1 CH 2: SOLUTIONS
2 SOLUTION, SOLVENT, SOLUTE Solutions are homogeneous mixtures of two or more than two components. i.e. composition and properties are uniform throughout the mixture. Eg: The component that is present in the largest quantity is known as solvent. It determines the physical state in which solution exists. One or more components present in the solution other than solvent are called solutes.
TYPES OF SOLUTIONS 3
EXPRESSING CONCENTRATIONS OF SOLUTIONS 4 1. Mass percentage (w/w) 2. Volume percentage (v/v) 3. Mass by volume percentage (w/v) 4. Parts per million (ppm) 5. Mole fraction 6. Molarity 7. Molality
1. MASS PERCENTAGE (w/w) 5 For example: 10% w/w glucose solution means 10g of Glucose + 90g of water = 100g of solution OR 100g of Glucose + 900g of water = 1000g of solution Commercial bleaching solution contains 3.62 mass percentage of sodium hypochlorite in water
2. VOLUME PERCENTAGE (v/v) 6 For example, 10% ethanol solution in water means: 10 ml of ethanol + 90 ml of water = 100 ml solution. A 35% (v/v) solution of ethylene glycol, an antifreeze, is used in cars for cooling the engine. [At this concentration the antifreeze lowers the freezing point of water to 255.4K ( 17.6 C)]
3. MASS BY VOLUME PERCENTAGE (w/v) 7 Mass by volume percentage = mass of solute x 100/volume of solution For example, 10% w/v means: 10g solute + v ml of solvent = 100 ml of solution Commonly used in medicine and pharmacy.
4. PARTS PER MILLION (PPM: 10 6 ) 8 When a solute is present in trace quantities. Can also be expressed as mass to mass, volume to volume and mass to volume. For example: A liter of sea water (which weighs 1030 g) contains about 6 10 3 g of dissolved oxygen (O 2 ). Or 5.8 g per 106 g (5.8 ppm)
5. MORE FRACTION 9 For a binary mixture A+B: For a mixture containing i components:
MOLARITY (M) 10 Molarity (M) is defined as number of moles of solute dissolved in one litre (or one cubic decimeter) of solution
11 MOLALITY (m) Molality: Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent
12 NUMERICALS 2.1 (SE) Calculate the mole fraction of ethylene glycol (C 2 H 6 O 2 ) in a solution containing 20% of C 2 H 6 O 2 by mass. 2.4 (intext) Calculate the mass of urea (NH 2 CONH 2 ) required in making 2.5 kg of 0.25 molal aqueous solution. 2.5 (intext) Calculate (a) molality (b) molarity and (c) mole fraction of KI if the density of 20% (mass/mass) aqueous KI is 1.202 g ml -1.
13 SOLUBILITY OF SOLIDS IN LIQUIDS Every solid does not dissolve in a given liquid. Sodium chloride & sugar dissolve readily in water. Naphthalene and anthracene do not dissolve in water but but they dissolve readily in benzene Polar solutes dissolve in polar solvents and non polar solutes in nonpolar Like dissolves like
14 DISSOLUTION, CRYSTALLIZATION & EQUILIBRIUM Dissolution: When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is known as dissolution. Crystallization: Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallization. A stage is reached when the two processes occur at the same rate. Solute + Solvent Solution (EQUILIBRIUM)
15 SATURATED SOLUTION, Saturated solution: Solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. Solubility: The solution which is in dynamic equilibrium with undissolved solute is the saturated solution and contains the maximum amount of solute dissolved in a given amount of solvent. Thus, the concentration of solute in such a solution is its solubility. (depends upon T & P)
FACTORS AFFECTING SOLUBILITY 16 Consider the equilibrium: Solute + Solvent Solution Effect of temperature It has significant effect on solubility According to Le Chateliers Principle If sol H > 0 Forward direction If sol H < 0 Backward direction Effect of pressure It does not have any significant effect on solubility of solids in liquids. ( solids and liquids are highly incompressible)
SOLUBILITY OF GAS IN LIQUIDS 17 Solubility of Oxygen in water very very less Solubility of HCl & NH 3 gas in water high Solubility of G in L is greatly affected by P and T. The solubility of gases increase with increase of pressure.
18 HENRY S LAW The solubility of a gas in a liquid is directly proportional to the pressure of the gas. (S α p) Dalton s Law: the solubility of a gas in a liquid solution is a function of partial pressure of the gas. (Sα p) In other words: mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution (χα p) Most commonly used Henry s Law: 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 = K H χ
Characteristics of K H 19 1. K H value depends upon Nature of gas, P & T. 2. Different gases different K H values at the same T. 3. the K H at a given P, is the S. (K H α 1/S) 4. the T, is K H & thus the S. (aquatic species are more comfortable in cold water than hot water)
APPS OF HENRY S LAW 20 1. S of CO 2 in soft drinks and soda water 2. Scuba divers bends 3. At high altitudes anoxia
EFFECT OF T ON S 21 Solubility of G in L is similar to condensation. Thus it is an endothermic process. Thus S α1/t. (Le-Chatelier s Principle)
22 VAPOUR PRESSURE OF LIQUID SOLUTIONS 1. Vapour pressure of liquid-liquid solutions 2. Rault s law as special case of Henry s law 3. Vapour pressure of liquid-solid solutions
23 VAPOUR PRESSURE OF LIQUID-LIQUID SOLUTIONS Raoult s law which 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. For a solution containing two liquids 1 & 2 p o 1 x 1 p 1 = p 1 x 1 similarly, p 2 x 2 p 2 = p o 2 x 2 where, p 1 & p 2 are partial pressures of comp. 1 & 2 in the solution p o 1 &p o 2 are partial pressures of pure components 1 & 2 x 1 & x 2 are mole fractions of 1 & 2 in the solution.
24 Now, according to Dalton s law of partial pressures.. Conclusions. p total =p 1 + p 2 = p o 1 x 1 + p o 2 x 2 = p o 1 (1-x 2 ) + p o 2 x 2 = p o 1 + (p o 2 - p o 1)x 2 1. p total can be related to the x or any one component. 2. p total over the solution x 2. 3. Depending on the p o 1 & p o 2, p total decreases or increases with the increase of the x 1.
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26 COMPOSTION IN THE VAPOUR PHASE The composition of vapour phase in equilibrium with the solution is determined by the partial pressures of the components. If y 1 and y 2 are the mole fractions of the components 1 and 2 respectively in the vapour phase then, using Dalton s law of partial pressures: p 1 = y 1 p total p 2 = y 2 p total In general, p i = y i p total
27 NUMERICAL 2.5 (SE) Vapour pressure of chloroform (CHCl 3 ) and dichloromethane (CH 2 Cl 2 ) at 298 K are 200 mm Hg and 415 mm Hg respectively. (i) Calculate the vapour pressure of the solution prepared by mixing 25.5 g of CHCl 3 and 40 g of CH 2 Cl 2 at 298 K and, (ii) mole fractions of each component in vapour phase.
28 RAOULT S LAW AS A SPECIAL CASE OF HENRY S LAW Rault s law: p i = p o i x i Henry s law: p i = K H x i In the solution of a gas in a liquid, when one of the components is so volatile that it exists as a gas, it Raoult s law becomes a special case of Henry s law. Comparing the two equations: p io is similar to K H.
29 VAPOUR PRESSURE OF SOLID IN LIQUID SOLUTIONS 1. When only solvent is present pressure exerted by solvent vapour molecules is p o. 2. When non-volatile solute is added solute molecules share the surface & affect the evaporation & hence the vp. Thus vp decreases to p. 3. If more solute is added the vp decreases even more.
30 RAOULT S LAW FOR S IN L SOLUTIONS For any solution the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction. p 1 x 1 p 1 = p o 1x 1
IDEAL & NON-IDEAL SOLUTIONS 31 Liquid-liquid solutions on the basis of Raoult s law can be classified into 1. Ideal solutions & 2. Non-ideal solutions.
IDEAL SOLUTIONS 32 Ideal solutions: The solutions which obey Raoult s law over the entire range of concentration,temperature and pressure are known as ideal solutions. Ideal solutions have two important properties: 1. Enthalpy of mixing mix H=0 (i.e. no heat change occurs on mixing) 2. Volume of mixing mix V=0 (i.e. vol. of solution is equal to sum of volumes of two liquids exactly)
Elaboration.. 33 Consider two liquids A & B: 1. Intermolecular forces of attraction in pure components will be A-A & B-B. 2. In the solution, IMF would be A-B 3. If A-A & B-B are nearly equal to A-B, ideal solution is formed. 4. A perfectly ideal solution is rare. 5. Examples: n-hexane & n-heptane, benzene & toluene, bromoethane & chloroethane are nearly ideal solutions.
NON-IDEAL SOLUTIONS 34 When a solution does not obey Raoult s law over the entire range of concentration, then it is called non-ideal solution. Thus the vp is either higher (+ve deviation) or lower (-ve deviation) contrary to as expected Raoult s law.
POSITIVE & NEGATIVE DEVIATIONS 35 p i p o ix i
+ve deviation -ve deviation 36 A-A & B-B > A-B mix H>0 mix H<0 mix V>0 mix V<0 A-A & B-B < A-B p 1 > p o 1x 1 p 1 < p o 1x 1 p 2 > p o 2x 2 p 2 < p o 2x 2 ethanol + water chloroform + acetone
AZEOTROPES 37 Some liquids on mixing, form azeotropes which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. the components cannot be separated by fractional distillation. There are two types of azeotropes Minimum boiling azeotrope (ethanol+water : approximately 95% by volume of ethanol) and Maximum boiling azeotrope (Nitric acid+water : 68% nitric acid and 32% water by mass)
COLLIGATIVE PROPERTIES 38 Vapour pressure of a solvent decreases when a nonvolatile solute is added. (1) Relative lowering of vapour pressure of the solvent (2) Depression of freezing point of the solvent (3) Elevation of boiling point of the solvent and (4) Osmotic pressure of the solution. The properties which depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution are called Colligative properties.
1. RELATIVE LOWERING OF VP 39 ex: 2.6 (SE)
2. ELEVATION OF BOILING POINT 40 EX: 2.7 (SE)
DEPRESSION IN FREEZING POINT 41
42 OSMOTIC PRESSURE The flow of solvent molecules through SPM from pure solvent to the solution is called osmosis. The pressure that just stops the flow of solvent is called osmotic pressure of the solution. Or The osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis semipermeable membranes (SPM).
Uses 43 Widely used to determine mol. masses of widely used to determine molar masses of proteins, polymers and other macromolecules. Advantage over other methods as 1. pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. 2. its magnitude is large even for very dilute solutions. Particularly particularly useful for biomolecules as they are generally not stable at higher temperatures and polymers have poor solubility.
44 Different solutions Two solutions having same osmotic pressure at a given temperature are called isotonic solutions. The solution having higher osmotic pressure is called hypertonic & one with lower osmotic pressure is called hypotonic solution. For example, the π associated with the fluid inside the blood cell is equivalent to that of 0.9% (mass/volume) sodium chloride solution, called normal saline solution and it is safe to inject intravenously. Less or more would be hypo or hypertonic solutions.
45 Examples A raw mango placed in concentrated salt solution loses water via osmosis and shrivel into pickle. Wilted flowers revive when placed in fresh water. A carrot that has become limp because of water loss into the atmosphere can be placed into the water making it firm once again. Water will move into them through osmosis. When placed in water containing less than 0.9% (mass/volume) salt, blood cells collapse due to loss of water by osmosis. People taking a lot of salt or salty food experience water retention in tissue cells and intercellular spaces because of osmosis. Edima. preservation of meat by salting and of fruits by adding sugar protects against bacterial action.
Reverse Osmosis and Water Purification 46 Application of pressure higher than osmotic pressure results into flow of solvent molecules from solution to pure solvent. This phenomenon is called reverse osmosis. Desalination of water.
ABNORMAL MOLAR MASS 47
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