Thermodynamics
Chetna
(Assistant prof. in chemistry)
Thermodynamics may be defined as follows :
Thermodynamics is the science that deals with the interaction between energy and material systems.
Thermodynamics, basically entails four laws or axioms known as Zeroth, First, Second and Third law of thermodynamics.
The First law throws light on concept of internal energy.
DEFINITION OF THERMODYNAMICS:
The Second law indicates the limit of converting heat into work and introduces the principle of increase of entropy.
The Third law defines the absolute zero of entropy.
The Zeroth law deals with thermal equilibrium and establishes a concept of temperature.
The System and the Surroundings:
A system in thermodynamics refers to that part of universe in which observations are made and remaining universe constitutes the surroundings. The surroundings include everything other than the system. System and
the surroundings together constitute the universe .
The universe = The system + The surroundings
Types of the System:
Open System:
In an open system, there is exchange of energy
and matter between system and surroundings
The presence of reactants in an open beaker is an example of an open system. Here the boundary is an imaginary surface enclosing the beaker and reactants.
Closed System:
In a closed system, there is no exchange of matter, but exchange of energy is possible between system and the surroundings. The presence of reactants in a closed vessel made of conducting material .
E.g.
copper or steel is an example of a closed system.
Isolated System
In an isolated system, there is no exchange of
energy or matter between the system and the
Surroundings.
Eg: The presence of reactants in a thermos flask or any other closed insulated vessel is an example of an isolated system.
Extensive and Intensive properties:
An extensive property is a property whose value depends on the quantity or size of matter present in the system.
For example, mass, volume, internal energy,
enthalpy, heat capacity, etc. are extensive
properties.
Those properties which do not depend on the quantity or size of matter present are known as intensive properties.
For example:
temperature, density, pressure etc. are
intensive properties. A molar property, χm, is
the value of an extensive property χ of the
system for 1 mol of the substance. If n is the
amount of matter,
Χm= χ/n
m is independent of amount of matter.
Heat capacity:
The heat appears as a rise in temperature of the system in case of heat absorbed by the system.
The increase of temperature is proportional to the heat transferred.
i.e q=coeff ×T
The magnitude of the coefficient depends on the size, composition and nature of the system.
We can also write it as q = C ∆T
The coefficient, C is called the heat capacity.
the molar heat capacity is the heat capacity one mole of the substance and is the quantity of heat needed to raise the temperature of one mole by one degree celsius (or one kelvin).
i.e
Cm =C/n
Specific heat, also called specific heat capacity
is the quantity of heat required to raise the
temperature of one unit mass of a substance
by one degree celsius (or one kelvin)
For finding out the heat q, required to raise the
temperatures of a sample,
we multiply the specific heat of the substance c, by the mass m, and temperatures change ∆T as:
q = c ×m ×∆ T
=C∆ T
The relationship between Cp and CV for an ideal gas:
At constant volume, the heat capacity, C is
denoted by CV and at constant pressure, this
is denoted by CP . Let us find the relationship
between the two.
We can write equation for heat, q
at constant volume as qV = = CV ∆T=∆U
at constant pressure as qp = = Cp ∆T =∆H
The difference between Cp and CV can be
derived for an ideal gas as:
For a mole of an ideal gas, ∆H = = ∆U + ∆(pV )
∆H= ∆U + ∆(pV )
= ∆U + ∆(RT )
= ∆U + R∆T
∴ ∆H = ∆U +R∆ T
On putting the values of ∆H and ∆U,
we have
CP∆T=CV∆T+R∆T
CP=CV+R
Cp−CV=R