An important feature of liquid water is its ability to spontaneously dissociate according to the reaction:

H 2 O (l) « H + (aq) + OH - (aq)

This process is also called self-ionization or autoprotolysis. The resulting H + protons and OH - anions are surrounded by a certain number of polar water molecules, i.e. hydrated: H + ×nH 2 O; OH - ×mH 2 O. Primary hydration can be represented by a number of aqua complexes: H 3 O + ; H 5 O 2 + ; H 7 O 3 + ; H 9 O 4 + , among which H 9 O 4 + ions predominate (H + ×4H 2 O). The lifetime of all these ions in water is very short, because protons constantly migrate from some molecules

water to others. Typically, in the equations, for simplicity, only the cation of the composition H 3 O + (H + ×H 2 O), called the hydronium ion, is used.

The process of water dissociation, taking into account the hydration of the proton and the formation of the hydronium ion, can be written: 2H 2 O « H 3 O + + OH -

Water is a weak electrolyte, the degree of dissociation of which is

Since à C equal (H 2 O)" C out (H 2 O) or [H 2 O] equal ≈ [H 2 O] out

– the number of moles contained in one liter of water. C out (H 2 O) in a dilute solution remains constant. This circumstance allows us to include C equals (H 2 O) in the equilibrium constant.

Thus, the product of two constants gives a new constant, which is called ionic product of water. At a temperature of 298 K.

¾- The constancy of the ionic product of water means that in any aqueous solution: acidic, neutral or alkaline, there are always both types of ions (H + and OH -)

¾- In pure water, the concentrations of hydrogen and hydroxyl ions are equal and under normal conditions are:

K w 1/2 = 10 -7 mol/l.

¾- When acids are added, the concentration of [H + ] increases, i.e. the equilibrium shifts to the left, and the concentration of [OH - ] decreases, but Kw remains equal to 10 -14.

In an acidic environment > 10 -7 mol/l, and< 10 -7 моль/л

In an alkaline environment< 10 -7 моль/л, а >10 -7 mol/l

In practice, for convenience, they use pH value and hydroxyl index (pOH) of the medium.

This is the reversed decimal logarithm of the concentrations (activities) of hydrogen ions or hydroxyl ions in solution, respectively: pH = - log, pH = - log

In aqueous solutions pH + pH = 14.

Table No. 14.

K w depends on temperature (since water dissociation is an endothermic process)

K w (25 o C) = 10 -14 Þ pH = 7

K w (50 o C) = 5.47×10 -14 Þ pH = 6.63

pH measurement is used extremely widely. In biology and medicine, the pH value of biological fluids is used to determine pathologies. For example, normal blood serum pH is 7.4±0.05; saliva – 6.35..6.85; gastric juice – 0.9..1.1; tears – 7.4±0.1. In agriculture, pH characterizes the acidity of soils, the ecological state of natural waters, etc.



Acid-base indicators are chemical compounds that change color depending on the pH of the environment in which they are found. You've probably noticed how the color of tea changes when you put lemon in it - this is an example of the action of an acid-base indicator.

Indicators are usually weak organic acids or bases and can exist in solutions in two tautomeric forms:

HInd « H + + Ind - , where HInd is the acidic form (this is the form that predominates in acidic solutions); Ind is the main form (predominant in alkaline solutions).

The behavior of the indicator is similar to the behavior of a weak electrolyte in the presence of a stronger one with the same ion. The more, therefore, the equilibrium shifts towards the existence of the acidic form of HInd and vice versa (Le Chatelier's principle).

Experience clearly shows the possibility of using some indicators:

Table No. 15

Special devices - pH meters - allow you to measure pH with an accuracy of 0.01 in the range from 0 to 14. The determination is based on measuring the emf of a galvanic cell, one of the electrodes of which is, for example, glass.

The most accurate concentration of hydrogen ions can be determined by acid-base titration. Titration is the process of gradually adding small portions of a solution of known concentration (titrant) to the titrated solution, the concentration of which we want to determine.

Buffer solutions- these are systems whose pH changes relatively little when diluting or adding small amounts of acids or alkalis to them. Most often they are solutions containing:

a) a) Weak acid and its salt (CH 3 COOH + CH 3 COONa) - acetate buffer

c) Weak base and its salt (NH 4 OH + NH 4 Cl) - ammonium-ammonium buffer

c) Two acid salts with different K d (Na 2 HPO 4 + NaH 2 PO 4) - phosphate buffer

Let us consider the regulatory mechanism of buffer solutions using the example of an acetate buffer solution.

CH 3 COOH « CH 3 COO - + H + ,

CH 3 COONa « CH 3 COO - + Na +

1. 1) if you add a small amount of alkali to the buffer mixture:

CH 3 COOH + NaOH « CH 3 COONa + H 2 O,

NaOH is neutralized by acetic acid to form the weaker electrolyte H 2 O. Excess sodium acetate shifts the equilibrium towards the resulting acid.

2. 2) if you add a small amount of acid:

CH 3 COONa + HCl « CH 3 COOH + NaCl

Hydrogen cations H + bind CH3COO - ions

Let's find the concentration of hydrogen ions in the buffer acetate solution:

The equilibrium concentration of acetic acid is C ref,k (since it is a weak electrolyte), and [CH 3 COO -- ] = C salt (since salt is a strong electrolyte), then . Henderson-Hasselbalch equation:

Thus, the pH of buffer systems is determined by the ratio of salt and acid concentrations. When diluted, this ratio does not change and the pH of the buffer does not change when diluted; this distinguishes buffer systems from a pure electrolyte solution, for which Ostwald’s dilution law is valid.

There are two characteristics of buffer systems:

1.Buffer force. The absolute value of the buffer force depends on

the total concentration of the components of the buffer system, i.e. The higher the concentration of the buffer system, the more alkali (acid) is required for the same pH change.

2.Buffer capacity (B). Buffer capacity is the limit within which the buffering effect occurs. The buffer mixture maintains the pH constant only under the condition that the amount of strong acid or base added to the solution does not exceed a certain limit value - B. The buffer capacity is determined by the number of g/equiv of strong acid (base) that must be added to one liter of the buffer mixture to change pH value per unit, i.e. . Conclusion: Properties of buffer systems:

1. 1. depends little on dilution.

2. 2.The addition of strong acids (bases) changes little within the limits of buffer capacity B.

3. 3.Buffer capacity depends on the buffering force (the concentration of the components).

4. 4. The buffer exhibits maximum effect when the acid and salt are present in the solution in equivalent quantities:

With salt = With salt; = K d,k; pH = pK d,k (pH is determined by the value of K d).

Hydrolysis is the chemical reaction of water with salts. Hydrolysis of salts is reduced to the process of proton transfer. As a result of its occurrence, a certain excess of hydrogen or hydroxyl ions appears, imparting acidic or alkaline properties to the solution. Thus, hydrolysis is the reverse of the neutralization process.

Hydrolysis of salts includes 2 stages:

a) Electrolytic dissociation of salt with the formation of hydrated ions:. KCl à K + + Cl - K + + xH 2 O à K + × xH 2 O (donor-acceptor bond, donor is an O atom having 2 lone electron pairs,

acceptor – cations with vacant orbitals)

Cl - + yH 2 O « Cl - ×yH 2 O (hydrogen bond)

c) Hydrolysis by anion. Cl - + HOH à HCl + OH -

c) Hydrolysis by cation. K + + HOH à KOH +

All salts formed with the participation of weak

electrolytes:

1.Salt formed by an anion of weak acids and a cation of strong bases

CH 3 COONa + HOH « CH 3 COOH + NaOH

CH 3 COO - + НОН « CH 3 COН + OH - , рН > 7

Anions of weak acids act as bases in relation to water, a proton donor, which leads to an increase in the concentration of OH -, i.e. alkalization of the environment.

The depth of hydrolysis is determined by: degree of hydrolysis a g:

– concentration of salt subjected to hydrolysis

– concentration of the original salt

a g is small, for example, for a 0.1 mol solution of CH 3 COONa at 298 K it is equal to 10 -4.

During hydrolysis, an equilibrium is established in the system, characterized by K p

Therefore, the lower the dissociation constant, the higher the hydrolysis constant. The degree of hydrolysis is related to the hydrolysis constant by the equation:

With increasing dilution, i.e. By decreasing C0, the degree of hydrolysis increases.

2. 2.Salt formed by a cation of weak bases and an anion of strong acids

NH 4 Cl + HOH ↔ NH 4 OH +

NH 4 + + HOH ↔ NH 4 OH + H + , pH< 7

The protolytic equilibrium is shifted to the left, the weak base cation NH 4 + acts as an acid in relation to water, which leads to acidification of the environment. The hydrolysis constant is determined by the equation:

The equilibrium concentration of hydrogen ions can be calculated: [H + ] equals = a g × C 0 (initial salt concentration), where

The acidity of the medium depends on the initial concentration of salts of this type.

3. 3.Salt formed by an anion of weak acids and a cation of weak bases. Hydrolyzes both cation and anion

NH 4 CN + HOH à NH 4 OH + HCN

To determine the pH of the solution medium, compare KD,k and KD,bas

K D,k > K D,bas à slightly acidic medium

K D,k< К Д,осн à среда слабо щелочная

K D,k = K D,bas à neutral environment

Consequently, the degree of hydrolysis of this type of salts does not depend on their concentration in solution.

because and [OH - ] are determined by K D,k and K D,bas, then

The pH of the solution is also independent of the salt concentration in the solution.

Salts formed by a multiply charged anion and a singly charged cation (sulfides, carbonates, ammonium phosphates) are almost completely hydrolyzed in the first step, i.e. are in solution in the form of a mixture of the weak base NH 4 OH and its salt NH 4 HS, i.e. in the form of an ammonium buffer.

For salts formed by a multiply charged cation and a singly charged anion (acetates, formates of Al, Mg, Fe, Cu), hydrolysis increases with heating and leads to the formation of basic salts.

Hydrolysis of nitrates, hypochlorites, hypobromites Al, Mg, Fe, Cu proceeds completely and irreversibly, i.e. salts are not isolated from solutions.

Salts: ZnS, AlPO 4, FeCO 3 and others are slightly soluble in water, however, some of their ions take part in the hydrolysis process, this leads to a slight increase in their solubility.

Chromium and aluminum sulfides hydrolyze completely and irreversibly to form the corresponding hydroxides.

4. 4.Salts formed by the anion of strong acids and strong bases do not undergo hydrolysis.

Most often, hydrolysis is a harmful phenomenon that causes various complications. Thus, during the synthesis of inorganic substances from aqueous solutions, impurities appear in the resulting substance - products of its hydrolysis. Some compounds cannot be synthesized at all due to irreversible hydrolysis.

· - if hydrolysis proceeds through the anion, then excess alkali is added to the solution

· - if hydrolysis proceeds through the cation, then excess acid is added to the solution

So, the first qualitative theory of electrolyte solutions was expressed by Arrhenius (1883 - 1887). According to this theory:

1. 1.Electrolyte molecules dissociate into opposite ions

2. 2. Between the processes of dissociation and recombination, a dynamic equilibrium is established, which is characterized by KD. This equilibrium obeys the law of mass action. The proportion of disintegrated molecules is characterized by the degree of dissociation a. D and a are connected by Ostwald's law.

3. 3.An electrolyte solution (according to Arrhenius) is a mixture of electrolyte molecules, its ions and solvent molecules, between which there is no interaction.

Conclusion: Arrhenius’s theory made it possible to explain many properties of solutions of weak electrolytes at low concentrations.

However, Arrhenius’s theory was only of a physical nature, i.e. did not consider the following questions:

· For what reason do substances in solutions disintegrate into ions?

· What happens to ions in solutions?

Arrhenius' theory was further developed in the works of Ostwald, Pisarzhevsky, Kablukov, Nernst, etc. For example, the importance of hydration was first pointed out by Kablukov (1891), initiating the development of the theory of electrolytes in the direction indicated by Mendeleev (i.e., he was the first to combine Mendeleev’s solvation theory with the physical theory of Arrhenius). Solvation is the process of electrolyte interaction

solvent molecules to form solvate complex compounds. If the solvent is water, therefore, the process of interaction of the electrolyte with water molecules is called hydration, and aqua complexes are called crystalline hydrates.

Let us consider an example of the dissociation of electrolytes in a crystalline state. This process can be represented in two stages:

1. 1.destruction of the crystal lattice of the substance DН 0 cr > 0, the process of formation of molecules (endothermic)

2. 2.formation of solvated molecules, DН 0 solv< 0, процесс экзотермический

The resulting heat of dissolution is equal to the sum of the heats of the two stages DH 0 dist = DH 0 cr + DH 0 solv and can be either negative or positive. For example, the energy of the KCl crystal lattice = 170 kcal/mol.

The heat of hydration of K + ions = 81 kcal/mol, Cl - = 84 kcal/mol, and the resulting energy is 165 kcal/mol.

The heat of hydration partially covers the energy required to release ions from the crystal. The remaining 170 - 165 = 5 kcal/mol can be covered by the energy of thermal motion, and dissolution is accompanied by the absorption of heat from the environment. Hydrates or solvates facilitate the endothermic dissociation process, making recombination more difficult.

But here is the situation when only one of the two named stages is present:

1. dissolution of gases - there is no first stage of destruction of the crystal lattice, exothermic solvation remains, therefore the dissolution of gases is, as a rule, exothermic.

2. when crystalline hydrates are dissolved, there is no solvation stage, only endothermic destruction of the crystal lattice remains. For example, a crystalline hydrate solution: CuSO 4 × 5H 2 O (t) à CuSO 4 × 5H 2 O (r)

DН dist = DН cr = + 11.7 kJ/mol

Anhydrous salt solution: CuSO 4 (t) à CuSO 4 (r) à CuSO 4 × 5H 2 O (r)

DN dist = DN solv + DN cr = - 78.2 + 11.7 = - 66.5 kJ/mol

Conducts electricity very poorly, but still has some measurable electrical conductivity, co which is explained by a slight dissociation of water into hydrogen and hydroxyl ions:

H2O ⇄ H + OH’

Based on the electrical conductivity of pure water, the concentration of hydrogen ions and hydroxyl ions in water can be calculated. It turns out to be equal to 10 -7 G-ion /l.

Applying the law of mass action to the dissociation of water, we can write:

Let's rewrite this equation as follows:

[OH'] = [H 2 O]K

Since there is very little water, the concentration of undissociated H 2 O molecules not only in water, but also in any dilute aqueous solution can be considered a constant value. Therefore, replacing [H 2 O] K with the new constant KH 2 O, we will have:

[H] [OH’] = TO H2O

The resulting equation shows that for water and dilute aqueous solutions at a constant temperature, the product of the concentrations of hydrogen and hydroxyl ions is a constant value. This constant is called the ionic product of water. Its numerical value can be easily obtained by substituting the concentrations of hydrogen and hydroxyl ions into the last equation

TO H2O = 10 -7 10 -7 = 10 -14

Solutions in which the concentration of hydrogen and the concentration of hydroxyl ions are the same and equal to every 10 —7 g-ion/l are called neutral solutions. In acidic solutions the concentration of hydrogen ions is higher, in alkaline solutions the concentration of hydroxyl ions is higher. But whatever the reaction of the solution, the product of the concentrations of H and OH' ions must remain constant.

If, for example, enough acid is added to pure water so that the concentration of hydrogen ions increases to 10 -3, the concentration of hydroxyl ions will have to decrease so that the product [H] [OH'] remains equal to 10 -14. Therefore, in this solution the concentration of hydroxyl ions will be:

10 -14: 10 -3 = 10 -11

On the contrary, if you add alkali to water and thereby increase the concentration of hydroxyl ions, for example, to 10 -5, the concentration of hydrogen ions will become equal to:

10 -14: 10 -5 = 10 -9

You are reading an article on the topic Dissociation of water

An extremely important role in biological processes is played by water, which is an essential component (from 58 to 97%) of all cells and tissues of humans, animals, plants and simple organisms. this is the environment in in which a wide variety of biochemical processes occur.

Water has good dissolving ability and causes electrolytic dissociation of many substances dissolved in it.

The process of water dissociation according to the Brønsted theory proceeds according to the equation:

N 2 0+H 2 0 N 3 ABOUT + + HE - ; ΔН dis = +56.5 KJ/mol

Those. one water molecule donates, and the other adds a proton, autoionization of water occurs:

N 2 0 N + + HE - - deprotonation reaction

N 2 0 + N + N 3 ABOUT + - protonation reaction

The dissociation constant of water at 298°K, determined by the electrical conductivity method, is equal to:

a(H +) - activity of H + ions (for brevity, instead of H3O + write H +);

a(OH -) - activity of OH - ions;

a(H 2 0) - water activity;

The degree of dissociation of water is very small, so the activity of hydrogen and hydroxide ions in pure water is almost equal to their concentrations. The concentration of water is constant and equal to 55.6 mol.

(1000g: 18g/mol= 55.6 mol)

Substituting this value into the expression for the dissociation constant Kd(H 2 0), and instead of the activities of hydrogen and hydroxide ions, their concentrations, a new expression is obtained:

K(H 2 0) = C (H +) × C (OH -) = 10 -14 mol 2 / l 2 at 298 K,

More precisely, K(H 2 0) = a(H +) × a(OH -) = 10 -14 mol 2 l 2 -

K(H 2 0) is called ionic product of water or autoionization constant.

In pure water or any aqueous solution at a constant temperature, the product of the concentrations (activities) of hydrogen and hydroxide ions is a constant value, called the ionic product of water.

The constant K(H 2 0) depends on temperature. As the temperature rises, it increases, because The process of water dissociation is endothermic. In pure water or aqueous solutions of various substances at 298K activity (concentration), hydrogen and hydroxide ions will be:

a(H +)=a(OH -)=K(H 2 0) = 10 -14 =10 -7 mol/l.

In acidic or alkaline solutions, these concentrations will no longer be equal to each other, but will change conjugately: as one of them increases, the other will correspondingly decrease and vice versa, for example,

a(H +)=10 -4, a(OH -)=10 -10, their product is always 10 -14

pH value

Qualitatively, the reaction of the medium is expressed through the activity of hydrogen ions. In practice, they do not use this value, but the hydrogen indicator pH - a value numerically equal to the negative decimal logarithm of the activity (concentration) of hydrogen ions, expressed in mol/l.

pH= -lga(H + ),

and for dilute solutions

pH= -lgC(H + ).

For pure water and neutral media at 298K pH=7; for acidic pH solutions<7, а для щелочных рН>7.

The reaction of the medium can also be characterized by the hydroxyl index:

pOH= -lga(OH - )

or approximately

pOH= -IgC(OH - ).

Accordingly, in a neutral environment pH = pH = 7; in an acidic environment pOH>7, and in an alkaline environment pOH<7.

If we take the negative decimal logarithm of the expression for the ionic product of water, we get:

pH + pH = 14.

Therefore, pH and pOH are also conjugate quantities. Their sum for dilute aqueous solutions is always equal to 14. Knowing the pH, it is easy to calculate pOH:

pH=14 – pH

and vice versa:

rOH= 14 - pH.

Solutions are distinguished between active, potential (reserve) and total acidity.

Active acidity measured by the activity (concentration) of hydrogen ions in a solution and determines the pH of the solution. In solutions of strong acids and bases, pH depends on the concentration of the acid or base, and the activity of H ions + and HE - can be calculated using the formulas:

a(N + )= C(l/z acid)×α each; pH= - log a(H + )

a(OH - )=C(l/z base)×α each; pH= - log a(OH - )

pH= - logC(l/z acid) – for extremely dilute solutions of strong acids

pOH= - logC(l/z base) - for extremely dilute solutions of bases

Potential acidity measured by the number of hydrogen ions bound in acid molecules, i.e. represents a “reserve” of undissociated acid molecules.

Total acidity- the sum of active and potential acidity, which is determined by the analytical concentration of the acid and is established by titration

One of the amazing properties of living organisms is acid-base

homeostasis - constancy of pH of biological fluids, tissues and organisms. Table 1 presents the pH values ​​of some biological objects.

Table 1

From the table data it is clear that the pH of various fluids in the human body varies over a fairly wide range depending on location. BLOOD, like other biological fluids, it strives to maintain a constant pH value, the values ​​of which are presented in Table 2

Table 2

Changes in pH from the indicated values ​​by only 0.3 towards an increase or decrease leads to a change in the exchange of enzymatic processes, which causes a severe painful condition in humans. A pH change of just 0.4 is no longer compatible with life. Researchers have found that the following blood buffer systems are involved in the regulation of acid-base balance: hemoglobin, bicarbonate, protein and phosphate. The share of each system in the buffer capacity is presented in Table 3.

Table 3

All buffer systems of the body have the same mechanism of action, because They consist of a weak acid: carbonic, dihydrophosphoric (dihydrogen phosphate ion), protein, hemoglobin (oxohemoglobin) and salts of these acids, mainly sodium, which have the properties of weak bases. But since the bicarbonate system in the body has no equal in terms of speed of response, we will consider the ability to maintain a constant environment in the body using this system.

Dissociation of water. Hydrogen index.

IN Oda is a very weak electrolyte. (Electrolyte is a substance whose solution or melt conducts electric current). Water dissociates (breaks up) into its constituent ions:

H 2 O ↔ H + + OH -

Ionic product of water K W = [H + ] · [OH - ] = 10 -14 = const (the molar concentration of ions mol/l is conventionally indicated in square brackets). In practice, the hydrogen index is used to determine the environment. Hydrogen exponent negative decimal logarithm of the molar concentration of hydrogen ions: pH= - log [H + ] and is within 0<рН<14

Ions in solution

Wednesday

pH

[H + ] > [OH - ]

Sour

pH< 7

[H + ] = [OH - ] = 10 -7 mol/l

Neutral

pH = - log [ H + ] = - log 10 -7 = - (- 7) = 7

[OH - ] > [H + ]

Alkaline

pH > 7

Where

pH

Where

pH

stomach

Rain

5,5-6,5

intestines

8,5 - 9

Tap water

6-6,5-7

leather

5,5 -6

sea ​​water

8-8,5

blood

7,35-7,45

Soil

4-10

Hydrolysis

The interaction of salt ions with water ions, which changes the pH value, is called hydrolysis. This is a reversible reaction.

If the pH does not change when the salt is dissolved (pH = 7 remains), then hydrolysis does not occur.

The presence of a weak ion in the salt causes hydrolysis - it is the weak ion that attaches to itself the oppositely charged water ion, thereby forming new particle (with or without charge), and the remaining water ion organizes the medium: H+ - acidic, OH − - alkaline.

Strong electrolytes.

Strong acids

Strong grounds

HCl ↔ H + + Cl −

NaOH↔ Na + + OH −

H 2 SO 4 ↔ 2 H + + SO 4 2−

KOH↔ K + + OH −

HNO 3 ↔ H + + NO 3 −

If there is no particle in the table, then it will be weak particle (weak ion).

A salt is made up of a cation (positive ion) and an anion (negative ion):

Me + n K.O. -n (acid residue)

There are 4 possible salt combinations: 1. strong + and strong −

2. strong + and weak −

3. weak + and strong −

4.weak + and weak −

Let's consider reactions with the following ion variations:

1. NaCl + H 2 O there is no hydrolysis, since there is no weak particle in the salt, and the pH does not change (equal to 7)

strong+strong

the reaction is neutral and goes dissociation into ions: NaCl + H 2 O ↔ Na + + Cl − + H 2 O

2. hydrolysis of soda (technical)

Na 2 CO 3 + H 2 O ↔

Strong+weak

CO 3 2− + H + OH - ↔ H + CO 3 2− − + OH - alkaline environment, pH>7, you need to further write in molecular form

Na 2 CO 3 + H 2 O ↔ Na + H + CO 3 2− O + Na + OH - O

3. Hydrolysis of zinc sulfate

ZnSO 4 + H 2 O ↔

Weak+strong

Zn +2 + H + OH - ↔ Zn +2 OH - + + H + acidic environment, pH<7, нужно далее написать в молекулярном виде

2 ZnSO 4 +2 H 2 O ↔ (Zn +2 OH - ) + 2 SO 4 2- O + H 2 + SO 4 2- O

4. hydrolysis of aluminum carbonate proceeds to completion, since the salt is composed of two weak particles.

Al 2 (С O 3 ) 3 + 6H 2 O ↔ 2Al(OH) 3 + 3H 2 CO 3

Electrochemistry

If a metal plate is placed in a solution of its salt, then a double electric layer is formed at the boundary of the solid and liquid phases, the value of which is estimated by the value of the electrode potential φ. For many metals, electrode potentials are determined using a hydrogen electrode, the potential of which is assumed to be zero φ=0. The electrode potential data is presented in Table No. 3 of the Appendix in Method 4/23/2 “Work program and task for the test”.

Standard electrode potentials ( 0 )

some metals (range of voltages) at 298K.

Electrode half-reaction

Electrode half-reaction

Li + (aq.) + 1 e - = Li (sol.)

3.045

Cd 2+ (aq) + 2 e - = Cd (sol)

0.403

Rb + (aq) + 1 e - = Rb (sol)

2.925

Co 2+ (aq) + 2e - = Co (sol)

0.277

K + (aq.) + 1 e - = K (sol.)

2.924

Ni 2+ (aq) + 2 e - = Ni (sol)

0.250

Cs + (aq.) + 1 e - = Cs (sol.)

2.923

Sn 2+ (aq) + 2 e - = Sn (sol)

0.136

Ba 2+ (aq) + 2 e - = Ba (sol)

2.905

Pb 2+ (aq.) + 2 e - = Pb (sol.)

0.126

Ca 2+ (aq) + 2 e - = Ca (sol)

2.866

Fe 3+ (aq) + 3 e - = Fe (sol)

0.037

Na + (aq.) + e - = Na (sol.)

2.714

2 H + (aq) + 2 e - = H 2 (g)

0.000

Mg 2+ (aq.) + 2 e - = Mg (sol.)

2.363

Sb 3+ (aq.) + 3 e - = Sb (sol.)

0.200

Al 3+ (aq.) + 3 e - = Al (sol.)

1.663

Bi 3+ (aq) + 3 e - = Bi (sol)

0.215

Ti 2+ (aq) + 2 e - = Ti (sol)

1.630

Cu 2+ (aq.) + 2 e - = C u (sol.)

0.337

Zr 4+ (aq.) + 4 e - = Zr (sol.)

1.539

Cu + (aq) + e - = Cu (sol)

0.520

Mn 2+ (aq.) + 2 e - = Mn (sol.)

1.179

Ag + (aq.) + e - = Ag (sol.)

0.799

V 2+ (aq.) + 2 e - = V (sol.)

1.175

Hg 2+ (aq) + 2 e - = Hg (l)

0.850

Cr 2+ (aq) + 2 e - = Cr (sol)

0.913

Pd 2+ (aq) + 2 e - = Pd (sol)

0,987

Zn 2+ (aq.) + 2 e - = Zn (sol.)

0.763

Pt 2+ (aq) + 2 e - = Pt (sol)

1,188

Cr 3+ (aq.) + 3 e - = Cr (sol.)

0.744

Au 3+ (aq) + 3 e - = Au (sol)

1,498

Fe 2+ (aq) + 2 e - = Fe (sol)

0.440

Au + (aq.) + e - = Au (sol.)

1,692

Electrode potentials with a minus sign refer to those metals that displace hydrogen from acids. In the title of table “Standard electrode potentials” correspond to the potentials determined under standard conditions: temperature t =25 0 C (T = 298 K), pressure P = 1 atm, concentration of the solution in which the electrode is immersed C = 1 mol/l. φ 0 --- st. conventional

The lower the electrode potential φ, the more active the metal, the greater the reducing agent it is.

Example . Which metal is more active, zinc or aluminum? Answer: Aluminum, since its potential (according to table No. 3) is less than that of zinc.

Galvanic elements.

A galvanic cell (GC) is a device in which the energy of a chemical reaction is directly converted into electrical energy. GE consists of interconnected metal electrodes immersed in solutions of their salt. The metal plates are connected through an indicating device. The half-cells are connected into an electrical circuit using a tube filled with a conductive solution (the so-called salt bridge). In Fig. 1. A diagram of a copper-zinc galvanic cell (Jacobi-Daniel) is shown. − Zn / Zn 2+ / / Cu 2+ / Cu +

Zn Cu

- +

ZnSO 4 CuSO 4

Rice. 1. Diagram of a galvanic cell: 1 - electrode (Zn); 2- vessel with ZnSO solution 4 ; 3 - salt bridge; 4- vessel with CuSO solution 4 ; 5- electrode (Ci).

We write down the values ​​of electrode potentials for zinc and copper from table No. 3:

 0 = 0.337 V  0 = −0.763 V

Cu 2+ / Cu 0 Zn 2+ / Zn 0

A metal having a lower electrode potential is considered anode and it oxidizes.

We see that the potential value for zinc is less than for copper, we conclude that zinc

anode (serves as a negative electrode) A Zn 0 - 2ē  Zn 2+

The metal having a higher electrode potential is considered to be atom and he is being restored.

The reaction at the right electrode for copper, since it is the cathode (functions as a positive electrode), corresponds to the reduction process:

K Cu 2+ + 2ē  Cu 0

Galvanic cells are represented by the following notation:

− Zn 0 / ZnSO 4 / / CuSO 4 / Cu 0 + or in ionic form: − Zn 0 / Zn 2+ / / Cu 2+ / Cu 0

in which the vertical lines symbolize the metal-solution boundary, and the double line symbolizes the boundary between electrolyte solutions.

Job The GE is assessed by the value of its E.M.F. (the highest voltage that the GE can produce). The EMF of a galvanic cell is the difference between the electrode potentials of the oxidizer and the reducer, that is, it is equal to the difference between the electrode potentials of the cathode and anode.

E =  K 0 −  A 0 (1) E theor =  K calculated −  A calculated

The electrode potential of a metal depends on the concentration of its ions in solution.

This dependence is expressed by the Nernst equation:

where  - standard metal potential, R - universal gas constant, T - absolute temperature, n - number working electrons , passing from anode to cathode, F - Faraday number 1 F = 96500 C, C - concentration of metal ions.

If in the above equation we replace the constants R and F with their numerical values, and the natural logarithm with a decimal one, then it will take the following form:

If the concentrations of solutions at the electrodes are not the same, then first calculate new potential values ​​for the cathode and anode, corrected for concentration according to the Nernst equation, and then substitute them into equation (1).

Concentration galvanic cell (CGE)consists of two plates of the same metal immersed in solutions of its salt, which differ only in concentration. − Zn 0 / Zn 2+ / / Zn 2+ / Zn 0 +

Zn Zn

- +

ZnSO 4 Zn SO 4

C 1 C 2 Rice. 2. Diagram of a galvanic cell: 1.5 - electrodes (Zn); 2, 4 - vessels with ZnSO solution 4 ; 3 - salt bridge.

An electrode that is immersed in a solution withlower concentration considered an anode.

Let's say C 1< С 2 , then the left electrode 1 is the anode and the right electrode 2 will be the cathode. The CGE operates until the concentrations of C level out. 1 = C 2.

Metal corrosion

This is the destruction (oxidation) of metals under the influence of the environment.

Polarization slowing down corrosion due to the formation on the metal surface of: 1) a thin film invisible to the eye, which prevents further penetration of the oxidizing agent; I have such a film Al, Ti, Zn, Sn, Pb, Mn, Cd, Tl.

2) a thick layer of corrosion products (visible), which makes it difficult to approach the metal itself. In this case, the degree of polarization depends on the porosity of this layer. For example, green patina on copper has the composition ( CuOH) 2 CO 3 and its porosity is less than that of iron (the product is rust Fe 2 O 3 nH 2 O ), therefore patina protects copper better than rust protects iron.

Depolarization acceleration of corrosion. There are hydrogen and oxygen.

1) Hydrogen depolarizationoccurs in acidic environments (dilute acids HCl, H2SO4, HNO3 etc.). During electrochemical corrosion, since alloying metal additives are introduced into many metals and microgalvanic cells are formed due to potential differences, the environment is restored at the cathode, that is, hydrogen is reduced from the acid:

K 2Н + + 2ē  Н 0 2,

and on anode A metal oxidation.

2) Oxygen depolarizationoccurs in neutral and slightly alkaline environments (we are considering atmospheric corrosion)

K 2H 2 O + + O 2 + 4ē  4OH − ,

A Fe 0 - 2ē  Fe 2+ oxidation of iron to Fe 2+ at the beginning of corrosion, only then, over time, does oxidation occur to Fe3+.

Corrosion product Fe (OH) 2 + O 2 → Fe (OH) 3 or Fe 2 O 3 · nH 2 O brown rye.

CONCLUSION: corrosion (oxidation) of a metal is always an anodic process, and the medium is restored at the cathode.

Many concentrated acids passivate (block, sharply reduce the corrosion rate) many metals. This is how concentrated sulfuric acid passivates iron: a dense thin film is formed on the surface FeSO4 , which prevents the penetration of sulfuric acid.

Influence of pH value on corrosion rate.

Chart 1 for metals Al, Zn, Sn, Pb . These metals are stable in a neutral environment due to amphotericity (they stand between true metals and non-metals in the periodic table) and corrosion products react with both acids and alkalis. Thus, it is necessary to prepare water for working with aluminum heat exchangers (correction: Al stable at pH=7; Pb at pH=8; Sn at pH=9; the nature of the curve is the same).

speed V KOR

corrosion

V KOR

| |

0 7 pH 0 7 pH

Graph 1. Graph 2.

Graph 2 shows the curve for gland: it is stable in highly alkaline environments.

Methods for protecting metals from corrosion.

  1. Alloyingmetals introduction of metal additives into the base metal in order to obtain new properties: a) increase in hardness rails, wheelsMn, W, Zn, Cr, Moetc.; b) increased corrosion resistance various types of stainless steel; c) the appearance of plasticity and softness; d) ferromagnetic properties.
  2. Introductioncorrosion inhibitorssubstances that reduce environmental aggression: oxygen absorbers in solutionNa2 SO3 ; cathodic moderators form a film on the metal (chromates, bichromatesK2 Cr2 O7 , nitrites, etc.); For acidic environments, organic compounds (catapin) are used.
  3. Non-metallic coatings: varnishes, paints, lubricants, waxes, pastes, polymers, rubbers, hard rubber. Protection with rubber and ebonite is called gumming.
  4. Electrochemical protection: A)metal coatings; b) tread protection; c) cathodic protection.
  5. Stray current protection: It is believed that 50% of corrosion in railway transport occurs due to stray currents; all parts of the rolling stock and what is in the ground are affected. The idea of ​​protection is to divert some of the current through guides in the ground, which are connected to a diode, which organizes the passage of current in one direction (suction).
  6. Protection frommicrobiological corrosion: polymer-based varnishes and paints, air exchange, temperature conditions not exceeding 200 C and humidity not more than 80%, preservatives using inhibitors, sacrificial and cathodic protection.

Tread protection:I- steel structure,Cathodic protection:I- coated pipe,

2- protector, 3- filler, 4- electrical 2- connecting wires, 3- source

contact with structure, 5 control DC, 4 anode.

measuring terminal (IPZProtective current Mechanism: electrolysis

protection). Mechanism: GE

Attread(anodic) electrochemical protection, a protector is attached to the protected metal structure - metal withmore negative value of the electrode potential. The activity of the metal chosen as protection can be assessed by the radius of action of the protector, i.e. the distance over which the action of the selected metal extends. For the tread protection of steel, zinc is most often used, as well as aluminum, cadmium and magnesium. The radius of the tread protection is approximately 50 m.

When protecting cables, pipelines and other structures located in the ground, zinc protectors are installed in a filler composition: 25% CaSO4 2H2 Oh, 28%Na2 SO4 · 10 N2 Oh, 50% clay. Protectors for installation in the ground are usually made in the form of cylinders. For contact with the connecting wire, which is usually soldered, the protector has a galvanized steel core.

The corrosion rate with anodic protection can be reduced to a minimum value corresponding to the full polarization current, but is never reduced to zero, as in the case of cathodic protection.

Cathodeelectrochemical protection is used to protect metal products located in the soil. It is carried out by connecting metal structures to the negative pole of an external direct current source. With cathodic protectionInsoluble materials (graphite, coal) or dissolving scrap metal (rails, old pipes) are used as an auxiliary electrode (anode), which must be periodically renewed. In the case of combating underground corrosion, the positive pole of the external current source is grounded. The range of cathodic protection is about 2 km.

Stray current protection: Irectifier substation, 2- overhead contact network, 3- rails, 4- soil, 5- stray current, 6- pipeline, 7- diode, 8- metal jumper.

To protect underground metal structures from destruction by stray currents, it is usedelectrical drainage protection. It is carried out by connecting the anode section of an underground structure (pipe) with a metal conductor to a source of stray currents, for example, a rail. The current passes through the metal conductor, as a result of which the ground-rail potential difference is eliminated, and therefore the danger of corrosion. Since on electrified railways the current can often change its direction, polarized electric drainage is used for greater reliability of protection. To do this, a rectifier, for example a silicon or germanium diode, is included in the metal connections, which ensures that current flows only in the desired direction.

ELECTROLYSIS

This is the transformation of a substance under the influence of electric current. At the same time, oncathodeare being restoredpositive particles (cations), and onanodeoxidizenegative particles (anions).

Used in electrolysissoluble(metal) andinsoluble(coal)electrodes.Electrode solubility is only important for the anodic process. By default, carbon electrodes are used.

Faraday's first law.

When passing an amount of electricity through a solution or melt of a substance 1F= 96500 C at the cathode and at the anode, one equivalent of electrolysis products is released.

Faraday's second law.

The mass or volume of the electrolysis product depends directly on the strength of the current, the time of passage of electricity and the nature of the electrolysis product.

And,

WhereI – current strength, A;t – time, s; Eprod – mass equivalent, G;EVprod – volumetric equivalent, l.Current output

Electrolysis is characterized by high current efficiency values: 97-99%.

Electrolysis is used for the production of high-purity substances, metals, for coating, electroplating, electroforming, separation of mixtures of substances, for electrocoagulation, for the production of hydrogen as an alternative fuel, in cathodic corrosion protection, etc.

Rules for writing electrolysis equations for aqueous solutions.

  1. Reduction of cations at the cathode.

a) If the salt metal is in the “voltage series” up toAlinclusive, then hydrogen is reduced from water at the cathode, and the metal remains in solution:

TO2H+ + 2ē → N0 2

b) If the salt metal is in the “voltage series” fromTiup to H inclusive, then both hydrogen from water and metal are reduced at the cathode:

TO2H+ + 2ē → N0 2 AndCr3+ + 3ē →Cr0

V)If the salt metal is in the “voltage series” after hydrogen, then one metal is reduced at the cathode:

TOAg+ + 1 ē → Ag0

  1. Oxidation of anions at the anode

A)for insoluble (carbon) electrodes:

S2- , I- , Br - ,Cl- OH- ,NO3 - ,SO4 2- , P.O.4 3-

increasing difficulty of anion oxidation.

b)for soluble (metal) electrodes:

the salt anions remain in solution, andsoluble metal anode material oxidizes.

PAGE 7


cation+

anion −

The ionic product of water is the product of the concentrations of hydrogen ions H+ and hydroxide ions OH? in water or in aqueous solutions, water autoprotolysis constant. Displaying the value of the ionic product of water

Water, although a weak electrolyte, dissociates to a small extent:

H2O + H2O - H3O+ + OH? or H2O - H+ + OH?

The equilibrium of this reaction is strongly shifted to the left. The dissociation constant of water can be calculated using the formula:

Concentration of hydronium ions (protons);

Hydroxide ion concentration;

Concentration of water (in molecular form) in water;

The concentration of water in water, taking into account its low degree of dissociation, is practically constant and amounts to (1000 g/l)/(18 g/mol) = 55.56 mol/l.

At 25 °C, the dissociation constant of water is 1.8×10×16 mol/l. Equation (1) can be rewritten as: Let us denote the product K· = Kw = 1.8×10?16 mol/l · 55.56 mol/l = 10?14mol/l = · (at 25 °C).

The constant Kw, equal to the product of the concentrations of protons and hydroxide ions, is called the ionic product of water. It is constant not only for pure water, but also for dilute aqueous solutions of substances. With increasing temperature, the dissociation of water increases, therefore, Kw also increases; with decreasing temperature, vice versa. Practical significance of the ionic product of water

The practical significance of the ionic product of water is great, since it allows, with a known acidity (alkalinity) of any solution (that is, at a known concentration or ), to find the corresponding concentration or . Although in most cases, for convenience of presentation, they do not use absolute values ​​of concentrations, but their decimal logarithms taken with the opposite sign - respectively, the hydrogen index (pH) and the hydroxyl index (pOH).

Since Kb is a constant, when acid (H+ ions) is added to a solution, the concentration of hydroxide ions OH? will fall and vice versa. In a neutral environment = = mol/l. At a concentration > 10?7 mol/l (respectively, the concentration< 10?7 моль/л) среда будет кислой; При концентрации >10?7 mol/l (respectively, concentration< 10?7 моль/л) -- щелочной.

Electrolytic dissociation of water. pH value

Water is a weak amphoteric electrolyte:

H2O H+ + OH- or, more precisely: 2H2O H3O+ + OH-

The dissociation constant of water at 25°C is equal to: This value of the constant corresponds to the dissociation of one out of one hundred million water molecules, therefore the concentration of water can be considered constant and equal to 55.55 mol/l (density of water 1000 g/l, mass of 1 liter 1000 g, amount of water substance 1000g: 18g/mol=55.55 mol, C=55.55 mol: 1 l = 55.55 mol/l). Then

This value is constant at a given temperature (25°C), it is called the ionic product of water KW:

The dissociation of water is an endothermic process, therefore, with increasing temperature, in accordance with Le Chatelier’s principle, the dissociation intensifies, the ionic product increases and reaches a value of 10-13 at 100°C.

In pure water at 25°C, the concentrations of hydrogen and hydroxyl ions are equal:

10-7 mol/l Solutions in which the concentrations of hydrogen and hydroxyl ions are equal are called neutral. If an acid is added to pure water, the concentration of hydrogen ions will increase and become greater than 10-7 mol/l, the medium will become acidic, and the concentration of hydroxyl ions will instantly change so that the ionic product of water retains its value of 10-14. The same thing will happen when adding alkali to clean water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ionic product, therefore, knowing the concentration of one of the ions, it is easy to calculate the concentration of the other. For example, if = 10-3 mol/l, then = KW/ = 10-14/10-3 = 10-11 mol/l, or if = 10-2 mol/l, then = KW/ = 10-14 /10-2 = 10-12 mol/l. Thus, the concentration of hydrogen or hydroxyl ions can serve as a quantitative characteristic of the acidity or alkalinity of the medium.

In practice, they do not use the concentrations of hydrogen or hydroxyl ions, but the hydrogen pH or hydroxyl pOH indicators. The hydrogen pH indicator is equal to the negative decimal logarithm of the concentration of hydrogen ions:

The hydroxyl index pOH is equal to the negative decimal logarithm of the concentration of hydroxyl ions:

pOH = - log

It is easy to show by taking the logarithm of the ionic product of water that

pH + pH = 14

If the pH of the environment is 7, the environment is neutral, if less than 7, it is acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH greater than 7 means the environment is alkaline; the higher the pH, the higher the concentration of hydroxyl ions. Pure water conducts electricity very poorly, but still has measurable electrical conductivity, which is explained by the slight dissociation of water into hydrogen ions and hydroxide ions. Based on the electrical conductivity of pure water, the concentration of hydrogen and hydroxide ions in water can be determined.

Since the degree of dissociation of water is very small, the concentration of undissociated molecules in water is practically equal to the total concentration of water, therefore, from the expression for the dissociation constant of water, half a half, that for water and dilute aqueous solutions at a constant temperature, the product of the concentrations of hydrogen ions and hydroxide ions is a constant value. This constant is called the ionic product of water.

Solutions in which the concentrations of hydrogen and hydroxide ions are the same are called neutral. Acidic solutions contain more hydrogen ions, while alkaline solutions contain more hydroxide ions. But the product of their concentrations is always constant. This means that if the concentration of hydrogen ions in an aqueous solution is known, then the concentration of hydroxide ions is also determined. Therefore, both the degree of acidity and the degree of alkalinity of a solution can be quantitatively characterized by the concentration of hydrogen ions:

The acidity or alkalinity of a solution can be expressed in a more convenient way: instead of the concentration of hydrogen ions, indicate its decimal logarithm, taken with the opposite sign. The last value is called the hydrogen index and is denoted pH:. From this it is clear that in a neutral solution pH = 7; in acidic solutions pH<7 и тем меньше, чем кислее раствор; в щелочных растворах рН>7, and the more, the greater the alkalinity of the solution.

There are various methods for measuring pH. The approximate reaction of a solution can be determined using special reactors called indicators, the color of which changes depending on the concentration of hydrogen ions. The most common are methyl orange, methyl red, phenolphthalein and litmus.