6. INDIVIDUAL ASSIGNMENTS. ELECTROLYTE SOLUTIONS

Task 1

1.List features and properties of the electrolyte aqueous solutions.

2.Identify the main criteria for separation of the electrolytes into “strong” and “weak”.

3.Draw the curves of the molar conductivity as a function of the dilution for the strong and weak electrolytes.

4.What is the solubility product constant?

5.What is the relationship between the anion and cation transport numbers?

6.Calculate the AgCl solubility in water by using the reference data on

the solubility product constant of the salt.

7. Calculate the limiting molar conductivity for acetic acid in water at 298К if values of the limiting molar conductivities for the NaCl, HCl ,

and CH3COONa aqueous solutions at this temperature are 140.80, 480.75, and 100.98Ω−1∙cm2∙mol−1, respectively.

Task 2

1.Explain mechanism of current passing through the electrolyte solution.

2.Give the explanation to the concept of weak and strong electrolytes. Describe in detail Wilhelm Ostwald’s dilution law.

3.Write the empirical relation between the molar conductivity and concentration of a solution of a strong electrolyte at a low concentration (Kohlrausch's Law).

4.Write down an equation determining the solubility of a product constant through the molality of a saturated solution and a mean activity coefficient of electrolyte.

5.What is the relationship between the transport numbers of ions, the limiting molar conductivities of the individual ions, and the molar conductivity?

6. Calculate a mean molality m± of the LaBr3 aqueous solution if

molality of the salt in water m is 0.2.

7. The molar conductivity of a chloroacetic acid solution with the 512 l∙mol−1 dilution at 298К is 219.4 Ω−1∙cm2∙mol−1. Determine a dissociation degree of the acid at this condition if the limiting molar conductivity of the aqueous sodium chloroacetate at 298К is 89.8Ω−1∙cm2∙mol−1. Given that

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the limiting molar conductivities of the sodium and hydrogen ions are 50.1 and 349.8 Ω−1∙cm2∙mol−1 respectively.

Task 3

1.What are the reasons for the dissociation of electrolytes in solution into ions?

2.What are the dissociation constant and the degree of dissociation α ?

3.Draw the curve of the specific conductivity as a function of the dilution for electrolyte.

4.What is the mean molality of electrolyte?

5.How does the radius of the ion atmosphere depend on temperature?

6.Calculate the mean activity of electrolyte a± for the FeCl3 aqueous

solution with molality m being equal to 0.2.

7. The molar conductivity (λ of cyanoacetic acid in water at room temperature and different concentrations (c) is the following:

Calculate an average value of the dissociation constant if the limiting molar conductivity is 386.1Ω−1∙cm2∙mol−1.

Task 4

1.Give an explanation to the basic concept of the Arrhenius theory of electrolytic dissociation.

2.Write down the Ostwald equation. In which cases would it be valid?

3.Which of the following factors affect the molar conductivity of the

electrolyte: concentration (c), the degree of dissociation (α ), temperature (Т ), the viscosity of solution (η ), the conductometric vessel constant ( к), the radius of ion ( r ), and the electrode surface area (A)?

4.What is the mean activity coefficient?

5.Explain the reason for the increase of the molar conductivity as a consequence of the increase in the magnitude of the electric field.

Cl − ions 0.491 and the limiting molar conductivity of the solution is 149

S∙cm2∙mol−1.

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Task 5

1.How will the Gibbs energy of the electrolyte solution change during the electrolytic dissociation process?

2.Formulate expression for the dissociation constant of the weak binary electrolyte.

3.How does the specific conductivity of the electrolyte solution vary with the concentration?

4.What is meant by the “activity of the electrolyte solution”? How can it be determined?

5.How will the radius of the ionic atmosphere be changed by transition from aqueous (dielectric permittivity is 81) solution of a strong electrolyte to alcoholic (dielectric permittivity is 24) solution with the same concentration at the constant temperature?

6.The ratio between the CaCl2 and NaCl activities in aqueous solutions

is 8/4. What is the ratio between the mean activities of these salts in the given solvents?

7. Calculate the molar conductivity of the 0.001 M LiCl aqueous solution and compare this value with the experimental one, which is 96.6 S

cm2∙mol−1. The molar conductivity values for the 0.001 M LiNO3 , NaNO3

and NaCl aqueous solutions at 291 K are 92.9, 102.9, and 106.5 S∙cm2∙mol−1, respectively.

Task 6

1.What is the hydration entropy ( ∆S ) of ions? What is the sign of the hydration ∆S and how it can be explained?

2.Can the degree of dissociation of the electrolyte be greater than one? Provide reasons for your answers?

3.What happens to the resistance of the weak electrolyte solution when it is diluted?

4.How the mean activity coefficient depends on concentration of electrolyte?

5.Explain the reason for the increase in the conductivity of an electrolyte solution when the applied voltage has a very high frequency (Debye– Falkenhagen effect).

6.Using the reference data on the thermodynamic quantities of compounds

and ions in aqueous solutions calculate the ∆H 0 , ∆S 0 , and ∆G0 values for the dissociation of formic acid in water at 298K.

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7. The solubility of sparingly soluble cobalt ferrocyanide Co2 Fe(CN )6

was determined by conductometric measurement in saturated solution. The specific conductivity of the saturated solution at 298 K was 2.06∙10−6Ω−1∙cm−1, while the specific conductivity of the used water was 4.1∙10-7Ω−1∙cm−1. The limiting molar conductivity values for the Co2+ and

Fe(CN)64+ ions (mobility of the ions) are 43 and 111 S∙cm2∙mol−1, respectively. Determine the solubility of Co2 Fe(CN )6 in water at 298К .

Task 7

1.What property of the solvent is the determining factor in its ability to ionize the solute?

2.What is a values area of the dissociation degree (α ) for weak

electrolytes?

3.What happens to the conductivity of the electrolyte solution in case of increasing the distance between the electrodes?

4.Show the ionic strength rule. For what kind of solutions it is valid?

5.What are the concepts of “central” ion and ionic “atmosphere”, according to the Debye–Hückel theory of strong electrolytes?

6. Calculate and compare the ionic strength values for FeCl3 and Fe2 (SO4 )3 aqueous solutions with molality m = 0.5.

7. Calculate the limiting molar conductivity for the monochloroacetic acid aqueous solution at 298К if the dissociation constant (Kd) value is 1.55∙10- 3 mol∙l-1and the molar conductivity value is 77.2 S∙cm2∙mol−1at the 32 l∙mol- 1 dilution.

Task 8

1.What is ”electrolyte”?

2.How is the dissociation degree of the weak electrolyte changing with dilution of the solution?

3.Spell two properties of diluted solutions of the strong 1:1 electrolytes which are changing in proportion to the square root of the molar concentration?

4.What is ”Ionic strength”? Write the expression.

5.How does the radius of the ionic atmosphere depend on the concentration of the strong electrolyte?

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6. Match the ionic strength of the two solutions at the same concentration;

(a) the AgNO3 solution and (b) the Co(ClO4 )2 solution. For which solution

(How much?)is ionic strength higher?

7. The molar conductivity of the 0.02 M acetic acid aqueous solution is 11.58S∙cm2∙mol−1at 291 K. Calculate the dissociation degree and the dissociation constant of the acid.

Task 9

1.What kind of chemical compounds are characterized by ionic conductivity?

2.What is the dissociation degree? What values can it take?

3.For what kind of electrolytes a concentration dependence of the molar

conductivity satisfies following equation λ = λ0 − a С ?

4.Express the Debye–Hückel limiting law.

5.What is the cataphoretic (electrophoretic) effect?

6.Write the mathematical expression of the Debye–Hückel limiting law

for the AlCl3 solution.

7. The specific conductivity of the 0.05 M CH3COOH aqueous solution

at 298 K is 0.000362 Ω−1∙cm−1. Calculate the dissociation degree by using the reference data on the limiting molar conductivity.

Task 10

1.Identify the reasons for the electrolytic dissociation.

2.Give examples of strong and weak electrolytes.

3.How does the viscosity of the solvent affect the conductivity of the electrolyte solution?

4.At what concentration region of the solution, can you use Debye– Hückel limiting law to calculate the activity coefficient of a strong electrolyte?

5.List two effects that explain the inhibition of ion motion.

6.Calculate the solubility product constant of Ag2CrO4 if solubility in

water of the salt is 8∙10-5 mol∙l-1 at 298 K. The mean activity coefficient is taken equal to unity.

7. Calculate the molar conductivity of the 0.05 M benzoic acid aqueous solution at 298 K if the acid dissociation constant at this temperature is

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6.14∙10-5. The limiting molar conductivity can be determined using the reference data.

Task 11

1.What is the role of the solvent in the process of dissociation into ions of electrolytes in solution?

2.What happens to the dissociation degree value (α ) of a weak

electrolyte solution with increasing of concentration?

3.Write down the law of regardless motion of ions at infinite dilution. For what kind of electrolyte solutions is it valid?

4.Does the presence of foreign ions affect the activity coefficient of the ion in the solution? Why?

5.Explain what is the relaxation effect?

6.Calculate pKd of formic acid at 298К if Kd=1.772∙10-4.

7.Resistance of the 0.01 M KNO3 aqueous solution is 423 Ω. Calculate

values of the specific and molar conductivities if the conductometric vessel constant is 0.5cm−1.

Task 12

1.What are the main differences between the properties of electrolyte solutions and non-electrolyte solutions?

2.How can you distinguish "strong" electrolyte from the "weak" on the base of the conductometry data of the electrolyte solution?

3.Write down the equation for the concentration dependence of the molar conductivity (Kohlrausch's Law). For what kind of electrolyte solutions is it valid?

4.Write the equation relating the mean activity of the electrolyte with its activity.

5.How does the mobility of the ion in solution change with increase of its own ionic radius if we compare a number of similar charged ions with the same properties?

6.Calculate the NO3− ion mobility (the limiting molar conductivity of the

individual ion)in a highly diluted solution using the following values of the limiting molar conductivity (S∙cm2∙mol−1at 298К ):KCl – 149.9; KNO3 – 144.9; HCl – 426.1.

The H + transport number in HCl is equal to 0.821.

7. Define λ0 for lithium chloride at 250С using following data:

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Task 13

1.What is an “electrolytic dissociation”?

2.Underline the theory of the strong electrolytes, principles used in Debye–Hückel theory.

3.Write two Kohlrausch's equations. First equation is expression for the conductivity of a solution of a strong electrolyte at low concentration (so called Kohlrausch's Law) and the second one is expression for the limiting conductivity of a solution(so called Law of the additive contributions from anions and cations).

4.What is the relationship between the mean activity of the electrolyte solution and its molar concentration?

5.What kind of ions have the highest mobility in aqueous solutions, and why?

6.The limiting molar conductivity of potassium picrate at 298K is 103.97

S∙cm2∙mol−1, the limiting molar conductivity of the potassium ion (the mobility of ion) is 73.58 S∙cm2∙mol−1. Calculate the limiting molar conductivity and the transport number for the picrate ion.

7. The specific conductivity of a saturated silver chloride aqueous solution at 291 K is 1.37∙10-6Ω−1∙cm−1; the specific conductivity of water is 4∙10- 8Ω−1∙cm−1. Calculate solubility of silver chloride in water at this temperature by using the reference data on the limiting molar conductivity of individual ions and by approaching that the limiting diluted solution condition is valid for the saturated solution.

Task 14

1.Is the process of electrolytic dissociation reversible?

2.How does temperature affect the dissociation degree of a weak electrolyte in aqueous solution?

3.What is the specific conductivity of the electrolyte solution?

4.How is the mean molality of the electrolyte solution associated with its molality?

5. What is the cause of the abnormal mobility of hydronium ions ( H3O+ )?

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6. The limiting molar conductivity of the KCl aqueous solution at 250Сis 149.9 S∙cm2∙mol−1. Calculate the Cl − ion velocity (or mobility of the ion,

cm2∙V−1∙s−1) in this solution if the transport number of the K + ion is 0.497. 7. The limiting molar conductivity for the propionic acid aqueous solution

at 298К is 285.6 Ω−1∙cm2∙mol−1 and the dissociation constant (Kd) value of this acid is 1.34∙10-5 mol∙l-1. Calculate the molar conductivity value of the 0.05 M propionic acid at the same temperature.

Task 15

1.What determines the completeness of the electrolyte dissociation in solution?

2.What is the "coefficient of electric conductivity" for the solution of a strong electrolyte? What kind of factors have effect on it?

3.What is the molar conductivity of the electrolyte solution?

4.What is an Van't Hoff isotonic coefficient and what kind of properties of electrolyte solutions does it describe?

5.What is a transport number?

6.The limiting molar conductivity of the aqueous ammonium chloride is

149.7S∙cm2∙mol−1. The limiting molar conductivity values for the OH −

and Cl − ions (mobility of the ions) are 198.0 and 76.3 S∙cm2∙mol−1, respectively. Determine the limiting molar conductivity of ammonium hydroxide.

7. The limiting molar conductivity ( λ0 ) of sodium chloroacetate at 298К is 89.8 S∙cm2∙mol−1. Calculate the λ0 value for chloroacetic acid at the same temperature.

Task 16

1.What is the theory put forward by the Swedish scientist S. Arrhenius to explain the specific properties of electrolyte solutions?

2.What happens to the activity coefficient of strong electrolyte during dilution of the solution?

3.For what kind of solutions the Debye-Hückel-Onsager equation is valid?

4.How does the solubility multiplicity of the hard soluble salt expressed through the activity will be affected by the adding of an outsider electrolyte into the solution?

5.What is the relationship between the drift velocity of ion and its mobility?

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6. The solubility product constant of the AgBr in water at 298К is 5.0∙10−13. Calculate the AgBr solubility in pure water (a) and in the 0.01

M aqueous NaBr (b).Use the Debye–Hückel limiting law.

7. Calculate the molar conductivity of the 0.05 M acetic acid at 298К on the base of following data, λ0 =391 Ω−1∙cm2∙mol−1and Kd = 1.8∙10-5.

Task 17

1.Which of the two categories of chemical compounds are able to conduct the electricity in solution?

2.How does a dielectric permittivity of solvent affect the dissociation constant of electrolyte (if to compare solvents with close properties)?

3.What is the specific electric resistance?

4.Give the definition of acids and bases in the light of protolytic BronstedLowry theory.

5.What is meant by the term “absolute velocity of the ion”? What is the dimension of this parameter?

6.The pKд values for perchloric and nitric acids in acetic acid as solvent

are 4.95 and 9.39, respectively. Calculate the dissociation constant values for these acids. Which of these two acids in the solvent is stronger?

7. Calculate the molar conductivity of the 0.005 M sodium chloride aqueous solution at 298 K by using the Debye-Hückel-Onsager equation

and compare the obtained value with the reference one, λ0 =126.5

Ω−1∙cm2∙mol−1.

Task 18

1.What type of electric conductivity is characteristic for the electrolyte solutions?

2.How does temperature affect the dissociation constant of a weak acids?

3.What is the specific conductivity of the electrolyte solution? What is the dimension of this parameter?

4.What is the Protolytic theory of acids and bases by Bronsted-Lowry?

5.Describe the ability of electrolyte solutions to conduct electric current in terms of the absolute velocity of the ions, the ion mobility, and the transport numbers.

6.Calculate the pH value of the 0.05 M phenol aqueous solution at 298 K by using reference data.

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7. The specific conductivity of the 0.059 M acetic acid aqueous solutions 20.96 Ω−1∙cm2∙mol−1. Calculate the dissociation degree and the dissociation constant values.

Task 19

1.What kind of data on the electrical conductivity of the electrolyte solution is required in order to calculate the heat of the electrolytic dissociation?

2.Summarize of the electrostatic theory of strong electrolytes.

3.What is the molar conductivity of the electrolyte solution? What is the dimension of this parameter?

4.What is the ionic product for water, Kw? What kind of factors does it depend on?

5.What is the relationship between the transport number and the velocity of the anion and cation?

6.Calculate the ionic strength value for aqueous solution consisting of 0.5 M NaCl and 0.5 M Na2SO4.

7.The specific conductivity of a saturated thallium bromide aqueous solution at 293 K is 2.158∙10-4 Ω−1∙cm−1; the specific conductivity of water is 4.4∙10-8 Ω−1∙cm−1. The limiting molar conductivity of this salt in water is 138.3 Ω−1∙cm2∙mol−1. Calculate the solubility of thallium bromide in water.

Task 20

1.What is the dissociation heat? What is the sign of this parameter?

2.Does the dissociation constant of weak electrolyte depend on concentration of the electrolyte solution?

3.What is the relationship between molar conductivity and specific conductivity? Write the equation.

4.What is the pH. What values it takes for acids and bases?

5.What is the mobility of ion?

6.Calculate the pH value for the 0.1 M HCl aqueous solution at 298 K by using reference data.

7.Calculate λ0 for aqueous silver chlorate on the base of following data

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6.1.REFERENCES

1.A.G. Stromberg, D.P. Semchenko, Fizicheskaya Khimiya, M.:Vysshaya shkola, 2001, 527 p. [In Russian]

2.G. V. Bulidorova, Electrokhimiya i khimicheskaya kinetika, Kazan: KNRTU, 2014, 372 p. [In Russian]

3.V. Kudryashov, G.S Karetnikov, Sbornik primerov i zadach po fizicheskoj khimii, M.:Vysshaya shkola, 1991, 527 p. [In Russian]

4.Kontrolnye zadaniya po fizichesloj khimii, Chast III, 4-16 p. [In Russian]

5.Bairamov V.M. Osnovi electrokhimii, M.: Izdatelskii zentr «Academiya», 2005, 240 p. [In Russian]

6.Kratkij spravochnik fiziko-khimicheskih velichin, (Short Handbook of Physicochemical Values (Eds.: A.A. Ravdel and A.M. Ponomareva), St.P: Ivan Fedorov, 2003, 240 p. [In Russian]

7.Theoretical and experimental methods of chemical solutions, М.: Prospect, 2011., 684 p. [In Russian]

8.CRC Handbook of Chemistry and Physics, 95th Edition (26 Jun 2014) ISBN 1-4822-0867-9, Editor-in-Chief W. M. Haynes, CRC Press, Inc., 2693 p.

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