There are two main reasons for the passage of electric current through conductors: either due to the transfer of electrons, or due to the transfer of ions. Electronic conductivity is inherent primarily in metals. Ionic conductivity is inherent in many chemical compounds that have an ionic structure, for example, salts in solid or molten states, as well as many aqueous and non-aqueous solutions.
All substances by their behavior in solutions usually divided into two categories:
a) substances whose solutions have ionic conductivity (electrolytes);
b) substances whose solutions do not have ionic conductivity (non-electrolytes).
Electrolytes include most inorganic acids, bases and salts. Non-electrolytes include many organic compounds eg alcohols and carbohydrates.
It turned out that electrolyte solutions have lower melting points and higher boiling points compared to the corresponding values for a pure solvent or for a solution of a non-electrolyte in the same solvent. To explain these facts, Arrhenius proposed theory of electrolytic dissociation.
Under electrolytic dissociation refers to the breakdown of electrolyte molecules in solution with the formation of positively and negatively charged ions - cations and anions. For example, a molecule acetic acid dissociates in an aqueous solution like this:
CH 3 COOH CH 3 COO - + H +
The dissociation process in all cases is reversible, therefore, when writing the equations for the dissociation reaction, the reversibility sign is used. Different electrolytes dissociate into ions to varying degrees. The completeness of decomposition depends on the nature of the electrolyte, its concentration, the nature of the solvent, and temperature.
Strong and weak electrolytes. Degree of dissociation. Dissociation constant. Degree of dissociation α called - the ratio of the number of molecules disintegrated into ions (n) to the total number of dissolved molecules (n 0).
α = (n/n 0)?100
The degree of dissociation can vary from 0 to 1, from no dissociation to complete dissociation. Depending on the degree of dissociation, weak and strong electrolytes are distinguished. TO weak electrolytes include substances whose degree of dissociation in 0.1 M solutions is less than 3%; if the degree of dissociation in a 0.1 M solution exceeds 30%, then such an electrolyte is called strong. Electrolytes, the degree of dissociation of which lies in the range from 3% to 30%, are called electrolytes medium strength.
Strong electrolytes include most salts, some acids - HCl, HBr, HI, HNO 3, HClO 4, H 2 SO 4 and bases alkaline and alkaline earth metals- alkalis LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2.
The equation for the dissociation reaction of the electrolyte AA into K + cations and A - anions can be found in general view present as follows:
KA K + + A -
and degree of dissociation α in this case can be expressed as the ratio of the molar concentration of the formed ions [K + ] or [A - ] to original molar concentration of the electrolyte [AK] o, i.e.
With increasing solution concentration, the degree of electrolyte dissociation decreases.
Polybasic acids and bases dissociate stepwise - first one of the ions is split off from the molecule, then another, etc. Each dissociation step is characterized by its own dissociation constant.
Stage I: H 2 SO 4 → H + + HSO 4 -
Stage II: НSO 4 - Н + + SO 4 2-
General equation: H 2 SO 4 2H + + SO 4 2-
The process of electrolytic dissociation is characterized by dissociation constant (K) . So, for the reaction KA K + + A is the dissociation constant:
K = [K + ] ? [A - ]/[KA]
There is a quantitative relationship between the constant and the degree of electrolytic dissociation. In the example given, we denote the total concentration of the dissolved substance With , and the degree of dissociation α . Then [K + ] = [A - ] = α?с and accordingly the concentration of undissociated particles [CA] = (1 - α )With .
Substituting the values into the expression for the dissociation constant, we obtain the relation
Since the molar concentration is C = 1/V, then
This equation is a mathematical expression of Ostwald's dilution law: the dissociation constant of the electrolyte does not depend on the dilution of the solution.
Ionic product of water. pH solution. Water dissociation constant value K H2O = 1·10 -14 . This constant for water is called ionic product of water, which depends only on temperature.
According to the reaction H 2 OH + + OH -, during the dissociation of water, one OH - ion is formed for each H + ion, therefore, in pure water the concentrations of these ions are the same: [H + ] = [OH - ] = 10 -7 .
pH = -log[H + ]
Aqueous solutions have a pH value in the range from 1 to 14. Based on the ratio of the concentrations of these ions, three types of media are distinguished: neutral, acidic and alkaline.
Neutral environment- an environment in which the ion concentrations [H + ] = [OH - ] = 10 -7 mol/l (pH = 7).
Acidic environment- a medium in which the concentration of [H + ] ions is greater than the concentration of [OH - ] ions, i.e. [H + ] > 10 -7 mol/l (pH< 7).
Alkaline environment- an environment in which the concentration of [H + ] ions is less than the concentration of [OH - ] ions, i.e. [H+]< 10 -7 моль/л (рН > 7).
Qualitatively, the reaction of the medium and the pH of aqueous solutions of electrolytes are determined using indicators and a pH meter.
For example, if the ion concentration = 10 -4 mol/l, then pH = - log10 -4 = 4 and the solution medium is acidic, and if the ion concentration is [OH - ] = 10 -4 mol/l, then [H + ] = TO(H 2 O) - [OH - ] = 10 -14 - 10 -4 = 10 -10, and pH = - log10 -10 = 10 and the solution is alkaline.
Product of solubility. The dissolution of a solid in water stops when a saturated solution is formed, i.e. a balance is established between solid and particles of the same substance in solution. So, for example, in a saturated solution of silver chloride the equilibrium is established:
AgCl solid Ag + aq + Cl - aq
In a saturated electrolyte solution, the product of the concentrations of its ions is a constant value at a given temperature and this value quantitatively characterizes the ability of the electrolyte to dissolve, it is called solubility product(PR).
PR(AgCl) = [Ag + ]
Solubility product - this is a constant value equal to the product of the concentrations of ions of a poorly soluble electrolyte in its saturated solution. In general, for a slightly soluble electrolyte of composition A m B n we can write: A m B n ↔ mA + nB
PR AmBn = [A] m ? [B]n
Knowing the values of solubility products, it is possible to solve issues related to the formation or dissolution of precipitation during chemical reactions, which is especially important for analytical chemistry.
Spontaneous partial or complete disintegration of dissolved electrolytes (see) into ions is called electrolytic dissociation. The term “ions” was introduced by the English physicist M. Faraday (1833). The theory of electrolytic dissociation was formulated by the Swedish scientist S. Arrhenius (1887) to explain the properties of aqueous solutions of electrolytes. Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of the atom and chemical bonds. Modern content This theory can be reduced to the following three provisions:
1. Electrolytes, when dissolved in water, dissociate (break up) into ions - positively and negatively charged. (“Ion” is Greek for “wandering.” In a solution, ions move randomly in different directions.)
2. Under the influence of electric current, ions acquire directional movement: positively charged ones move towards the cathode, negatively charged ones move towards the anode. Therefore, the former are called cations, the latter - anions. The directional movement of ions occurs as a result of the attraction of their oppositely charged electrodes.
3. Dissociation is a reversible process. This means that a state of equilibrium occurs in which as many molecules break up into ions (dissociation), so many of them are formed again from ions (association).
Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used.
For example:
where KA is an electrolyte molecule, is a cation, A is an anion.
The doctrine of chemical bonding helps answer the question of why electrolytes dissociate into ions. Substances with ionic bonds dissociate most easily, since they already consist of ions (see Chemical bonding). When they dissolve, the water dipoles are oriented around the positive and negative ions. Mutual attractive forces arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. Electrolytes, whose molecules are formed according to the type of covalent bond, dissociate similarly. polar connection. Dissociation polar molecules can be complete or partial - it all depends on the degree of polarity of the bonds. In both cases (during the dissociation of compounds with ionic and polar bonds), hydrated ions are formed, that is, ions chemically bonded to water molecules (see figure on p. 295).
The founder of this view of electrolytic dissociation was honorary academician I. A. Kablukov. In contrast to the Arrhenius theory, which did not take into account the interaction of the solute with the solvent, I. A. Kablukov applied the chemical theory of solutions of D. I. Mendeleev to explain electrolytic dissociation. He showed that upon dissolution occurs chemical reaction dissolved substance with water, which leads to the formation of hydrates, and then they dissociate into ions. I. A. Kablukov believed that an aqueous solution contains only hydrated ions. Currently, this idea is generally accepted. So, ion hydration is the main cause of dissociation. In others, not aqueous solutions In electrolytes, the chemical bond between particles (molecules, ions) of a solute and particles of a solvent is called solvation.
Hydrated ions have both a constant and variable number of water molecules. A hydrate of constant composition forms hydrogen ions that hold one molecule, this is a hydrated proton. IN scientific literature it is usually represented by a formula and called the hydronium ion.
Since electrolytic dissociation is a reversible process, in solutions of electrolytes, along with their ions, there are also molecules. Therefore, electrolyte solutions are characterized by the degree of dissociation (denoted Greek letter A). The degree of dissociation is the ratio of the number of molecules dissociated into ions n to the total number of dissolved molecules:
The degree of electrolyte dissociation is determined experimentally and is expressed in fractions of a unit or as a percentage. If there is no dissociation, and if or 100%, then the electrolyte completely disintegrates into ions. Different electrolytes have different degrees of dissociation. With dilution of the solution it increases, and with the addition of ions of the same name (the same as the electrolyte ions) it decreases.
However, to characterize the ability of an electrolyte to dissociate into ions, the degree of dissociation is not very convenient size, since it depends on the electrolyte concentration. More general characteristic is the dissociation constant K. It can be easily derived by applying the law of mass action to the electrolyte dissociation equilibrium:
where KA is the equilibrium concentration of the electrolyte, and are the equilibrium concentrations of its ions (see Chemical equilibrium). K does not depend on concentration. It depends on the nature of the electrolyte, solvent and temperature.
For weak electrolytes, the greater the K (dissociation constant), the stronger the electrolyte, the more ions in the solution.
Strong electrolytes do not have dissociation constants. Formally, they can be calculated, but they will not be constant as the concentration changes.
Polybasic acids dissociate in steps, which means that such acids will have several dissociation constants - one for each step. For example:
First stage:
Second stage:
Third stage:
Always, i.e., a polybasic acid, when dissociated in the first step, behaves as a stronger acid than in the second or third.
Polyacid bases also undergo stepwise dissociation. For example:
Acidic and basic salts also dissociate stepwise. For example:
In this case, in the first step, the salt completely disintegrates into ions, which is due to the ionic nature of the bond between and; and dissociation in the second stage is insignificant, since charged particles (ions) undergo further dissociation as very weak electrolytes.
From the point of view of the theory of electrolytic dissociation, definitions are given and the properties of such classes are described. chemical compounds as acids, bases, salts.
Acids are electrolytes whose dissociation produces only hydrogen ions as cations. For example:
All general characteristic properties acids - sour taste, change in color of indicators, interaction with bases, basic oxides, salts - are caused by the presence of hydrogen ions, more precisely.
Bases are electrolytes whose dissociation produces only hydroxide ions as anions:
According to the theory of electrolytic dissociation, all common alkaline properties solutions - soapiness to the touch, change in color of indicators, interaction with acids, acid anhydrides, salts - are due to the presence of hydroxide ions.
True, there are electrolytes, during the dissociation of which both hydrogen ions and hydroxide ions are simultaneously formed. These electrolytes are called amphoteric or ampholytes. These include water, zinc, aluminum, chromium hydroxides and a number of other substances. Water, for example, in small quantities dissociates into ions and:
Since all reactions in aqueous solutions of electrolytes represent the interaction of ions, the equations for these reactions can be written in ionic form.
The significance of the theory of electrolytic dissociation is that it explained numerous phenomena and processes occurring in aqueous solutions of electrolytes. However, it does not explain the processes occurring in non-aqueous solutions. So, if ammonium chloride in an aqueous solution behaves like a salt (dissociates into ions and ), then in liquid ammonia it exhibits the properties of an acid - it dissolves metals with the release of hydrogen. Nitric acid behaves as a base when dissolved in liquid hydrogen fluoride or anhydrous sulfuric acid.
All these factors contradict the theory of electrolytic dissociation. They are explained by the protolytic theory of acids and bases.
The term “dissociation” itself means the disintegration of molecules into several more simple particles. In chemistry, in addition to electrolytic dissociation, thermal dissociation is distinguished. This is a reversible reaction that occurs when the temperature increases. For example, thermal dissociation of water vapor:
calcium carbonate:
iodine molecules:
The equilibrium of thermal dissociation obeys the law of mass action.
During the dissociation of acids, the role of cations is played by hydrogen ions(H +), no other cations are formed during the dissociation of acids:
HF ↔ H + + F - HNO 3 ↔ H + + NO 3 -
It is hydrogen ions that give acids their characteristic properties: sour taste, coloring of the indicator red, etc.
Negative ions (anions) split off from an acid molecule make up acid residue.
One of the characteristics of the dissociation of acids is their basicity - the number of hydrogen ions contained in an acid molecule that can be formed during dissociation:
- monobasic acids: HCl, HF, HNO 3;
- dibasic acids: H 2 SO 4, H 2 CO 3;
- tribasic acids: H 3 PO 4.
The process of elimination of hydrogen cations in polybasic acids occurs in stages: first one hydrogen ion is eliminated, then another (third).
Stepwise dissociation of a dibasic acid:
H 2 SO 4 ↔ H + + HSO 4 - HSO 4 - ↔ H + + HSO 4 2-
Stepwise dissociation of a tribasic acid:
H 3 PO 4 ↔ H + + H 2 PO 4 - H 2 PO 4 - ↔ H + + HPO 4 2- HPO 4 2- ↔ H + + PO 4 3-
When dissociating polybasic acids, the highest degree of dissociation occurs in the first step. For example, during dissociation phosphoric acid the degree of first-stage dissociation is 27%; second - 0.15%; third - 0.005%.
Base dissociation
During the dissociation of bases, the role of anions is played by hydroxide ions(OH -), no other anions are formed during the dissociation of bases:
NaOH ↔ Na + + OH -
The acidity of a base is determined by the number of hydroxide ions formed during the dissociation of one molecule of the base:
- monoacid bases - KOH, NaOH;
- diacid bases - Ca(OH) 2;
- triacid bases - Al(OH) 3.
Polyacid bases, by analogy with acids, also dissociate stepwise - at each stage one hydroxide ion is split off:
Some substances, depending on the conditions, can act both as acids (dissociate with the elimination of hydrogen cations) and as bases (dissociate with the elimination of hydroxide ions). Such substances are called amphoteric(See Acid-base reactions).
Dissociation of Zn(OH) 2 as bases:
Zn(OH) 2 ↔ ZnOH + + OH - ZnOH + ↔ Zn 2+ + OH -
Dissociation of Zn(OH) 2 as an acid:
Zn(OH) 2 + 2H 2 O ↔ 2H + + 2-
Dissociation of salts
Salts dissociate in water into anions of acidic residues and cations of metals (or other compounds).
Classification of salt dissociation:
- Normal (medium) salts are obtained by complete simultaneous replacement of all hydrogen atoms in the acid with metal atoms - these are strong electrolytes, completely dissociate in water with the formation of metal catoins and a one-acid residue: NaNO 3, Fe 2 (SO 4) 3, K 3 PO 4.
- Acid salts contain in their composition, in addition to metal atoms and an acidic residue, one more (several) hydrogen atoms - they dissociate stepwise with the formation of metal cations, anions of the acidic residue and a hydrogen cation: NaHCO 3, KH 2 PO 4, NaH 2 PO 4.
- Basic salts contain in their composition, in addition to metal atoms and an acidic residue, one more (several) hydroxyl groups - they dissociate with the formation of metal cations, anions of the acidic residue and hydroxide ion: (CuOH) 2 CO 3, Mg(OH)Cl.
- Double salts are obtained by simultaneous replacement of hydrogen atoms in the acid with atoms of various metals: KAl(SO 4) 2.
- Mixed salts dissociate into metal cations and anions of several acidic residues: CaClBr.
Electrolytes and non-electrolytes
From physics lessons we know that solutions of some substances are capable of conducting electric current, but others do not.
Substances whose solutions conduct electric current are called electrolytes.
Substances whose solutions do not conduct electric current are called non-electrolytes. For example, solutions of sugar, alcohol, glucose and some other substances do not conduct electricity.
Electrolytic dissociation and association
Why do electrolyte solutions conduct electric current?
Swedish scientist S. Arrhenius, studying electrical conductivity various substances, came to the conclusion in 1877 that the cause of electrical conductivity is the presence in solution ions, which are formed when an electrolyte is dissolved in water.
The process of electrolyte breaking down into ions is called electrolytic dissociation.
S. Arrhenius, who adhered to the physical theory of solutions, did not take into account the interaction of the electrolyte with water and believed that there were free ions in solutions. In contrast, Russian chemists I.A. Kablukov and V.A. Kistyakovsky applied the chemical theory of D.I. Mendeleev to explain electrolytic dissociation and proved that when an electrolyte is dissolved, a chemical interaction of the dissolved substance with water occurs, which leads to the formation hydrates, and then they dissociate into ions. They believed that solutions contained not free, not “naked” ions, but hydrated ones, that is, “dressed in a coat” of water molecules.
Water molecules are dipoles(two poles), since the hydrogen atoms are located at an angle of 104.5°, due to which the molecule has an angular shape. The water molecule is shown schematically below.
As a rule, substances dissociate most easily with ionic bond and, accordingly, with ionic crystal lattice, since they already consist of ready-made ions. When they dissolve, the water dipoles are oriented with oppositely charged ends around the positive and negative ions of the electrolyte.
Mutual attractive forces arise between electrolyte ions and water dipoles. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. It is obvious that the sequence of processes occurring during the dissociation of substances with ionic bonds (salts and alkalis) will be as follows:
1) orientation of water molecules (dipoles) near the ions of the crystal;
2) hydration (interaction) of water molecules with ions of the surface layer of the crystal;
3) dissociation (decay) of the electrolyte crystal into hydrated ions.
Simplified processes can be reflected using the following equation:
Electrolytes dissociate similarly, in the molecules of which covalent bond(for example, hydrogen chloride molecules HCl, see below); only in this case, under the influence of water dipoles, the transformation of a covalent polar bond into an ionic one occurs; The sequence of processes occurring in this case will be as follows:
1) orientation of water molecules around the poles of electrolyte molecules;
2) hydration (interaction) of water molecules with electrolyte molecules;
3) ionization of electrolyte molecules (conversion of a covalent polar bond into an ionic one);
4) dissociation (decay) of electrolyte molecules into hydrated ions.
In a simplified way, the process of dissociation of hydrochloric acid can be reflected using the following equation:
It should be taken into account that in electrolyte solutions, chaotically moving hydrated ions can collide and recombine with each other. This reverse process is called association. Association in solutions occurs in parallel with dissociation, therefore the reversibility sign is put in the reaction equations.
The properties of hydrated ions differ from those of non-hydrated ions. For example, the unhydrated copper ion Cu 2+ is white in anhydrous crystals of copper (II) sulfate and has a blue color when hydrated, i.e., bound to water molecules Cu 2+ nH 2 O. Hydrated ions have both constant and variable number of water molecules.
Degree of electrolytic dissociation
In electrolyte solutions, along with ions, there are also molecules. Therefore, electrolyte solutions are characterized degree of dissociation, which is denoted by the Greek letter a (“alpha”).
This is the ratio of the number of particles broken up into ions (N g) to the total number of dissolved particles (N p).
The degree of electrolyte dissociation is determined experimentally and is expressed in fractions or percentages. If a = 0, then there is no dissociation, and if a = 1, or 100%, then the electrolyte completely disintegrates into ions. Different electrolytes have different degrees of dissociation, i.e. the degree of dissociation depends on the nature of the electrolyte. It also depends on the concentration: as the solution is diluted, the degree of dissociation increases.
Based on the degree of electrolytic dissociation, electrolytes are divided into strong and weak.
Strong electrolytes- these are electrolytes that, when dissolved in water, almost completely dissociate into ions. For such electrolytes, the degree of dissociation tends to unity.
Strong electrolytes include:
1) all soluble salts;
2) strong acids, for example: H 2 SO 4, HCl, HNO 3;
3) all alkalis, for example: NaOH, KOH.
Weak electrolytes- these are electrolytes that, when dissolved in water, almost do not dissociate into ions. For such electrolytes, the degree of dissociation tends to zero.
Weak electrolytes include:
1) weak acids - H 2 S, H 2 CO 3, HNO 2;
2) aqueous solution of ammonia NH 3 H 2 O;
4) some salts.
Dissociation constant
In solutions of weak electrolytes, due to their incomplete dissociation, dynamic equilibrium between undissociated molecules and ions. For example, for acetic acid:
You can apply the law of mass action to this equilibrium and write down the expression for the equilibrium constant:
The equilibrium constant characterizing the process of dissociation of a weak electrolyte is called dissociation constant.
The dissociation constant characterizes the ability of an electrolyte (acid, base, water) dissociate into ions. The larger the constant, the easier the electrolyte breaks down into ions, therefore, the stronger it is. The values of dissociation constants for weak electrolytes are given in reference books.
Basic principles of the theory of electrolytic dissociation
1. When dissolved in water, electrolytes dissociate (break up) into positive and negative ions.
Ions is one of the forms of existence of a chemical element. For example, sodium metal atoms Na 0 vigorously interact with water, forming alkali (NaOH) and hydrogen H 2, while sodium ions Na + do not form such products. Chlorine Cl 2 has a yellow-green color and a pungent odor, and is poisonous, while chlorine ions Cl are colorless, non-toxic, and odorless.
Ions- these are positively or negatively charged particles into which atoms or groups of atoms of one or more are transformed chemical elements as a result of the donation or gain of electrons.
In solutions, ions move randomly in different directions.
According to their composition, ions are divided into simple- Cl - , Na + and complex- NH 4 + , SO 2 - .
2. The reason for the dissociation of an electrolyte in aqueous solutions is its hydration, i.e., the interaction of the electrolyte with water molecules and the breaking of the chemical bond in it.
As a result of this interaction, hydrated ions are formed, i.e. associated with water molecules. Consequently, according to the presence of a water shell, ions are divided into hydrated(in solutions and crystalline hydrates) and unhydrated(in anhydrous salts).
3. Under the influence of an electric current, positively charged ions move to the negative pole of the current source - the cathode and are therefore called cations, and negatively charged ions move to the positive pole of the current source - the anode and are therefore called anions.
Consequently, there is another classification of ions - according to the sign of their charge.
The sum of the charges of cations (H +, Na +, NH 4 +, Cu 2+) is equal to the sum of the charges of anions (Cl -, OH -, SO 4 2-), as a result of which electrolyte solutions (HCl, (NH 4) 2 SO 4, NaOH, CuSO 4) remain electrically neutral.
4. Electrolytic dissociation- the process is reversible for weak electrolytes.
Along with the dissociation process (decomposition of the electrolyte into ions), the reverse process also occurs - association(combination of ions). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used, for example:
5. Not all electrolytes dissociate into ions to the same extent.
Depends on the nature of the electrolyte and its concentration. The chemical properties of electrolyte solutions are determined by the properties of the ions that they form during dissociation.
The properties of weak electrolyte solutions are determined by the molecules and ions formed during the dissociation process, which are in dynamic equilibrium with each other.
The smell of acetic acid is due to the presence of CH 3 COOH molecules, the sour taste and color change of indicators are associated with the presence of H + ions in the solution.
The properties of solutions of strong electrolytes are determined by the properties of the ions that are formed during their dissociation.
For example, the general properties of acids, such as sour taste, changes in the color of indicators, etc., are due to the presence of hydrogen cations (more precisely, oxonium ions H 3 O +) in their solutions. General properties alkalis, such as soapiness to the touch, changes in the color of indicators, etc., are associated with the presence of hydroxide ions OH - in their solutions, and the properties of salts are associated with their decomposition in solution into metal (or ammonium) cations and anions of acid residues.
According to the theory of electrolytic dissociation all reactions in aqueous solutions of electrolytes are reactions between ions. This accounts for the high speed of many chemical reactions in electrolyte solutions.
Reactions occurring between ions are called ionic reactions
, and the equations of these reactions are ionic equations.
Ion exchange reactions in aqueous solutions can occur:
1. Irreversible, to the end.
2. Reversible, that is, to flow simultaneously in two opposite directions. Exchange reactions between strong electrolytes in solutions proceed to completion or are practically irreversible when the ions combine with each other to form substances:
a) insoluble;
b) low dissociating (weak electrolytes);
c) gaseous.
Here are some examples of molecular and abbreviated ionic equations:
The reaction is irreversible, because one of its products is an insoluble substance.
The neutralization reaction is irreversible, because a low-dissociating substance is formed - water.
The reaction is irreversible, because CO 2 gas and a low-dissociating substance - water - are formed.
If among the starting substances and among the reaction products there are weak electrolytes or poorly soluble substances, then such reactions are reversible, that is, they do not proceed to completion.
IN reversible reactions the equilibrium shifts towards the formation of the least soluble or least dissociated substances.
For example:
The equilibrium shifts towards the formation of a weaker electrolyte - H 2 O. However, such a reaction will not proceed to completion: undissociated molecules of acetic acid and hydroxide ions remain in the solution.
If the starting substances are strong electrolytes, which upon interaction do not form insoluble or slightly dissociating substances or gases, then such reactions do not occur: when mixing solutions, a mixture of ions is formed.
Reference material for taking the test:
Periodic table
Solubility table
LAB 3
ELECTROLYTES
Purpose of the work: studying chemical properties strong and weak electrolytes.
Job Objectives: establish the shift of ionic equilibrium in solutions of weak electrolytes; learn to compose ion-molecular exchange reactions in electrolyte solutions.
Reagents: CH 3 COONa (sol.), NaCl (sol.), solutions: HCl (0.1 M), CH 3 COOH (0.1 M), H 2 SO 4, Pb(NO 3) 2, K 2 CrO 4, BaCl 2, Na 2 SO 4, CuSO 4, NiSO 4, Na 2 CO 3, NH 4 Cl, NaOH (conc.), methyl orange; additionally: NH 4 OH (dil.), NH 4 OH (conc.).
Equipment: test tubes, pipettes.
THEORETICAL PART
Inorganic substances, solutions of which conduct electric current are called electrolytes. Solutions of many organic matter(sugars, alcohols) do not conduct electricity and are non-electrolytes.
The term “electrolytes” was introduced by Faraday, who believed that the conductivity of solutions is due to the decomposition of substances in an electric field (translated from Greek, electrolyte means “decomposed by electricity”). However, in late XIX V. S. A. Arrhenius based on the properties of solutions ∆T deputy, ∆T kip, P Osm (Raoult's and van't Hoff's laws) showed that decay occurs outside the field - at the stage of dissolution of substances. Arrhenius is the author of the foundations of the theory of electrolytic dissociation.
Modern theory solutions (I.A. Kablukov and D.I. Mendeleev participated in its development) is based on the decisive role of the solvent (polar water molecules) in dissociation. The process of appearance of hydrated ions in aqueous solutions is called electrolytic dissociation.
Electrolytes– these are substances that dissociate (break up) into ions. Ions in solutions are not free, but form weak compounds with water - hydrates. Electrolytes include acids, bases, salts and complex compounds.
To explain the differences in properties (electrical conductivity and others) between electrolytes and non-electrolytes, Arrhenius introduced the concepts of the degree of dissociation and strength of electrolytes. Degree of electrolyte dissociation α is the fraction of molecules that break up into ions. Based on the degree of dissociation, electrolytes are divided into strong and weak (sometimes electrolytes of medium strength are isolated).
Strong electrolytes(α ≈ 1 or 100%) in solutions almost completely dissociate into ions. Solutions consist only of ions (more precisely, hydrates of ions). This process is irreversible. Strong electrolytes include:
Acids: HCl, HBr, HI, HNO 3, HClO 3, HClO 4, H 2 Cr 2 O 7, HMnO 4, H 2 SO 4;
Bases (alkalis): LiOH, NaOH, KOH, Ca(OH) 2, Ba(OH) 2, Sr(OH) 2;
Salts: almost everything;
Complex connections(majority): K 3, SO 4, Na 2, etc.
Weak electrolytes (α< 0,3 или 30 %) диссоциируют на ионы незначительно и обратимо, к ним применим закон действующих масс. Их растворы состоят преимущественно из молекул. К слабым электролитам относятся многие неорганические и все органические кислоты, раствор аммиака, малорастворимые основания. Величина α слабых электролитов зависит от их концентрации: с разбавлением степень диссоциации увеличивается. Ее рассчитывают по закону разбавления Оствальда:
Where TO d – dissociation constant of a weak electrolyte (reference value);
C– molar concentration of the substance, mol/l.
Calculations are accurate only in dilute solutions at WITH < 0,1 моль/л. При больших концентрациях величину WITH replaced by activity A:
a = f C, (2)
Where f– activity coefficient ( f < 1 – справочная величина).
The reason is that with increasing concentration in solutions, a strong ion-ion interaction begins to appear: the Coulomb interaction of cations and anions with each other. For example, in a 0.1 M solution of fcndjht NaCl solution f= 0.78, and in 1 M NaCl solution f= 0.66; in 0.001 M CaCl 2 solution f= 0.84, and in a 1 M solution of CaCl 2 f = 0,5.
Examples of dissociation of substances without taking into account hydration (in a simplified form)
Dissociation of acids
Strong acids immediately completely dissociate into ions
HClO 4 → H + + ClO 4 – (α ≈ 100%).
Solution composition: H + and ClO 4 – ions.
Weak acids dissociate into ions reversibly
HClO ↔ H + + ClO – ( α 0.1 M solution = 0.05%),
Composition of the solution: HClO molecules (> 99%) and H +, ClO – ions.
Weak polybasic acids dissociate into ions reversibly and stepwise
H 2 S ↔ H + + HS – (1st stage of dissociation, α0.1 M solution = 0.07%),
HS – ↔ H + + S 2 – (2nd stage of dissociation, α<< 1).
Composition of the solution: H 2 S molecules (> 99%), some H + and HS – ions and very little S 2 – .
Base dissociation
Strong bases immediately completely dissociate into ions
Ba(OH) 2 → Ba 2+ + 2OH – (α ≈ 100%).
Solution composition: Ba 2+ and OH – ions.
Weak bases dissociate into ions reversibly (polyacid bases dissociate stepwise)
NH 4 OH ↔ NH 4 + + OH – (α0.1 M solution = 1.3%).
Composition of the solution: predominantly NH 4 OH molecules and partially NH 4 + and OH – ions.
Dissociation of salts
Medium salts immediately completely dissociate into ions:
Na 2 S → 2Na + + S 2 – ( α ≈ 100 %).
Composition of the solution: Na + and S 2 - ions.
Acidic and basic salts dissociate into ions in the first step as strong electrolytes - irreversibly, and in the second and third steps as weak electrolytes - reversibly
NaHS → Na + + HS – (1st dissociation step, α ≈ 100 %),
CuONНNO 3 → CuOH + + NO 3 – (1st dissociation stage, α ≈ 100 %).
The second stage of dissociation can be neglected ( α << 1).
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