ch 2 solutions - chemzblog – the ultimate resource … example, 10% ethanol solution in water...

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CH 2: SOLUTIONS

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

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� “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.

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TYPES OF SOLUTIONS3


EXPRESSING CONCENTRATIONS OF SOLUTIONS

1. Mass percentage (w/w)

2. Volume percentage (v/v)

3. Mass by volume percentage (w/v)

4. Parts per million (ppm)

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4. Parts per million (ppm)

5. Mole fraction

6. Molarity

7. Molality


1. MASS PERCENTAGE (w/w)

For example:10% w/w glucose solution means

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10% w/w glucose solution means

10g of Glucose + 90g of water = 100g of solutionOR

100g of Glucose + 900g of water = 1000g of solution

� Commercial bleaching solution contains 3.62 mass percentage of sodium hypochlorite in water www.chemzblog.wordpress.com

2. VOLUME PERCENTAGE (v/v)

� For example, 10% ethanol solution in water means:

10 mL of ethanol + 90 mL of water = 100 mL solution.

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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)

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

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10g solute + v mL of solvent = 100 mL of solution

� Commonly used in medicine and pharmacy.


4. PARTS PER MILLION (PPM: 106)

� When a solute is present in trace quantities.

� Can also be expressed as mass to mass, volume to

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� 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 (O2). Or 5.8 g per 106 g (5.8 ppm)


5. MORE FRACTION

For a binary mixture A+B:

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For a binary mixture A+B:

For a mixture containing ‘i’ components:


MOLARITY (M)

“Molarity (M) is defined as number of moles of solute

dissolved in one litre (or one cubic decimeter) of solution”

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MOLALITY (m)

“Molality: Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent”

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NUMERICALS

2.1 (SE) Calculate the mole fraction of ethylene glycol (C2H6O2) in a solution containing 20% of C2H6O2 by mass.

2.4 (intext) Calculate the mass of urea (NH2CONH2)

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2 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.


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

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but they dissolve readily in benzene

� “Polar solutes dissolve in polar solvents and non polar solutes in nonpolar”

� “Like dissolves like”


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

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� 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)www.chemzblog.wordpress.com

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

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� 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

Consider the equilibrium: Solute + Solvent ⇌ ⇌ ⇌ ⇌ Solution

Effect of temperature

� It has significant effect on solubility

� According to Le Chateliers Principle…

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� 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) www.chemzblog.wordpress.com

SOLUBILITY OF GAS IN LIQUIDS

� Solubility of Oxygen in water ↣ very very less

� Solubility of HCl & NH3 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.

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↣

� The solubility of gases increase with increase of pressure.


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. (S α p)

In other words:

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In other words:

“mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution” (χ α 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 = KH χwww.chemzblog.wordpress.com

Characteristics of KH

1. KH value depends upon Nature of gas, P & T.

2. Different gases → different KH values at the same T .

3. ↑ the KH at a given P, ↓ is the S. (KH α 1/S)

4. ↑ the T, ↑ is KH & thus ↓ the S.

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(aquatic species are more comfortable in cold water than hot water)


APPS OF HENRY’S LAW

1. S of CO2 in soft drinks and soda water

2. Scuba divers → bends

3. At high altitudes → anoxia

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EFFECT OF T ON S

� Solubility of G in L is similar to condensation. Thus it is an endothermic process.

� Thus S α1/T. (Le-Chatelier’s Principle)

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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

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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…

p1 ∝ x1 ∴ p1 = po1 x1

∝ ∴

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p1 ∝ x1 ∴ p1 = p 1 x1

similarly, p2 ∝ x2 ∴ p2 = po2 x2

where,

p1 & p2 are partial pressures of comp. 1 & 2 in

the solution

po1 &po

2 are partial pressures of pure

components 1 & 2

x1 & x2 are mole fractions of 1 & 2 in the solution.www.chemzblog.wordpress.com

Now, according to Dalton’s law of partial pressures..

ptotal=p1 + p2

= po1 x1 + po

2 x2

= po (1-x ) + po x

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= po1 (1-x2) + po

2 x2

= po1 + (po

2 - po1)x2

Conclusions….

1. ptotal can be related to the x or any one component.

2. ptotal over the solution ∝ x2 .

3. Depending on the po1 & po

2 , ptotal decreases or increases with the increase of the x1 .


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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 y1 and y2 are the mole fractions of the components 1 and

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� 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

p2 = y2 ptotal

In general,

pi = yi ptotal www.chemzblog.wordpress.com

NUMERICAL

2.5 (SE) Vapour pressure of chloroform (CHCl3) and dichloromethane (CH2Cl2) 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 CHCl3 and 40 g of CH Cl at 298 K and, (ii) mole fractions of each

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3

40 g of CH2Cl2 at 298 K and, (ii) mole fractions of each component in vapour phase.


RAOULT’S LAW AS A SPECIAL CASE OF HENRY’S LAW

Rault’s law: pi = poi xi

Henry’s law: pi = KHxi

� 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

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components is so volatile that it exists as a gas, it Raoult’slaw becomes a special case of Henry’s law.

� Comparing the two equations: pio is similar to KH.


VAPOUR PRESSURE OF SOLID IN LIQUID SOLUTIONS

1. When only solvent is present pressure exerted by solvent vapour molecules is po.

2. When non-volatile solute is

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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.www.chemzblog.wordpress.com

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.”

∝

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to its mole fraction.”

p1 ∝ x1

p1 = po1x1


IDEAL & NON-IDEAL SOLUTIONS

Liquid-liquid solutions on the basis of Raoult’s law can be classified into…

1. Ideal solutions &

2. Non-ideal solutions.

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2. Non-ideal solutions.


IDEAL SOLUTIONS

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:

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Ideal solutions have two important properties:

1. Enthalpy of mixing ∆mixH=0 (i.e. no heat change occurs on mixing)

2. Volume of mixing ∆mixV=0 (i.e. vol. of solution is equal to sum of volumes of two liquids exactly)


Elaboration..

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

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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

“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 (-vedeviation) contrary to as expected Raoult’s law.

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deviation) contrary to as expected Raoult’s law.


POSITIVE & NEGATIVE DEVIATIONS35

pi ≠ poixi


+ve deviation -ve deviation

A-A & B-B > A-B A-A & B-B < A-B

∆mixH>0 ∆mixH<0

∆mixV>0 ∆mixV<0

p > po x p < po x

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p1 > po1x1 p1 < po

1x1

p2 > po2x2 p2 < po

2x2

ethanol + water chloroform + acetone


AZEOTROPES

“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.

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� 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)www.chemzblog.wordpress.com

COLLIGATIVE PROPERTIES

Vapour pressure of a solvent decreases when a non-volatile solute is added.

(1) Relative lowering of vapour pressure of the solvent

(2) Depression of freezing point of the solvent

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(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.” www.chemzblog.wordpress.com

1. RELATIVE LOWERING OF VP39

ex: 2.6 (SE)www.chemzblog.wordpress.com

2. ELEVATION OF BOILING POINT40

EX: 2.7 (SE)www.chemzblog.wordpress.com

DEPRESSION IN FREEZING POINT41


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.”

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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…

� Widely used to determine mol. masses of widely used to determine molar masses of proteins, polymers and other macromolecules.

� Advantage over other methods as

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� 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.


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

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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.


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.

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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.www.chemzblog.wordpress.com

Reverse Osmosis and Water Purification

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.

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� Desalination of water.


ABNORMAL MOLAR MASS47


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