Entry Test Preparation 2015
Chemistry
Chapter # 9
Solutions
A homogeneous mixture of two or more components, composition of which
can be varied at will, is known as solution.
Solute
Solvent
Example
Gas
Gas
Air
Gas
Liquid
O2 or CO2
in water
Gas
Solid
H2 in
palladium
Liquid
Gas
Mist, fog, clouds
Liquid
Solid
Mercury in silver
Solid
Liquid
Sugar in water,
jellies
Solid
Gas
Smoke
Solid
Solid
Metal alloys,
pearls
Liquid
Liquid
Milk, alcohols in
water
- If the forces of interactions between the molecules
of different components of the solute are same as when they were in the
pure state, they are ideal solutions otherwise non-ideal.
- If the volume of solution is not equal to the sum of
the individual volumes of the components, the solution is non-ideal.
- Ideal solutions have zero enthalpy change as their
heat of solution.
- If the solutions obey Raoult’s law, then they are
ideal. This is one of the best criterions for checking the ideality of a
solution.
Substance
Heat of solution (kJ mol-1
of solute)
Liquids Practically Immiscible:
Some typical ideal solutions
forming liquid pairs are: benzene toluene, benzene-ether, chlorobenzene and
bromobenzene, ethyl iodide-ethyl bromide, etc.
Zeotropic Mixtures:
Hsolu. The Hsolu gives the difference between the energy
possessed by the solution after its formation and the original energy of the
components before their mixing i.e.
Salts of weak acids with strong
base react with water to produce basic solutions whereas salts of weak bases
with strong acids react to give acidic solutions. Such reactions are called by
hydrolytic reactions and the salts are said to be hydrolyzed.
Chapter # 9
Solutions
A homogeneous mixture of two or more components, composition of which
can be varied at will, is known as solution.
Type of solution:
A. Gas in gas B. Gas in liquid
C. Liquid in liquid D. Solid
in liquid
·
A solution is consisted of
only two components is known as a binary solution. In a binary solution, the
component which is present in relatively larger amount is known as solvent
while the other component present in relatively smaller amount is called
solute.
·
A solution made up of three
components is called a ternary solution.
·
A solution is said to be
unsaturated if it can still dissolve more solute at a given temperature.
Phase:
Every sample of matter with
uniform properties and a fixed composition is called a phase.
Composition Expressions
The amounts of solute and solvent
can be expressed in percentage composition by four different ways:
Percentage weight / weight:
%
by weight =
Percentage weight / volume:
It is the weight of a solute
dissolved per 100 parts by volume of solution. 10 g of glucose dissolved in 100
cm3 of solution is 10% w/v solution of glucose.
Percentage volume / weight:
It is the number of cm3
of a solution dissolved per 100 g of solution. If we dissolve 10 cm3
of alcohol in water and the total weight of the solution is 100 g, then it is
10% v/w solution of alcohol in water.
Percentage volume / volume:
It is the volume of a solute
dissolved per 100 cm3 of the solution.
Molarity (M):
Molarity is the number of mole of
solute dissolved per dm3 of the solution. To prepare one molar
solution of glucose in water, we take 180 g of glucose and add sufficient water
to make the total volume 1 dm3 (a litre) in a measuring flask
Molarity (M) =
M =
A solution in which concentration
is equal to 10-7 M is called neutral solution.
Molality (m):
Molality is the number of moles
of solute in 1000 g (1 kg) of the solvent. 180 g of glucose when dissolved in
1000 g of water gives one molal solution of glucose. The total mass of the
solution is 1180 g. For one molal sucrose solution, 342 g of sourose are
dissolved in 1000 g of H2O
Molality (M) =
The molality of a solution is
indirect expression of the ratio of the moles of the solute to the moles of the
solvent.
The value of
concentration given in the units of molality does not change with temperature
but that of molarity does.
Mole Fraction (x):
The mole fraction of any
component in a mixture is the ratio of the number of moles of it to the total
number of moles of all the components present. The sum of the mole fractions of
all the components of a solution must be equal to one.
Normality (N):
Normality is the number of
gram-equivalent of solute in a dm3 of solution. The symbol of
normality is N.
Parts per Million (ppm) =
Mass of Solute
Types of Solutions:
Solute
|
Solvent
|
Example
|
Gas
|
Gas
|
Air
|
Gas
|
Liquid
|
O2 or CO2
in water
|
Gas
|
Solid
|
H2 in
palladium
|
Liquid
|
Gas
|
Mist, fog, clouds
|
Liquid
|
Solid
|
Mercury in silver
|
Solid
|
Liquid
|
Sugar in water,
jellies
|
Solid
|
Gas
|
Smoke
|
Solid
|
Solid
|
Metal alloys,
pearls
|
Liquid
|
Liquid
|
Milk, alcohols in
water
|
- If the forces of interactions between the molecules of different components of the solute are same as when they were in the pure state, they are ideal solutions otherwise non-ideal.
- If the volume of solution is not equal to the sum of the individual volumes of the components, the solution is non-ideal.
- Ideal solutions have zero enthalpy change as their heat of solution.
- If the solutions obey Raoult’s law, then they are ideal. This is one of the best criterions for checking the ideality of a solution.
Substance
|
Heat of solution (kJ mol-1
of solute)
|
Liquids Practically Immiscible:
Those liquids which do not
dissolve into each other in any proportion are immiscibe. For example. Water
and benzene, carbon disulphide and water.
Completely miscible liquids:
Water and ethyl alcohol, ethyl
alcohol and ether, benzene and toluene
Partially miscible liquids:
Conjugate Solutions:
On shaking equal volumes of water
and ether, two layers are formed. Each liquid layer is a saturated solution of
the other liquid. Such solutions are called conjugate solutions. Solubility of
these conjugate solutions is affected by temperature changes. Typical examples
of such systems are:
Phenol-water system
Triethylamine-water system
Nicotine-water system
Raoult’s Law:
p
= po x X1
∆p / po = X2
The total vapour pressure of a
mixture of two or more volatile liquids is equal to the sum of their partial
vapour pressures in that mixture.
where
and PA and PB
are the vapour pressures of liquids A and B in the pure states.
Ideal and non ideal Solutions:
Some typical ideal solutions
forming liquid pairs are: benzene toluene, benzene-ether, chlorobenzene and
bromobenzene, ethyl iodide-ethyl bromide, etc.
Zeotropic Mixtures:
A zoetrope is a liquid mixture
that obeys Raoult’s law. Such liquid mixture distill with a change in
composition.Methyl alcohol-water solutions.
Solubility and solubility curves:
The concentration of solute in
solution when it is in equilibrium with solid substance at a particular
temperature. OR
g
of solute / 100 g of solvent
If excess of solid is present in
solution then
Dissolution
= crystallization
Effect of temperature on solubility
Continuous solubility curves
Discontinuous solubility curves
Fractional crystallization
Separation of solid substance from
their solutions one by one depending upon their solubilities at different
temperatures is called fractional crystallization.
Colligative properties of Solutions
Colligative properties are those
properties, which depend only upon the number of solute particles and are
independent of the nature of the solute.
Relative Lowering of Vapour Pressure:
On adding a non-volatile solute
particle, the vapour pressure of the solution decreases because a part of the
total surface of solvent is covered by non-volatile solute particles.
∆p
/ po = X2
M2
= po W2 M1/
∆p W1
Elevation of Boiling Point:
Boiling point of a liquid is
defined as that temperature at which its vapour pressure becomes equal to
atmospheric pressure, that is, 760mm of Hg.
∆Tb
∞ m
M2
= KbW2 1000/ ∆Tb W1
If a non-volatile solute is added
to the solvent, there is a decrease in vapour pressure of the solution and thus
boiling point of the solution (T) is higher than that of pure solvent (T0).
This increase in boiling point is called elevation of boiling point.
Depression in Freezing Point:
Freezing point of a liquid is
defined as the temperature, at which it is in equilibrium with its solid state, ∆Tb ∞
m
M2
= KfW2 1000/ ∆Tf W1
At freezing point, liquid and
solid states will have same vapour pressure. If a non-volatile solute is added
to the solvent, there is decrease in vapour pressure of the solution and thus
the freezing point of the solution is less than that of pure solvent. This
decrease in freezing point is called depression of freezing point.
Enthalpy or Heat of Solution:
The enthalpy or heat of solution
of substance is defined as the heat change when one mole of the substance is
dissolved in a specific number of moles of solvent at a given temperature. It
is given the symbol Hsolu. The Hsolu gives the difference between the energy
possessed by the solution after its formation and the original energy of the
components before their mixing i.e.
Hsolu. The Hsolu gives the difference between the energy
possessed by the solution after its formation and the original energy of the
components before their mixing i.e.Hsolu = Hsolution – Hcomponents
Here Hsolu is the energy content of solution after its
formation while Hcomponents represents the energy contents of
components before their mixing.
Hsolu is the energy content of solution after its
formation while Hcomponents represents the energy contents of
components before their mixing.
Heat of solution of some ionic solids:
NaCl 4.98
NH4NO3 26.0
KCl 17.8
Kl 21.4
NH4Cl 16.2
LiCl -37.0
AlCl3 -321.0
Hydration:
The process in which water
molecules surround and interact with solute ions or molecules is called
hydration.
Hydrates:
The crystalline substances, which
contain chemically combined water in definite proportions is called a hydrate.
Hydrates are mostly produced when aqueous solution of soluble salt is
evaporated. The formation of hydrates is not limited to salts but is common
with acids, bases, and elements. For example, copper sulphate is pentahydrated.
Four water molecules are attached with Cu2+ and one with SO42-.
The reason is that Cu2+ ha a greater charge density.
Water Molecules of Crystallization or Water of Hydration:
Those water molecules, which
combine with compounds as they are crystallized from aqueous solutions, are
called water molecules of crystallization or water of hydration.
Lattice Energy:
The amount of energy needed to
separate a crystalline compound into isolated ions (or atoms) is known as
lattice energy. The lattice energy of ionic solids is always higher than
molecular solids.
Hydrolysis:
Salts of weak acids with strong
base react with water to produce basic solutions whereas salts of weak bases
with strong acids react to give acidic solutions. Such reactions are called by
hydrolytic reactions and the salts are said to be hydrolyzed.
EmoticonEmoticon