As anions, only oh ions are formed upon dissociation of naoh. Soluble in water – alkalis insoluble in water

Electrolyte - substance which conducts electricity due to dissociation on ions what's happening in solutions And melts, or the movement of ions in crystal lattices solid electrolytes. Examples of electrolytes include aqueous solutions acids, salts And reasons and some crystals(For example, silver iodide, zirconium dioxide). Electrolytes - conductors of the second kind, substances whose electrical conductivity is determined by the mobility of ions.

Based on the degree of dissociation, all electrolytes are divided into two groups

Strong electrolytes- electrolytes, the degree of dissociation of which in solutions is equal to unity (that is, they dissociate completely) and does not depend on the concentration of the solution. This includes the vast majority of salts, alkalis, as well as some acids (strong acids, such as: HCl, HBr, HI, HNO 3, H 2 SO 4).

Weak electrolytes- the degree of dissociation is less than unity (that is, they do not dissociate completely) and decreases with increasing concentration. These include water, a number of acids (weak acids such as HF), bases p-, d-, and f-elements.

There is no clear boundary between these two groups; the same substance can exhibit the properties of a strong electrolyte in one solvent, and a weak electrolyte in another.

Isotonic coefficient(Also van't Hoff factor; denoted by i) is a dimensionless parameter characterizing the behavior of a substance in solution. It is numerically equal to the ratio of the value of a certain colligative property of a solution of a given substance and the value of the same colligative property of a non-electrolyte of the same concentration, with other parameters of the system unchanged.

Basic principles of the theory of electrolytic dissociation

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

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

3. Directed movement occurs as a result of attraction by their oppositely charged electrodes (the cathode is negatively charged, and the anode is positively charged).

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

Based on the theory of electrolytic dissociation, the following definitions can be given for the main classes of compounds:

Acids are electrolytes whose dissociation produces only hydrogen ions as cations. For example,

HCl → H + + Cl - ; CH 3 COOH H + + CH 3 COO - .

The basicity of an acid is determined by the number of hydrogen cations that are formed during dissociation. Thus, HCl, HNO 3 are monobasic acids, H 2 SO 4, H 2 CO 3 are dibasic, H 3 PO 4, H 3 AsO 4 are tribasic.

Bases are electrolytes whose dissociation produces only hydroxide ions as anions. For example,

KOH → K + + OH - , NH 4 OH NH 4 + + OH - .

Bases soluble in water are called alkalis.

The acidity of a base is determined by the number of its hydroxyl groups. For example, KOH, NaOH are one-acid bases, Ca(OH) 2 is two-acid, Sn(OH) 4 is four-acid, etc.

Salts are electrolytes whose dissociation produces metal cations (as well as the NH 4 + ion) and anions of acidic residues. For example,

CaCl 2 → Ca 2+ + 2Cl - , NaF → Na + + F - .

Electrolytes, during the dissociation of which, depending on the conditions, can simultaneously form both hydrogen cations and anions - hydroxide ions are called amphoteric. For example,

H 2 OH + + OH - , Zn(OH) 2 Zn 2+ + 2OH - , Zn(OH) 2 2H + + ZnO 2 2- or Zn(OH) 2 + 2H 2 O 2- + 2H + .

Cation- positive charged and he. Characterized by the amount of positive electric charge: for example, NH 4 + is a singly charged cation, Ca 2+

Doubly charged cation. IN electric field cations move to negative electrode - cathode

Derived from the Greek καθιών “descending, going down.” Term introduced Michael Faraday V 1834.

Anion - atom, or molecule, electric charge which is negative, which is due to an excess electrons compared to the number of positive elementary charges. Thus, the anion is negatively charged and he. Anion charge discrete and is expressed in units of elementary negative electric charge; For example, Cl− is a singly charged anion, and the remainder sulfuric acid SO 4 2− is a doubly charged anion. Anions are present in solutions of most salts, acids And reasons, V gases, For example, H− , as well as in crystal lattices connections with ionic bond, for example, in crystals table salt, V ionic liquids and in melts many inorganic substances.

In the magical world of chemistry, any transformation is possible. For example, you can get a safe substance that is often used in everyday life from several dangerous ones. Such an interaction of elements, which results in a homogeneous system in which all reacting substances break down into molecules, atoms and ions, is called solubility. In order to understand the mechanism of interaction of substances, it is worth paying attention to solubility table.

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A table showing the degree of solubility is one of the aids for studying chemistry. Those who are learning science may not always remember how certain substances dissolve, so you should always have a table handy.

It helps in solving chemical equations that involve ionic reactions. If the result is an insoluble substance, then the reaction is possible. There are several options:

• The substance is highly soluble;
• Slightly soluble;
• Practically insoluble;
• Insoluble;
• Hydralizes and does not exist in contact with water;
• Does not exist.

Electrolytes

These are solutions or alloys that conduct electric current. Their electrical conductivity is explained by the mobility of ions. Electrolytes can be divided into 2 groups:

1. Strong. They dissolve completely, regardless of the degree of concentration of the solution.
2. Weak. Dissociation is partial and depends on concentration. Decreases at high concentrations.

During dissolution, electrolytes dissociate into ions with different charges: positive and negative. When exposed to current, positive ions are directed towards the cathode, while negative ions are directed towards the anode. The cathode is a positive charge, the anode is a negative charge. As a result, ion movement occurs.

Simultaneously with dissociation, the opposite process takes place - the combination of ions into molecules. Acids are electrolytes whose decomposition produces a cation - a hydrogen ion. Bases - anions - are hydroxide ions. Alkalis are bases that dissolve in water. Electrolytes that are capable of forming both cations and anions are called amphoteric.

Ions

This is a particle in which there are more protons or electrons, it will be called an anion or cation, depending on what is more: protons or electrons. As independent particles, they are found in many states of aggregation: gases, liquids, crystals and plasma. The concept and name were introduced into use by Michael Faraday in 1834. He studied the effect of electricity on solutions of acids, alkalis and salts.

Simple ions carry a nucleus and electrons. The nucleus makes up almost all of the atomic mass and is made up of protons and neutrons. The number of protons coincides with the atomic number in the periodic table and the charge of the nucleus. The ion has no definite boundaries due to the wave motion of electrons, so it is impossible to measure their sizes.

Removing an electron from an atom requires, in turn, energy expenditure. It's called ionization energy. When an electron is added, energy is released.

Cations

These are particles that carry a positive charge. They can have different amounts of charge, for example: Ca2+ is a doubly charged cation, Na+ is a singly charged cation. They migrate to the negative cathode in an electric field.

Anions

These are elements that have a negative charge. It also has different amounts of charge, for example, CL- is a singly charged ion, SO42- is a doubly charged ion. Such elements are found in substances that have an ionic crystal lattice, in table salt and many organic compounds.

• Sodium. Alkali metal. By giving up one electron located in the outer energy level, the atom will turn into a positive cation.
• Chlorine. An atom of this element takes one electron to the last energy level, it will turn into a negative chloride anion.
• Salt. The sodium atom gives an electron to chlorine, as a result of which in the crystal lattice the sodium cation is surrounded by six chlorine anions and vice versa. As a result of this reaction, a sodium cation and a chlorine anion are formed. Due to mutual attraction, sodium chloride is formed. A strong ionic bond is formed between them. Salts are crystalline compounds with ionic bonds.
• Acid residue. It is a negatively charged ion found in a complex inorganic compound. It is found in acid and salt formulas and usually appears after the cation. Almost all such residues have their own acid, for example, SO4 - from sulfuric acid. Acids of some residues do not exist and are written formally, but they form salts: phosphite ion.

Chemistry is a science where it is possible to create almost any miracle.

Acid-base properties of organic compounds, ionization. The role of ionization in the manifestation of biological activity

According to the theory of electrolytic dissociation by Arrhenius (1887), acids are substances that dissociate in aqueous solutions to form only hydrogen cations H + as cations, bases are substances whose dissociation produces only hydroxide anions OH - as anions. These definitions are valid for those reactions that occur in aqueous solutions. At the same time, a large number of reactions leading to the formation of salts were known, but the reactants were not acids and bases according to the Arrhenius theory. In 1923, two theories of acids and bases were proposed: the protolytic theory of Brønsted and Lowry, and the electron theory of Lewis.

According to the protolytic theory, acids – These are ions or molecules capable of donating a hydrogen cation, i.e. proton donor substances . Reasons – these are molecules or ions capable of attaching a hydrogen cation, i.e. substances that are proton acceptors or donors of a pair of electrons necessary for the addition of a proton. According to this theory, an acid and a base form a conjugate pair and are related by the equation: acid ↔ base + H +.

In protolytic theory, the concepts of acids and bases refer only to the function performed by a substance in a given reaction. The same substance, depending on the reaction partner, can act as both an acid and a base:

Typically, acidity is defined in relation to water as a base. Quantitative assessment of acidity (acid strength) is carried out by comparing the equilibrium constants of reactions involving proton transfer from acid to base.

The water concentration practically does not change, therefore, multiplying the right and left sides of this equality by [H 2 O], we obtain the following expression:

K a – acidity constant, The higher the value of the acidity constant, the stronger the acid. In practice, for convenience, they often use not the acidity constant, but the negative decimal logarithm of the acidity constant, called the acidity index pK a = – log K a. For acetic acid, the acidity constant K a = 1.75 · 10 -5, and the acidity index pK a = 4.75. The lower the pKa value, the stronger the acid. For stronger formic acid, these values ​​are equal, respectively: K a = 1.7 · 10 -4, pK a = 3.77.

A comparative analysis of the strength of acids (qualitative assessment) is carried out by comparing the stability of the conjugate bases (anions) corresponding to the acids. The more stable the anion (base) conjugate to an acid, the stronger the conjugate acid. The stability of anions depends on the degree of delocalization of the negative charge - the more delocalized the negative charge, the more stable the anion, the stronger the conjugate acid.

The degree of delocalization of the negative charge depends on the following factors:

from the nature of the atom of the acid center, i.e. on its electronegativity and radius (polarizability);

on the nature of the radical associated with it;

on the electronic structure of the anion;

4) from the influence of the solvent.

Influence of the nature of the acid center atom

Depending on the nature of the acid center, they are distinguished: OH-acids (alcohols, phenols, carboxylic acids), SH-acids (thiols), NH-acids (amides, amines), CH-acids (hydrocarbons). To consider the effect of the electronegativity of an acid center atom, let us take compounds in which the acid center atoms are connected to the same substituents: CH 4, NH 3, H 2 O. All atoms of the acid centers are located in the same period, electronegativity increases from carbon to oxygen, in the same direction there is an increase in the polarity of bonds and a decrease in the strength of bonds between the atoms of acid centers and the hydrogen atom. Thus, we can say that when moving from methane to water, the ability of compounds to eliminate the hydrogen cation increases, i.e. be proton donors. At the same time, in the series of emerging anions H 3 C - , H 2 N - , HO - their stability increases, since with an increase in the electronegativity of the atom of the acid center, its ability to retain a negative charge increases. In the series of compounds methane - ammonia - water, the acidic properties increase. When comparing the H 2 S molecule with these three molecules, it is necessary to take into account not only the electronegativity of the sulfur atom, but also the atomic radius of the sulfur and the polarizability of this atom. In terms of electronegativity, sulfur occupies an intermediate position between carbon and nitrogen. Based on the above reasoning, one would expect that the acidic properties of H 2 S would be more pronounced than that of methane, but weaker than that of ammonia. But the sulfur atom among the acid centers under consideration has the largest atomic radius (as an element of the third period), which determines the long bond length with the hydrogen atom and its lower strength. In addition, the atomic radius, larger than that of other acidic centers, provides greater polarizability of the sulfur atom, i.e., the ability of the HS anion to disperse electron density and negative charge over a larger volume, which increases the stability of this anion in comparison with those discussed above. Thus, these acids and their corresponding conjugate bases (anions) can be arranged in order of increasing acidic properties and increasing the stability of anions:

A similar picture is observed for compounds in which the acid center atom is bonded to the same organic radical:

CH acids exhibit the weakest acidic properties, although alkanes, alkenes and alkynes vary somewhat in acidity.

The increase in acidity in this series is due to an increase in the electronegativity of the carbon atom during the transition from sp 3 - to sp hybridization.

Effect of substituents associated with the acid center

Electron-withdrawing substituents increase acidity connections. By shifting the electron density towards themselves, they contribute to an increase in polarity and a decrease in the strength of the bond between the acid center atom and the hydrogen atom, and facilitate proton abstraction. A shift in electron density to the electron-withdrawing substituent leads to greater delocalization of the negative charge in the anion and an increase in its stability.

Electron-donating substituents reduce the acidity of compounds, since they shift the electron density away from themselves, which leads to the localization of a negative charge on the atom of the acid center in the anion and a decrease in its stability, an increase in its energy, which complicates its formation.

Influence of the electronic structure of anions

The degree of delocalization of the negative charge in the anion and its stability are strongly influenced by the presence of a conjugated system and the manifestation of the mesomeric effect. Delocalization of the negative charge along the conjugation system leads to stabilization of the anion, i.e., to an increase in the acidic properties of the molecules.

Molecules of carboxylic acids and phenol form more stable anions and exhibit stronger acidic properties than aliphatic alcohols and thiols, which do not exhibit a mesomeric effect.

Effect of solvent

The influence of the solvent on the manifestation of the acidic properties of the compound can be significant. For example, hydrochloric acid, which is a strong acid in an aqueous solution, exhibits practically no acidic properties in a benzene solution. Water, as an effective ionizing solvent, solvates the resulting ions, thereby stabilizing them. Benzene molecules, being nonpolar, cannot cause significant ionization of hydrogen chloride molecules and cannot stabilize the resulting ions through solvation.

In the protolytic theory of acids and bases, there are two types of bases - p-base and n-base(onium bases).

p-Bases are compounds that provide a pair of p-bond electrons to form a bond with a proton. These include alkenes, dienes, and aromatic compounds. They are very weak bases, since a pair of electrons is not free, but forms a p-bond, i.e. belongs to both atoms. For education s-bond with a proton first needs to break the p-bond, which requires energy.

n-Bases (onium bases) – These are molecules or ions that provide a lone pair of p electrons to form a bond with a proton. Based on the nature of the main center, they are divided into: ammonium bases, oxonium bases and sulfonium bases.

Ammonium bases – these are compounds in which the center of basicity is a nitrogen atom with a lone pair of p-electrons (amines, amides, nitriles, nitrogen-containing heterocycles, imines, etc.)

Oxonium bases– these are compounds in which the center of basicity is an oxygen atom with a lone pair of p-electrons (alcohols, ethers and esters, aldehydes, ketones, carboxylic acids, etc.)

Sulfonium bases – these are compounds in which the center of basicity is a sulfur atom with a lone pair of p-electrons (thioalcohols, thioethers, etc.).

The strength of base B in water can be estimated by considering the equilibrium:

For convenience, the basicity constant K B, as well as the acidity constant K a, is expressed by the value pK B, numerically equal to the negative decimal logarithm of the basicity constant. The greater the basicity constant KB and the smaller the pKB, the stronger the base.

To quantify the strength of bases, the acidity index pK a of the conjugate acid BH +, denoted pK BH +, is also used:

The lower the value of K VN + and the greater the value of pK VH +, the stronger the base. The values ​​of pK B in water can be converted to pK BH + , using the ratio: pK B + pK VN + = 14.

The strength of the bases depends on: 1) the nature of the atom of the main center - electronegativity and polarizability (from the radius of the atom); 2) from the electronic effects of substituents associated with the main center; 3) from the influence of the solvent.

Influence of the nature of the main center atom

As the electronegativity of the atom of the main center increases, the strength of the bases decreases, since the greater the electronegativity, the stronger the atom holds on to its lone pair of electrons, and thus makes it more difficult to provide it for the formation of a bond with a proton. Based on this, oxonium bases are weaker than ammonium bases, which contain identical substituents at the main center:

Sulfonium bases containing identical substituents at the main center exhibit even weaker basic properties. The sulfur atom, although less electronegative than oxygen and nitrogen atoms, has a larger atomic radius and is more polarizable, making it more difficult to provide a lone pair of outer shell electrons to form a bond with a proton.

Influence of substituents associated with the main center

Electron-donating substituents, shifting the electron density to the atom of the main center, facilitate the addition of a proton, thereby enhancing the basic properties. Electron-withdrawing substituents, shifting the electron density towards themselves, reduce it at the main center, thereby complicating the addition of a proton and weakening the basic properties:

Solvent influence:

Since an increase in base strength is associated with an increase in the ability to attach a proton and, consequently, with an increase in the partial negative charge on the main center, one can expect an increase in basicity in the series of ammonium bases NH 3< RNH 2 < R 2 NH < R 3 N в результате усиления индуктивного эффекта при последовательном увеличении числа алкильных групп. В действительности, однако, ряд аминов имеет следующие значения рК ВН + :

As would be expected, the introduction of an alkyl group into the ammonia molecule significantly increases the basicity of the compounds, with the ethyl group having a slightly greater effect than the methyl group. The introduction of a second alkyl group leads to a further increase in basicity, but the effect of its introduction is much less pronounced. The introduction of a third alkyl group leads to a noticeable decrease in basicity. This picture is explained by the fact that the basicity of the amine in water is determined not only by the magnitude of the negative charge arising on the nitrogen atom, but also by the ability of the cation formed after the addition of a proton to solvation, and, consequently, its stabilization. The more hydrogen atoms are bonded to a nitrogen atom, the stronger the solvation due to the occurrence of intermolecular hydrogen bonds and the more stable the cation becomes. In the given series of compounds, the basicity increases, but the stabilization of the cation as a result of hydration in the same direction decreases and reduces the manifestation of basicity. A similar change is not observed if basicity measurements are carried out in solvents in which there are no hydrogen bonds: the basicity of butylamines in chlorobenzene increases in the series: C 4 H 9 NH 2< (С 4 Н 9) 2 NH < (С 4 Н 9) 3 N.

Lecture No. 5

Competitive reactions of nucleophilic substitution and elimination at a saturated carbon atom

In nucleophilic substitution reactions, alcohols, thiols, amines, and halogen derivatives act as substrates, i.e. compounds whose molecules contain sp 3 -hybridized carbon atoms linked by a covalent polar bond to a more electronegative atom of the functional group. The nucleophilic particles in these reactions are anions and neutral molecules having an atom with one or more pairs of electrons.

Acids are complex compounds that, upon dissociation, form only hydrogen ions as cations.

Equilibrium in systems containing complex compounds. Stability of complex compounds.

The outer sphere with the complex ion is connected primarily by electrostatic forces (ionogenic). Therefore, in solutions, complex compounds easily undergo dissociation with the elimination of the outer sphere, similar to the dissociation of strong electrolytes. This dissociation is called primary dissociation complex connection.

From the point of view of electrolytic dissociation, complex compounds are divided into acids, bases and salts.

For example:

For example:

Salts are complex compounds that, when dissociated, do not form hydrogen ions and hydroxide ions.

For example:

Neutral complexes are nonelectrolytes and do not undergo primary dissociation.

In exchange reactions, complex ions move from one compound to another without changing their composition.

EXAMPLE 12. draw up molecular and ionic equations for the exchange reactions between copper (II) nitrate and a complex iron compound, as a result of which an insoluble complex salt is formed.

EXAMPLE 13. When lead (II) nitrate reacts with a complex compound, a precipitate of lead chloride precipitates. Write molecular and ionic equations for exchange reactions.

Ligands are connected to the complexing agent by a covalent bond, which is much stronger than an ionic bond. Therefore, the decomposition of the inner sphere of the complex compound is observed to an insignificant extent and is characteristic. The reversible disintegration of the inner sphere is called secondary dissociation of the complex compound.

For example, a complex base is a strong electrolyte and easily dissociates into a complex ion and hydroxide ions.

At the same time, using sensitive methods of analysis, it is possible to detect in the solution a very low concentration of ammonia ions and molecules, which are formed as a result of dissociation of the inner sphere and the establishment of equilibrium.

The dissociation of complex ions, as well as the dissociation of weak electrons, occurs to an insignificant extent and can be quantitatively characterized by the dissociation constant, which is usually called instability constant of a complex compound (TO nest.). The instability constant of a complex ion can be expressed as follows:

The dissociation of complex ions occurs in steps and each dissociation step is characterized by its own instability constant. When ions dissociate, the following equilibria are established:

In calculations, in most cases, the general instability constant of the complex ion is used, which is equal to the product of the step constants.

The relative stability of a complex ion is judged by the value of its instability constant. The smaller this value, the more stable the complex; the more, the more unstable. Thus, comparing the instability constants of complex ions of the same type.

we can conclude that the most stable of these ions is the latter, and the least stable is the first.

A comparison of the instability constants of complexes of the same type also makes it possible in some cases to determine the direction of the equilibrium shift.

Reasons: classification, properties based on the concepts of the theory of electrolytic dissociation. Practical use.

Bases are complex substances that contain metal atoms (or an ammonium group NH 4) connected to one or more hydroxyl groups (OH).

In general, bases can be represented by the formula: Me(OH)n.

From the point of view of the theory of electrolytic dissociation(TED), bases are electrolytes whose dissociation produces only hydroxide anions (OH –) as anions. For example, NaOH = Na + + OH – .

Classification. BASES

Soluble in water – alkalis insoluble in water

For example, for example,

NaOH – sodium hydroxide Cu(OH) 2 – copper (II) hydroxide

Ca(OH) 2 – calcium hydroxide Fe(OH) 3 – iron (III) hydroxide

NH 4 OH – ammonium hydroxide

Physical properties. Almost all bases are solids. They are soluble in water (alkali) and insoluble. Copper (II) hydroxide Cu(OH) 2 is blue, iron (III) hydroxide Fe(OH) 3 is brown, most others are white. Alkali solutions feel soapy to the touch.

Chemical properties.

Soluble bases - alkalis	Insoluble bases (most of them)
1. Change the color of the indicator: red litmus - blue, colorless phenolphthalein - crimson.	---–– Indicators are not affected.
2. React with acids (neutralization reaction). Base + acid = salt + water 2KOH + H 2 SO 4 = K 2 SO 4 + 2H 2 O In ionic form: 2K + + 2OH – +2H + + SO 4 2– = 2K + + SO 4 2– + 2H 2 O 2H + + 2OH – = 2H 2 O	1. React with acids: Cu(OH) 2 + H 2 SO 4 = CuSO 4 + 2H 2 O Base + acid = salt + water.
3. React with salt solutions: alkali + salt = new. alkali + new salt (condition: formation of precipitate ↓or gas). Ba(OH) 2 + Na 2 SO 4 = BaSO 4 ↓ + 2 NaOH In ionic form: Ba ​​2+ + 2OH – + 2Na + + SO 4 2– = BaSO 4 ↓ + 2Na + +2OH – Ba 2+ + SO 4 2– = BaSO 4 .↓	2. When heated, they decompose into oxide and water. Cu(OH) 2 = CuO + H 2 O Reactions with salt solutions are not typical.
4. React with acid oxides: alkali + acid oxide = salt + water 2NaOH + CO 2 = Na 2 CO 3 + H 2 O In ionic form: 2Na + + 2OH – + CO 2 = 2Na + + CO 3 2– + H 2 O 2OH – + CO 2 = CO 3 2– + H 2 O	Reactions with acid oxides are not typical.
5. React with fats to form soap.	They do not react with fats.

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