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2, , SOLUTIONS

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Solutions ?, Solutions are homogeneous mixtures containing, two or more components., , • Solutions containing only two components are called, binary solutions. These components are called solvent and, solute., , • Generally, the component that is present in larger quantity, is called solvent., • The substance which is dissolved is called solute and the, , substance in which solute is dissolved is called solvent., • Solvent determines the physical state of the solution.

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SOLUBILITY, ▫Solubility of a substance is its maximum amount that can be, dissolved in a specified amount of solvent., ▫It depends upon the nature of solute and solvent,, , temperature and pressure., , -Solubility of a Gas in a Liquid, ▫Solubility of gases in liquids is greatly affected by pressure, and temperature., , ▫The solubility of a gas increases with increase of pressure.

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Henry’s Law :, The relation between pressure and solubility of a gas, in a liquid was first given by Henry, which is known as, Henry’s law., “The law states that at a constant, temperature, the solubility of a gas in a liquid is, directly proportional to the pressure of the gas”., Or,, “the partial pressure of the gas in vapour phase, (p) is proportional to the mole fraction of the gas (x) in, the solution”, and is expressed as:, , p = KH x

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Here KH is the Henry’s law constant., • The value of KH depends on the nature of the gas and, temperature., • As the value of KH increases, the solubility of the gas in, the liquid decreases., A graph of partial pressure (p) of the, gas against mole fraction (x) of the, gas in solution is a straight line as, follows. The slope of the graph gives, the value of KH.

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-As the temperature increases solubility of a gas in a liquid, decreases. It is due to this reason that aquatic species are, more comfortable in cold waters rather than in warm, waters., , ▪ Applications of Henry’s law, 1. In the preparation of soda water, the bottle is sealed, at high pressure., 2. A condition known as Bends in Scuba divers., 3. A medical condition known as Anoxia in people living, at high altitudes or climbers., , Effect of Temperature: Since the dissolution of a gas, in liquid is an exothermic process, solubility decreases, with increase in temperature.

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Vapour Pressure of Liquid-Liquid Solutions, – Raoult’s Law :, The relationship between the vapour pressure and, mole fraction of solute in a solution was first given by, , F.M Raoult and it is known as Raoult’s Law., “It states that for a solution of, volatile liquids, the partial vapour pressure of each, component in the solution is directly proportional to, its mole fraction”.

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Graphical representation :, Plots of p1 or p2 against the mole fractions x1 and x2 give straight, lines (I and II). Similarly the plot of ptotal versus x2 (line III) is also, linear., •The composition of vapour phase, in equilibrium with the solution is, determined from the partial, pressures of the components., • If y1 and y2 are the mole fractions, of the components 1 and 2, respectively in the vapour phase, then, using Dalton’s law of, partial pressures:, , p1 = y1 ptotal and, p2 = y2 ptotal

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Ideal and non-ideal solutions Liquid:, – Liquid solutions can be classified into ideal and non-ideal, solutions on the basis of Raoult’s law., Non-ideal solutions, , IDEAL SOLUTIONS, , 1. It obeys Raoult’s law., 2. ΔHmixing =0 , ΔVmixing =0., 3. the solute-solute interactions, and solvent-solvent interactions, are nearly equal to solutesolvent interaction., 4. n-hexane and n-heptane,, bromoethane and chloroethane,, benzene and toluene ., , 1.Solutions which do not obey, Raoult’s law ., 2. ΔHmixing ≠0 , ΔVmixing≠0, 3.PA≠POA XA, 4.Classified as two types:, 1., 2., , Non ideal solutions showing, positive deviations, Non ideal solutions showing, negative deviations

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In the case of positive deviation from Raoult’s law,, • the solute-solvent interactions are weaker than solute-solute and solventsolvent interactions., • So more molecules are escaped to vapour phase and hence the vapour, pressure of the solution increases., • E.g. solutions of ethanol and acetone, acetone and CS 2, acetone and CCl4, etc., , Non ideal, solutions, In case of negative deviation from Raoult’s law,, • the solute-solvent interactions are stronger than solute-solute interaction, and solvent-solvent interaction., • So number of molecules escaped to vapour phase decreases and hence the, vapour pressure of the solution decreases., • E.g. solution of phenol and aniline, chloroform and acetone etc.

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--Azeotropes, , •They are binary mixtures having the same composition, in liquid phase and vapour phase and boil at a constant, , temperature., , ▫For such solutions, it is not possible to separate the components, by fractional distillation., , •There are two types of azeotropes:, minimum boiling azeotrope and, maximum boiling azeotrope., , ▫The solutions which show a large positive deviation from, Raoult’s law form minimum boiling azeotrope at a, particular composition., ▫ E.g. 95% ethanol solution by volume., ▫The solutions which show large negative deviation from, Raoult’s law form maximum boiling azeotrope at a, particular composition., ▫ E.g. a mixture of 68% Nitric acid and 32% water by mass, forms a maximum boiling azeotrope at 393.5 K.

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COLLIGATIVE PROPERTIES AND, DETERMINATION OF MOLARMASS, The properties which depend only on the number of, solute particles and not on their nature are called, Colligative properties., i) Relative lowering of Vapour pressure, ii) Elevation of Boiling point, iii)Depression of Freezing point and, iv)Osmotic Pressure.

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RELATIVE LOWERING OF VAPOUR PRESSURE :, •When a non-volatile solute is added to a pure solvent, the vapour, pressure (V.P) of the resulting solution is lower than that of the pure, solvent., •The difference between the vapour pressure of pure solvent and that, of the solution is called lowering of vapour pressure (∆P) ., •The relative lowering of vapour pressure (∆P/P1 0 ) is given by,, Where P10 is the vapour pressure of pure solvent,, w1 is the mass of the solvent,, w2 is the mass of the solute,, M1 is the molar masses of solvent and, M2 is that of the solute respectively.., ∆P is the lowering of vapour pressure of the solvent, which is given by, ∆P = P10 – P1, Where P1 is the vapour pressure of the solution

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2. Elevation of Boiling Point (∆Tb), Boiling point of a liquid is the temperature at which its vapour, pressure becomes equal to the atmospheric pressure., -When a non-volatile solute is added to a pure solvent, the, , boiling point of the resulting solution is always greater than that, of the pure solvent., , -The difference between the boiling point of solution (∆Tb) and that, of the pure solvent (Tb0 ) is called elevation of boiling point (Tb)., i.e., ∆Tb = Boiling point of solution – Boiling point of pure solvent, Or,, ∆Tb = Tb – Tb0

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For dilute solutions, the elevation, of boiling point is directly, proportional to molality (m). i.e., ∆Tb α m, or,, ∆Tb = Kb.m, • Where K b is a constant called, Boiling Point Elevation, Constant or Molal Elevation, Constant or Ebullioscopic, Constant., •It is defined as the elevation of, boiling point for 1 molal solution., •The unit of Kb is K kg/mol., • For water, Kb = 0.52K kg/mol., Where w1 = mass of solvent, w2 = mass of solute,, M2 = molar mass of solute.

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3. Depression of Freezing point (∆Tf), -Freezing point is the temperature at which the solid phase and, liquid phase of a substance has the same vapour pressure., -When a non-volatile solute is added to a pure solvent, the freezing, point of the resulting solution is always less than that of the pure, , solvent., , -The difference between the freezing point (f.p) of pure solvent, (Tf0 ) and that of the solution (Tf) is called, depression of freezing point (∆Tf)., , i.e., ∆Tf = Tf 0 - T

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•For dilute solutions, it is found that, the depression of freezing point (∆Tf), is directly proportional to molality, (m) of the solution., •Thus ∆Tf α m Or, ∆Tf = K f .m, Where Kf is a constant called, Freezing Point ., Depression Constant or Molal, Depression Constant or Cryoscopic, Constant., •It is defined as the depression of, freezing point for 1 molal solution., The unit of Kf is K kg/mol. For, water, K f = 1.86 K kg/mol., , The vapour pressure – Temperature, graph representing the freezing, point of pure solvent and solution.

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4. Osmosis and Osmotic Pressure :, Osmosis is the process of flow of solvent molecules from, , pure solvent to solution through a semipermeable membrane., , Or,, It is the flow of solvent molecules from lower, , concentration side to a higher concentration side through a semipermeable membrane (SPM)., •A membrane that allows the passage of solvent molecules only, , is called a semi-permeable membrane., -E.g. egg membrane, all animal and plant membrane. Cellulose, acetate is an example for artificial SPM.

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Osmotic pressure is defined as the excess pressure that, must be applied on solution side to stop osmosis., Or,, it is the pressure that just stops the flow of solvent, molecules., -It is denoted by π., -It is a colligative property, since it depends on the number of, solute molecules and not on their nature., •For dilute solutions, osmotic, pressure is proportional to the, molarity (C) and temperature, (T). i.e., π = CRT, •Here R is the universal gas, constant.

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But C = n2/V, the concentration of the solution., , Advantages of osmotic pressure, 1., 2., , 3., , 4., , Osmotic pressure measurement can, be done at room temperature., Here molarity of the solution is used, instead of molality, which can be, determined easily., The magnitude of osmotic pressure, is large even for very dilute, solutions., This method can be used for the, determination of molar masses of, Biomolecules (which are generally, not stable at higher temperatures), and for polymers (which have poor, solubility, , Examples for osmosis:, a) Raw mango placed in concentrated salt, solution loses water and shrink., b) Wilted flowers revive when placed in, fresh water, c) Blood cells collapse when suspended in, saline water., d) The preservation of meat by salting and, fruits by adding sugar protect against, bacterial action.

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Reverse osmosis and water purification:, , -The direction of osmosis can be reversed if a pressure, larger than the osmotic pressure is applied to the solution, side., -Now the pure solvent flows out of the solution through, the semi permeable membrane. This phenomenon is called, , reverse osmosis and is used in desalination of sea water.

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Isotonic, hypertonic and hypotonic solutions, Two solutions having same osmotic, pressure at a given temperature are called, isotonic solutions., -When such solutions are separated by a, semi-permeable membrane, no osmosis, occurs., For e.g. our blood cells, are isotonic with 0.9%, ., (mass/volume), sodium, chloride solution, called, normal saline solution., So it is safe to inject, intravenously, , A solution having higher, osmotic pressure than, another is called hypertonic, solution., While a solution having lower, osmotic pressure than another, is called hypotonic solution., , • If we place our blood cells in a solution, containing more than 0.9% (mass/volume), sodium chloride solution, water will flow out of, the cells and they would shrink., • On the other hand, if they are placed in a solution, containing less than 0.9% (mass/volume) NaCl,, water will flow into the cells and they would, swell.