Thermodynamics deals with heat changes during a physical or chemical process. It also deals with different forms of energy and the quantitative relationship between them.
Some Basic Concepts
System
The part of the universe which is under experimental study is called system.
There are three types of system:
Open System
A system that can exchange energy and matter with the surrounding.
Closed System
A system which can exchange energy but not matter with the surrounding.
Isolated System
A system which cannot exchange energy and matter with the surrounding.
Surrounding
The part of the universe except the system is called surrounding.
Universe
Universe= System + Surrounding.
State Function
Those microscopic properties which can change the state of a system are called state function or state variables. Example: pressure, volume, temperature, internal energy, entropy, etc.
Path Function
There are two types of path functions in chemical thermodynamics :
Intensive Variables
Those properties which do not depend on quantity. Example: temperature, viscosity, surface tension, pressure, etc.
Extensive Variables
Those properties which depend on quantity. Example: mass, volume, no. of moles, entropy, internal energy, etc.
Thermodynamic Processes
Isothermal
In this process of chemical thermodynamics, the temperature remains constant.
Isobaric
In this process of chemical thermodynamics, pressure remains constant.
Isochoric
In this process of chemical thermodynamics, volume remains constant.
Cyclic
In this process, a system comes back to its original state after undergoing a number of changes.
Adiabatic
In this process of chemical thermodynamics, no heat change takes place.
Thermodynamic Equilibrium
A system is said to thermodynamic equilibrium if it obeys all three (mechanical, thermal, chemical) equilibrium.
Mechanical Equilibrium
When the position or velocity of the system does not change with time.
Thermal Equilibrium
When the temperature of the system does not change with time.
Chemical Equilibrium
When the composition of a system does not change with time.
Some Thermodynamic Quantities
Heat
When there is a difference in the temperature of the system and surrounding then heat exchange takes place. For this either heat is absorbed by the system or given out by the system. It is denoted by q.
Heat absorbed by the system (q) = +ve
Heat given out by system (q) = -ve
Work
This type of work is involved in a system containing gases expand or contract by applying pressure.
Work is done on the system = +ve
Work is done by the system = -ve
Internal Energy
It is the energy stored within a substance or a system. It is a state function. It is denoted by U.
Energy of the system increases = +ve
Energy of the system decreases = -ve
First Law Of Thermodynamics
Energy can neither be created nor be destroyed but one form of energy can be converted into another form of energy.
∆U= q+w
Where,
∆U= change in internal energy
q= heat
w= work
Enthalpy and Enthalpy Change
It is equal to the heat absorbed or evolved by the system at constant pressure and constant temperature.
H= ∆U+P∆V
Heat Capacity
Amount of heat required to raise the temperature of a system through 1°C.
C= q/∆t
Specific Heat Capacity (c)
The amount of heat required to raise the temperature of 1g of a substance through 1°C.
c= C/m
Molar Heat Capacity
It is the amount of heat required to raise the temperature of 1 mole of the substance through 1°C.
Cm= C/n
Or
Cm= q/n∆t
Molar heat capacities are of two types:
M.H.C. at Constant Volume
It is at constant volume (Cv).
Cv= ∆U/∆T
M.H.C. at Constant Pressure
It is at constant pressure (Cp).
Cp= ∆H/∆T
Relation between Cp and Cv
Cp – Cv = nR
Relation between Cp/Cv and atomicity of gas
If Cp/Cv = 1.66, then the gas is monoatomic.
If Cp/Cv = 1.40, then the gas is diatomic.
