CH 2: SOLUTIONS

Transcription

CH 2: SOLUTIONS

www.chemzblog.wordpress.com
CH 2: SOLUTIONS
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SOLUTION, SOLVENT, SOLUTE
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“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
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EXPRESSING CONCENTRATIONS OF SOLUTIONS
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1.
2.
3.
4.
5.
6.
7.
Mass percentage (w/w)
Volume percentage (v/v)
Mass by volume percentage (w/v)
Parts per million (ppm)
Mole fraction
Molarity
Molality
1. MASS PERCENTAGE (w/w)
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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 sodiumwww.chemzblog.wordpress.com
hypochlorite in water
2. VOLUME PERCENTAGE (v/v)
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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)
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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:
6
10 )
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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 (O2).
Or 5.8 g per 106 g (5.8 ppm)
5. MORE FRACTION
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For a binary mixture A+B:
For a mixture containing ‘i’ components:
MOLARITY (M)
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“Molarity (M) is defined as number of moles of solute
dissolved in one litre (or one cubic decimeter) of solution”
MOLALITY (m)
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“Molality: Molality (m) is defined as the number of moles of
the solute per kilogram (kg) of the solvent”
NUMERICALS
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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)
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
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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”
DISSOLUTION, CRYSTALLIZATION &
EQUILIBRIUM
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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)
SATURATED SOLUTION,
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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
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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
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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.
HENRY’S LAW
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“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:
“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 = KHwww.chemzblog.wordpress.com
χ
Characteristics of KH
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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.
(aquatic species are more comfortable in cold water than hot water)
APPS OF HENRY’S LAW
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1. S of CO2 in soft drinks and soda water
2. Scuba divers → bends
3. At high altitudes → anoxia
EFFECT OF T ON S
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Solubility of G in L is similar to condensation. Thus it is an
endothermic process.
Thus S α1/T. (Le-Chatelier’s Principle)
VAPOUR PRESSURE OF LIQUID
SOLUTIONS
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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
VAPOUR PRESSURE OF LIQUID-LIQUID
SOLUTIONS
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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
similarly, p2 ∝ x2
∴ p2 = po2 x2
where,
p1 & p2 are partial pressures of comp. 1 & 2 in
the solution
po1 &po2 are partial pressures of pure
components 1 & 2
x1 & x2 are mole fractions of 1 & 2 in the solution.
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Now, according to Dalton’s law of partial pressures..
ptotal=p1 + p2
= po1 x1 + po2 x2
= po1 (1-x2) + po2 x2
= po1 + (po2 - 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 & po2 , ptotal decreases or increases
with the increase of the x1 .
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COMPOSTION IN THE VAPOUR PHASE
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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
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
NUMERICAL
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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 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
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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
law becomes a special case of Henry’s law.
Comparing the two equations: pio is similar to KH.
VAPOUR PRESSURE OF SOLID IN LIQUID
SOLUTIONS
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1. When only solvent is
present pressure exerted
by solvent vapour molecules
is po.
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.
RAOULT’S LAW FOR S IN L SOLUTIONS
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“For any solution the partial
vapour pressure of each
volatile component in the
solution is directly proportional
to its mole fraction.”
p1 ∝ x1
p1 = po1x1
IDEAL & NON-IDEAL SOLUTIONS
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Liquid-liquid solutions on the basis of Raoult’s law can be
classified into…
1. Ideal solutions &
2. Non-ideal solutions.
IDEAL SOLUTIONS
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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 ∆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..
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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
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“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
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p i ≠ p oi x i
+ve deviation
-ve deviation
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A-A & B-B > A-B
∆mixH>0
∆mixV>0
p1 > po1x1
p2 > po2x2
ethanol + water
A-A & B-B < A-B
∆mixH<0
∆mixV<0
p1 < po1x1
p2 < po2x2
chloroform + acetone
AZEOTROPES
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“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
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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.” www.chemzblog.wordpress.com
1. RELATIVE LOWERING OF VP
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ex: 2.6 (SE)
2. ELEVATION OF BOILING POINT
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EX: 2.7 (SE)
DEPRESSION IN FREEZING POINT
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OSMOTIC PRESSURE
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“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…
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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.
2.
pressure measurement is around the room temperature and
the molarity of the solution is used instead of molality.
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
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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.
Examples…
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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
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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
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CH 2: SOLUTIONS

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