients, related to the same product for the starting substances, is a constant value at a given temperature and pressure. Numerical value of the chemical equilibrium constant TO does not depend on the concentration of reactants. For example, the equilibrium constant for the dissociation of nitrous acid in accordance with the law of mass action can be written as:

HNO 2 + H 2 OD H 3 O + + NO 2 -

Size K a is called the dissociation constant of an acid, in this case nitrous.

The dissociation constant of a weak base is expressed similarly. For example, for the ammonia dissociation reaction:

NH 3 + H 2 O DNH 4 + + OH -

Size K b is called the dissociation constant of a base, in this case ammonia. The higher the dissociation constant of the electrolyte, the more strongly the electrolyte dissociates and the higher the concentration of its ions in the solution at equilibrium. There is a relationship between the degree of dissociation and the dissociation constant of a weak electrolyte:

This is a mathematical expression of Ostwald's dilution law: when a weak electrolyte is diluted, the degree of its dissociation increases. For weak electrolytes at TO≤1∙ 10 -4 and WITH≥0.1 mol/l use a simplified expression:

TO= α 2 ∙WITH or α

Example1. Calculate the degree of dissociation and concentration of ions and [NH 4 + ] in a 0.1 M ammonium hydroxide solution, if TO NH 4 OH =1.76∙10 -5

Given: NH 4 OH

TO NH 4 OH =1.76∙10 -5

Solution:

Since the electrolyte is quite weak ( To NH 4 OH =1,76∙10 –5 <1∙ 10 - 4) и раствор его не слишком разбавлен, можно принять, что:

or 1.33%

The concentration of ions in a binary electrolyte solution is equal to C∙α, since the binary electrolyte ionizes to form one cation and one anion, then = [ NH 4 + ]=0.1∙1.33∙10 -2 =1.33∙10 -3 (mol/l).

Answer:α=1.33%; = [NH 4 + ]=1.33∙10 -3 mol/l.

Strong electrolyte theory

Strong electrolytes in solutions and melts completely dissociate into ions. However, experimental studies of the electrical conductivity of solutions of strong electrolytes show that its value is somewhat underestimated compared to the electrical conductivity that should be at 100% dissociation. This discrepancy is explained by the theory of strong electrolytes proposed by Debye and Hückel. According to this theory, in solutions of strong electrolytes there is electrostatic interaction between ions. Around each ion, an “ionic atmosphere” is formed of ions of opposite charge sign, which inhibits the movement of ions in the solution when a direct electric current is passed. In addition to the electrostatic interaction of ions, in concentrated solutions it is necessary to take into account the association of ions. The influence of interionic forces creates the effect of incomplete dissociation of molecules, i.e. apparent degree of dissociation. The experimentally determined value of α is always slightly lower than the true α. For example, in a 0.1 M solution of Na 2 SO 4 the experimental value is α = 45%. To take into account electrostatic factors in solutions of strong electrolytes, the concept of activity is used (A). The activity of an ion is the effective or apparent concentration at which the ion acts in solution. Activity and true concentration are related by the expression:

Where f – activity coefficient, which characterizes the degree of deviation of the system from the ideal due to electrostatic interactions of ions.

Ion activity coefficients depend on the value µ, called the ionic strength of the solution. The ionic strength of a solution is a measure of the electrostatic interaction of all ions present in the solution and is equal to half the sum of the products of concentrations (With) each of the ions present in the solution per square of its charge number (z):

In dilute solutions (µ<0,1М) коэффициенты активности меньше единицы и уменьшаются с ростом ионной силы. Растворы с очень низкой ионной силой (µ < 1∙10 -4 М) можно считать идеальными. В бесконечно разбавленных растворах электролитов активность можно заменить истинной концентрацией. В идеальной системе a = c and the activity coefficient is 1. This means that there are practically no electrostatic interactions. In very concentrated solutions (µ>1M), ion activity coefficients can be greater than unity. The relationship between the activity coefficient and the ionic strength of the solution is expressed by the formulas:

at µ <10 -2

at 10 -2 ≤ µ ≤ 10 -1

+ 0,1z 2 µ at 0.1<µ <1

The equilibrium constant expressed in terms of activity is called thermodynamic. For example, for the reaction

a A+ b B d D+ e E

The thermodynamic constant has the form:

It depends on temperature, pressure and the nature of the solvent.

Since the activity of the particle is

Where TO C is the concentration equilibrium constant.

Meaning TO C depends not only on temperature, the nature of the solvent and pressure, but also on ionic strength m. Since thermodynamic constants depend on the smallest number of factors, they are therefore the most fundamental characteristics of equilibrium. Therefore, it is thermodynamic constants that are given in reference books. The thermodynamic constants of some weak electrolytes are given in the appendix of this manual. =0.024 mol/l.

As the charge of the ion increases, the activity coefficient and activity of the ion decreases.

Questions for self-control:

What is an ideal system? Name the main reasons for the deviation of a real system from an ideal one.
What is the degree of dissociation of electrolytes called?
Give examples of strong and weak electrolytes.
What relationship exists between the dissociation constant and the degree of dissociation of a weak electrolyte? Express it mathematically.
What is activity? How are the activity of an ion and its true concentration related?
What is the activity coefficient?
How does the charge of an ion affect the activity coefficient?
What is the ionic strength of a solution, its mathematical expression?
Write down formulas for calculating the activity coefficients of individual ions depending on the ionic strength of the solution.
Formulate the law of mass action and express it mathematically.
What is the thermodynamic equilibrium constant? What factors influence its value?
What is the concentration equilibrium constant? What factors influence its value?
How are thermodynamic and concentration equilibrium constants related?
Within what limits can the activity coefficient values change?
What are the main principles of the theory of strong electrolytes?

Salts, their properties, hydrolysis

8th grade student B of school No. 182

Petrova Polina

Chemistry teacher:

Kharina Ekaterina Alekseevna

MOSCOW 2009

In everyday life, we are accustomed to dealing with only one salt - table salt, i.e. sodium chloride NaCl. However, in chemistry, a whole class of compounds is called salts. Salts can be considered as products of the replacement of hydrogen in an acid with a metal. Table salt, for example, can be obtained from hydrochloric acid by a substitution reaction:

2Na + 2HCl = 2NaCl + H2.

acid salt

If you take aluminum instead of sodium, another salt is formed - aluminum chloride:

2Al + 6HCl = 2AlCl3 + 3H2

Salts- These are complex substances consisting of metal atoms and acidic residues. They are the products of complete or partial replacement of hydrogen in an acid with a metal or a hydroxyl group in a base with an acid residue. For example, if in sulfuric acid H 2 SO 4 we replace one hydrogen atom with potassium, we get the salt KHSO 4, and if two - K 2 SO 4.

There are several types of salts.

Types of salts	Definition	Examples of salts
Average	The product of complete replacement of acid hydrogen with metal. They contain neither H atoms nor OH groups.	Na 2 SO 4 sodium sulfate CuCl 2 copper (II) chloride Ca 3 (PO 4) 2 calcium phosphate Na 2 CO 3 sodium carbonate (soda ash)
Sour	A product of incomplete replacement of acid hydrogen by metal. Contain hydrogen atoms. (They are formed only by polybasic acids)	CaHPO 4 calcium hydrogen phosphate Ca(H 2 PO 4) 2 calcium dihydrogen phosphate NaHCO 3 sodium bicarbonate (baking soda)
Basic	The product of incomplete replacement of the hydroxyl groups of a base with an acidic residue. Includes OH groups. (Formed only by polyacid bases)	Cu(OH)Cl copper (II) hydroxychloride Ca 5 (PO 4) 3 (OH) calcium hydroxyphosphate (CuOH) 2 CO 3 copper (II) hydroxycarbonate (malachite)
Mixed	Salts of two acids	Ca(OCl)Cl – bleach
Double	Salts of two metals	K 2 NaPO 4 – dipotassium sodium orthophosphate
Crystalline hydrates	Contains water of crystallization. When heated, they dehydrate - they lose water, turning into anhydrous salt.	CuSO4. 5H 2 O – pentahydrate copper(II) sulfate (copper sulfate) Na 2 CO 3. 10H 2 O – sodium carbonate decahydrate (soda)

Methods for obtaining salts.

1. Salts can be obtained by acting with acids on metals, basic oxides and bases:

Zn + 2HCl ZnCl 2 + H 2

zinc chloride

3H 2 SO 4 + Fe 2 O 3 Fe 2 (SO 4) 3 + 3H 2 O

iron(III) sulfate

3HNO 3 + Cr(OH) 3 Cr(NO 3) 3 + 3H 2 O

chromium(III) nitrate

2. Salts are formed by the reaction of acidic oxides with alkalis, as well as acidic oxides with basic oxides:

N 2 O 5 + Ca(OH) 2 Ca(NO 3) 2 + H 2 O

calcium nitrate

SiO 2 + CaO CaSiO 3

calcium silicate

3. Salts can be obtained by reacting salts with acids, alkalis, metals, non-volatile acid oxides and other salts. Such reactions occur under the conditions of evolution of gas, precipitation of a precipitate, evolution of an oxide of a weaker acid, or evolution of a volatile oxide.

Ca 3 (PO4) 2 + 3H 2 SO 4 3CaSO 4 + 2H 3 PO 4

calcium orthophosphate calcium sulfate

Fe 2 (SO 4) 3 + 6NaOH 2Fe(OH) 3 + 3Na 2 SO 4

iron (III) sulfate sodium sulfate

CuSO 4 + Fe FeSO 4 + Cu

copper (II) sulfate iron (II) sulfate

CaCO 3 + SiO 2 CaSiO 3 + CO 2

calcium carbonate calcium silicate

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

sulfate chloride sulfate chloride

aluminum barium barium aluminum

4. Salts of oxygen-free acids are formed by the interaction of metals with non-metals:

2Fe + 3Cl 2 2FeCl 3

iron(III) chloride

Physical properties.

Salts are solids of various colors. Their solubility in water varies. All salts of nitric and acetic acids, as well as sodium and potassium salts, are soluble. The solubility of other salts in water can be found in the solubility table.

Chemical properties.

1) Salts react with metals.

Since these reactions occur in aqueous solutions, Li, Na, K, Ca, Ba and other active metals that react with water under normal conditions cannot be used for experiments, or reactions cannot be carried out in a melt.

CuSO 4 + Zn ZnSO 4 + Cu

Pb(NO 3) 2 + Zn Zn(NO 3) 2 + Pb

2) Salts react with acids. These reactions occur when a stronger acid displaces a weaker one, releasing gas or precipitating.

When carrying out these reactions, they usually take dry salt and act with concentrated acid.

BaCl 2 + H 2 SO 4 BaSO 4 + 2HCl

Na 2 SiO 3 + 2HCl 2NaCl + H 2 SiO 3

3) Salts react with alkalis in aqueous solutions.

This is a method of obtaining insoluble bases and alkalis.

FeCl 3 (p-p) + 3NaOH(p-p) Fe(OH) 3 + 3NaCl

CuSO 4 (p-p) + 2NaOH (p-p) Na 2 SO 4 + Cu(OH) 2

Na 2 SO 4 + Ba(OH) 2 BaSO 4 + 2NaOH

4) Salts react with salts.

The reactions take place in solutions and are used to obtain practically insoluble salts.

AgNO 3 + KBr AgBr + KNO 3

CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl

5) Some salts decompose when heated.

A typical example of such a reaction is the firing of limestone, the main component of which is calcium carbonate:

CaCO 3 CaO + CO2 calcium carbonate

1. Some salts are capable of crystallizing to form crystalline hydrates.

Copper (II) sulfate CuSO 4 is a white crystalline substance. When it is dissolved in water, it heats up and a blue solution is formed. The release of heat and color changes are signs of a chemical reaction. When the solution is evaporated, crystalline hydrate CuSO 4 is released. 5H 2 O (copper sulfate). The formation of this substance indicates that copper (II) sulfate reacts with water:

CuSO 4 + 5H 2 O CuSO 4 . 5H 2 O + Q

white blue-blue

Use of salts.

Most salts are widely used in industry and in everyday life. For example, sodium chloride NaCl, or table salt, is indispensable in cooking. In industry, sodium chloride is used to produce sodium hydroxide, soda NaHCO 3, chlorine, sodium. Salts of nitric and orthophosphoric acids are mainly mineral fertilizers. For example, potassium nitrate KNO 3 is potassium nitrate. It is also part of gunpowder and other pyrotechnic mixtures. Salts are used to obtain metals, acids, and in glass production. Many plant protection products from diseases, pests, and some medicinal substances also belong to the class of salts. Potassium permanganate KMnO 4 is often called potassium permanganate. Limestone and gypsum – CaSO 4 – are used as building materials. 2H 2 O, which is also used in medicine.

Solutions and solubility.

As stated earlier, solubility is an important property of salts. Solubility is the ability of a substance to form with another substance a homogeneous, stable system of variable composition, consisting of two or more components.

Solutions- These are homogeneous systems consisting of solvent molecules and solute particles.

So, for example, a solution of table salt consists of a solvent - water, a dissolved substance - Na +, Cl - ions.

Ions(from Greek ión - going), electrically charged particles formed by the loss or gain of electrons (or other charged particles) by atoms or groups of atoms. The concept and term “ion” was introduced in 1834 by M. Faraday, who, while studying the effect of electric current on aqueous solutions of acids, alkalis and salts, suggested that the electrical conductivity of such solutions is due to the movement of ions. Faraday called positively charged ions moving in solution towards the negative pole (cathode) cations, and negatively charged ions moving towards the positive pole (anode) - anions.

Based on the degree of solubility in water, substances are divided into three groups:

1) Highly soluble;

2) Slightly soluble;

3) Practically insoluble.

Many salts are highly soluble in water. When deciding the solubility of other salts in water, you will have to use the solubility table.

It is well known that some substances, when dissolved or molten, conduct electric current, while others do not conduct current under the same conditions.

Substances that disintegrate into ions in solutions or melts and therefore conduct electric current are called electrolytes.

Substances that, under the same conditions, do not disintegrate into ions and do not conduct electric current are called non-electrolytes.

Electrolytes include acids, bases and almost all salts. Electrolytes themselves do not conduct electricity. In solutions and melts, they break up into ions, which is why current flows.

The breakdown of electrolytes into ions when dissolved in water is called electrolytic dissociation. Its content boils down to the following three provisions:

1) Electrolytes, when dissolved in water, break up (dissociate) into ions - positive and negative.

2) Under the influence of an electric current, ions acquire directional movement: positively charged ions move towards the cathode and are called cations, and negatively charged ions move towards the anode and are called anions.

3) Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.

reversibility

Strong and weak electrolytes.

To quantitatively characterize the ability of an electrolyte to disintegrate into ions, the concept of the degree of dissociation (α), t . E. The ratio of the number of molecules disintegrated into ions to the total number of molecules. For example, α = 1 indicates that the electrolyte has completely disintegrated into ions, and α = 0.2 means that only every fifth of its molecules has dissociated. When a concentrated solution is diluted, as well as when heated, its electrical conductivity increases, as the degree of dissociation increases.

Depending on the value of α, electrolytes are conventionally divided into strong (dissociate almost completely, (α 0.95)) medium strength (0.95

Strong electrolytes are many mineral acids (HCl, HBr, HI, H 2 SO 4, HNO 3, etc.), alkalis (NaOH, KOH, Ca(OH) 2, etc.), and almost all salts. Weak ones include solutions of some mineral acids (H 2 S, H 2 SO 3, H 2 CO 3, HCN, HClO), many organic acids (for example, acetic acid CH 3 COOH), an aqueous solution of ammonia (NH 3. 2 O), water, some mercury salts (HgCl 2). Electrolytes of medium strength often include hydrofluoric HF, orthophosphoric H 3 PO 4 and nitrous HNO 2 acids.

Hydrolysis of salts.

The term "hydrolysis" comes from the Greek words hidor (water) and lysis (decomposition). Hydrolysis is usually understood as an exchange reaction between a substance and water. Hydrolytic processes are extremely common in the nature around us (both living and nonliving), and are also widely used by humans in modern production and household technologies.

Salt hydrolysis is the reaction of interaction between the ions that make up the salt and water, which leads to the formation of a weak electrolyte and is accompanied by a change in the solution environment.

Three types of salts undergo hydrolysis:

a) salts formed by a weak base and a strong acid (CuCl 2, NH 4 Cl, Fe 2 (SO 4) 3 - hydrolysis of the cation occurs)

NH 4 + + H 2 O NH 3 + H 3 O +

NH 4 Cl + H 2 O NH 3 . H2O + HCl

The reaction of the medium is acidic.

b) salts formed by a strong base and a weak acid (K 2 CO 3, Na 2 S - hydrolysis occurs at the anion)

SiO 3 2- + 2H 2 O H 2 SiO 3 + 2OH -

K 2 SiO 3 +2H 2 O H 2 SiO 3 +2KOH

The reaction of the medium is alkaline.

c) salts formed by a weak base and a weak acid (NH 4) 2 CO 3, Fe 2 (CO 3) 3 - hydrolysis occurs at the cation and at the anion.

2NH 4 + + CO 3 2- + 2H 2 O 2NH 3. H2O + H2CO3

(NH 4) 2 CO 3 + H 2 O 2NH 3. H2O + H2CO3

Often the reaction of the environment is neutral.

d) salts formed by a strong base and a strong acid (NaCl, Ba(NO 3) 2) are not subject to hydrolysis.

In some cases, hydrolysis proceeds irreversibly (as they say, it goes to the end). So, when mixing solutions of sodium carbonate and copper sulfate, a blue precipitate of hydrated basic salt precipitates, which, when heated, loses part of the water of crystallization and acquires a green color - it turns into anhydrous basic copper carbonate - malachite:

2CuSO 4 + 2Na 2 CO 3 + H 2 O (CuOH) 2 CO 3 + 2Na 2 SO 4 + CO 2

When mixing solutions of sodium sulfide and aluminum chloride, hydrolysis also proceeds to completion:

2AlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl

Therefore, Al 2 S 3 cannot be isolated from an aqueous solution. This salt is obtained from simple substances.

Strong and weak electrolytes

In solutions of some electrolytes, only a portion of the molecules dissociate. To quantitatively characterize the strength of the electrolyte, the concept of the degree of dissociation was introduced. The ratio of the number of molecules dissociated into ions to the total number of molecules of the solute is called the degree of dissociation a.

where C is the concentration of dissociated molecules, mol/l;

C 0 is the initial concentration of the solution, mol/l.

According to the degree of dissociation, all electrolytes are divided into strong and weak. Strong electrolytes include those whose degree of dissociation is more than 30% (a > 0.3). These include:

· strong acids (H 2 SO 4, HNO 3, HCl, HBr, HI);

· soluble hydroxides, except NH 4 OH;

· soluble salts.

Electrolytic dissociation of strong electrolytes is irreversible

HNO 3 ® H + + NO - 3 .

Weak electrolytes have a degree of dissociation less than 2% (a< 0,02). К ним относятся:

· weak inorganic acids (H 2 CO 3, H 2 S, HNO 2, HCN, H 2 SiO 3, etc.) and all organic ones, for example, acetic acid (CH 3 COOH);

· insoluble hydroxides, as well as soluble hydroxide NH 4 OH;

· insoluble salts.

Electrolytes with intermediate values of the degree of dissociation are called electrolytes of medium strength.

The degree of dissociation (a) depends on the following factors:

on the nature of the electrolyte, that is, on the type of chemical bonds; dissociation most easily occurs at the site of the most polar bonds;

from the nature of the solvent - the more polar the latter, the easier the dissociation process occurs in it;

from temperature - increasing temperature enhances dissociation;

on the concentration of the solution - when the solution is diluted, the dissociation also increases.

As an example of the dependence of the degree of dissociation on the nature of chemical bonds, consider the dissociation of sodium hydrogen sulfate (NaHSO 4), the molecule of which contains the following types of bonds: 1-ionic; 2 - polar covalent; 3 - the bond between the sulfur and oxygen atoms is low-polar. Breaking occurs most easily at the site of the ionic bond (1):

Na 1 O 3 O S 3 H 2 O O

1. NaHSO 4 ® Na + + HSO - 4, 2. then at the site of a polar bond of a lesser degree: HSO - 4 ® H + + SO 2 - 4. 3. The acid residue does not dissociate into ions.

The degree of electrolyte dissociation strongly depends on the nature of the solvent. For example, HCl dissociates strongly in water, less strongly in ethanol C 2 H 5 OH, and almost does not dissociate in benzene, in which it practically does not conduct electric current. Solvents with high dielectric constant (e) polarize the solute molecules and form solvated (hydrated) ions with them. At 25 0 C e(H 2 O) = 78.5, e(C 2 H 5 OH) = 24.2, e(C 6 H 6) = 2.27.

In solutions of weak electrolytes, the dissociation process occurs reversibly and, therefore, the laws of chemical equilibrium apply to the equilibrium in solution between molecules and ions. So, for the dissociation of acetic acid

CH 3 COOH « CH 3 COO - + H + .

The equilibrium constant Kc will be determined as

K c = K d = CCH 3 COO - · C H + / CCH 3 COOH.

The equilibrium constant (K c) for the dissociation process is called the dissociation constant (K d). Its value depends on the nature of the electrolyte, solvent and temperature, but it does not depend on the concentration of the electrolyte in the solution. The dissociation constant is an important characteristic of weak electrolytes, since it indicates the strength of their molecules in solution. The smaller the dissociation constant, the weaker the electrolyte dissociates and the more stable its molecules. Considering that the degree of dissociation, in contrast to the dissociation constant, changes with the concentration of the solution, it is necessary to find the relationship between K d and a. If the initial concentration of the solution is taken to be equal to C, and the degree of dissociation corresponding to this concentration is a, then the number of dissociated molecules of acetic acid will be equal to a · C. Since

CCH 3 COO - = C H + = a C,

then the concentration of undissolved molecules of acetic acid will be equal to (C - a · C) or C(1- a · C). From here

K d = aС · a С /(С - a · С) = a 2 С / (1- a). (1)

Equation (1) expresses Ostwald's dilution law. For very weak electrolytes a<<1, то приближенно К @ a 2 С и

a = (K/C). (2)

As can be seen from formula (2), with a decrease in the concentration of the electrolyte solution (when diluted), the degree of dissociation increases.

Weak electrolytes dissociate in stages, for example:

1st stage H 2 CO 3 « H + + HCO - 3,

Stage 2 HCO - 3 « H + + CO 2 - 3 .

Such electrolytes are characterized by several constants, depending on the number of stages of decomposition into ions. For carbonic acid

K 1 = CH + CHCO - 2 / CH 2 CO 3 = 4.45 × 10 -7; K 2 = CH + · CCO 2- 3 / CHCO - 3 = 4.7 × 10 -11.

As can be seen, the decomposition into carbonic acid ions is determined mainly by the first stage, and the second can only appear when the solution is highly diluted.

The total equilibrium of H 2 CO 3 « 2H + + CO 2 - 3 corresponds to the total dissociation constant

K d = C 2 n + · CCO 2- 3 / CH 2 CO 3.

The quantities K 1 and K 2 are related to each other by the relation

K d = K 1 · K 2.

The bases of polyvalent metals dissociate in a similar stepwise manner. For example, two stages of dissociation of copper hydroxide

Cu(OH) 2 « CuOH + + OH - ,

CuOH + « Cu 2+ + OH -

correspond to the dissociation constants

K 1 = СCuOH + · СОН - / СCu(OH) 2 and К 2 = Сcu 2+ · СОН - / СCuOH + .

Since strong electrolytes are completely dissociated in solution, the very term dissociation constant for them has no meaning.

Dissociation of different classes of electrolytes

From the point of view of the theory of electrolytic dissociation acid is a substance whose dissociation produces only the hydrated hydrogen ion H3O (or simply H+) as a cation.

The basis is a substance that, in an aqueous solution, forms hydroxide ions OH - and no other anions - as an anion.

According to Brønsted theory, an acid is a proton donor and a base is a proton acceptor.

The strength of bases, like the strength of acids, depends on the value of the dissociation constant. The larger the dissociation constant, the stronger the electrolyte.

There are hydroxides that can interact and form salts not only with acids, but also with bases. Such hydroxides are called amphoteric. These include Be(OH) 2 , Zn(OH) 2 , Sn(OH) 2 , Pb(OH) 2 , Cr(OH) 3 , Al(OH) 3. Their properties are due to the fact that they weakly dissociate as acids and as bases

H + + RO - « ROH « R + + OH -.

This equilibrium is explained by the fact that the bond strength between the metal and oxygen differs slightly from the bond strength between oxygen and hydrogen. Therefore, when beryllium hydroxide reacts with hydrochloric acid, beryllium chloride is obtained

Be(OH) 2 + HCl = BeCl 2 + 2H 2 O,

and when interacting with sodium hydroxide - sodium beryllate

Be(OH) 2 + 2NaOH = Na 2 BeO 2 + 2H 2 O.

Salts can be defined as electrolytes that dissociate in solution to form cations other than hydrogen cations and anions other than hydroxide ions.

Medium salts, obtained by completely replacing the hydrogen ions of the corresponding acids with metal cations (or NH + 4), dissociate completely Na 2 SO 4 « 2Na + + SO 2- 4.

Acid salts dissociate step by step

1 stage NaHSO 4 « Na + + HSO - 4 ,

2nd stage HSO - 4 « H + + SO 2- 4 .

The degree of dissociation in the 1st step is greater than in the 2nd step, and the weaker the acid, the lower the degree of dissociation in the 2nd step.

Basic salts obtained by incomplete replacement of hydroxide ions with acid residues, also dissociate in stages:

1st stage (CuОH) 2 SO 4 « 2 CuОH + + SO 2- 4,

Stage 2 CuОH + « Cu 2+ + OH - .

Basic salts of weak bases dissociate mainly in the 1st step.

Complex salts, containing a complex complex ion that retains its stability upon dissolution, dissociate into a complex ion and outer sphere ions

K 3 « 3K + + 3 - ,

SO 4 « 2+ + SO 2 - 4 .

At the center of the complex ion is a complexing atom. This role is usually performed by metal ions. Polar molecules or ions, and sometimes both together, are located (coordinated) near the complexing agents; they are called ligands. The complexing agent together with the ligands constitutes the inner sphere of the complex. Ions located far from the complexing agent, less tightly bound to it, are located in the external environment of the complex compound. The inner sphere is usually enclosed in square brackets. The number indicating the number of ligands in the inner sphere is called coordination. Chemical bonds between complex and simple ions are relatively easily broken during the process of electrolytic dissociation. Bonds leading to the formation of complex ions are called donor-acceptor bonds.

Outer sphere ions are easily split off from the complex ion. This dissociation is called primary. Reversible disintegration of the inner sphere is much more difficult and is called secondary dissociation

Cl « + + Cl - - primary dissociation,

+ « Ag + +2 NH 3 - secondary dissociation.

secondary dissociation, like the dissociation of a weak electrolyte, is characterized by an instability constant

K nest. = × 2 / [ + ] = 6.8 × 10 -8 .

The instability constants (K inst.) of various electrolytes is a measure of the stability of the complex. The less K nest. , the more stable the complex.

So, among similar compounds:

-	+	+	+
K nest = 1.3×10 -3	K nest =6.8×10 -8	K nest =1×10 -13	K nest =1×10 -21

The stability of the complex increases upon transition from - to +.

The values of the instability constant are given in chemistry reference books. Using these values, it is possible to predict the course of reactions between complex compounds, with a strong difference in instability constants, the reaction will go towards the formation of a complex with a lower instability constant.

A complex salt with a low-stable complex ion is called double salt. Double salts, unlike complex salts, dissociate into all the ions included in their composition. For example:

KAl(SO 4) 2 « K + + Al 3+ + 2SO 2- 4,

NH 4 Fe(SO 4) 2 « NH 4 + + Fe 3+ + 2SO 2- 4.

Instructions

The essence of this theory is that when melted (dissolved in water), almost all electrolytes are decomposed into ions that are both positively and negatively charged (which is called electrolytic dissociation). Under the influence of electric current, negative ones ("-") move towards the anode (+), and positively charged ones (cations, "+") move towards the cathode (-). Electrolytic dissociation is a reversible process (the reverse process is called “molarization”).

The degree of (a) electrolytic dissociation depends on the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that broke up into ions to the total number of molecules introduced into the solution (N). You get: a = n / N

Thus, strong electrolytes are substances that completely disintegrate into ions when dissolved in water. Strong electrolytes are usually substances with highly polar or bonds: these are salts that are highly soluble (HCl, HI, HBr, HClO4, HNO3, H2SO4), as well as strong bases (KOH, NaOH, RbOH, Ba(OH)2 , CsOH, Sr(OH)2, LiOH, Ca(OH)2). In a strong electrolyte, the substance dissolved in it is mostly in the form of ions ( ); There are practically no molecules that are undissociated.

Weak electrolytes are substances that dissociate into ions only partially. Weak electrolytes, together with ions in solution, contain undissociated molecules. Weak electrolytes do not produce a strong concentration of ions in solution.

The weak ones include:
- organic acids (almost all) (C2H5COOH, CH3COOH, etc.);
- some of the acids (H2S, H2CO3, etc.);
- almost all salts that are slightly soluble in water, ammonium hydroxide, as well as all bases (Ca3(PO4)2; Cu(OH)2; Al(OH)3; NH4OH);
- water.

They practically do not conduct electric current, or conduct, but poorly.

note

Although pure water conducts electricity very poorly, it does have measurable electrical conductivity due to the fact that water dissociates slightly into hydroxide and hydrogen ions.

Helpful advice

Most electrolytes are aggressive substances, so when working with them, be extremely careful and follow safety regulations.

A strong base is an inorganic chemical compound formed by the hydroxyl group -OH and an alkaline (elements of group I of the periodic table: Li, K, Na, RB, Cs) or alkaline earth metal (elements of group II Ba, Ca). Written in the form of the formulas LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH) ₂, Ba(OH) ₂.

You will need

evaporation cup
burner
indicators
metal rod
N₃PO₄

Instructions

Strong reasons are manifested, characteristic of all. The presence in the solution is determined by the change in color of the indicator. Add phenolphthalein to the sample with the test solution or omit the litmus paper. Methyl orange produces a yellow color, phenolphthalein produces a purple color, and litmus paper turns blue. The stronger the base, the more intense the color of the indicator.

If you need to find out which alkalis are presented to you, then conduct a qualitative analysis of the solutions. The most common strong bases are lithium, potassium, sodium, barium and calcium. Bases react with acids (neutralization reactions) to form salt and water. In this case, Ca(OH) ₂, Ba(OH) ₂ and LiOH can be distinguished. When combined with acid, insoluble compounds are formed. The remaining hydroxides will not produce precipitation, because All K and Na salts are soluble.
3 Ca(OH) ₂ + 2 H₃PO₄ --→ Ca₃(PO₄)₂↓+ 6 H₂O

3 Ba(OH) ₂ +2 Н₃PO₄ --→ Ba₃(PO₄)₂↓+ 6 H₂О

3 LiOH + H₃PO₄ --→ Li₃PO₄↓ + 3 H₂O
Strain them and dry them. Add the dried sediment to the burner flame. By changing the color of the flame, lithium, calcium and barium ions can be qualitatively determined. Accordingly, you will determine which hydroxide is which. Lithium salts color the burner flame carmine red. Barium salts are green, and calcium salts are crimson.

The remaining alkalis form soluble orthophosphates.

3 NaOH + H₃PO₄--→ Na₃PO₄ + 3 H₂O

3 KOH + H₃PO₄--→ K₃PO₄ + 3 H₂O

It is necessary to evaporate the water to a dry residue. Place the evaporated salts on a metal rod one by one into the burner flame. There, sodium salt - the flame will turn bright yellow, and potassium - pink-violet. Thus, having a minimal set of equipment and reagents, you have identified all the strong reasons given to you.

An electrolyte is a substance that in its solid state is a dielectric, that is, it does not conduct electric current, but when dissolved or molten it becomes a conductor. Why does such a sharp change in properties occur? The fact is that electrolyte molecules in solutions or melts dissociate into positively charged and negatively charged ions, due to which these substances in such an aggregate state are capable of conducting electric current. Most salts, acids, and bases have electrolytic properties.

Instructions

What substances are considered strong? Such substances, in solutions or melts of which almost 100% of molecules are exposed, regardless of the concentration of the solution. The list includes the absolute majority of soluble alkalis, salts and some acids, such as hydrochloric, bromide, iodide, nitric, etc.

How do weak ones behave in solutions or melts? electrolytes? Firstly, they dissociate to a very small extent (no more than 3% of the total number of molecules), and secondly, their dissociation becomes worse and slower the higher the concentration of the solution. Such electrolytes include, for example, (ammonium hydroxide), most organic and inorganic acids (including hydrofluoric acid - HF) and, of course, familiar water to all of us. Since only a negligible fraction of its molecules breaks down into hydrogen ions and hydroxyl ions.

Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on factors: the nature of the electrolyte itself, the solvent, and temperature. Therefore, this division itself is to a certain extent arbitrary. After all, the same substance can, under different conditions, be both a strong electrolyte and a weak one. To assess the strength of the electrolyte, a special value was introduced - the dissociation constant, determined on the basis of the law of mass action. But it is applicable only to weak electrolytes; strong electrolytes do not obey the law of mass action.

Sources:

strong electrolytes list

Salts- these are chemical substances consisting of a cation, that is, a positively charged ion, a metal and a negatively charged anion - an acid residue. There are many types of salts: normal, acidic, basic, double, mixed, hydrated, complex. This depends on the cation and anion compositions. How can you determine base salt?