SOLUTIONS
THEORY OF ELECTROLYTIC DISSOCIATION
ELECTROLYTIC DISSOCIATION
ELECTROLYTES AND NON-ELECTROLYTES
Electrolytic dissociation theory
(S. Arrhenius, 1887)
1. When dissolved in water (or melted), electrolytes break down into positively and negatively charged ions (subject to electrolytic dissociation).
2. Under the influence of electric current, cations (+) move towards the cathode (-), and anions (-) move towards the anode (+).
3. Electrolytic dissociation is a reversible process (the reverse reaction is called molarization).
4. Degree of electrolytic dissociation ( a ) depends on the nature of the electrolyte and solvent, temperature and concentration. It shows the ratio of the number of molecules broken up into ions ( n ) to the total number of molecules introduced into the solution ( N).
a = n / N 0< a <1
Mechanism of electrolytic dissociation of ionic substances
When dissolving compounds with ionic bonds ( for example NaCl ) the hydration process begins with the orientation of water dipoles around all the protrusions and faces of the salt crystals.
Orienting around the ions of the crystal lattice, water molecules form either hydrogen or donor-acceptor bonds with them. This process releases a large amount of energy, which is called hydration energy.
The energy of hydration, the magnitude of which is comparable to the energy of the crystal lattice, is used to destroy the crystal lattice. In this case, the hydrated ions pass layer by layer into the solvent and, mixing with its molecules, form a solution.
Mechanism of electrolytic dissociation of polar substances
Substances whose molecules are formed according to the type of polar covalent bond (polar molecules) dissociate similarly. Around each polar molecule of matter ( for example HCl ), water dipoles are oriented in a certain way. As a result of interaction with water dipoles, the polar molecule becomes even more polarized and turns into an ionic molecule, then free hydrated ions are easily formed.
Electrolytes and non-electrolytes
The electrolytic dissociation of substances, which occurs with the formation of free ions, explains the electrical conductivity of solutions.
The process of electrolytic dissociation is usually written down in the form of a diagram, without revealing its mechanism and omitting the solvent ( H2O ), although he is the main participant.
CaCl 2 « Ca 2+ + 2Cl -
KAl(SO 4) 2 « K + + Al 3+ + 2SO 4 2-
HNO 3 « H + + NO 3 -
Ba(OH) 2 « Ba 2+ + 2OH -
From the electrical neutrality of molecules it follows that the total charge of cations and anions should be equal to zero.
For example, for
Al 2 (SO 4) 3 ––2 (+3) + 3 (-2) = +6 - 6 = 0
KCr(SO 4) 2 ––1 (+1) + 3 (+3) + 2 (-2) = +1 + 3 - 4 = 0
Strong electrolytes
These are substances that, when dissolved in water, almost completely disintegrate into ions. As a rule, strong electrolytes include substances with ionic or highly polar bonds: all highly soluble salts, strong acids ( HCl, HBr, HI, HClO4, H2SO4, HNO3 ) and strong bases ( LiOH, NaOH, KOH, RbOH, CsOH, Ba (OH) 2, Sr (OH) 2, Ca (OH) 2).
In a strong electrolyte solution, the solute is mainly in the form of ions (cations and anions); undissociated molecules are practically absent.
Weak electrolytes
Substances that partially dissociate into ions. Solutions of weak electrolytes contain undissociated molecules along with ions. Weak electrolytes cannot produce a high concentration of ions in solution.
Weak electrolytes include:
1) almost all organic acids ( CH 3 COOH, C 2 H 5 COOH, etc.);
2) some inorganic acids ( H 2 CO 3, H 2 S, etc.);
3) almost all salts, bases and ammonium hydroxide that are slightly soluble in water(Ca 3 (PO 4) 2; Cu (OH) 2; Al (OH) 3; NH 4 OH);
4) water.
They conduct electricity poorly (or almost not at all).
СH 3 COOH « CH 3 COO - + H +
Cu(OH) 2 «[CuOH] + + OH - (first stage)
[CuOH] + « Cu 2+ + OH - (second stage)
H 2 CO 3 « H + + HCO - (first stage)
HCO 3 - « H + + CO 3 2- (second stage)
Non-electrolytes
Substances whose aqueous solutions and melts do not conduct electric current. They contain covalent non-polar or low-polar bonds that do not break down into ions.
Gases, solids (non-metals), and organic compounds (sucrose, gasoline, alcohol) do not conduct electric current.
Degree of dissociation. Dissociation constant
The concentration of ions in solutions depends on how completely a given electrolyte dissociates into ions. In solutions of strong electrolytes, the dissociation of which can be considered complete, the concentration of ions can be easily determined from the concentration (c) and the composition of the electrolyte molecule (stoichiometric indices), For example :
The concentrations of ions in solutions of weak electrolytes are qualitatively characterized by the degree and dissociation constant.
Degree of dissociation (a) - the ratio of the number of molecules disintegrated into ions ( n ) to the total number of dissolved molecules ( N):
a=n/N
and is expressed in fractions of a unit or in % ( a = 0.3 – conventional limit of division into strong and weak electrolytes).
Example
Determine the molar concentration of cations and anions in 0.01 M solutions KBr, NH 4 OH, Ba (OH) 2, H 2 SO 4 and CH 3 COOH.
Degree of dissociation of weak electrolytes a = 0.3.
Solution
KBr, Ba(OH)2 and H2SO4 - strong electrolytes that dissociate completely(a = 1).
KBr « K + + Br -
0.01 M
Ba(OH) 2 « Ba 2+ + 2OH -
0.01 M
0.02M
H 2 SO 4 « 2H + + SO 4
0.02M
[ SO 4 2- ] = 0.01 M
NH 4 OH and CH 3 COOH – weak electrolytes(a = 0.3)
NH 4 OH + 4 + OH -
0.3 0.01 = 0.003 M
CH 3 COOH « CH 3 COO - + H +
[H + ] = [ CH 3 COO - ] = 0.3 0.01 = 0.003 M
The degree of dissociation depends on the concentration of the weak electrolyte solution. When diluted with water, the degree of dissociation always increases, because the number of solvent molecules increases ( H2O ) per molecule of solute. According to Le Chatelier’s principle, the equilibrium of electrolytic dissociation in this case should shift in the direction of the formation of products, i.e. hydrated ions.
The degree of electrolytic dissociation depends on the temperature of the solution. Typically, as the temperature increases, the degree of dissociation increases, because bonds in molecules are activated, they become more mobile and are easier to ionize. The concentration of ions in a weak electrolyte solution can be calculated by knowing the degree of dissociationaand initial concentration of the substancec in solution.
Example
Determine the concentration of undissociated molecules and ions in a 0.1 M solution NH4OH , if the degree of dissociation is 0.01.
Solution
Molecular concentrations NH4OH , which at the moment of equilibrium will disintegrate into ions, will be equal toac. Ion concentration NH 4 - and OH - - will be equal to the concentration of dissociated molecules and equalac(according to the electrolytic dissociation equation)
|
NH4OH |
NH4+ |
OH- |
||
|
c - a c |
A c = 0.01 0.1 = 0.001 mol/l
[NH 4 OH] = c - a c = 0.1 – 0.001 = 0.099 mol/l
Dissociation constant ( K D ) is the ratio of the product of equilibrium ion concentrations to the power of the corresponding stoichiometric coefficients to the concentration of undissociated molecules.
It is the equilibrium constant of the electrolytic dissociation process; characterizes the ability of a substance to disintegrate into ions: the higher K D , the greater the concentration of ions in the solution.
Dissociations of weak polybasic acids or polyacid bases occur in steps; accordingly, each step has its own dissociation constant:
First stage:
H 3 PO 4 « H + + H 2 PO 4 -
K D 1 = () / = 7.1 10 -3
Second stage:
H 2 PO 4 - « H + + HPO 4 2-
K D 2 = () / = 6.2 10 -8
Third stage:
HPO 4 2- « H + + PO 4 3-
K D 3 = () / = 5.0 10 -13
K D 1 > K D 2 > K D 3
Example
Derive an equation relating the degree of electrolytic dissociation of a weak electrolyte ( a ) with dissociation constant (Ostwald dilution law) for a weak monoprotic acid ON .
HA « H + + A +
K D = () /
If the total concentration of a weak electrolyte is denotedc, then the equilibrium concentrations H + and A - are equal ac, and the concentration of undissociated molecules ON - (c - a c) = c (1 - a)
K D = (a c a c) / c(1 - a ) = a 2 c / (1 - a )
In the case of very weak electrolytes ( a £ 0.01)
K D = c a 2 or a = \ é (K D / c )
Example
Calculate the degree of dissociation of acetic acid and the ion concentration H + in 0.1 M solution, if K D (CH 3 COOH) = 1.85 10 -5
Solution
Let's use Ostwald's dilution law
\é (K D / c ) = \é((1.85 10 -5) / 0.1 )) = 0.0136 or a = 1.36%
[H+] = a c = 0.0136 0.1 mol/l
Solubility product
Definition
Place some sparingly soluble salt in a beaker, for example AgCl and add distilled water to the sediment. In this case, the ions Ag+ and Cl- , experiencing attraction from the surrounding water dipoles, gradually break away from the crystals and go into solution. Colliding in solution, ions Ag+ and Cl- form molecules AgCl and deposited on the surface of the crystals. Thus, two mutually opposite processes occur in the system, which leads to dynamic equilibrium, when the same number of ions pass into the solution per unit time Ag+ and Cl- , how many of them are deposited. Ion accumulation Ag+ and Cl- stops in solution, it turns out saturated solution. Consequently, we will consider a system in which there is a precipitate of a sparingly soluble salt in contact with a saturated solution of this salt. In this case, two mutually opposite processes occur:
1) Transition of ions from precipitate to solution. The rate of this process can be considered constant at a constant temperature: V 1 = K 1 ;
2) Precipitation of ions from solution. The speed of this process V 2 depends on ion concentration Ag + and Cl - . According to the law of mass action:
V 2 = k 2
Since this system is in a state of equilibrium, then
V 1 = V 2
k 2 = k 1
K 2 / k 1 = const (at T = const)
Thus, the product of ion concentrations in a saturated solution of a sparingly soluble electrolyte at a constant temperature is constant size. This quantity is calledsolubility product(ETC ).
In the given example ETC AgCl = [Ag + ] [Cl - ] . In cases where the electrolyte contains two or more identical ions, the concentration of these ions must be raised to the appropriate power when calculating the solubility product.
For example, PR Ag 2 S = 2; PR PbI 2 = 2
In general, the expression for the product of solubility for an electrolyte is A m B n
PR A m B n = [A] m [B] n .
The values of the solubility product are different for different substances.
For example, PR CaCO 3 = 4.8 10 -9; PR AgCl = 1.56 10 -10.
ETC easy to calculate, knowing ra c solubility of a compound at a given t°.
Example 1
The solubility of CaCO 3 is 0.0069 or 6.9 10 -3 g/l. Find the PR of CaCO 3.
Solution
Let's express the solubility in moles:
S CaCO 3 = ( 6,9 10 -3 ) / 100,09 = 6.9 10 -5 mol/l
MCaCO3
Since every molecule CaCO3 gives one ion when dissolved Ca 2+ and CO 3 2-, then
[Ca 2+ ] = [ CO 3 2- ] = 6.9 10 -5 mol/l ,
hence, PR CaCO 3 = [Ca 2+ ] [CO 3 2- ] = 6.9 10 –5 6.9 10 -5 = 4.8 10 -9
Knowing the PR value , you can, in turn, calculate the solubility of a substance in mol/l or g/l.
Example 2
Solubility product PR PbSO 4 = 2.2 10 -8 g/l.
What is solubility? PbSO 4 ?
Solution
Let's denote solubility PbSO 4 via X mol/l. Having gone into solution, X moles of PbSO 4 will give X Pb 2+ and X ions ionsSO 4 2- , i.e.:
= = X
ETCPbSO 4 = = = X X = X 2
X =\ é(ETCPbSO 4 ) = \ é(2,2 10 -8 ) = 1,5 10 -4 mol/l.
To go to the solubility expressed in g/l, we multiply the found value by the molecular weight, after which we get:
1,5 10 -4 303,2 = 4,5 10 -2 g/l.
Precipitation formation
If
[ Ag + ] [ Cl - ] < ПР AgCl- unsaturated solution
[ Ag + ] [ Cl - ] = PRAgCl- saturated solution
[ Ag + ] [ Cl - ] > PRAgCl- supersaturated solution
A precipitate is formed when the product of concentrations of ions of a poorly soluble electrolyte exceeds the value of its solubility product at a given temperature. When the ionic product becomes equal to the valueETC, precipitation stops. Knowing the volume and concentration of the mixed solutions, it is possible to calculate whether a precipitate of the resulting salt will precipitate.
Example 3
Does a precipitate form when mixing equal volumes 0.2MsolutionsPb(NO 3
)
2
AndNaCl.
ETCPbCl 2
= 2,4 10
-4
.
Solution
When mixed, the volume of the solution doubles and the concentration of each substance decreases by half, i.e. will become 0.1 M or 1.0 10 -1 mol/l. These are there will be concentrationsPb 2+ AndCl - . Hence,[ Pb 2+ ] [ Cl - ] 2 = 1 10 -1 (1 10 -1 ) 2 = 1 10 -3 . The resulting value exceedsETCPbCl 2 (2,4 10 -4 ) . Therefore part of the saltPbCl 2 precipitates. From all of the above, we can conclude about the influence of various factors on the formation of precipitation.
Effect of solution concentration
A sparingly soluble electrolyte with a sufficiently large valueETCcannot be precipitated from dilute solutions.For example, sedimentPbCl 2 will not fall out when mixing equal volumes 0.1MsolutionsPb(NO 3 ) 2 AndNaCl. When mixing equal volumes, the concentrations of each substance will become0,1 / 2 = 0,05 Mor 5 10 -2 mol/l. Ionic product[ Pb 2+ ] [ Cl 1- ] 2 = 5 10 -2 (5 10 -2 ) 2 = 12,5 10 -5 .The resulting value is lessETCPbCl 2 , therefore, precipitation will not occur.
Influence of the amount of precipitant
For the most complete precipitation possible, an excess of precipitant is used.
For example, precipitate saltBaCO 3 : BaCl 2 + Na 2 CO 3 ® BaCO 3 ¯ + 2 NaCl. After adding an equivalent amountNa 2 CO 3 ions remain in solutionBa 2+ , the concentration of which is determined by the valueETC.
Increasing ion concentrationCO 3 2- caused by the addition of excess precipitant(Na 2 CO 3 ) , will entail a corresponding decrease in the concentration of ionsBa 2+ in solution, i.e. will increase the completeness of precipitation of this ion.
Influence of the same ion
The solubility of sparingly soluble electrolytes decreases in the presence of other strong electrolytes that have ions of the same name. If to an unsaturated solutionBaSO 4 add solution little by littleNa 2 SO 4 , then the ionic product, which was initially smaller ETCBaSO 4 (1,1 10 -10 ) , will gradually reachETCand will exceed it. Precipitation will begin to form.
Effect of temperature
ETCis a constant value at constant temperature. With increasing temperature ETC increases, so precipitation is best carried out from cooled solutions.
Dissolution of sediments
The solubility product rule is important for converting poorly soluble precipitates into solution. Suppose we need to dissolve the precipitateBaWITHO 3
. The solution in contact with this precipitate is relatively saturatedBaWITHO 3
.
It means that[
Ba 2+
] [
CO 3
2-
] = PRBaCO 3
.
If you add an acid to a solution, the ionsH + will bind the ions present in the solutionCO 3 2- into molecules of fragile carbonic acid:
2H + + CO 3 2- ® H 2 CO 3 ® H 2 O+CO 2
As a result, the ion concentration will sharply decreaseCO 3 2- , the ionic product will become less thanETCBaCO 3 . The solution will be unsaturated relativelyBaWITHO 3 and part of the sedimentBaWITHO 3 will go into solution. By adding enough acid, the entire precipitate can be brought into solution. Consequently, the dissolution of the precipitate begins when, for some reason, the ionic product of the poorly soluble electrolyte becomes less thanETC. In order to dissolve the precipitate, an electrolyte is introduced into the solution, the ions of which can form a slightly dissociated compound with one of the ions of the sparingly soluble electrolyte. This explains the dissolution of sparingly soluble hydroxides in acids
Fe(OH) 3 + 3HCl® FeCl 3 + 3H 2 O
IonsOH - bind into slightly dissociated moleculesH 2 O.
Table.Solubility product (SP) and solubility at 25AgCl
1,25 10 -5
1,56 10 -10
AgI
1,23 10 -8
1,5 10 -16
Ag 2 CrO4
1,0 10 -4
4,05 10 -12
BaSO4
7,94 10 -7
6,3 10 -13
CaCO3
6,9 10 -5
4,8 10 -9
PbCl 2
1,02 10 -2
1,7 10 -5
PbSO 4
1,5 10 -4
2,2 10 -8
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. The modern content of 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 polar bond, dissociate similarly. The dissociation of 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 during dissolution, a chemical interaction of the solute with water occurs, 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 other, non-aqueous electrolyte solutions, the chemical bond between the particles (molecules, ions) of the solute and the solvent particles 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 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 by the 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 a very convenient value, since it depends on the concentration of the electrolyte. A 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 of chemical compounds as acids, bases, and salts are described.
Acids are electrolytes whose dissociation produces only hydrogen ions as cations. For example:
All common characteristic properties of acids - sour taste, change in color of indicators, interaction with bases, basic oxides, salts - are due to 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 general alkaline properties of 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 breakdown of molecules into several simpler 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.
Lecture. Theory of electrolytic dissociation.
Electrolytes, non-electrolytes. Electrolytic dissociation.
The reason for the deviation from van't Hoff and Raoult's laws was first established in 1887 by the Swedish scientist Svante Arrhenius, proposing the theory of electrolytic dissociation, which is based on two postulates:
Substances whose solutions are electrolytes (i.e., they conduct electric current), when dissolved, they disintegrate into particles (ions), which are formed as a result of the dissociation of the dissolved substance. The number of particles increases. Positively charged ions are called cations , because under the influence of an electric field they move towards the cathode. Negatively charged ions - anions , because under the influence of an electric field they move towards the anode. Electrolytes include salts, acids and bases.
Al(NO3)3 ® Al ³ + + NO3ֿ
· Electrolytes do not dissociate completely. The ability of a substance to dissociate is characterized by the degree of electrolytic dissociation - a. The degree of electrolytic dissociation is the ratio of the amount of electrolyte substance disintegrated into ions to the total amount of dissolved electrolyte.
a = ionized / Ndissolved
n is the number of molecules broken up into ions
N is the total number of molecules in solution
a - degree of electrolytic dissociation
The value of a can range from 0 to 1, and a is often expressed as a percentage (from 0 to 100%). The degree of dissociation shows what part of the dissolved amount of electrolyte under given conditions is in solution in the form of hydrated ions.
The reasons for electrolytic dissociation are due to:
· the nature of chemical bonds in compounds (electrolytes include substances with ionic or covalent highly polar bonds)
· the nature of the solvent: the water molecule is polar, i.e. is a dipole
Thus, electrolytic dissociation is the process of decomposition of ionic or polar compounds into ions under the influence of polar solvent molecules.
The mechanism of electrolytic dissociation.
The Arrtius theory was significantly developed by Russian scientists I.A. Kablukov and V.A. Kistyakovsky, they proved that when an electrolyte is dissolved, a chemical interaction of the dissolved substance with water occurs, which leads to the formation of hydrates, and then they dissociate into ions, i.e. There are hydrated ions in solution.
The easiest way to dissociate a substance is with an ionic bond. The sequence of processes occurring during the dissociation of substances with ionic bonds (salts, alkalis) will be as follows:
orientation of water dipole molecules near crystal ions
· hydration (interaction) of water molecules with ions of the surface layer of the crystal
· dissociation (decay) of the electrolyte crystal into hydrated ions.
Taking into account the hydration of ions, the dissociation equation looks like this:
NaCl + X H2O ® Na + n H2O + Cl - n H2O
Since the composition of hydrated ions is not always constant, the equation is written abbreviated:
NaCl ® Na + + Cl -
The process of dissociation of substances with a polar bond occurs similarly, the sequence of processes occurring is as follows:
orientation of water molecules around the poles of an electrolyte molecule
· hydration (interaction) of water molecules with electrolyte molecules
· ionization of electrolyte molecules (conversion of a covalent polar bond into an ionic one)
· dissociation (decay) of electrolyte molecules into hydrated ions.
HCl + H2O ® H3O + + Cl -
HCl ® H + + Cl -
During the dissociation process, the hydrogen ion does not occur in free form, only in the form of hydronium ion H3O +.
THEORY OF ELECTROLYTIC DISSOCIATION
Solutions of all substances can be divided into two groups: they conduct electric current or are not conductors.
You can get acquainted with the characteristics of the dissolution of substances experimentally by studying the electrical conductivity of solutions of these substances using the device shown in the figure.
Observe the following experiment " Study of electrical conductivity of substances."
To explain the characteristics of aqueous solutions of electrolytes to a Swedish scientist S. Arrhenius in 1887 it was proposed electrolytic dissociation theory . Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of atoms and chemical bonds. The modern content of this theory can be reduced to the following three provisions:
1. Electrolytes when dissolved in water or melted break apart (dissociate) to ions – positive (cations) and negative (anions) charged particles.
Ions are in more stable electronic states than atoms. They can consist of one atom - this is simple ions ( Na + , Mg 2+ , Al 3+ etc.) - or from several atoms - this is complex ions ( NO 3 - ,SO 2- 4 , RO Z-4, etc.).
2. In solutions and melts electrolytes conduct electricity .
Under the influence of an electric current, ions acquire directional movement: positively charged ions move towards the cathode, negatively charged ions move towards the anode. Therefore, the former are called cations, the latter - anions. The directional movement of ions occurs as a result of their attraction by oppositely charged electrodes.
TESTING SUBSTANCES FOR ELECTRICAL CONDUCTIVITY
|
SUBSTANCES |
|
|
ELECTROLYTES |
NON-ELECTROLYTES |
|
Electrolytes– these are substances whose aqueous solutions or melts conduct electric current |
Non-electrolytes– these are substances whose aqueous solutions or melts do not conduct electric current |
|
Substances with ionic chemical bond or covalent highly polar chemical bond - acid, salt, base |
Substances with covalent nonpolar chemical bond or covalent weakly polar chemical communication |
|
In solutions and melts ions are formed |
In solutions and melts no ions are formed |
REMINDER
ELECTROLYTES AND NON-ELECTROLYTES
THERMAL EFFECTS WHEN DISSOLVING SUBSTANCES IN WATER
3. Dissociation - reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.
Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is put. For example, the dissociation equation of the electrolyte molecule K Ainto the K + cation and the A - anion, in general it is written as follows:
KA ↔K + + A -
Consider the process of dissolving electrolytes in water
In general, a water molecule is not charged. But inside the moleculeH 2 O The hydrogen and oxygen atoms are arranged so that positive and negative charges are at opposite ends of the molecule (Fig. 1). Therefore, a water molecule is a dipole.
Dissolution of substances with ionic chemical bonds in water
(using the example of sodium chloride - table salt)
Mechanism of electrolytic dissociationNaCl when table salt is dissolved in water (Fig. 2), it consists of the sequential elimination of sodium and chlorine ions by polar water molecules. Following the transition of ions Na + and Сl – From the crystal to the solution, hydrates of these ions are formed.
Dissolution of substances with covalent highly polar chemical bonds in water
(using the example of hydrochloric acid)
When hydrochloric acid is dissolved in water (in moleculesHCl the bond between atoms is covalent, highly polar), the nature of the chemical bond changes. Under the influence of polar water molecules, a covalent polar bond turns into an ionic one. The resulting ions remain bound to water molecules - hydrated. If the solvent is non-aqueous, then the ions are called solvated (Fig. 3).
Key points:
Electrolytic dissociation - This is the process of decomposition of an electrolyte into ions when it is dissolved in water or melted.
Electrolytes– these are substances that, when dissolved in water or in a molten state, disintegrate into ions.
Ions are atoms or groups of atoms that have a positive ( cations) or negative ( anions) charge.
Ions differ from atoms both in structure and properties
Example 1. Let's compare the properties of molecular hydrogen (consists of two neutral hydrogen atoms) with the properties of the ion.
|
Hydrogen atom |
Hydrogen ion |
|
1 Н 0 1 s 1 |
1 N + 1 s 0 |
Example 2. Let's compare the properties of atomic and molecular chlorine with the properties of the ion.
|
Chlorine atom |
Chlorine ion |
|
17 Cl 0 1s 2 2s 2 2p 6 3s 2 3p 5 |
17 Cl - 1s 2 2s 2 2p 6 3s 2 3p 6 |
|
Chlorine atoms have an incomplete outer level, so they are very chemically active, accepting electrons and being reduced. That is why chlorine gas is poisonous; inhaling it causes poisoning of the body. |
Chlorine ions have a complete external level, so they are chemically inactive and are in a stable electronic state. Chlorine ions are part of table salt, the consumption of which does not cause poisoning to the body. |
Remember!
1. Ions differ from atoms and molecules in structure and properties;
2. A common and characteristic feature of ions is the presence of electrical charges;
3. Solutions and melts of electrolytes conduct electric current due to the presence of ions in them.
Electrolytes and non-electrolytes
Taken individually, water, salts, alkalis and acids do not conduct current. But aqueous solutions of acids, alkalis and salts conduct electric current. What groups can all substances be divided into in relation to electric current?
Substances that conduct electric current - electrolytes; substances that do not conduct electric current – non-electrolytes.
Properties of electrolytes
Electrolytes are conductors of the second kind. In a solution or melt, they break up into ions, which is why they conduct electric current.
To explain this property, in 1887, the Swedish scientist S. Arenius proposed the theory of electrolytic dissociation.
The breakdown of electrolytes into ions when dissolved in water or melted is called electrolytic dissociation.
Basic principles of the theory of electrolytic dissociation.
1) Electrolytes, when dissolved in water, break up (dissociate) into ions - positive and negative: NaCl ↔ Na + + Cl -
2) When exposed to electric current, the ions acquire directional movement: positively charged ions move towards the cathode, negatively charged ions move towards the anode. Therefore, the former are called cations, and the latter – anions. The directional movement of ions occurs as a result of their attraction to oppositely charged electrodes.
3) Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions occurs (association). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used.
S. Arrhenius could not indicate why electrolytes, when dissolved in water, disintegrate into ions, since he considered the dissociation of electrolytes to be a physical process. The reason for the dissociation was discovered by the Russian scientist I.A. Kablukov, who, based on the theory of D.I. Mendeleev about the chemical nature of dissolution, began to consider electrolytic dissociation as a chemical interaction of electrolytes with water. The main reason for dissociation is the hydration of ions, which occurs with the release of a large amount of energy than is spent on the ionization of the solute.
Mechanism of electrolytic dissociation
Dissociation of electrolytes occurs in water and does not occur, for example, in kerosene. How to explain this?
In a water molecule, the bonds between hydrogen atoms and oxygen atoms are polar covalent. Electron pairs connecting atoms. Shifted from the hydrogen atom to the oxygen atom. Therefore, a positive charge is concentrated on the hydrogen atoms, and a negative charge on the oxygen atom.
To consider the mechanism of dissociation of electrolytes, it is necessary to take into account not only the polarity of the bonds between hydrogen and oxygen atoms in the water molecule. But also the polarity of the water molecule itself. A polar water molecule - a dipole - can be depicted as an ellipse indicating the charges at the poles with the charges at the poles indicated by the signs “+” and “–”.
Let us consider the mechanism of dissociation of substances with an ionic bond using the example of sodium chloride. It consists of three stages:
a) orientationpolar water molecules (dipoles) around the crystal and loosening of the crystal lattice under the influence of the chaotic movement of water molecules; (when a salt crystal is immersed in water, water molecules are attracted to the ions located on the surface of the crystal: to positive ions with their negative poles (oxygen atoms), and to negative ions with their positive poles (hydrogen atoms).
b) hydration– surrounding sodium and chlorine ions by water molecules (formation of hydrated ions);
c)destruction of the crystal lattice – dissociation of sodium chloride.
(being attracted to the ions of the dissolved salt, water molecules weaken the attraction of the ions to each other many times over. The bonds between positive and negative ions in the crystal lattice are broken. The hydrated ions are separated)
Water molecules attracted to the ions when the crystal dissolves remain bound to them in solution.
The mechanism of dissociation of substances with a polar covalent bond includes an additional step:
orientation of polar water molecules around a polar electrolyte molecule;
changing the type of bond from polar covalent to ionic;
electrolyte dissociation;
hydration of ions.
4) Not all electrolytes dissociate equally into ions. In electrolyte solutions, along with ions, molecules can also be present. The degree of dissociation a is the ratio of molecules dissociated into ions to the total number of molecules in the solutiona= n/N,
where n is the number of dissociated molecules, N is the total number of molecules in the solution.
Strong electrolytes, when dissolved in water, almost completely dissociate into ions. They have a tends to unity. Strong electrolytes include: all soluble salts, acids H2SO4, HNO3, HCl, all alkalis.
Weak electrolytes, when dissolved in water, almost do not dissociate into ions. They have a tends to zero. Weak electrolytes include: weak acids - H 2 S, H 2 CO 3, H 2 SO 3, HNO 2, NH 3 ·H 2 O, water.
Dissociation of acids, salts and bases.
Dissociation occurs in solutions and melts.
Soluble acids - These are electrolytes that dissociate in aqueous solutions and melts into a hydrogen cation and an anion of an acidic residue.
H 2 SO 4 ↔ 2 H + + SO 4 2-
Reasons– these are electrolytes that dissociate in aqueous solutions and melts into a metal cation and a hydroxide anion.
NaOH ↔ Na + + OH –
Soluble bases
– these are hydroxides formed by ions of active metals: monovalent: Li +, Na +, K +, Rb +, Cs +, Fr +; divalent: Ca 2+, Sr 2+, Ba 2+.
Salts – these are electrolytes that dissociate in aqueous solutions and melts into a metal cation and an anion of an acidic residue.
Na 2 SO 4 ↔ 2Na + + SO 4 2-
Self-test task:
Write dissociation equations for the following electrolytes: zinc nitrate, sodium carbonate, calcium hydroxide, strontium chloride, lithium sulfate, sulfurous acid, copper(II) chloride, iron(III) sulfate, potassium phosphate, hydrosulfide, calcium bromide, calcium hydroxychloride, sodium nitrate , lithium hydroxide.