2. General introduction:
The branch of science which deals with the study of different form of
energy and the quantitative relationship between them is known as
thermodynamics.
When we confine our study to chemical changes and chemical
substances only, then that branch of thermodynamics is known as
chemical thermodynamics.
3. Some basic terms and concepts:
1. System: A system means that part of Universe which is under study.
For example: a certain amount of solid, liquid or gas contain in a
system.
2. Surrounding: The rest of the universe or environment around the
system is called surrounding.
So, the surrounding means all other things which can interact
with the system.
3. Boundary: Anything that separates the system from the
surrounding is called boundary.
Thus, universe = system + surrounding
4. Depending upon how a system interacts with the surrounding it is classified
into 3 categories.
1. Closed system: A closed system is one which can’t exchange matter with
surrounding. For example, a close container in a closed system, because
no matter can enter or escape from it. However, the closed system can
exchange heat or work with surrounding.
2. Open system: An open system is one which can exchange both matter
and energy( heat or work) with the surrounding.
Hot water contained in an open beaker is an example of open system.
5. 3. Isolated system: An isolated system is one which can exchange neither
matter nor energy with the surrounding. If the boundary is closed and
insulated, no interaction is possible with the surrounding. Hence hot or cold
water contained in a thermos is an isolated system.
4. State of a system and state variable: The state of a system means the
condition of the system which is described in terms of certain measurable
properties such as temperature (T), pressure(P), volume(V), etc. of the
system. If any of these properties of the system changes, the system is said
to be in different state i.e. the state of the system changes. That is why these
properties of the system are called state variable.
The properties is said to occur when the state of the system changes. The
first and last state of the system are initial sate and final state respectively.
6. 5. State function: A physical quantity is said to be state function if its value
depends only upon the state of the system and doesn’t depend upon the
path by which this state has been attained.
For example: potential energy, internal energy, enthalpy, free energy etc.
Explanation:
let us consider two points at the peak and a base of a hill. The climbers can
move from base to the peak of the hill through different path. Work done
depend upon the distance travelled by the climber (may be longer and a
shorter route). So, it is not a state function. However a climbers standing on
the peak of hill has a fixed value of PE, irrespective of the fact that whether
he reached by stairs or lift. Thus PE of the person is a state function
7. 6. Thermodynamic properties:
i. Intensive properties: The properties of the system which do not
depend upon the amount of the substance present is called
intensive properties. For example: temperature, pressure, viscosity,
density, surface tension, etc.
The boiling point of water is 100oc at one atmosphere whatever
its mass.
ii. Extensive properties: The properties of the system which depend
upon the amount of substance present is called extensive
properties. For example, mass, volume, energy, work, entropy, etc.
let us consider a glass of water, if we double the mass of water,
the volume, no. of moles, internal energy etc. also get doubled so
they are extensive properties.
8. Types of thermodynamic process:
Isothermal and Adiabatic process:
A process is said to be isothermal, if the temperature of the system
remains constant during each stage of the process. Such type of
process may be achieved by placing the system in thermostat.
A process is said to be adiabatic, if no heat enters or leaves the system
during any step of the process i.e. the system is completely insulated
from the surrounding.
9. Isochoric and isobaric process:
A process is said to be isochoric, if the volume of the system remains
constant during each step of the process.
A process is said to be isobaric if the pressure of the system remains
constant during each step of the process.
10. Exchange of energy between system and
surrounding:
a. As work: If the system and the surrounding are at two different pressure, then
the energy is exchange between the system and surrounding in the form of
work.
Suppose there is a gaseous system enclosed in a cylinder fitted with a piston. If the
pressure of the gas is higher than that of the surrounding, the gas will expand, push
the piston and does a work against the surrounding. Here energy is transformed
from the system to surrounding in the form of work. If the surrounding is at higher
pressure, the gas contracts and work is done on the system by the surrounding. In
this case energy is transferred from the surrounding to the system in the form of
work.
11. b. As heat: If the system and the surrounding are at two different
temperature, energy is transformed or exchanged between the system
and surrounding in the form of heat.
For example: When ice is kept at room temperature, flow of heat
energy takes place from the surrounding to the system (ice) unless both
of them will have same temperature. Similarly if hot water in a vessel is
kept at room temperature, flow of heat energy takes place from hot
water( system) to surrounding unless both of them will have the same
temperature.
12. Internal energy (E) :
The energy of the thermodynamic system is called internal energy. It
includes all the possible forms of energy of the system. The sum of different
forms of energy associated with the molecule is called internal energy.
Internal energy = Translational energy + rotational energy + vibrational
energy + electrical energy + nuclear energy + bonding energy……..
The internal energy of the system depends upon the state of the system but
not upon the path on which the system attains another state. So, internal
energy is a state function. Therefore, the absolute value internal energy
cannot be determined.
13. In a chemical reaction,
ΔE = Eproduct – Ereactant
If Eproduct > Ereactant, heat is absorbed by the system and hence reaction is
endothermic.
If Eproduct < Ereactant, heat is released by the system and hence reaction is
exothermic.
14. First law of thermodynamics:
It states that, “ Energy can be neither be created nor destroyed
although it may be converted from one form to another”
OR
The total amount of energy in the Universe remains constant although
it may undergo transformation from one form to another.
15. Mathematical formulation of first law of
thermodynamics:( relation between internal
energy, work and heat)
The internal energy of the system can be increased in 2 ways.
1. By supplying heat to the system
2. By doing work on the system
