Last updated : September 9, 2024

Thermodynamics : Heat, Work, Internal energy, Heat Capacity, First law, Thermodynamic Processes, Entropy, Second law, Gibbs Free Energy, Thermochemistry

Thermodynamic Terms

System and Surroundings

System	Surrounding	Boundary
A specified portion of universe under observation.	Rest of universe is surrounding (for practical purposees, the surroundings are the portion of remaining universe that can interact (affected) by changes in system).	Real or imaginary surface sapearating sytem from surrounding.
A reaction taking place in a beaker.	The room where the beaker is kept.

Types of System :

Open	Closed	Isolated
Mass and heat both can be transfered to its surroundings.	Heat can be transfered to its surroundings, but mass can not.	Neither mass nor het can be transfered to its surroundings.
Reactants in an open beaker.	Reactants in a closed vessel made of conducting material.	Reactants in a thermos flask or any other insulated vessel.

Properties of a System

Intensive	Extensive
Does not depend upon amount (mass) of the system.	Depend upon amount (mass) of system.
Not additive in nature.	Additive in nature.
Example : Mass, Length, Mole, Volume, pH etc.	Example : Density, Temperature, Concentration, Atomic/Molecular mass etc.

Process

An operation by which, one or more property of a system undergoes a chage from one state to another.

Example : Change in

Temperature (T),
Amount (mol),
Volume (V),
Pressure (P),
Internal energy (U),
Enthalpy (H),
Entropy (S),
Gibbs free energy (G)

Types of Process

Based on a specified condition of a process, it can of several types -

Isothermal process - During the hole process temperature (T) remains/kept constant, and any other one or more property of the system changes from one state to another.
Isobaric process - During the hole process pressure (P) remains/kept constant, and any other one or more property of the system changes from one state to another.
Isochoric process - During the hole process volume (V) remains/kept constant, and any other one or more property of the system changes from one state to another.
Adibatic process - During the hole process heat (Q) remains/kept constant, and any other one or more property of the system changes from one state to another.
Cyclic process - In a process, if the initial and final state of all the properties of the system becomes same.

Mode of Process

Reversible	Irreversible
A process in which the system and surroundings are always in near equilibrium, the changes take place in infinite equilibrium states, and the process could be reversed by very small change during the process.	Any process other than reversible process is known as irreversible process. The process completes in finite number of steps.

State of the System

The state of a thermodynamic system is described by its measurable or macroscopic (bulk) properties.

Example : State of a gas described by its pressure (P), volume (V), temperature (T), moles (n), etc.

Variables like P, V, T are called state variables or state functions because their values only depend on initial and final state of the system and not on how it is reached.

State Function	Path Function
Only depend on initial and final state of the system.	Depend on path of the process.
Any number of steps results in same value.	Different steps results in different value.
Path independent.	Path dependent.
P, V, T, U, H, G, S	Work, Heat, Heat capacity

Internal Energy

When a chemical system lose or gain energy, we use a special quantity to represent the total energy of the system called Internal Energy. It is the sum of all chemical, electrical, kinetic, mechanical, etc. energies of the system.

Internal Energy may change, when

Work is done on or by the system (W),
Heat passes into or out of the system (Q),
Matter enters or leaves the system (mole).

Work

Work = Force x distance

Work sign convention by IUPAC
W = +ve, work is done on the system
W = -ve, work is done by the system

Physics book vs Chemistry book on sign convention of work
A quote from Class 11th NCERT Chemistry part 1 book

Earlier negative sign was assigned when the work is done on the system and positive sign when work is done by the system. This is still followed in physics books, although IUPAC has recommanded the use of new sign convention.

(a) Change in internal energy by doing work

Let’s take a cylindrical insulated apparatus contaning 1 mol of ideal gas, we will change its internal energy by two methods

(i) Mechanical work : i.e., pressure-volume work
Initial volume = V_i
Pressure of gas = P
External pressure = P_ex > P
Piston moved inward till presure inside bacomes = P_ex
If piston moves distance = l
Cross-section area = A
Change in volume (ΔV) = V_f - V_i = l x A

Force on piston = pressure x area = P_ex x A

Work (W) = force x distance = P_ex . A . l

\begin{array}{rclcl} Work (W) & = & force \times distance & = & P_{ex} \cdot A \cdot l \\ = & - P_{ex} ΔV & = & - P_{ex} (V_{f} - V_{i}) \end{array}

The negative sign of this expression is required to obtain conventional sign for W.

(ii) Electrical work :

Heat

Change in Amount of Matter

1st Law of Thermodynamics

DOF

Heat Capacity

Enthalpy

Entropy

2nd Law of Thermodynamics

Gibbs Free Energy

Thermodynamic Processes

Thermochemistry