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Solutions and electrochemistry

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Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni
1 | P a g e
SOLUTIONS
CONTENTS
WAYS OF EXPRESSING C...
Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni
2 | P a g e
SOLUTION
Solution: It is a homogeneous ...
Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni
3 | P a g e
TYPES OF SOLUTIONS:
WAYS OF EXPRESSING ...
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Solutions and electrochemistry

  1. 1. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 1 | P a g e SOLUTIONS CONTENTS WAYS OF EXPRESSING CONCENTRATION: Molarity Molality Normality SOLUTIONS OF GASES IN GASES: Henry‟s Law SOLUTIONS OF LIQUIDS IN LIQUIDS: SOLUBILITY OF COMPLETELY MISCIBLE LIQUIDS SOLUBILITY OF PARTIALLY MISCIBLE LIQUIDS VAPOUR PRESSURES OF LIQUID LIQUID SOLUTIONS DISTILLATION: STEAM DISTILLATION SOLUTIONS OF SOLIDS IN LIQUIDS SOLUBILITY–EQUILIBRIUM CONCEPT DETERMINATION OF SOLUBILITY
  2. 2. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 2 | P a g e SOLUTION Solution: It is a homogeneous mixture of two or more substances. Solution consists of solute and solvent. The constituent of the mixture present in a smaller amount is called the Solute and the one present in a larger amount is called the Solvent. For example, when a smaller amount of sugar (solute) is mixed with water (solvent). SOLUTION CONCENTRATION: The concentration of a solution is defined as : the amount of solute present in a given amount of solution. Concentration is generally expressed as: A solution containing low concentration of solute is called DILUTE SOLUTION. A solution of high concentration is called CONCENTRATED SOLUTION.
  3. 3. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 3 | P a g e TYPES OF SOLUTIONS: WAYS OF EXPRESSING CONCENTRATION: (1) Per cent by weight (wt%) (2) Mole fraction (x) (3) Molarity (M) (4) Molality (m)
  4. 4. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 4 | P a g e (5) Normality (N) (1) Per cent by weight (wt%) For example: if a solution of HCl contains 36 per cent HCl by weight, it has 36 g of HCl for 100 g of solution. (2) Mole fraction (x)
  5. 5. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 5 | P a g e (3) Molarity (M) For 1 mole of solute dissolved in one litre of solution: M = 1 i . e., molarity is one. Such a solution is called 1 M (read “1 molar”). A solution containing 2 moles of solute in one litre is 2M (“two molar”); and so on. Unit of molarity is mol litre–1
  6. 6. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 6 | P a g e . (4) Molality (m) A solution obtained by dissolving one mole of the solute in 1000 g of solvent is called one molal or 1 m solution.
  7. 7. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 7 | P a g e (5) Normality (N) Thus, if 40 g of NaOH (eq. wt. = 40) be dissolved in one litre of solution, normality of the solution is one and the solution is called 1 N (one- normal). A solution containing 4.0 g of NaOH is 0.1 N.
  8. 8. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 8 | P a g e SOLUTIONS OF GASES IN GASES: When a gas is mixed with another gas a completely homogeneous solution results. PROPERTIES OF GASEOUS SOLUTIONS: (1) Complete miscibility. When one gas is dissolved in another gas they form a homogeneous solution.
  9. 9. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 9 | P a g e (2) Dalton‟s law of Partial pressures. The total pressure exerted by a gaseous mixture is the sum of the partial pressures of the component gases. If p1, p2 ,p3 are the partial pressures of the constituents, the total pressure PT of the mixture is given by: PT = p1 + p2 + p3 HENRY‟S LAW: The solubility of a gas in a solvent depends on the pressure and the temperature. When a gas is in saturated solution, the following equilibrium exists: gas  gas in solution If pressure is increased on the system, the equilibrium will move in the direction which will reduce the pressure (Le Chatelier Principle). The pressure can be reduced by more gas dissolving in solvent. Thus solubility or concentration of a gas in a given solvent is increased with increase of pressure.
  10. 10. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 10 | P a g e Henry‟s Law: for a gas in contact with a solvent at constant temperature, concentration of the gas that dissolves in the solvent is directly proportional to the pressure of the gas. Mathematically, Henry‟s Law may be expressed as: Cα P C = k P Where k is Henry‟s Law constant depends on the nature of the gas and solvent, and the units of P and C used.
  11. 11. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 11 | P a g e
  12. 12. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 12 | P a g e SOLUTIONS OF LIQUIDS IN LIQUIDS: 1. SOLUBILITY OF COMPLETELY MISCIBLE LIQUIDS: Liquids like alcohol and water mix in all proportions and they could be compared to gases. 2. SOLUBILITY OF PARTIALLY MISCIBLE LIQUIDS: A large number of liquids are known which dissolve in one another only to a limited extent e . g ., ether and water. Ether dissolves about 1.2% water; and water also dissolves about 6.5% ether. When equal volumes of ether and water are shaken together, two layers are formed, one of a saturated solution of ether in water and the other of a saturated solution of water in ether. STUDY THE EFFECT OF TEMPERATURE ON THE COMPOSITION OF SUCH MIXTURES WITH REFERENCE TO THREE TYPICAL SYSTEMS: (1) Phenol-Water system (2) Triethylamine-Water system (3) Nicotine-Water system
  13. 13. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 13 | P a g e The temperature at which the two solutions layers merge into one another to from one layer, is called the Critical Solution Temperature (CST) VAPOUR PRESSURES OF LIQUID–LIQUID SOLUTIONS: The study of the vapour pressures of mixtures of completely miscible liquids at constant temperature help in the separation of the liquids by fractional distillation.
  14. 14. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 14 | P a g e By plotting the vapour pressure against composition it has been revealed that mixtures of the miscible liquids are of three types: 1. First Type of Mixtures of Miscible Liquids: complete separation of this type of solutions into components is impossible. 2. Second Type of Mixtures of Miscible Liquids: it is not possible to effect a complete separation by fractional distillation 3. Third Type of Mixtures of Miscible Liquids: we can completely separate the components by fractional distillation. THEORY OF FRACTIONAL DISTILLATION: Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to atemperature at which one or more fractions of the compound will vaporize. It is a special type of distillation. Generally the
  15. 15. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 15 | P a g e component parts boil at less than 25 °C from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is used. The efficiency of fractional distillation is considerably enhanced by the use of the Fractionating columns.
  16. 16. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 16 | P a g e
  17. 17. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 17 | P a g e STEAM DISTILLATION: Distillation carried in a current of steam is called steam distillation. Steam distillation is used for purification of organic liquids which are steam volatile and immiscible with water ( e. g., aniline). THEORY OF STEAM DISTILLATION: The vapour pressure of a liquid increases with increase of temperature. When the vapour pressure equals the atmospheric pressure, the temperature recorded is the boiling point of the given liquid. In case of a mixture of two immiscible liquids, each component exert its own vapour pressure as if it were alone. The total vapour pressure over the mixture (P) is equal to the sum of the individual vapour pressures (p1, p2) at that temperature. P = p1 + p2 Hence the mixture will boil at a temperature when the combined vapour pressure P, equals the atmospheric pressure. Since P > p1 or p2, the boiling point of the mixture of two liquids will be lower than either of the pure components.
  18. 18. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 18 | P a g e SOLUTIONS OF SOLIDS IN LIQUIDS: The process of solution of a solid substance in a solvent is explained by the electrical forces operating between the molecules or ions of the solute and the molecules of the solvent. Polar solutes dissolve easily in polar solvents while they remain insoluble in non-polar solvents. For example, sodium chloride (an electrolyte) is soluble in water which is highly polar solvent, while it is insoluble in a non-polar solvent like chloroform. The sodium chloride (NaCl) dissolves in water to give Na+ and Cl– ions. The Na+ ion is solvated (hydrated) to have around a layer of water molecules.
  19. 19. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 19 | P a g e DISSOLUTION DUE TO HYDROGEN BONDING: SOLUBILITY–ITS EQUILIBRIUM CONCEPT: (1) Dissolution – the particles of the solute leaving the solid and passing into solution. ( 2) Recrystallisation – the particles of the solute returning from the solution and precipitating on the solid. (dynamic equilibrium)
  20. 20. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 20 | P a g e THE SOLUBILITY is defined as the concentration of the solute in solution when it is in equilibrium with the solid substance at a particular temperature. THE SATURATED SOLUTION is defined as one which is in equilibrium with the excess of solid at a particular temperature. THE SUPERSATURATED SOLUTION is a solution which would contain more solute than the saturated solution at that temperature. DETERMINATION OF SOLUBILITY: The solubility of a substance is determined by preparing its saturated solution and then finding the concentration by evaporation or a suitable chemical method. SOLUBILITY CURVES: A curve drawn between solubility and temperature is termed Solubility Curve. THE SOLUBILITY CURVES ARE TWO TYPES: (1) Continuous solubility curves (2) Discontinuous solubility curves
  21. 21. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 21 | P a g e THEORY OF DILUTE SOLUTIONS CONTENTS COLLIGATIVE PROPERTIES: LOWERING OR VAPOUR PRESSURE: RAOULT‟S LAW MEASUREMENT OF LOWERING OF VAPOUR PRESSURE: (1) Barometric Method (2) Manometric Method (3) Ostwald and Walker‟s Dynamic Method BOILING POINT ELEVATION: Relation between Boiling-point elevation and Vapour- pressure lowering MEASUREMENT OF BOILING POINT ELEVATION: (1) Landsberger-Walker Method (2) Cottrell‟s Method FREEZING-POINT DEPRESSION MEASUREMENT OF FREEZING-POINT DEPRESSION: (1) Beckmann‟s Method (2) Rast‟s Camphor Method COLLIGATIVE PROPERTIES OF ELECTROLYTES
  22. 22. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 22 | P a g e COLLIGATIVE PROPERTIES Dilute solutions containing non-volatile solute exhibit the following properties: (1) Lowering of the Vapour Pressure (2) Elevation of the Boiling Point (3) Depression of the Freezing Point (4) Osmotic Pressure Colligative Properties: (Greek colligatus = Collected together). Its defined as property which depends on the number of particles in solution and not on the size or chemical nature of the particles  Each colligative property is related to any other.  The colligative properties are used to measure the molecular weights of the dissolved substances. LOWERING OF VAPOUR PRESSURE: RAOULT‟S LAW
  23. 23. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 23 | P a g e P is the vapour pressure of the pure solvent and P is the pressure of the solution. (P-Ps) is the lowering of vapour pressure. Raoult‟s Law: the relative lowering of the vapour pressure of a dilute solution is equal to the mole fraction of the solute present in dilute solution. IDEAL SOLUTIONS AND DEVIATIONS FROM RAOULT‟S LAW:  A solution which obeys Raoult‟s law strictly is called an Ideal solution.  A solution which shows deviations from Raoult‟s law is called a Nonideal or Real solution.
  24. 24. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 24 | P a g e Determination of Molecular Mass from Vapour Pressure Lowering: If in a determination w grams of solute are dissolved in W grams of the solvent, m and M are molecular masses of the solute and solvent respectively, we have:
  25. 25. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 25 | P a g e
  26. 26. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 26 | P a g e MEASUREMENT OF LOWERING OF VAPOUR PRESSURE: (1) Barometric Method  Measuring the individual vapour pressure of a liquid and then the solution  This method is not accurate as the lowering of vapour pressure is too small. (2) Manometric Method  The air in the connecting tube removed by a vacuum pump.  The manometric liquid can be mercury or n-butyl phthalate which has low density and lowvolatility .
  27. 27. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 27 | P a g e ELEVATION OF BOILING POINT: Relation between Elevation of Boiling Point and Lowering of Vapour- pressure:  When a liquid is heated, its vapour pressure rises and when it equals the atmospheric pressure, the liquid boils.  The addition of a non volatile solute lowers the vapour pressure and consequently elevates the boiling point as the solution has to be heated to a higher temperature to make its vapour pressure become equal to atmospheric pressure.  If Tb is the boiling point of the solvent and T is the boiling point of the solution, the difference in the boiling points (ΔT) is called the elevation of boiling point. Determination of Molecular Mass from Elevation of Boiling Point:
  28. 28. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 28 | P a g e where Kb is a constant called Boiling point constant or Ebulioscopic constant of molal elevation constant. If w/ m = 1, W = 1, Kb = ΔT . and the units of Kb are :
  29. 29. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 29 | P a g e MEASUREMENT OF BOILING–POINT ELEVATION: (1) Landsberger-Walker Method (2) Cottrell‟s Method
  30. 30. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 30 | P a g e FREEZING–POINT DEPRESSION The difference of the freezing point of the pure solvent and the solution is referred to as the Depression of freezing point. It is represented by the symbol ΔT or ΔTf. Determination of Molecular Weight from Depression of Freezing point: Where Kf is a constant called Freezing-point constant or Cryoscopic constant or Molal depression constant. If w /m = 1 and W = 1, Kf = ΔT.
  31. 31. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 31 | P a g e MEASUREMENT OF FREEZING–POINT DEPRESSION (1) Beckmann‟s Method (1903): (2) Rast‟s Camphor Method
  32. 32. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 32 | P a g e COLLIGATIVE PROPERTIES OF ELECTROLYTES  The electrolytes ionize and yield more than one particle per formula unit in solution. Therefore, the colligative effect of an electrolyte solution is always greater than that of a nonelectrolyte of the same molal concentration. Van‟t Hoff factor (i): its the ratio of the colligative effect produced by an electrolyte solution to the effect for the same concentration of a nonelectrolyte solution. is the freezing-point depression for the electrolyte. is the value of depression of freezing-point of the electrolyte solution assuming no ionization (nonelectrolyte).
  33. 33. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 33 | P a g e ABNORMAL MOLECULAR MASSES OF ELECTROLYTES: Since the value of (i) is always greater than 1 the experimental molecular weight will always be less than the theoretical value calculated from the formula. Relation Between van‟t Hoff Factor (i) and Degree of Dissociation (α): v is number of ions produced
  34. 34. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 34 | P a g e CONCEPT OF ACTIVITY AND ACTIVITY COEFFICIENT:  The experimentally determined value of concentration whether of is less than the actual concentration. The „activity‟: its the effective concentration of a molecule or ion in a solution. Activity coefficient (γ) is:
  35. 35. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 35 | P a g e
  36. 36. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 36 | P a g e OSMOSIS AND OSMOTIC PRESSURE CONTENTS WHAT IS OSMOSIS? The Egg Experiment Silica Garden SEMIPERMEABLE MEMBRANES Preparation of Cupric ferrocyanide membrane OSMOTIC PRESSURE Pfeffer‟s Method Berkeley and Hartley‟s Method A Modern Osmometer ISOTONIC SOLUTIONS THEORIES OF OSMOSIS Molecular Sieve Theory Membrane Solution Theory Vapour Pressure Theory Membrane Bombardment Theory REVERSE OSMOSIS Desalination of Sea Water LAWS OF OSMOTIC PRESSURE Boyle-van‟t Hoff Law for Solutions Charles‟-van‟t Hoff Law for Solutions Van‟t Hoff Equation for Solutions Avogadro-van‟t Hoff Law for Solutions VAN‟T HOFF THEORY OF DILUTE SOLUTIONS CALCULATION OF OSMOTIC PRESSURE DETERMINATION OF MOLECULAR WEIGHT RELATION BETWEEN VAPOUR PRESSURE AND OSMOTIC PRESSURE OSMOTIC PRESSURE OF ELECTROLYTES
  37. 37. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 37 | P a g e WHAT IS OSMOSIS?  The flow of the solvent through a semipermeable membrane from pure solvent to solution, or from a dilute solution to concentrated solution, is termed Osmosis (Greek Osmos = to push). COMPARISON BETWEEN DIFFUSION AND OSMOSIS
  38. 38. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 38 | P a g e OSMOSIS EXPERIMENTS 1. Thistle Funnel Experiment:  The phenomenon of osmosis can be demonstrated by fastening a piece of cellophane over a thistle funnel as shown in Fig. 16.3. A concentrated aqueous sugar solution is placed inside the thistle funnel which is then immersed in water. The osmosis takes place through the semipermeable membrane from water to the sugar solution. The flow of water into the funnel shows up as the solution is seen rising in the tube remarkably. 2. The Egg Experiment:
  39. 39. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 39 | P a g e  The outer hard shell of two eggs of the same size is removed by dissolving in dilute HCl. One of these is placed in distilled water and the other in saturated salt solution. After a few hours it will be noticed that the egg placed in water swells and the one in salt solution shrinks. In the first case, water diffuses through the skin (a semipermeable membrane) into the egg material which swells. In the second case, the concentration of the salt solution being higher than the egg material, the egg shrinks. WHAT IS OSMOTIC PRESSURE?
  40. 40. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 40 | P a g e  The hydrostatic pressure built up on the solution which just stops the osmosis of pure solvent into the solution through a semipermeable membrane, is called Osmotic Pressure. Another definition:  Osmotic pressure may be defined as the external pressure applied to the solution in order to stop the osmosis of solvent into solution separated by a semipermeable membrane.
  41. 41. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 41 | P a g e DETERMINATION OF OSMOTIC PRESSURE  The osmotic pressure of a given solution can be determined experimentally by the methods detailed below. The apparatus used for the purpose is often referred to as osmometer. (1) Pfeffer‟s Method: (2) Berkeley and Hartley‟s Method:
  42. 42. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 42 | P a g e (3) A Modern Osmometer: ISOTONIC (ISO-OSMOTIC) SOLUTIONS
  43. 43. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 43 | P a g e  When two solutions are separated by a semipermeable membrane and there is no flow of water across the membrane, the solutions are said to be Isotonic. THEORIES OF OSMOSIS (1) The Molecular Sieve Theory:  Smaller solvent molecules can pass through the pores but the larger molecules cannot.
  44. 44. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 44 | P a g e (2) Membrane Solution Theory  Membrane proteins bearing functional groups such as – COOH, – OH, – NH2, etc., dissolve water molecules by hydrogen bonding or chemical interaction. Thus membrane dissolves water from the pure water (solvent) forming what may be called „membrane solution‟. (3) Vapour Pressure Theory: REVERSE OSMOSIS Desalination of Sea Water by Hollow-fibre Reverse Osmosis:
  45. 45. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 45 | P a g e LAWS OF OSMOTIC PRESSURE (1) Boyle-van‟t Hoff Law for Solutions: (2) Charles-van‟t Hoff Law for Solutions: at constant pressure (3) Van‟t Hoff Equation for Solutions:
  46. 46. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 46 | P a g e (4) Avogadro-van‟t Hoff Law for Solutions: CALCULATION OF OSMOTIC PRESSURE  All gas laws may be considered to apply to dilute solutions rigidly. This gives an easy solution to problems on osmotic pressure.
  47. 47. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 47 | P a g e DETERMINATION OF MOLECULAR WEIGHT FROM OSMOTIC PRESSURE
  48. 48. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 48 | P a g e RELATION BETWEEN VAPOUR PRESSURE AND OSMOTIC PRESSURE
  49. 49. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 49 | P a g e
  50. 50. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 50 | P a g e OSMOTIC PRESSURE OF ELECTROLYTES  Like other colligative properties, observed osmotic pressure of electrolytes (π ) in aqueous solutions is higher than the value calculated using van‟t Hoff equation, π0V = nRT . Expressing in terms of van‟t Hoff factor (i): Van‟t Hoff Equation for Solutions of Electrolytes:  For example, if NaCl is ionized completely as at infinite dilution, 1 mole of NaCl will give 2 moles of particles (Na+ and Cl– , one mole each). Therefore, the observed osmotic pressure will be twice the calculated value for no ionization. That is,  Similarly, the value of i for CaCl2 at infinite dilution will be 3; for FeCl3 it will be 4.  If electrolytes are partially ionized in aqueous solution. α be the degree of ionization for a salt AB,
  51. 51. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 51 | P a g e
  52. 52. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 52 | P a g e ELECTROLYSIS AND ELECTRICAL CONDUCTANCE CONTENTS Electrolysis and Electrical Conductance MECHANISM OF ELECTROLYSIS ELECTRICAL UNITS FARADAY’S LAWS OF ELECTROLYSIS FARADAY’S FIRST LAW FARADAY’S SECOND LAW IMPORTANCE OF THE FIRST LAW OF ELECTROLYSIS IMPORTANCE OF THE SECOND LAW OF ELECTROLYSIS CONDUCTANCE OF ELECTROLYTES SPECIFIC CONDUCTANCE EQUIVALENT CONDUCTANCE SUMMARY OF ELECTROCHEMICAL QUANTITIES STRONG ELECTROLYTES WEAK ELECTROLYTES MEASUREMENT OF ELECTROLYTIC CONDUCTANCE DETERMINATION OF THE CELL CONSTANT
  53. 53. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 53 | P a g e  Water-soluble substances are distinguished as electrolytes or nonelectrolytes.  Electrolytes are electrovalent substances that form ions in solution which conduct an electric current. NaCl, CuSO4 and KNO3 are examples of electrolytes.  Nonelectrolytes are covalent substances which furnish neutral molecules in solution. Their water-solutions do not conduct an electric current. Sugar, alcohol and glycerol are typical nonelectrolytes.  Electrolysis The decomposition of an electrolyte by passing electric current through its solution is termed Electrolysis ( lyo = breaking).  Electrolytic cell. The cell contains water-solution of an electrolyte in which two metallic rods (electrodes) are dipped .These rods are connected to the two terminals of a battery (source of electricity). The electrode connected to the positive terminal of the battery attracts the negative ions (anions) and is called anode. The other electrode connected to the negative end of the battery attracts the positive ions (cations) and is called cathode. MECHANISM OF ELECTROLYSIS  Example: Let us consider the electrolysis of hydrochloric acid as an example. In solution, HCl is ionised
  54. 54. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 54 | P a g e  In the electrolytic cell Cl– ions will move toward the anode and H+ ions will move toward the cathode. At the electrodes, the following reactions will take place.  At cathode:  At anode:  Overall reaction: ELECTRICAL UNITS Coulomb
  55. 55. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 55 | P a g e 1. A coulomb is a unit of electricity. It is the amount of electricity which will deposit 0.001118 gram of silver from a 15 per cent solution of silver nitrate in a coulometer. 2. Ampere is a unit rate of flow of electricity. It is that current which of silver in one second. In other words, an ampere is a current of one coulomb per second. 3. Ohm is a unit of electrical resistance. 4. Volt is a unit of electromotive force. It is the difference in electrical potential required to send a current of one ampere through a resistance of one ohm. FARADAY‟S LAWS OF ELECTROLYSIS  If m is the mass of substance (in grams) deposited on electrode by passing Q coulombs ofelectricity, then (First law)  We know that Q = I × t, where I is the strength of current in amperes and t is the time in second. Therefore, (Second law)  Where Z is the constant known as the Electrochemical equivalent of the substance (electrolyte).  THE ELECTRICAL UNIT FARADY:
  56. 56. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 56 | P a g e VERIFICATION OF THE SECOND LAW OF ELECTROLYSIS  According to this law when the same quantity of electricity is passed through different electrolyte solutions, the masses of the substances deposited on the electrodes are proportional to their chemical equivalents.
  57. 57. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 57 | P a g e  To verify the law, let us take an arrangement of the type shown in Fig. 24.2. Pass the same quantity of electricity through the three coulometers (the term „coulometer‟ is now in practice replaced by the older term „voltameter‟) containing solution of dilute H2SO4, CuSO4 and AgNO3.
  58. 58. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 58 | P a g e CONDUCTANCE OF ELECTROLYTES  Electrolyte solutions conduct electric current by movement of the ions to the electrodes. The power of electrolytes to conduct electric currents is termed conductivity or conductance.  Like metallic conductors, electrolytes obey Ohm‟s law. According to this law, the current I flowing through a metallic conductor is given by the relation:
  59. 59. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 59 | P a g e  Where E is the potential difference at two ends (in volts); and R is the resistance measured in ohms (or Ω). The resistance R of a conductor is directly proportional to its length, l, and inversely proportional to the area of its cross-section, A . That is,  Where ρ “rho” is a constant of proportionality and is called resistivity or specific resistance. Its value depends upon the material of the conductor. From above we can write:  Specific resistance: It is the resistance in ohms which one centimetre cube of it offers to the passage of electricity Specific conductance or Specific conductivity.  It is defined as: the conductance of one centimetre cube (cc) of a solution of an electrolyte.
  60. 60. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 60 | P a g e Units of Specific conductance: or  The specific conductance increases with : (i) ionic concentration, and (ii) speeds of the ions concerned. Equivalent Conductance ():  It is defined as the conductance of an electrolyte obtained by dissolving one gram-equivalent of it in V cc of water.
  61. 61. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 61 | P a g e
  62. 62. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 62 | P a g e Variation of Equivalent conductance with Concentration (or Dilution):  Strong electrolytes are completely ionised at all concentrations (or dilutions). But weak electrolytes are partially ionised.  The increase in equivalent conductance is not due to the increase in the number of current carrying species. This is, in fact, due to the decrease in forces of attraction between the ions of opposite charges with the decrease in concentration (or increase in dilution). At higher concentration, the forces of attraction between the opposite ions increase  Consequently, it affects the speed of the ions with which they move towards oppositely charged electrodes. This phenomenon is called ionic interference.  As the solution becomes more and more dilute, the equivalent conductance increases, till it reaches a limitary value. This value
  63. 63. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 63 | P a g e is known as equivalent conductance at infinite dilution (zero concentration) and is denoted by 0.  Weak electrolytes have low ionic concentrations and hence interionic forces are negligible. Ionic speeds are not affected with decrease in concentration (or increase in dilution). The increase in equivalent conductance with increasing dilution is due to the increase in the number of current-carrier species. In other words, the degree of ionisation (α) increases.  Thus increase in equivalent conductance (Λ) in case of a weak electrolyte is due to the increase in the number of ions.  In case of a weak electrolyte Λ∝ is the equivalent conductance when ionisation is complete. So, the conductance ratio Λ / Λ∝ is the degree of ionisation. That is,
  64. 64. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 64 | P a g e Molar Concentration (μ):  It is defined as : the conductance of all ions produced by one mole of an electrolyte when dissolved in a certain volume V cc.  Upon dilution specific conductance decreases, while Equivalent conductance and Molar conductance increases.
  65. 65. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 65 | P a g e Variation of Conductance with Temperature:  The conductance of a solution of an electrolyte generally increases with rise in temperature. For example, the conductances of 0.1 M KCl at two different temperatures are
  66. 66. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 66 | P a g e  The conductance of a given electrolyte depends on two factors : 1. The number of ions present in unit volume of solution 2. The speed at which ions move towards the electrodes  At a given temperature, the first factor remains the same for a particular electrolyte. Thus the increase in conductance with rise in temperature is due to the influence of factor (2). With rise in temperature the viscosity of the solvent (water) decreases which makes the ions to move freely toward the electrodes.  For weak electrolytes, the influence of temperature on conductance depends upon the value of ΔH accompanying the process of ionisation. If the ionisation is exothermic (–ΔH), the degree of ionisation is less at higher temperature (Le Chatelier‟s principle) and conductance decreases.Conversely, if the ionisation is endothermic (+ ΔH), the degree of ionisation is more at higher temperature and conductance increases. STRONG AND WEAK ELECTROLYTES  Electrolytes may be divided into two classes : 1. Strong electrolytes 2. Weak electrolytes 1. Strong Electrolytes  A strong electrolyte is a substance that gives a solution in which almost all the molecules are ionised. The solution itself is called a
  67. 67. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 67 | P a g e strong electrolytic solution. Such solutions are good conductors of electricity and have a high value of equivalent conductance even at low concentrations. The strong electrolytes are : 2. Weak Electrolytes:  A weak electrolyte is a substance that gives a solution in which only a small proportion of the solute molecules are ionised. Such a solution is called a weak electrolytic solution, that has low value of equivalent conductance. The weak electrolytes are: MEASUREMENT OF ELECTROLYTIC CONDUCTANCE  The conductance is the reciprocal of resistance. Therefore it can be determined by measuring the resistance of the electrolytic solution. This can be done in the laboratory with the help of a Wheatstone bridge.The solution whose conductance is to be determined is placed in a special type of cell known as the conductance cell.
  68. 68. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 68 | P a g e
  69. 69. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 69 | P a g e DETERMINATION OF THE CELL CONSTANT
  70. 70. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 70 | P a g e  To determine the cell constant, a standard solution of KCl whose specific conductance at a given temperature is known, is used. Then a solution of KCl of the same strength is prepared and its conductance determined experimentally at the same temperature. Substituting the two values in the above expression, the cell constant can be calculated.  For example, according to Kohlrausch the specific conductance of N/50 solution at 25ºC is 0.002765 mho. Now, an N/50 solution of KCl is prepared by dissolving 0.372 g pure KCl in 250 cc „extra- pure‟ water (conductance water) and its conductance determined at 25ºC. The cell constant is then calculated by substituting the observed conductance in the expression. 
  71. 71. Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni 71 | P a g e

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