Bpharm 2nd semester Physical chemistry Behaviour of gases, Kinetic theory of gases

1. Snb College of pharmacy
and management
By
Mr. ANAND KUMAR (LECTURER)
Faculty of Pharmaceutical Chemistry
SNB College of Pharmacy and Management
PHARMACEUTICAL CHEMISTRY – II
(PHYSICAL CHEMISTRY)
B.Pharm 2nd
semester

2. Chapter-1
Behaviour of Gases
Syllabus:- Behaviour of Gases: Kinetic theory of gases, deviation from behaviours and
explanation.
 Behaviour of Gases:
i. The Behaviour of gase molecules is dependent on the properties & laws obeyed by the
molecules of the gase. The distribution of molecules in a gase is very different from the
distribution of molecules in liquids & solids.
ii. Matter is made of molecules, atoms & ions. Matter exist in one of the three state gas,
liquid & solid.
iii. Gase: A gase is a homogeneous fluid, generally of low density &low viscosity & the
volume of gase is assumed to have the volume of the vessel.
iv. A gase consist of molecules separated wide apart in empty space. The molecules are
free to move about through the container.
 Properties of Gases:-
i. Shape = No
ii. Volume = No
iii. Intermolecular space = High
iv. Molecular attraction = low
v. Intermolecular force = low
vi. Able to flow
vii. Indefinite shape & size
viii. Highly compressible
 You can predict the behaviour of gases based on the following properties:
 Pressure
 Temperature
 Volume
 Number of particles

3.  Definitions of some importance terms:
1. Sublimation: - It is a physical process in which, conversion of a substance from the
solid state to the gaseous state without its becoming liquid.
2. Deposition: - It is a physical process or the phase transition in which gase
transformation into solid state without passing through the liquid phase.
3. Melting (Fusion):- It is a physical process in which solid can be heated to the point
where the molecules holding their bonds together break apart & form a liquid.
4. Freezing: - The process in which, the change in the liquid state to the solid state at a
constant temperature.
5. Condensation: - It is a physical process in which phase transform into liquids state
from vapor or gaseous state.
6. Vaporization: - It is a physical process, it can be defined as the process in which
liquid state change into the vapour state.
MELTING
FREEZING
LIQUID
SOLID
GASES

4. KINETIC THEORY OF GASES
 Introduction:
i. This type of theory also known as Kinetic Molecular Theory.
ii. The kinetic molecular theory is a single set of descriptive characteristics of a substance
known as the Ideal Gas. Based on experimental & microscopic studies to explain the
behaviour of gases.
Ideal Gas Equation: PV=nRT (R is a Gase constant)
iii. As results of experimental studies from 17th
to 19th
century, scientists derived the
relationships among the pressure, temperature & volume of a given mass of a gase.
These relationships which describe the general behaviour of gases are called the Gase Law.
iv. The Kinetic theory of gases attempts to explain the microscopic properties of a gase in
terms of the motion of its molecules.
 Gase Laws:-The gase laws are a group of laws that govern the behaviour of gases by
providing relationships between the following:
 The volume occupied by a gase.
 The pressure exerted by a gase on the walls of its container.
 The absolute temperature of the gase.
 The amount of gaseous substance or the number of moles of gase.
The gase laws were developed towards the end of the 18th
century by
numerous scientists. These are following:
1. Boyle’s Law
2. Charles’s Law
3. Gay-Lussac’s law
4. Avogadro’s law

5. Boyle’s law
 Introduction: - In 1660 Robert Boyle found out experimentally the change in volume of
a given sample of gase with pressure at room temperature, Form his observations the
formulated a generalization known as Boyle’s law.
 Law: - It state that at constant temperature, the volume of a fixed mass of a gase is
inversely proportional to its pressure. If the pressure is doubled, the volume is halfed.
 Derivation:-
The Boyle’s law may be expressed mathematically as,
V 𝜶
𝟏
𝑷
(T, n are constant)
, V=K/P (where K is a proportionally constant)
, PV = K
If P1, V1 are the initial pressure & volume of a given sample of gase & P2, V2 are the changed
pressure & volume.

6. We can write as,
P1V1 = K = P2V2
Or, P1V1 = P2V2
 Explanation: - The Boyle’s law can be demonstrated by adding liquid mercury. To the
open end of a J-tube. As the pressure is Increase by addition of liquid mercury, the volume
of the sample of trapped gas decreases. Gas pressure & volume are inversely related 1
increase when the other decrease.

7. Charles’s law
 Introduction: - In 1787 Jacques Charles investigated the effect of change of temperature
on the volume of a fixed amount of gase at constant pressure. He established a generalization
which is called the Charles’s Law.
 Law: - It state that at constant pressure, the volume of a fixed mass of gase is directly
proportional to the temperature. If the temperature is doubled, the volume is doubled.
 Derivation:- Charles’s law may be expressed mathematically as,
V 𝜶 𝑻 (P, n are constant)
, V=KT (where K is a proportionally constant)
, V/T = K
If V1, T1 are the initial volume & temperature of a given mass of a gase at constant pressure &
V2, T2 be the change values. We can write as,
V1/T1 = K = V2/T2
Or, V1/T1 = V2/T2

8. The Combined Gas Law
Boyle’s law & Charles’s law can be combined into a single relationship called the Combined
Law.
Boyle’s Law , V 𝜶
𝟏
𝑷
(T, n are constant)
Charles’s Law, V 𝜶 𝑻 (P, n are constant)
Therefore, V 𝜶
𝑻
𝑷
(n are constant)
The combined law can be state as for a fixed mass of gase, the volume is directly
proportional to temperature & inversely proportional to the pressure.
If K be the proportionally constant,
V = KT/P
Or, PV = KT (n are constant)
If the pressure, volume & temperature of gase be changed from V1, P1, T1 to V2, P2, T2. Then,
P1V1T1 = K = P2V2/T2
This is the form of combination law for two set of combination. It can be used to solve
problems involving a change in three variables P, V & T for a fixed mass of gase.

9. Gay-lussaC’s law
 Introduction: - In 1802 Joseph Gay-Lussac named scientist was given experimental
studies law. After result of his experiments established a general relation between the
pressure & temperature of a gas. This is known as Gay-Lussac’s law or Pressure-
Temperature Law.
 Law: - It states that at constant volume, the pressure of a fixed mass of a gase is directly
proportional to the temperature.
 Derivation:- The law may be expressed mathematically as,
P 𝜶 𝑻 (V, n are constant)
, P=KT (where K is a proportionally constant)
, P/T = K
For different conditions of pressure & temperature.
P1/T1 = K = P2/T2
Or, P1/T1 = P2/T2
Knowing P1, T1 & P2, and T2 can be calculated.

10. aVoGaDro’s law
It state that as equal volumes of gases at the same temperature & pressure contain equal number
of moles or molecules.
If the molar amount is doubled, the volume is doubled.
 Derivation: - Let us take a balloon containing a certain mass of gase. If we add to it
more mass of gase, holding the temperature & pressure constant, the volume of gase will
increase. It was found experimentally that the amount of gase in moles is proportional to
the volume.
That is, V 𝜶 𝒏 (P, T are constant)
, V=An (where A is a proportionally constant)
, V/n =A
For any two gases with volume V1, V2 & moles n1, n2 at constant T&P.
V1/n1 = A = V2/n2
If V1=V2, n1=n2
Thus for equal volume of the two gases at fixed T&P, number of more is also equal.