ChemistryPhysics

Chemical Thermodynamics

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.

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