PHYSICAL CHEMISTRY,B.Sc.I

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2. GASEOUS LAW
PART I ,B.Sc.I PAPER III,UNIT II
BY
-Neha Tripathee
Assistant Professor
K.M.G.G.P.G.COLLEGE BADALPUR

3. Kinetic theory of gases-
• given by Maxwell and Boltzmann .
• every gas is made up of extremely small particles known as
molecules.
• actual volume of the gas molecules is negligible as compared
to the total volume of the gas
• the molecules are so far from each other that the force of
attraction among them is also negligible .
• molecules keep on colliding on the walls of the container and
this exerts a pressure.

4. COTND…..
• no energy is lost while collision on the wall.
• the force of gravitation is also supposed to be
negligible.
• the kinetic energy of the gas molecules is proportional
to the Absolute temperature of the gas.

5. Difference between ideal gas and real gas
•Ideal gas obeys Boyle's law and Charles law under
all the conditions of temperature and pressure
•This obeys the equation PV =nRT
•Real gas follows the gas equation on a very low
pressure and high temperature .
•All gases are real gases.

6. DEVIATION OF REAL GASES FROM IDEAL BEHAVIOUR-
PLOT OF PV Vs P-

7. PLOT OF Z Vs P FOR N2 GAS AT DIFFERENT
TEMPERATURES

8. VANDERWAAL EQUATION
• Two postulates taken under kinetic theory of gases –
• the intermolecular force of attraction among the
molecules is negligible
• and the volume of gas molecules is also negligible
as compared to the whole volume of the gas.

9. Pressure correction–
• gas molecules present in the centre of the container is surrounded
from all the sides by other molecules.
• Thus the overall force exerted on them is NIL.
• But the molecules present just next to the wall of the container
are surrounded only by the molecules present in the back side of
the molecule.
• Thus the pressure exerted by these molecules is reduced.
• Thus we can say the real pressure is less than the ideal pressure
• i.e
. P(ideal) = P(real) + p

10. p= pressure correction
• p depends on the the force of attraction on striking
molecules,which itself depends on no.of molecules present per
unit volume.
P α n/V (here n/V is the density of gas)
• p also depends on no. of striking molecules,per unit volume.
p α n/V
Thus
• p α n2/V2
• p=a n2/V2 Here ‘’a ‘is proportionality constant
• Pi =Pr + a n2/V2

11. Volume correction
• since at high pressure the volume of the molecules does not remain negligible as
compared to the whole volume of the gas.
• Thus in real gas the volume is less than the ideal gas.
• ideal gas volume
• Real gas volume
• volume of gas molecules
Image A Image B
• Thus the ideal volume Vi = Vr – v
• v α n therefore v = nb
• Thus
• a and b are known as vanderwaal constant.
Vi = Vr - nb

12. Contd…..
• Putting these values of real gas in ideal gas
equation-PiVi=nRT
• Pi =Pr + a n2/v2
• Vi = Vr - nb
• (Pr + a n2/v2)(Vr - nb) =nRT
(P +
𝒂𝒏𝟐
𝑽𝟐
)(V-nb) =nRT

13. Explanation of real gas behaviour-
• More the values of vanderwaals constants a and b ,more is the tendency of
liquification of gases.
• in four parts --- (i) At low pressure
(ii) at moderate pressure
(iii) at high pressure
(iv) at high temperature

14. • (i)At very low pressure-V is very large- ֶtherefore (a/V2)
• and b are negligible , as compared to P and V
respectively. Equation becomes –PV=RT
• (ii) At moderate pressure – b can be neglected as V is still
high

15. (iii)At high pressure-
• V is very small , and P very large.So , a/V2 can be neglected but
not b.
• Thus-
• (iv)At high temperature-
V is very large,so equation again
reduces to PV=RT,i.e.ideal behaviour
is shown.

16. Anomoulous behaviour of hydrogen and helium
gases-
• Gas molecules are very small in size and thus masses.
• Therefore intermolecular force of attraction and a/V2
• is negligible.
• Thus
P(V-b) = RT
PV=RT + Pb
References-
Chemistry of degree students by Dr.R.L.Madan
Physical chemistry by Puri and Sharma

17. •Thank you