Dissociation of water. pH value
Pure water, although poorly (compared to electrolyte solutions), can conduct electric current. This is caused by the ability of a water molecule to disintegrate (dissociate) into two ions, which are conductors of electric current in pure water (below, dissociation means electrolytic dissociation - decomposition into ions): H 2 O ↔ H + + OH -
For approximately 556,000,000 non-dissociated water molecules, only 1 molecule dissociates, but this is 60,000,000,000 dissociated molecules in 1mm3. Dissociation is reversible, that is, the H + and OH - ions can form a water molecule again. As a result, dynamic equilibrium occurs in which the number of decayed molecules is equal to the number of H + and OH - ions formed. In other words, the speeds of both processes will be equal. For our case, the equation for the rate of a chemical reaction can be written as follows:
υ 1 = κ 1 (for water dissociation)
υ 2 = κ 2 (for the reverse process)
where υ is the reaction rate; κ is the reaction rate constant (depending on the nature of the reactants and temperature); , and - concentrations (mol/l).
In a state of equilibrium υ 1 = υ 2, therefore: κ 1 = κ 2
Since, at a certain temperature, the quantities used in calculating the ionic product of water (K, ) are constant, the value of the ionic product of water is also constant. And since the dissociation of a water molecule produces the same number of ions and , it turns out that for pure water the concentrations and will be equal to 10 -7 mol/l. From the constancy of the ionic product of water, it follows that if the number of H + ions becomes larger, then the number of HO - ions becomes smaller. For example, if a strong acid HCl is added to pure water, it, as a strong electrolyte, will completely dissociate into H + and Cl -, as a result, the concentration of H + ions will increase sharply, and this will lead to an increase in the rate of the process opposite to dissociation, since it depends on the concentration of ions H + and OH -: υ 2 = κ 2
During the accelerated process opposite to dissociation, the concentration of HO - ions will decrease to a value corresponding to the new equilibrium, at which there will be so few of them that the rates of dissociation of water and the reverse process will again be equal. If the concentration of the resulting HCl solution is 0.1 mol/l, the equilibrium concentration will be equal to: = 10 -14 /0.1 = 10 -13 mol/l
Ionic product of wateŕ is the product of the concentrations of hydrogen ions H + and hydroxyl ions OH − in water or in aqueous solutions, the autoprotolysis constant of water.
Water, although a weak electrolyte, dissociates to a small extent:
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);
· - concentration of hydroxide ions;
· - 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:
The constant K in, 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, Kv also increases, with decreasing temperature - vice versa. 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 the 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 sour; At a concentration > 10 −7 mol/l (respectively, the concentration< 10 −7 моль/л) - alkaline.
27. Buffer solutions: their composition, properties, mechanism of action. Buffer capacity
Buffer solutions- these are solutions containing buffer systems. Buffer systems are mixtures that contain weak acids and their salts with strong bases or weak bases and their salts with strong acids in a certain quantitative ratio. Such solutions have a stable concentration of H+ ions when diluted with a neutral solvent (water) and a certain amount of strong acids or bases is added to them.
Buffer solutions are found in the waters of the world's oceans, soil solutions and living organisms. These systems perform the functions of regulators that maintain an active reaction of the environment at a certain value necessary for the successful occurrence of metabolic reactions. Buffer solutions are classified into acidic and basic. An example of the former would be an acetate buffer system, and an example of the latter would be an ammonium buffer system. There are natural and artificial buffer solutions. A natural buffer solution is blood, which contains bicarbonate, phosphate, protein, hemoglobin and acid buffer systems. An artificial buffer solution can be an acetate buffer consisting of CH3COOH.
Let us consider the features of the internal composition and mechanism of action of buffer systems using the example of an acetate buffer system: acetate acid/sodium acetate. In an aqueous environment, the components of the buffer system undergo electrolytic dissociation. Sodium acetate, as a salt of a weak acid and a strong base, completely dissociates into ions. The presence of anions in such a buffer mixture depends on the concentration of salt in it and the degree of its dissociation. The concentration of H+ ions in the buffer system is directly proportional to the concentration of the acid in it and inversely proportional to the content of the salt of this acid in it.
Thus, the concentration of H+ ions in the main buffer is directly proportional to the concentration of salt in it and inversely proportional to the concentration of base.
For example, it is necessary to prepare an acetate buffer with several pH values. First, prepare 5M solutions of acetate acid and sodium acetate. To prepare the first solution, take 50 ml of each component. Guided by the formula, determine the concentration of H+ ions in the resulting solution.
For the next buffer solution, take 80 ml of acid solution and 20 ml of salt solution prepared earlier. There are a number of recipes for various buffer solutions used in chemical analysis and laboratory practice.
Buffer solutions are characterized by certain properties. These, first of all, include buffering - the ability to maintain a constant concentration of H+ ions when a certain amount of a strong acid or strong base is added to a buffer solution. For example, if a small amount of chloride acid is added to the acetate buffer, the pH will not shift to the acidic side, since the chloride acid will undergo an exchange decomposition reaction with the salt of the weak acid. As a result of the reaction, a strong acid that can shift the pH to the acidic side is replaced by a weak acid and a neutral salt. The degree of dissociation of a weak electrolyte solution decreases as its concentration increases, tends to zero, and a pH shift does not occur.
Buffer solution capacity(from English buffer- shock absorber, English buff- soften shocks) - the amount of acid or base needed to change the pH of the buffer solution by exactly 1.
Buffer mixture, buffer solution, buffer system- a combination of substances, a system that maintains a constant pH.
Water- weak amphoteric electrolyte.
The equation for the ionization of water taking into account the hydration of hydrogen ions H + is as follows:
Without taking into account the hydration of H + ions, the water dissociation equation has the form:
As can be seen from the second equation, the concentrations of hydrogen ions H + and hydroxide ions OH - in water are the same. At 25 o C [H + ] = [OH - ] = 10 -7 mol/l.
The product of the concentrations of hydrogen ions and hydroxide ions is called ionic product of water(KH2O).
K H 2 O = ∙
K H 2 O is a constant value, and at a temperature of 25 o C
K H 2 O = 10 -7 ∙10 -7 = 10 -14
In dilute aqueous solutions of electrolytes, as in water, the product of the concentrations of hydrogen ions H + and hydroxide ions OH - is a constant value at a given temperature. The ionic product of water makes it possible for any aqueous solution to calculate the concentration of hydroxide ions OH - if the concentration of hydrogen ions H + is known, and vice versa.
The environment of any aqueous solution can be characterized by the concentration of hydrogen ions H + or hydroxide ions OH -.
There are three types of media in aqueous solutions: neutral, alkaline and acidic.
Neutral environment is a medium in which the concentration of hydrogen ions is equal to the concentration of hydroxide ions:
[H + ] = = 10 -7 mol/l
Acidic environment is a medium in which the concentration of hydrogen ions is greater than the concentration of hydroxide ions:
[H + ] > [OH - ], > 10 -7 mol/l
Alkaline environment is a medium in which the concentration of hydrogen ions is less than the concentration of hydroxide ions:
< , < 10 -7 моль/л
To characterize solution environments, it is convenient to use the so-called pH value (pH).
pH value is called the negative decimal logarithm of the concentration of hydrogen ions: pH = -log.
For example, if = 10 -3 mol/l, then pH = 3, the solution medium is acidic; if [H + ] = 10 -12 mol/l, then pH = 12, the solution medium is alkaline: 
The pH is less than 7, the more acidic the solution. The pH is greater than 7, the greater the alkalinity of the solution.
The relationship between the concentration of H + ions, the pH value and the solution environment is shown in the following diagram:
There are various methods for measuring pH. Qualitatively, the nature of the medium of aqueous solutions of electrolytes is determined using indicators.
Indicators are substances that reversibly change their color depending on the solution environment, i.e., the pH of the solution.
In practice, indicators are used litmus, methyl orange (methyl orange) and phenolphthalein. They change their color in a small pH range: litmus - in the pH range from 5.0 to 8.0; methyl orange - from 3.1 to 4.4 and phenolphthalein - from 8.2 to 10.0.
The change in color of the indicators is shown in the diagram: 
The shaded areas show the range of changes in the color of the indicator.
In addition to the above indicators, a universal indicator is also used, which can be used to approximately determine pH in a wide range from 0 to 14.
The pH value is of great importance in chemical and biological processes, since, depending on the nature of the environment, these processes can occur at different speeds and in different directions.
Therefore, determining the pH of solutions is very important in medicine, science, technology, and agriculture. Changing the pH of blood or gastric juice is a diagnostic test in medicine. Deviations of pH from normal values, even by 0.01 units, indicate pathological processes in the body. The constancy of the concentrations of hydrogen ions H + is one of the important constants of the internal environment of living organisms.
Thus, with normal acidity, gastric juice has a pH of 1.7; The pH of human blood is 7.4; saliva - 6.9. Each enzyme functions at a certain pH value: blood catalase at pH 7 gastric juice pepsin - at pH 1.5-2; etc.
Pure water, although poorly (compared to electrolyte solutions), can conduct electric current. This is caused by the ability of a water molecule to disintegrate (dissociate) into two ions, which are conductors of electric current in pure water (below, dissociation means electrolytic dissociation - disintegration into ions):
H 2 O ↔ H + + OH -
For approximately 556,000,000 non-dissociated water molecules, only 1 molecule dissociates, but this is 60,000,000,000 dissociated molecules in 1mm3. Dissociation is reversible, that is, the H + and OH - ions can form a water molecule again. Eventually it comes dynamic equilibrium in which the number of decayed molecules is equal to the number of H + and OH - ions formed. In other words, the speeds of both processes will be equal. For our case, the equation for the rate of a chemical reaction can be written as follows:
υ 1 = κ 1 (for water dissociation)
υ 2 = κ 2 (for the reverse process)
Where υ - speed reaction; κ - reaction rate constant (depending on the nature of the reactants and temperature); , And - concentration (mol/l).
In a state of balance υ 1 = υ 2, hence:
κ 1 = κ 2
Let's do some simple math and get:
κ 1 /κ 2 = /
κ 1 /κ 2 = K
K- equilibrium constant, and in our case, dissociation constant, which depends on the temperature and nature of the substances, and does not depend on concentrations (as well as κ 1 and κ 2). K for water 1.8 10 -16 at 25 °C (reference value).
Due to the very small number of dissociated molecules, the concentration can be taken to be equal to the total concentration of water, and the total concentration of water in dilute solutions as a constant value:
=1000(g/l)/18(g/mol)=55.6 mol/l.
Replacing κ 1 / κ 2 on K and using the value , we determine what the product of concentrations is equal to And which is called - ionic product of water:
K = /55.6 mol/l
1.8 10 -16 55.6 mol/l =
10 -14 =
Since, at a certain temperature, the quantities used in calculating the ionic product of water ( K, ) are constant, the value of the ionic product of water just the same all the time. And since the dissociation of a water molecule produces the same number of ions And , it turns out that for pure water the concentration And will be equal 10 -7 mol/l. From the constancy of the ionic product of water, it follows that if the number of H + ions becomes larger, then the number of HO - ions becomes smaller. For example, if a strong acid HCl is added to pure water, it, as a strong electrolyte, will completely dissociate into H + and Cl -, as a result, the concentration of H + ions will increase sharply, and this will lead to an increase in the rate of the process opposite to dissociation, since it depends on the concentration of ions H+ and OH-:
υ 2 = κ 2
During the accelerated process opposite to dissociation, the concentration of HO - ions will decrease to a value corresponding to the new equilibrium, at which there will be so few of them that the rates of dissociation of water and the reverse process will again be equal. If the concentration of the resulting HCl solution is 0.1 mol/l, the equilibrium concentration will be equal to:
= 10 -14 /0.1 = 10 -13 mol/l
When adding the strong base NaOH, the shift will be towards a decrease in the H + concentration.
End of work -
This topic belongs to the section:
Electron clouds of orbitals with different values of l have different configurations, and those with the same l have a similar configuration
Modern quantum mechanical theory states that an atom of any element has a complex structure; the positive part of the atom is a positive charge.. quantum theory implies that the energy of an electron can only be received.. so at l s, the orbital for an electron with any value of the principal quantum number n, the electron cloud is limited..
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The structure of atoms and the Pauli principle
The Pauli principle helps explain a variety of physical phenomena. A consequence of the principle is the presence of electron shells in the structure of the atom, from which, in turn, follows a variety of chemical
Basic types of chemical bonds. Covalent bond. Basic principles of the valence bond method. Sigma and Picovalent bonds
atoms can combine with each other to form both simple and complex substances. In this case, various types of chemical bonds are formed: ionic, covalent (non-polar and polar), metal
Sp hybridization
Occurs when one s- and one p-orbital mix. Two equivalent sp-atomic orbitals are formed, located linearly at an angle of 180 degrees and directed in different directions from the nucleus of the atom
Geometric shape and polarity of molecules
Hybridization Geometric shape Angle between bonds sp Linear 180° sp
Ionic bond as a limiting case of covalent bond polarization. Electrostatic interaction of ions
An ionic bond is a very strong chemical bond formed between atoms with a large difference (>1.5 on the Pauling scale) of electronegativity, at which the shared electron pair
Chemical properties of basic oxides
1. Water-soluble basic oxides react with water to form bases: Na2O + H2O → 2NaOH. 2. Interact with acid oxides, causing
Chemical properties of acid oxides
1. React with water to form an acid: SO3 + H2O → H2SO4. But not all acid oxides react directly with water (SiO
Chemical properties of amphoteric oxides
1. React with acids, forming salt and water: ZnO + 2HCl → ZnCl2 + H2O. 2. React with solid alkalis (during fusion), resulting in the formation
Grounds. Chemical properties of bases. Amphoteric states, reactions of their interaction with acids and alkalis
A base is a chemical compound capable of forming a covalent bond with a proton (Brønsted base
Characteristic reactions
Amphoteric oxides react with strong acids, forming salts of these acids. Such reactions are a manifestation of the basic properties of amphoteric oxides, for example: ZnO + H2SO4
Acids. Anoxic and oxygenic acids. Properties of acids (sulfuric, hydrochloric, nitric)
Acids are complex substances whose molecules consist of substitutable hydrogen atoms and acidic residues. The acid residue has a negative charge.
Sulfuric acid
Sulfuric acid H2SO4 is a strong dibasic acid corresponding to the highest oxidation state of sulfur (+6). Under normal conditions, concentrated sulfuric acid
Nitrates
Nitric acid is a strong acid. Its salts - nitrates - are obtained by the action of HNO3 on metals, oxides, hydroxides or carbonates. All nitrates are highly soluble in water. Nitrate ion in
Homogeneous catalysis
An example of homogeneous catalysis is the decomposition of hydrogen peroxide in the presence of iodine ions. The reaction occurs in two stages: H2O2 + I → H2O + IO
Heterogeneous catalysis
In heterogeneous catalysis, the acceleration of the process usually occurs on the surface of a solid body - the catalyst, therefore the activity of the catalyst depends on the size and properties of its surface. In practice
The effect of concentration on the rate of a chemical reaction. Law of mass action
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The influence of temperature on the rate of a chemical reaction. Activation energy
The effect of temperature on the number of molecular collisions can be shown using a model. To a first approximation, the effect of temperature on the reaction rate is determined by the van’t Hoff rule (formulated
Reactions without and with the participation of electrons. Ion exchange and redox reactions
Valence electrons determine the behavior of a chemical element in chemical reactions. The fewer valence electrons an element has, the more easily it gives up these electrons (it exhibits the properties of reducing
Image of ion exchange reactions
An exchange reaction in a solution is usually represented by three equations: molecular, full ionic and abbreviated ionic. In the ionic equation, weak electrolytes, gases and poorly soluble substances are represented by m
Rules for writing ion exchange reactions
When writing ionic equations, you should be sure to follow the table of solubility of acids, bases and salts in water, that is, be sure to check the solubility of the reagents and products
Oxidation
Oxidation is the process of losing electrons, with an increase in the degree of oxidation. When a substance is oxidized, its oxidation state increases as a result of the loss of electrons. At
Recovery
Reduction is the process of adding electrons to an atom of a substance, while its oxidation state decreases. When reducing atoms or ions I add
Redox couple
An oxidizing agent and its reduced form, or a reducing agent and its oxidized form constitute a conjugated redox pair, and their interconversions are oxidation-in
Types of redox reactions
Intermolecular - reactions in which oxidizing and reducing atoms are located in molecules of different substances, for example: H2S + Cl2 → S + 2HCl Int.
Oxidation, reduction
In redox reactions, electrons are transferred from one atom, molecule, or ion to another. The process of losing electrons is oxidation. During oxidation, the oxidation state increases:
Interaction with simple substances
Interaction with metals: 2Na + Cl2 = 2NaCl, Fe + S = FeS, 6Li + N2 = 2Li3N, 2Ca + O2
Mass fraction
Mass fraction is the ratio of the mass of the solute to the mass of the solution. Mass fraction is measured in fractions of a unit or as a percentage:
Molality (molar weight concentration, molal concentration)
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Solution titer
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Solubility. Equilibrium in heterogeneous systems. Product of solubility of poorly soluble inorganic substances
Solubility is the ability of a substance to form homogeneous systems with other substances - solutions in which the substance is in the form of individual atoms, ions, molecules or
Aqueous solutions of electrolytes. Strong and weak electrolytes. Constant and degree of dissociation. Ostwald's dilution law
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Water pH
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Dissociation of strong electrolytes. Activity of ions in solutions. Activity coefficient. Understanding the ionic strength of solutions
Strong electrolytes are chemical compounds whose molecules in dilute solutions are almost completely dissociated into ions. The degree of dissociation of such electrolytes is close to
Degree of hydrolysis
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Electrode potential. The appearance of a potential jump at the interphase boundary. Hydrogen electrode. Standard hydrogen electrode
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Electrochemical corrosion of metals in various environments
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Chemical interaction of metals with solutions of ordinary acids and oxidizing acids
Hydrochloric acid is the technical name for hydrochloric acid. It is obtained by dissolving hydrogen chloride gas – HCl – in water. Due to its low solubility in water, the concentration with
Dilute sulfuric acid
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Concentrated sulfuric acid
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Electrolysis process. Cathode and anodic processes. The order of particle discharge at the anode and cathode depending on the value of their electrode potential
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Cathode reactions
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Mathematical view
Faraday's laws can be written as the following formula: where:
Configuration of atoms and ions
The solubility of salts and hydroxides of cations, which underlies the analytical classification, like all other properties of cations, is functionally related to the position of the corresponding elements in the periodic cycle.
Types of titration
There are direct, reverse and substituent titrations. During direct titration to a solution of the analyte (aliquot or sample, titrated substance) d
Types of titrimetric analysis
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Equivalence number
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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 |
The 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.
- 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.
- 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.
- Non-metallic coatings: varnishes, paints, lubricants, waxes, pastes, polymers, rubbers, hard rubber. Protection with rubber and ebonite is called gumming.
- Electrochemical protection: A)metal coatings; b) tread protection; c) cathodic protection.
- 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 currents through guides in the ground, which are connected to a diode, which organizes the passage of current in one direction (suction).
- Defence frommicrobiological corrosion: polymer-based varnishes and paints, air exchange, temperature conditions not higher than 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 renewed periodically. 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 the current on electrified railways 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, one equivalent of electrolysis products is released at the cathode and anode.
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.
- 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
- 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.
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