Electrical conductivity of electrolyte’s solutions Plan Weak electrolytes. Strong electrolytes. Electric conductance of electrolytes solutions. Conductometry. AssistantKozachok S.S.prepared
Molecules of certain substances dissociate in a solvent to give two or more particles. For example: Consequently, the total number of particles increases in solution and, therefore, the colligative properties of such solutions will be large. Van’t Hoff factor ‘i’ to express the extent of association or dissociation of solutes in solution. It is the ratio of the normal and observed molar masses of the solute… In case of association, observed molar mass being more than the normal, the factor ‘i’ has a value less than 1. But in case of dissociation, the Van’t Hoff factor is more than 1 because the observed molar mass has a lesser value.
In case of solutes which do not undergo any association or dissociation in a solvent. Van’t Hoff factor ‘i’ will be equal to 1 because the observed and normal molar masses will be same. Since the molar mass are inversely proportional to the colligative property, Van’t Hoff factor may also be expressed as: i = Observed value of colligative property/Normal value of colligative propert Inclusion of Van’t Hoff (i) modifies the equation for colligative properties as follows:
The Arrhenius theory ACID A substance that provides H+ ions in water BASEA substance that provides OH- ions in water The symbol H+ does not really represent the structure of the ion present in aqueous solution. As a bare hydrogen nucleus (proton) with no electron nearby, H+ is much too reactive to exist by itself. Rather, the attaches to a water molecule, giving the more stable hydroniumion, H3O+ .We’ll sometimes write H+ for convenience, particularly when balancing equations.
The main principles Arrhenius’s theory of electrolytic dissociation:1. Compounds dissociate into ions when dissolved in water.2. This process is reverse.3. The ions don’t interact among themselves.
Degree of dissociation It is defined as the fraction of total substance that undergoes dissociated into ions
where m is the number of particles in solution, i is Van’t Hoff factor For example: the electrolytes of the type AB, such as KCl, NaCl, etc., the the number of particles in solution m = 2 • Degree of dissociation depends into: • Concentration • Temperature • Nature of the substance
According to the Degree of dissociation (α) electrolytes can be classified into the following: • strong electrolytes are compounds that dissociate to a large extent ( α> 30%) into ions when dissolved in water. For example, HCl, H2SO4, HNO3, HI, NaOH, KOH, KCl. • medium strong electrolytes α = 2 - 30%. H3PO4, H3PO3. • weak electrolytes are compounds that dissociate to only a small extent α<2%. For example, NH4OH, H2S, HCN, H2CO3. • nonelectrolytes α = 0 are compounds that don’t dissociate when dissolved in water.
Debye-Hückel theory of strong electrolytes: • Ions of electrolytes interact with themselves according to the electrostatic’s law. • The nature of solvent influences on the interaction between ions (inductivity).The dielectric permeability of a solvent shows the difference between the ion’s attraction in a solvent and in a vacuum. • A central ion is surrounded by the ion’s atmosphere. 4. The size of the central ion is like a point charge. 5. Decreasing of the active concentration of the strong electrolyte’s solution in the comparing with its general analytic concentration. a < c
The thickness of the ion’s atmosphere decreases with the increasing of the charge value and ion’s concentration and the ionic strength of the solution. The general interaction of the ions increasing with the increasing of solution’s concentration according to the reducing of the average distance between the ions. Increasing of the ion’s interaction coursing the reducing of the ionic activity.
The properties of the strong electrolytes solution. Activity When the concentration of a solute is greater than about 0.1 mol/m-3, or we have strong electrolytes solution (NaCl, HCl, etc.), interactions between the solute molecules or ions are significant, and the effective and real concentrations are no longer equal. It becomes necessary to define a new quantity called the activity, which is a measure of concentration but takes into account the interactions between the solution species. The relative activity, ai, of a component i is dimensionless and is defined by equation 6.6 where μi is the chemical potential of component i or ionic strength (I) , μi0 is the standard chemical potential of i, R is the molar gas constant, and T is the temperature in kelvin.
or μi = μi0 + R T ln ci + R T ln fi For the ideal solution: μi = μi0 + R T ln ci The relative activity of a solute is related to its molarity by the following equation where fi is the activity coefficient of the solute, and CM is the molarity. ai = fi CM Thermodynamics dissociation constant of strong electrolyte’s solution is calculated by:
lg f = -0.5 Z i2√ μiμi (I) = 0.5 (CM1 Z12 + CM2 Z22+..... CMi Zi2)The activity coefficient of the electrolytes depends only upon the ionic strength of the solution and in dilution solutions of strong electrolytes has the same value if this solutions have equal ionic strength.
Electrochemistry is the branch of science which deals with the relationship between electrical energy and chemical energy and interconversion to one from into another. Electrolysis is the changes in which electrical energy causes chemical reaction to occur. The changes in which electrical energy is produced as a result of chemical change. The devices used to produce electrical energy from chemical reactions are called electrical cells, galvanic or voltic cells.
Conductors are the substances which allow the passage of electric current. N.B. The best conductors are metals such as copper, silver, tin. Non-conductors or insulators are the substances which don’t allow the passage of electrical current through them. Examples are rubber, wax, wood. Types of conductors • Metallic conductors. There are metallic substances which allow the electricity to pass trough them without undergoing any chemical change. • Electrolytes. There are substances which allow the electricity to pass through them in their molten states or in the form of their aqueous solutions and undergo chemical decomposition.
When the voltage is applied to the electrodes dipped into an electrolytic solution, ions of the electrolyte move and, therefore, electric current flows through the electrolytic solution. The power that the electrolytes to conduct electric current is termed conductance or conductivity. Ohm’s law. The current flowing through a conductor is directly proportional to the potential difference across it. Or the strength of current flowing through a conductor is directly proportional to the potential difference applied across the conductor and inversely proportional to the resistance . I=V/R, where I is the current strength (in amperes) and V is the potential difference applied across the conductor (in volts), R is the resistance of the conductor ( in ohms)
The conductometry cell l/S – the constant of the cell
Resistance It measures the obstruction to the flow of current. The resistance of a conductor is proportional to the length (l) and inversely proportional to the area of cross-section. where ρ (rho) is the constant of proportionality and is called specific resistance or resistivity. The resistance depends upon the nature of the material. Its units are ohm (Ω) R= ρ , if l =1 cm, a=1 In other words, specific resistance is the resistance between opposite faces of one centimetre cube of the conductor.
Conductance. It is a measure of the ease with which current flows through a conductor. Specific conductance or conductivity. It may be defined as the conductance of a solution of 1 cm length and having 1 sq. cm as the area of cross-section. In other words, specific conductance is the conductance of one centimetre cube of a solution of an electrolyte.
It’s generally denoted by κ (kappa) Units. The units of specific conductance are In SI units, Κ = C l/a, where С is electrical conductance, a-area
Specific conductance is defined by the number of the ions and the their velocity. The more ion’s concentration and more their velocity the more will be conductance. Therefore there are some factors that influence on the value of κ: the nature of solvent and solute, the concentration of electrolyte’s solution, temperature. Dependence of the specific conductance from the upper factors is expressed by the following equation: Κ= (u+ + u- ) F c α = (u+ + u- ) F c α where u+,u- are the mobility of the cation and the anion (at the V is the potential difference applied across the conductor = 1 V, and the length = 1 m). C is a molarconcentration, α is the degree of dissociation, F is Faraday’s number (1F=96485 Kl/mol)
Equivalent conductance or molar conductance It is defined as the conducting power of all the ions produced by dissolving one gram equivalent of an electrolyte in solution. It is denoted by the symbol (lambda).
where C is the concentration of the solution in equivalent per litre or is the molar concentration of the solution Units of the equivalent conductance are: Molar conductance. It’s defined as the conducting power of all the ions produced by dissolving one gram mole of an electrolyte in solution.
where M is the concentration in moles per litre Factors for variation of molar conductance: • Nature of electrolyte • Concentration of the solution • Temperature 1.Nature of electrolyte. The conductance of an electrolyte depends upon the number of ions present in the solution. Therefore, the greater the number of ions in the solution, the greater is the conductance. The number of ions produced by an electrolyte depends upon its nature.
2. Concentration of the solution The molar conductance of electrolytic solution varies with the concentration of the electrolyte. In general, the molar conductance of an electrolyte increases with decreases in concentration or increases in dilution.
Variation of Molar Conductance with Concentration for Strong Electrolytes In case of strong electrolytes, there is the tendency for molar conductance to approach a certain limiting value when the concentration approaches zero, when the dilution is infinite. The molar conductance when the concentration approaches zero (infinite dilution) is called molar conductance at infinite dilution. It has been observed that the variation of molar conductance with concentration may be given by the expression where b is a constant depending upon ion charge, viscosity of solvent, temperature, dielectric permeability (of a solvent, and is called molar conductivity at infinite dilution.
Dependence of the specific electric conduction from an electrolyte’s concentration С, mol/м3
The variation of molar conductance with concentration can be studied by plotting the values of against square root of concentration
Variation of Molar Conductance with Concentration of Weak Electrolytes The weak electrolytes dissociate to a much lesser extent as compared to strong electrolytes. Therefore, the molar conductance is low as compared to that of strong electrolytes.
Conductance behaviour of weak electrolytes. The variation of with dilution can be explained on the basis of number of ions in solution. The number of ions furnished by an electrolyte in solution depends upon the degree of dissociation with dilution. With the increases in dilution, the degree of dissociation increases and as a result molar conductance increases.
Conduction behavior of strong electrolytes. For strong electrolytes, there is no increase in the number of ions with dilution because strong electrolytes are completely ionised in solution at all concentrations (by definition). However, in concentrated solutions of strong electrolytes there are strong forces of attraction between the ions of opposite charges called inter-ionic forces. As a result, the molar conductance increases with dilution. 3. Temperature The conductance of an electrolyte depends upon the temperature with increase n temperature, the conductance of an electrolyte increases.
Thus, it may be concluded that each ion makes definite contribution to the molar conductance at infinite dilution irrespective of the other ions. Kohlrausch’s law of independent migration of ions states that: At infinite dilution when the dissociation is complete, each ion makes a definite contribution towards molar conductance of the electrolyte irrespective of the nature of the other ion with which it’s associated. If molar conductivity of the cation is denoted by and that of anion bythen the law of independent migration of ions is: _
Application of Kohlrausch’s law 1. Calculation of Molar Conductance at Infinite Dilution for Weak Electrolytes
2. Calculation of Degree of Dissociation of Weak Electrolytes. Molar conductance of a weak electrolyte depends upon its degree of dissociation.
Velocity of the ion’s mobility and the number of the ion transfer. As a rule the ion’s mobility is from 4*10-8till 8*10-8in aqueous infinite dilution solution, except ion of hydroxonium ion (u=36,3 *10-8 m2V-1c-1) and hydroxyl ion (u =20,5*10-8 m2V-1c-1). It’s explained by the special serial transmission mechanism of their conductance. Velocity of a cation and an anion in each solution in general doesn’t equal, therefore there is not equal quantity of electricity that is transferred by ions.
The number of the ion transfer is the relation of an amount of electricity is carried by the one type ions to the general quantity of electricity, which passed through electrolyte: ti = Qi/Q where Qi is the quantity of electricity which is carried by the ions of i type through the cross-section of the electrolyte’s solution, which is calculated from the next formula: Qi=ziFciuiaτ where, zi is valence, ci is concentration, mol/m, ui is the ion’s mobility, a is the cross section, m , τis time, s. t+ = u+/u- + u+, t- = u-/u- + u+, t+ + t- = 1 3 2
At infinite dilution, λ∞+ + λ∞-= λ∞, Therefore, λ∞+ = λ∞ t+ and λ∞- = λ∞ t- Conductometric titration (conductometry) The main concept of this method is the changing of the electrical conductance of an electrolyte’s solution during a titration. The change of the electrical conductance is grounded on the displacing of an one ions by the another, which have other mobility. Equivalent point is defined accurate by the graph.
The curves of conductometry titration λ=1/R 1. The titration of НCl NaOH 2. The titration of CH3COOH NaOH 3. The titration of the mixture of (НCl (а) і CH3COOH (б) NaOH V titrant
Usage of direct conductometry: • For definition of individual electrolytes in solution • For analysis of medicines: the determination of weak acid and the substances with weak-acid property: phenobarbital, sulfadimine, thymol. Weak base - caffeine • For definition of electrolytes in mix when impurities concentration don’t change • For continuous control of manufactures • For control of water treatment process • For sewage pollution assessment • For definition of general content of salts in mineral, ocean and fluvial water • For control of operations filter washing and ion-exchange material regeneration • For definition of cleanliness slightly soluble precipitate or organic drugs • For definition of dampness of organic solvent, gases, crystal salts, paper • For detecting in chromatography