Which are in dynamic equilibrium with undissociated molecules. Weak electrolytes include most organic acids and many organic bases in aqueous and non-aqueous solutions.
Weak electrolytes are:
- almost all organic acids and water;
- some inorganic acids: HF, HClO, HClO 2, HNO 2, HCN, H 2 S, HBrO, H 3 PO 4, H 2 CO 3, H 2 SiO 3, H 2 SO 3, etc.;
- some poorly soluble metal hydroxides: Fe(OH) 3, Zn(OH) 2, etc.; as well as ammonium hydroxide NH 4 OH.
Literature
- M. I. Ravich-Sherbo. V.V. Novikov “Physical and colloidal Chemistry” M: graduate School, 1975
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See what “Weak electrolytes” are in other dictionaries:
weak electrolytes- – electrolytes that slightly dissociate into ions in aqueous solutions. Process of dissociation weak electrolytes reversible and obeys the law of mass action. general chemistry: textbook / A. V. Zholnin ... Chemical terms
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- (from Electro... and Greek lytos decomposed, soluble) liquid or solid substances and systems in which ions are present in any noticeable concentration, causing the passage of electric current. In the narrow sense, E.... ... Great Soviet Encyclopedia
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Measuring the degree of dissociation of various electrolytes showed that individual electrolytes at the same normal concentration of solutions dissociate into ions very differently.
The difference in the degree of dissociation of acids is especially large. For example, nitrogen and hydrochloric acid in 0.1 n. solutions almost completely disintegrate into ions; carbonic, hydrocyanic and other acids dissociate under the same conditions only to a small extent.
Of the water-soluble bases (alkalis), ammonium oxide hydrate is weakly dissociable; other alkalis dissociate well. All salts, with a few exceptions, also dissociate well into ions.
The difference in the degree of dissociation of individual acids is determined by the nature of the valence bond between the atoms that form their molecules. The more polar the bond between the hydrogen and the rest of the molecule, the easier it is to split off, the more the acid will dissociate.
Electrolytes that dissociate well into ions are called strong electrolytes, in contrast to weak electrolytes, which form only a small number of ions in aqueous solutions. Solutions of strong electrolytes retain high electrical conductivity even at very high concentrations. On the contrary, the electrical conductivity of solutions of weak electrolytes decreases rapidly with increasing concentration. Strong electrolytes include acids such as hydrochloric, nitric, sulfuric and some others, then alkalis (except NH 4 OH) and almost all salts.
Polyonic acids and polyacid bases dissociate stepwise. For example, sulfuric acid molecules first dissociate according to the equation
H 2 SO 4 ⇄ H + HSO 4 ‘
or more precisely:
H 2 SO 4 + H 2 O ⇄ H 3 O + HSO 4 ‘
Abstraction of the second hydrogen ion according to the equation
HSO 4 ‘ ⇄ H + SO 4 »
or
HSO 4 ' + H 2 O ⇄ H 3 O + SO 4 "
is already much more difficult, since it has to overcome the attraction from the doubly charged SO 4 ion, which, of course, attracts the hydrogen ion more strongly than the singly charged HSO 4 ion. Therefore, the second stage of dissociation or, as they say, secondary dissociation occurs in a much smallerdegree than primary, and ordinary solutions of sulfuric acid contain only a small number of SO 4 ions "
Phosphoric acid H 3 PO 4 dissociates in three steps:
H 3 PO 4 ⇄ H + H 2 PO 4 ‘
H2PO4⇄H + HPO 4"
HPO 4 » ⇄ H + PO 4 »’
H 3 PO 4 molecules strongly dissociate into H and H 2 PO 4 ' ions. H 2 PO 4 ' ions behave like a weaker acid and dissociate into H and HPO 4 ' to a lesser extent. The HPO 4 ions dissociate like a very weak acid and produce almost no H ions
and P.O. 4 "'
Bases containing more than one hydroxyl group in the molecule also dissociate stepwise. For example:
Ba(OH) 2 ⇄ BaOH + OH’
VaON ⇄ Ba + OH'
As for salts, normal salts always dissociate into metal ions and acidic residues. For example:
CaCl 2 ⇄ Ca + 2Cl’ Na 2 SO 4 ⇄ 2Na + SO 4 "
Acid salts, like polybasic acids, dissociate stepwise. For example:
NaHCO 3 ⇄ Na + HCO 3 ‘
HCO 3 ‘ ⇄ H + CO 3 »
However, the second stage is very small, so that the acid salt solution contains only a small number of hydrogen ions.
Basic salts dissociate into basic and acidic ions. For example:
Fe(OH)Cl 2 ⇄ FeOH + 2Сl"
Almost no secondary dissociation of basic residue ions into metal and hydroxyl ions occurs.
In table 11 shows the numerical values of the degree of dissociation of some acids, bases and salts in 0 , 1 n. solutions.
It decreases with increasing concentration. Therefore, in very concentrated solutions, even strong acids are relatively weakly dissociated. For
Table 11
Acids, bases and salts in 0.1 N.solutions at 18°
| Electrolyte | Formula | Degree of dissociation in % |
| Acids | ||
| Solyanaya | HCl | 92 |
| Hydrobromic | HBr | 92 |
| Hydroiodide | H.J. | . 92 |
| Nitrogen | HNO3 | 92 |
| Sulfuric | H 2 SO 4 | 58 |
| Sulphurous | H 2 SO 3 | 34 |
| Phosphorus | H 3PO 4 | 27 |
| Hydrofluoric | HF | 8,5 |
| Vinegar | CH3COOH | 1,3 |
| Ugolnaya | H 2 CO3 | 0,17 |
| Hydrogen sulfide | H2S | 0,07 |
| Sinilnaya | HCN | 0,01 |
| Bornaya | H 3 BO 3 | 0,01 |
| Grounds | ||
| Barium hydroxide | Ba(OH)2 | 92 |
| Caustic potassium | con | 89 |
| Sodium hydroxide | NaON | 84 |
| Ammonium hydroxide | NH4OH | 1,3 |
| Salts | ||
| Chloride | KCl | 86 |
| Ammonium chloride | NH4Cl | 85 |
| Chloride | NaCl | 84 |
| Nitrate | KNO 3 | 83 |
| AgNO3 | 81 | |
| Acetic acid | NaCH3COO | 79 |
| Chloride | ZnCl2 | 73 |
| Sulfate | Na 2 SO 4 | 69 |
| Sulfate | ZnSO4 | 40 |
| Sulfate |
Electrolytes are substances, alloys of substances or solutions that have the ability to electrolytically conduct galvanic current. It is possible to determine which electrolytes a substance belongs to using the theory of electrolytic dissociation.
Instructions
1. The essence of this theory is that when melted (dissolved in water), virtually all electrolytes are decomposed into ions, which are both positively and negatively charged (which is called electrolytic dissociation). Under the influence of electric current, negative ones (anions, “-”) move towards the anode (+), and positively charged ones (cations, “+”) move towards the cathode (-). Electrolytic dissociation is a reversible process ( reverse process is called “molarization”).
2. The degree of (a) electrolytic dissociation depends on the nature of the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that have broken up into ions to all total number molecules (N) introduced into the solution. You get: a = n / N
3. Thus, powerful electrolytes are substances that completely disintegrate into ions when dissolved in water. Strong electrolytes, as usual, include substances with highly polar or ionic bonds: these are salts that are highly soluble, strong acids (HCl, HI, HBr, HClO4, HNO3, H2SO4), as well as powerful bases (KOH, NaOH, RbOH, Ba (OH)2, CsOH, Sr(OH)2, LiOH, Ca(OH)2). In a strong electrolyte, the substance dissolved in it is mostly in the form of ions (anions and cations); There are actually no molecules that are undissociated.
4. Weak electrolytes are substances that only partially dissociate into ions. Weak electrolytes, together with ions in solution, contain undissociated molecules. Weak electrolytes do not provide a strong concentration of ions in solution. Weak ones include: - organic acids (approximately all) (C2H5COOH, CH3COOH, etc.); - some of the inorganic acids (H2S, H2CO3, etc.); - virtually all salts, sparingly soluble in water, ammonium hydroxide, as well as all bases (Ca3(PO4)2; Cu(OH)2; Al(OH)3; NH4OH); - water. They actually do not conduct electric current, or they conduct, but poorly.
Strong base - inorganic chemical compound, formed by the hydroxyl group -OH and alkaline (elements of group I periodic table: Li, K, Na, RB, Cs) or alkaline earth metal (group II elements Ba, Ca). Written in the form of the formulas LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH)?, Ba(OH)?.
You will need
- evaporation cup
- burner
- indicators
- metal rod
- N?RO?
Instructions
1. Powerful grounds manifest Chemical properties, characteristic of all hydroxides. The presence of alkalis in a solution is determined by a change in the color of the indicator. Add methyl orange, phenolphthalein or omit the litmus paper to the sample with the test solution. Methyl orange gives a yellow color, phenolphthalein gives a purple color, and litmus paper turns Blue colour. The stronger the base, the more saturated the color of the indicator.
2. If you need to find out which alkalis are presented to you, then conduct a good review of the solutions. Particularly common powerful bases are lithium, potassium, sodium, barium and calcium hydroxides. Bases react with acids (neutralization reactions) to form salt and water. In this case, it is possible to isolate Ca(OH)?, Ba(OH)? and LiOH. When interacting with orthophosphoric acid, insoluble precipitates are formed. The remaining hydroxides will not produce precipitation, because all K and Na salts are soluble.3 Ca(OH) ? + 2 N?RO? –? Ca?(PO?)??+ 6 H?O3 Ba(OH) ? +2 N?RO? –? Ba?(PO?)??+ 6 H?O3 LiOH + H?PO? –? Li?PO?? + 3 H?О Strain them and dry them. Add the dried sediment to the burner flame. By changing the color of the flame, it is possible to accurately determine the ions of lithium, calcium and barium. Accordingly, you will determine which hydroxide is which. Lithium salts color the burner flame a carmine-scarlet color. Barium salts are green, and calcium salts are red.
3. The remaining alkalis form soluble orthophosphates.3 NaOH + H?PO?–? Na?PO? + 3 H?O3 KOH + H?PO?–? K?RO? + 3 H?ОIt is necessary to evaporate the water to a dry residue. Evaporated salts metal rod add one at a time to the burner flame. Where the sodium salt is located, the flame will turn clear yellow, and potassium orthophosphate - pink-violet. Thus, having the smallest set of equipment and reagents, you have identified all the powerful bases given to you.
An electrolyte is a substance that in its solid state is a dielectric, that is, it does not conduct electric current, but when dissolved or molten it becomes a conductor. Why does such a sharp change in properties occur? The fact is that electrolyte molecules in solutions or melts dissociate into positively charged and negatively charged ions, as a result of which these substances are in such state of aggregation capable of conducting electric current. Many salts, acids, and bases have electrolytic properties.
Instructions
1. Is that all electrolytes identical in strength, that is, they are excellent conductors of current? No, because many substances in solutions or melts dissociate only to a small extent. Consequently electrolytes are divided into strong, medium strength and weak.
2. What substances are considered powerful electrolytes? Such substances in solutions or melts of which virtually 100% of the molecules undergo dissociation, regardless of the concentration of the solution. The list of strong electrolytes includes an absolute variety of soluble alkalis, salts and some acids, such as hydrochloric, bromide, iodide, nitric, etc.
3. How are they different from them? electrolytes medium strength? The fact that they dissociate to a much lesser extent (from 3% to 30% of molecules disintegrate into ions). Typical representatives of such electrolytes are sulfuric and phosphoric acids.
4. How do weak compounds behave in solutions or melts? electrolytes? Firstly, they dissociate to a very small extent (no more than 3% of the total number of molecules), and secondly, their dissociation is the more clumsy and leisurely, the higher the saturation of the solution. Such electrolytes include, say, ammonia(ammonium hydroxide), many organic and inorganic acids (including hydrofluoric acid - HF) and, of course, water familiar to us all. Because only a pitifully small fraction of its molecules breaks down into hydrogen ions and hydroxyl ions.
5. Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on many factors: the nature of the electrolyte itself, the solvent, and temperature. Consequently, this distribution itself is to a certain extent arbitrary. Tea the same substance can different conditions be both a powerful electrolyte and a weak one. To assess the strength of the electrolyte, a special value was introduced - the dissociation constant, determined on the basis of the law of mass action. But it is applicable only to weak electrolytes; powerful electrolytes do not obey the law of mass action.
Salts- This chemical substances, consisting of a cation, that is, a positively charged ion, a metal and a negatively charged anion - acid residue. There are many types of salts: typical, acidic, basic, double, mixed, hydrated, complex. This depends on the cation and anion compositions. How is it possible to determine base salt?
Instructions
1. Let's imagine you have four identical containers with burning solutions. You know that these are solutions of lithium carbonate, sodium carbonate, potassium carbonate and barium carbonate. Your task: determine what salt is contained in the entire container.
2. Recall the physical and chemical properties of compounds of these metals. Lithium, sodium, potassium are alkali metals of the first group, their properties are very similar, activity increases from lithium to potassium. Barium is a group 2 alkaline earth metal. Its carbonic salt dissolves perfectly in hot water, but dissolves poorly in cold water. Stop! This is the first chance to immediately determine which container contains barium carbonate.
3. Cool the containers, say by placing them in a container with ice. Three solutions will remain clear, but the fourth will quickly become cloudy and a white precipitate will begin to form. This is where the barium salt is found. Set this container aside.
4. You can quickly determine barium carbonate using another method. Alternately, pour a little of the solution into another container with a solution of some sulfate salt (say, sodium sulfate). Only barium ions, binding with sulfate ions, instantly form a dense white precipitate.
5. It turns out that you have identified barium carbonate. But how do you differentiate between the 3 alkali metal salts? This is quite easy to do, you will need porcelain evaporation cups and an alcohol lamp.
6. Pour a small amount of the entire solution into a separate porcelain cup and evaporate the water over the fire of a spirit lamp. Small crystals form. Place them in the flame of an alcohol lamp or a Bunsen burner - supported by steel tweezers or a porcelain spoon. Your task is to notice the color of the blazing “tongue” of flame. If it is a lithium salt, the color will be clear red. Sodium will color the flame intense yellow, and potassium will color the flame purple-violet. By the way, if barium salt had been tested in the same way, the color of the flame should have been green.
Helpful advice
One famous chemist in his youth exposed the greedy hostess of a boarding house in much the same way. He sprinkled the remains of a half-eaten dish with lithium chloride - a substance that is certainly harmless in small numbers. The next day, at lunch, a slice of meat from the dish served to the table was burned in front of a spectroscope - and the boarding house residents saw a clear red stripe. The hostess was preparing food from yesterday's leftovers.
Note!
Is it true pure water conducts electric current very poorly, it still has measurable electrical conductivity, explained by the fact that water slightly dissociates into hydroxide ions and hydrogen ions.
Helpful advice
Many electrolytes are hostile substances, so when working with them, be extremely careful and follow safety regulations.
Electrolytes are classified into two groups depending on the degree of dissociation - strong and weak electrolytes. Strong electrolytes have a dissociation degree greater than one or more than 30%, weak electrolytes less than one or less than 3%.
Process of dissociation
Electrolytic dissociation is the process of breakdown of molecules into ions - positively charged cations and negatively charged anions. Charged particles carry electric current. Electrolytic dissociation is possible only in solutions and melts.
The driving force for dissociation is the disintegration of polar covalent bonds under the action of water molecules. Polar molecules are attracted by water molecules. IN solids Ionic bonds are destroyed during heating. High temperatures cause vibrations of ions at the nodes of the crystal lattice.
Rice. 1. The process of dissociation.
Substances that easily disintegrate into ions in solutions or melts and, therefore, conduct electric current are called electrolytes. Non-electrolytes do not conduct electricity because do not break down into cations and anions.
Depending on the degree of dissociation, strong and weak electrolytes are distinguished. Strong ones dissolve in water, i.e. completely, without the possibility of recovery, disintegrate into ions. Weak electrolytes partially break down into cations and anions. The degree of their dissociation is less than that of strong electrolytes.
The degree of dissociation shows the proportion of disintegrated molecules in the total concentration of substances. It is expressed by the formula α = n/N.
Rice. 2. Degree of dissociation.
Weak electrolytes
List of weak electrolytes:
- dilute and weak inorganic acids - H 2 S, H 2 SO 3, H 2 CO 3, H 2 SiO 3, H 3 BO 3;
- some organic acids (most organic acids are non-electrolytes) - CH 3 COOH, C 2 H 5 COOH;
- insoluble bases - Al(OH) 3, Cu(OH) 2, Fe(OH) 2, Zn(OH) 2;
- Ammonium hydroxide - NH 4 OH.
Rice. 3. Solubility table.
The dissociation reaction is written using the ionic equation:
- HNO 2 ↔ H + + NO 2 – ;
- H 2 S ↔ H + + HS – ;
- NH 4 OH ↔ NH 4 + + OH – .
Polybasic acids dissociate stepwise:
- H 2 CO 3 ↔ H + + HCO 3 – ;
- HCO 3 – ↔ H + + CO 3 2- .
Insoluble bases also decompose in stages:
- Fe(OH) 3 ↔ Fe(OH) 2 + + OH – ;
- Fe(OH) 2 + ↔ FeOH 2+ + OH – ;
- FeOH 2+ ↔ Fe 3+ + OH – .
Water is classified as a weak electrolyte. Water practically does not conduct electric current, because... weakly decomposes into hydrogen cations and hydroxide ion anions. The resulting ions are reassembled into water molecules:
H 2 O ↔ H + + OH – .
If water easily conducts electricity, it means there are impurities in it. Distilled water is non-conductive.
The dissociation of weak electrolytes is reversible. The resulting ions reassemble into molecules.
What have we learned?
Weak electrolytes include substances that partially disintegrate into ions - positive cations and negative anions. Therefore, such substances do not conduct electricity well. These include weak and dilute acids, insoluble bases, and slightly soluble salts. The weakest electrolyte is water. Dissociation of weak electrolytes is a reversible reaction.
All substances can be divided into electrolytes and non-electrolytes. Electrolytes include substances whose solutions or melts conduct electric current (for example, aqueous solutions or melts of KCl, H 3 PO 4, Na 2 CO 3). Non-electrolyte substances do not conduct electric current when melted or dissolved (sugar, alcohol, acetone, etc.).
Electrolytes are divided into strong and weak. Strong electrolytes in solutions or melts completely dissociate into ions. When writing equations chemical reactions this is emphasized by an arrow in one direction, for example:
HCl→ H + + Cl -
Ca(OH) 2 → Ca 2+ + 2OH -
Strong electrolytes include substances with a heteropolar or ionic crystal structure (Table 1.1).
Table 1.1 Strong electrolytes
Weak electrolytes only partially disintegrate into ions. Along with ions, melts or solutions of these substances contain overwhelmingly undissociated molecules. In solutions of weak electrolytes, in parallel with dissociation, the reverse process occurs - association, that is, the combination of ions into molecules. When writing the reaction equation, this is emphasized by two oppositely directed arrows.
CH 3 COOH D CH 3 COO - + H +
Weak electrolytes include substances with a homeopolar type of crystal lattice (Table 1.2).
Table 1.2 Weak electrolytes
The equilibrium state of a weak electrolyte in an aqueous solution is quantitatively characterized by the degree of electrolytic dissociation and the electrolytic dissociation constant.
The degree of electrolytic dissociation α is the ratio of the number of molecules dissociated into ions to the total number of molecules of the dissolved electrolyte:
The degree of dissociation shows what part of the total amount of dissolved electrolyte disintegrates into ions and depends on the nature of the electrolyte and solvent, as well as on the concentration of the substance in the solution, has a dimensionless value, although it is usually expressed as a percentage. With infinite dilution of the electrolyte solution, the degree of dissociation approaches unity, which corresponds to complete, 100%, dissociation of the molecules of the dissolved substance into ions. For solutions of weak electrolytes α<<1. Сильные электролиты в растворах диссоциируют полностью (α =1). Если известно, что в 0,1 М растворе уксусной кислоты степень электрической диссоциации α =0,0132, это означает, что 0,0132 (или 1,32%) общего количества растворённой уксусной кислоты продиссоциировало на ионы, а 0,9868 (или 98,68%) находится в виде недиссоциированных молекул. Диссоциация слабых электролитов в растворе подчиняется закону действия масс.
In general, a reversible chemical reaction can be represented as:
a A+ b B D d D+ e E
The reaction rate is directly proportional to the product of the concentration of reacting particles in powers of their stoichiometric coefficients. Then for the direct reaction
V 1 = k 1 [A] a[B] b,
and the speed of the reverse reaction
V 2 = k 2 [D] d[E] e.
At some point in time, the rates of the forward and reverse reactions will level out, i.e.
This state is called chemical equilibrium. From here
k 1 [A] a[B] b=k 2 [D] d[E] e
Grouping constants on one side and variables on the other, we get:
Thus, for a reversible chemical reaction in a state of equilibrium, the product of the equilibrium concentrations of the reaction products in powers of their stoichiometric coefficients, related to the same product for the starting substances, is a constant value at a given temperature and pressure. Numerical value of the chemical equilibrium constant TO does not depend on the concentration of reactants. For example, the equilibrium constant for the dissociation of nitrous acid in accordance with the law of mass action can be written as:
HNO 2 + H 2 OD H 3 O + + NO 2 -
.
Size K a is called the dissociation constant of an acid, in this case nitrous.
The dissociation constant of a weak base is expressed similarly. For example, for the ammonia dissociation reaction:
NH 3 + H 2 O DNH 4 + + OH -
.
Size K b is called the dissociation constant of a base, in this case ammonia. The higher the dissociation constant of the electrolyte, the more strongly the electrolyte dissociates and the higher the concentration of its ions in solution at equilibrium. There is a relationship between the degree of dissociation and the dissociation constant of a weak electrolyte:
This is a mathematical expression of Ostwald's dilution law: when a weak electrolyte is diluted, the degree of its dissociation increases. For weak electrolytes at TO≤1∙ 10 -4 and WITH≥0.1 mol/l use a simplified expression:
TO= α 2 ∙WITH or α
Example1. Calculate the degree of dissociation and concentration of ions and [NH 4 + ] in a 0.1 M ammonium hydroxide solution, if TO NH 4 OH =1.76∙10 -5
Given: NH 4 OH
TO NH 4 OH =1.76∙10 -5
Solution:
Since the electrolyte is quite weak ( To NH 4 OH =1,76∙10 –5 <1∙ 10 - 4) и раствор его не слишком разбавлен, можно принять, что:
or 1.33%
The concentration of ions in a binary electrolyte solution is equal to C∙α, since the binary electrolyte ionizes to form one cation and one anion, then = [ NH 4 + ]=0.1∙1.33∙10 -2 =1.33∙10 -3 (mol/l).
Answer:α=1.33%; = [NH 4 + ]=1.33∙10 -3 mol/l.
Strong electrolyte theory
Strong electrolytes in solutions and melts completely dissociate into ions. However, experimental studies of the electrical conductivity of solutions of strong electrolytes show that its value is somewhat underestimated compared to the electrical conductivity that should be at 100% dissociation. This discrepancy is explained by the theory of strong electrolytes proposed by Debye and Hückel. According to this theory, in solutions of strong electrolytes there is electrostatic interaction between ions. Around each ion, an “ionic atmosphere” is formed of ions of opposite charge sign, which inhibits the movement of ions in the solution when a direct electric current is passed. In addition to the electrostatic interaction of ions, in concentrated solutions it is necessary to take into account the association of ions. The influence of interionic forces creates the effect of incomplete dissociation of molecules, i.e. apparent degree of dissociation. The experimentally determined value of α is always slightly lower than the true α. For example, in a 0.1 M solution of Na 2 SO 4 the experimental value is α = 45%. To take into account electrostatic factors in solutions of strong electrolytes, the concept of activity is used (A). The activity of an ion is the effective or apparent concentration at which the ion acts in solution. Activity and true concentration are related by the expression:
Where f – activity coefficient, which characterizes the degree of deviation of the system from the ideal due to electrostatic interactions of ions.
Ion activity coefficients depend on the value µ, called the ionic strength of the solution. The ionic strength of a solution is a measure of the electrostatic interaction of all ions present in the solution and is equal to half the sum of the products of concentrations (With) each of the ions present in the solution per square of its charge number (z):
.
In dilute solutions (µ<0,1М) коэффициенты активности меньше единицы и уменьшаются с ростом ионной силы. Растворы с очень низкой ионной силой (µ < 1∙10 -4 М) можно считать идеальными. В бесконечно разбавленных растворах электролитов активность можно заменить истинной концентрацией. В идеальной системе a = c and the activity coefficient is 1. This means that there are practically no electrostatic interactions. In very concentrated solutions (µ>1M), ion activity coefficients can be greater than unity. The relationship between the activity coefficient and the ionic strength of the solution is expressed by the formulas:
at µ
<10 -2
at 10 -2 ≤ µ
≤ 10 -1
+ 0,1z 2 µ at 0.1<µ
<1
The equilibrium constant expressed in terms of activity is called thermodynamic. For example, for the reaction
a A+ b B d D+ e E
The thermodynamic constant has the form:
It depends on temperature, pressure and the nature of the solvent.
Since the activity of the particle is
Where TO C is the concentration equilibrium constant.
Meaning TO C depends not only on temperature, the nature of the solvent and pressure, but also on ionic strength m. Since thermodynamic constants depend on the smallest number of factors, they are therefore the most fundamental characteristics of equilibrium. Therefore, it is thermodynamic constants that are given in reference books. The thermodynamic constants of some weak electrolytes are given in the appendix of this manual. =0.024 mol/l.
As the charge of the ion increases, the activity coefficient and activity of the ion decreases.
Questions for self-control:
- What is an ideal system? Name the main reasons for the deviation of a real system from an ideal one.
- What is the degree of dissociation of electrolytes called?
- Give examples of strong and weak electrolytes.
- What relationship exists between the dissociation constant and the degree of dissociation of a weak electrolyte? Express it mathematically.
- What is activity? How are the activity of an ion and its true concentration related?
- What is the activity coefficient?
- How does the charge of an ion affect the activity coefficient?
- What is the ionic strength of a solution, its mathematical expression?
- Write down formulas for calculating the activity coefficients of individual ions depending on the ionic strength of the solution.
- Formulate the law of mass action and express it mathematically.
- What is the thermodynamic equilibrium constant? What factors influence its value?
- What is the concentration equilibrium constant? What factors influence its value?
- How are thermodynamic and concentration equilibrium constants related?
- Within what limits can the activity coefficient values vary?
- What are the main principles of the theory of strong electrolytes?