Entry Test Preparation 2015, Chemistry BOOK 1
Chapter # 2
gases
States of matter:
Matter exists in four states i.e., solid, liquid, gas and plasma.
Properties of Gases:
·
Gases don’t have definite shape
and volume.
·
Their molecules move rapidly in all directions.
·
They occupy the whole space available to them in the container.
·
There are large empty spaces between molecules of a gas in normal
state.
·
Gases can diffuse and effuse.
·
Gases can be compressed by applying pressure.
·
Gases expand on heating.
·
Low temperature is produced by expansion of gases. (Joule Thomson
effect). For example, the escaping CNG from the cylinder makes the cylinder
cool.
·
In gases molecules collide with one another and with the walls of
container elastically.
·
The gases have low densities as compared to the densities of liquids
and solids.
·
Gases form homogeneous mixtures.
·
The intermolecular forces in gases are very weak.
Units of pressure:
Atmospheric pressure:
The force exerted by the
atmosphere of the earth per unit area is called atmospheric pressure.
The force exerted by 76 cm long
column of mercury on an area of 1 cm2 at 0o C is called standard pressure or
atmospheric pressure.
SI unit for measurement of
pressure is Pascal (Pa). Other units are torr, mm of Hg, atmosphere and Nm-2
1 atm = 760 mm Hg
= 29.5230 in Hg
= 14.7 PSi
= 101325 Pa
= 101325 Nm-2
= 1.01325 bar
PSi used in engineering work and millibar used by meteorologists.
Scales of thermometry:
K= 0C + 273
0C=5/9(0F-32)
0F=9/5(0C) +32
Boyle’s Law:
Volume of a given mass of a gas is inversely proportional to the
pressure applied at constant temperature.
V ∞ 
(T is kept constant)
(T is kept constant)
PV = k
Charle’s Law:
The volume of a given mass of a
gas is directly proportional to the absolute temperature at constant pressure.
V ∞ T or 
= k
= k
(Pressure is kept constant)
Ideal Gas:
A gas which obeys gas laws completely at all conditions of temperature
and pressure is called ideal or perfect gas. Inert gases behave like ideal
gases up to some extent.
Absolute Zero:
The hypothetical temperature at which volume of a gas becomes zero OR
The temp. at which motion of molecules ceases.
O K =
-273.15 oC
Volume of a gas at various Temperatures is
Vt = Vo
(1 + 
) Where
) Where
Vt = volume
of gas at temperature t
Vo = volume
of a gas at temperature 0oC
t = temperature on Celsius scale
Density of an Ideal Gas:
The density of an ideal gas is
directly proportional to its molar mass.
Greater the pressure on the gas,
greater the density.
Higher temperature make density
falls.
General Gas Equation:
PV = nRT
Where
n
= number of moles of gas with the
molecular mass M, T = temperature in Kelvin, and R = gas constant.
|
Values R in various systems
|
|
8.314472 J . K-1 .mol-1
|
|
0.0820574587 L . atm. K-1
. mole-1
|
|
8.20574587 x 10-5 m3
. atm . K-1 . mol-1
|
|
8.314472 cm3 . MPa .
K-1 . mol -1
|
|
8.314472 L . kPa . K-1
. mol-1
|
|
8.314472 m3 . Pa . K-1
. mol-1
|
|
62.3637 L . mmHg . K-1 . mol-1
|
|
6.132439833 1bf .
ft . K-1 .
g . mol-1
|
|
10.7316 ft3 .
psi . oR-1 .
1b . mole-1
|
|
8.63 x 10-5 eV .
atom-1 . Kelvin-1
|
|
0.7302 ft3 .
atm . oR-1 . 1b
– mole-1
|
|
62.3637 L .
Torr . K-1 .
mol-1
|
|
83.14472 L . mbar
. K-1 .
mol-1
|
|
1.987 cal . K-1 .
mol-1
|
Avogardro’s Law:
According to this law, “Equal
volumes of ideal gases at the same temperature and pressure contain equal
number of molecules” 22.414 dm3 of a gas at 273.15 K and one atm. Pressure has
6.02 x 1023 molecules of the gas.
Brownian movement:
The continuous and random
movement of small solid particles, suspended in a fluid medium is called the
Brownian movement.
|
|
The total pressure exerted by the
mixture of gases is equal to the sum of the partial pressures of individual
gases.
Ptotal = P1
+ P2 + P3 …….
Where
P1 , P2
, P3 , …….
Are partial pressures of gases 1,2,3,…… respectively.
Important applications of Dalton
·
Some gases are collected
over water in the laboratory.
·
The process of respiration
depends upon the difference in the partial pressures.
·
At higher altitudes, a
person feels uncomfortable breathing because the partial pressure of oxygen in
the un-pressurized cabin is low, as compared to 159 g/cm3 on the
earth surface.
·
Deep sea divers take oxygen
mixed with an inert gas to adjust the partial pressure of oxygen according to
the requirement.
…………………………………
Aqueous tension:
The Partial pressure of water
vapours in a gas is called aqueous tension.
Diffusion of Gases:
The spontaneous mixing of
molecules of one gas with another at a given temperature and pressure is called
diffusion.
|
The spreading of fragrance of a rose or a scent is due to diffusion.
|
Effusion of Gases:
The movement of gas molecules
through pores into a region of low pressure of the gas is called Effusion. This
spreading of molecules is due to their collisions with the walls of the
container.
Graham’s Law of Diffusion and Effusion:
At constant temperature and
pressure, the rate of diffusion of a gas is inversely proportional to the
square root of its density.
The volume of a gas diffused per
unit time is called the rate of diffusion.
Rates of diffusion of two gases
can be found by the following relation
Where r1 , D1,
and M1 are rate of diffusion, Density, and Molar mass of the gas 1
respectively and r2, D2, and M2 are rate of
diffusion, Density , and Molar mass of the gas 2 respectively.
Kinetic Molecular Theory of Gases:
Following are the fundamental
postulates of the kinetic theory of gases:
·
Every gas consists of a
large number of very small particles called molecules.
·
The molecules of a gas move
randomly, colliding among themselves and with the walls of the container
elastically and change their direction.
·
The pressure exerted by a
gas is due to the collisions of its molecules with the walls of the container.
·
The collisions among the
molecules are perfectly elastic.
·
The molecules of a gas have
no forces of attraction for each other.
·
The actual volume of
molecules of a gas is negligible as compared to the volume of the gas.
·
The effect of gravity is
negligible on the gas molecules.
·
The kinetic energy of the
gas molecules varies directly as the absolute temperature of the gas.
Critical Temperature and Critical Pressure:
The highest temperature, at which
a substance can exist as a liquid, is called its critical temperature.
There is a corresponding
pressure, which is required to bring about liquefaction at this critical
temperature. This is called critical pressure.
Liquefaction of Gases
Gases can be liquefied by the
following methods:
High Pressure:
At very high pressure the gas
molecules come close to each other and the force of attraction between them
become strong enough to make the gas liquid.
Low Temperature:
At very low temperatures the gas
molecules loses kinetic energy and the slight attractive forces becomes strong
enough to make the gas a liquid.
Joule Thomson Effect:
Linds method: H2 and He cannot be liquefied by this
method as these have less attractive forces, small sizes and have very low
critical temperatures.
………………………………..
Non Ideal Behavior of Real Gases:
Real gases, such as hydrogen,
oxygen etc., do not obey gas laws exactly because:
·
At low temperatures the gas
molecules have less kinetic energy i.e move around less so they do attract each
other.
·
At high pressure the gas
molecules are forced to come close to each other so that the volume they
occupy, decrease significantly.
·
Under ordinary conditions,
deviations from Ideal Gas behaviour are so slight that they can be neglected.
Real gases,
however, obey gas laws at low pressures and high temperature,
A real gas do not obey gas laws
at high pressures and low temperatures. This is called non ideal behavior of
gases.
A gas which
obey gas laws at all temperatures and pressure is called an ideal gas.
Vander Waal’s Equation.
VOLUME CORRECTION: If the effective volume of the molecules per mole of a gas is represented
by b, then the volume available to gas molecules is the volume of the vessel
minus the volume of the gas molecules
Vfree = Vvessel _ Vmolecules
Vfree = Vvessel _ b
Where,
Vfree = volume which
is available to gas molecules.
b = excluded volume which is constant and
characteristics of a gas.
Its
value depends upon the size of gas molecules.
b = 4Vm
Where,
b = volume of a gas which is occupied by 1
mole of a gas molecules in highly
compressed state
and not in the liquid state.
PRESSURE CORRECTION:
Observed
pressure is less than the actual pressure.
P = Pi _ P′
Pi = P + P′
P′ = a / V2
P′ α (n / V) x (n / V)
P′ = n2 / V2
P′ = an2 / V2
P′ = a / V2 (
where n=1 )
Pi = P + (a / V2)
PV = RT
(P + a / V2) (V – b) = RT
(P + an2 / V2) (V – nb) = nRT
Units
of “a”:
a = P′V2
/ n2
a = (atm)
x (dm3)2/ (mol)2 = atm
dm6 mol-2
In SI unit,
Nm-2
x (m3)2 / (mol)2 = Nm+4
(mol) -2
Units
of “a”:
dm3
mol -1 or m3 mol -1
The presence of intermolecular forces in
gases like Cl2 and SO2 increases their 'a' factor. Value
of “a” depends upon intermolecular forces and value of “b” depends upon yhe
size of gas molecules. H2 has less value of “a” than SO2
or Cl2 due to greater non-polar characters.
“An electricity neutral mixture of electrons,
ions, and atoms is called plasma”
It
is the fourth state of matter. Plasma was identified in 1879 by William Crook.
99% of the universe is plasma state and solid, liquid and gases are only 1%.
Molecule → Atomic
Gas
Atomic
Gas → positive
ions + electrons
Natural plasma exist only at very
high temperatures, or low temperatures vacuums. Natural plasma on the other
hand do not breakdown or react rapidly, but are extremely hot (over 20,000oC
minimum). Their energy is so high that they vaporize any material they touch.
Future Horizon:
Scientists
are working on putting plasma to effective use. Plasma would have to be low
energy and should be able to survive without instantly reacting and
degenerating. The application of magnetic, fields involves the use of plasma.
The magnetic fields create low energy plasma which create molecules that are in
what scientist call metastable state. The magnetic fields used to create the
low temperature plasma give the plasma molecules electrons, which do not react
until they collide with another molecule with just the right energy. This
enables these metastable molecules to survive long enough to react with a
designated molecule.
These metastable particles are
selective in their reactivity. It makes them a potentially unique solution to
problems like radioactive contamination.
Scientists are currently experimenting
with mixtures of gases to work as metastable agents on plutonium and uranium,
and this is just the beginning.
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