Thermodynamics – Class 12 Chemistry

Chapter 4: Thermodynamics

Grade 12th Chemistry, Chapter 4 Thermodynamics Complete notes, Exercise PDF 2081 Academic year.

Introduction of Thermodynamics

It is the branch of Science that deals with the study of the flow of heat or any form of Energy into or out of the system when it undergoes any physical or chemical transformation.

Let us consider the following two reactions.
N₂ + 3 H₂→ 2 NH₃ [ ∆H=- 22.4 K Cal ]
N₂ + O2 → 2 NO [ ∆H= + 43.2 K Cal ]
In the given examples, the first reaction is an exothermic reaction and second reaction is an endothermic reaction. The branch of chemistry which deals with chemical reaction and energy change associated with it is called CHEMICAL THERMODYNAMICS.

Some Important thermodynamics terms

System

A System is defined as that part of the universe that is under thermodynamics study.

Surroundings

The rest of the universe ( Except the system ) is known as its surroundings.

Boundary

A  real or imaginary line separating the surroundings and the system is called a boundary.

Types of Thermodynamics system

1. Open System
2. Closed System
3. Isolated System

1. Open system

An Open system is one that can transfer both Energy and matter to and from the surrounding.

Example: Hot water in a beaker.

The boundary of the open system isn’t sealed.

2. Closed system

A Closed system is one that can transfer only energy not matter to and from the surrounding. 

Example: Hot water in a Hot water bag.

The boundary of the closed system is sealed.

3. Isolated system

An Isolated system is one that cannot transfer both Energy and matter to and from the surrounding.

Example: A thermos flask

The boundary of the Isolated system is closed and insulated.

Phase In Thermodynamics 

1. Homogeneous System
2. Heterogeneous System

1. Homogeneous System

A Homogeneous System is one that is uniform throughout and it contains only one phase.

Examples: A pure solid or liquid or gas, a mixture of gases, etc.

2. Heterogeneous System

A Heterogeneous System is one that is not uniform throughout and it contains more than one phase.

Examples: Ice Water, CH3Cl Water, etc.

Intensive and Extensive Properties

Intensive properties

Those Properties that don’t depend upon the quantity of matter are called Intensive properties.

Example: Boiling point of water, Density of water or liquid, etc.

Extensive properties

Those properties which depend upon the quantity of matter are called Extensive properties.

Example: Volume, Weight, etc.

State Function or State Variables

A thermodynamics system is said to be in a state where all of its Fundamental properties are fixed.

The Fundamental properties which defined a state of a system are pressure, temperature, number of moles ( its composition ), and Volume.

The change in the value of these properties alters the state of a system. These Properties are called State functions or State Variables.

For a pure substance, its composition is fixed as it is 100%.

If a mole of pure gas is taken the remaining state variable are related to one another by an algebraic equation called Equation of State.

PV = RT
where R is constant( Universal Gas Constant )

If P and T are Specified V is automatically fixed.

so, Pressure(P) and Time(T) are Called Independent State Variables as their Volume is Depend upon Pressure and Temperature.

Thermodynamics Process

When a system undergoes from one state( initial ) to another state ( final ) then the Operation is called Process.

There are different thermodynamic processes given:

1. Isothermal Process
2. Adiabatic Process
3. Isobaric Process
4. IsoChoric Process
5. Cyclic process

a. Isothermal Process: The process which is carried out at constant temperature condition is called isothermal process. During isothermal process, ∆T=0 or ∂T=0.

b. Adiabatic Process : The process in which heat can neither enter nor leave the system is called adiabatic process. During adiabatic process , ∆q = 0 or ∂q=0 or q=0.

c. Isobaric Process : The process which is carried out at constant pressure condition is called isobaric process. During isobaric process , ∆P=0 or ∂P=0.

d. Isochoric Process : The process which is carried out at constant volume condition is called isochoric process. During isochoric process , ∆V=0 or ∂V=0.

e. Cyclic Process: The process in which system comes to its initial stage after a series of different stages is called cyclic process. During cyclic process, ∆E= 0 & ∆H=0.

f. Reversible Process: If the process occurs very slowly such that during every step driving force is only infinitesimally greater than opposing force and which can be reversed back is called reversible process. It is ideal process and has only theoretical importance.

g. Irreversible Process: The process which occurs rapidly or in a single step and which cannot be reversed is called irreversible process. All natural process are irreversible. It is a real process and has practical importance.

Question: Write the differences between reversible and irreversible process.

Exchange of energy between System and surrounding

Energy exchanges between system and surrounding in two forms.

• As Heat
• As Work

i. As heat: If the system and surrounding differs in temperature, then energy exchanges between them in the form of heat.

Suppose the system is at higher temperature than surrounding. In this case, energy exchanges from system to surrounding in the form of heat till both are at thermal equilibrium which is denoted by -q. If surrounding is at higher temperature than system, then energy exchanges from surrounding to the system in the form of heat till both are at thermal equilibrium which is denoted by +q.

ii. As work : If the system and surrounding are at different pressures, then energy is exchanged between them in the form of work.

Suppose , a gas is kept in a cylinder fitted with frictionless piston. If the pressure of gaseous system is greater than that of surrounding, the gas expands and the piston moves in upward direction. In this case, work is done by the system against surrounding which is denoted by +w. If the pressure of
surrounding is greater than that of gaseous system, then gas contracts and piston moves in downward direction. In this case, work is done on system by surrounding which is denoted by -w. In both cases of expansion and contraction of gas, mechanical work or Pressure- Volume work or PV work can be calculated as follows.

i.e. W = P∆V; where, P is opposing pressure and ∆V is change in volume.

Internal Energy ( Intrinsic Energy ) [ E ]

A chemical compound contains various forms of energy in it due to its chemical nature. These energies are kinetic energy, potential energy, chemical bond energy, nuclear energy, electrical energy, rotational energy, vibrational energy translational energy etc. The sum of all these molecular energies in the chemical compound which arises due to chemical nature is called internal energy. It is denoted by E.

A definite amount of compound under the given set of conditions contains definite amount of internal energy. When the chemical reaction occurs, the internal energy of product differs from that of reactant. This difference in internal energy between reactant and product is denoted by ∆E and can be written as follows.

i.e. ∆E = Ep – Er; 

where Ep is the internal energy of product & Er is the internal energy of reactant.

• If ∆E = +ve, the reaction is endothermic
• if ∆E = -ve, the reaction is exothermic.

Absolute value of internal energy (E) cannot be determined however , the change in internal energy (∆E) can be determined experimentally.

Thermodynamics PDF