Thermodynamics is the study of flow of heat between system and surrounding. It deals with energy changes accompanying all types of physical and chemical processes.
Thermodynamics helps to lay down the criteria for predicting feasibility or spontaneity of a process, including a chemical reaction, under a given set of conditions. It also helps to determine the extent to which a process, including a chemical reaction, can proceed before attainment of equilibrium.
The laws of thermodynamics can be applied with energy changes of macroscopic systems involving a large number of molecules rather than microscopic systems containing a few molecules. Thermodynamics is based on two generalizations called the first and second law of thermodynamics. These are based on human experience.
Basic terms of thermodynamics
System
A system is defined as any specified portion of Universe under study which is separated from the rest of the universe with a bounding surface. A system may consist of one or more substances, e.g., a room, an engine human body etc.
Surroundings
The rest part of the universe, adjacent to real or imaginary boundaries of the system. which might be in a position to exchange energy and matter with the system is called the surroundings.
Universe =System + Surrounding
Objectives of thermodynamics
(1) Interrelate various energy changes during physical or chemical transformation.
(2) predict the feasibility of given change.
(3) Deduce various laws thermodynamically, e.g. phase rule, distribution law, law of mass action etc.
(4) Derive at what conditions, the equilibrium is attained by a change.
Limitations of thermodynamics
(1) It’s laws are valid for bulk of matter and does not provide information about individual atom.
(2) It predicts feasibility of reaction but fails to suggest rate of reaction.
(3) It fails to explain the systems which are not in equilibrium
Types of system
(i) Isolated system: A system which can exchange neither energy nor matter with its surrounding is called an isolated system.
(ii)Open system: A system which can exchange matter as well as energy with its surroundings is said to be an open system.
(iii)Closed system: A system which can exchange energy but not matter with its surroundings is called a closed system.
“Universe is considered as an isolated system. So all laws applicable for the universe are applicable for isolated system”
Equilibrium: It is defined as when there is no change in thermodynamic property (P,V,Tetc) of system With time.
Types Of Equilibrium
System and surrounding equilibrium condition is considered in three broader terms:
Thermal equilibrium: Equality of temperature between system and surrounding
Mechanical equilibrium: Equality of pressure between system and surrounding
Material equilibrium: No. of moles of every substance in a definite phase remains constant with respect to time. equilibrium attained in closed vessel.
Reversible process:
A process which is carried out so slowly that the system and the surroundings are always in equilibrium during the process is known as a Reversible Process (quasi-static). If this condition does not hold good, the process is said to be Irreversible.
Irreversible process:
In a reversible process the driving force is infinitesimally larger than the opposing force. A reversible process in very slow and takes infinite time. Where as an irreversible process completes in finite time.
Point to note: This thermodynamic reversible process is different from the “reversible reactions”. The term “reversible reaction” only indicates that the reaction proceeds in both the directions, i.e., towards the products and away from products.
A process which proceeds without any external help is called a spontaneous process.
Difference between Reversible and Irreversible process
| S No. | Reversible Process | Irreversible Process |
|---|---|---|
| 1. | Driving force is infinitesimally small. | Driving force is large and finite. |
| 2. | PV work is done across pressure difference dP. | PV work is done across pressure difference ΔP. |
| 3. | A reversible heat transfer take place across temperature difference dT. | Irreversible heat transfer take place across temperature difference ΔT. |
| 4. | It is an ideal process. | It is a real process. |
| 5. | It takes infinite time for completion of process. | It takes finite time for completion of process. |
| 6. | It is an imaginary process and can not be realised in actual practice. | It is a natural process and occurs in particular direction under given set of conditions. |
| 7. | Throughout the process, the system remain infinitesimally closes to state of equilibrium and exact path of process can be drawn. | The system is far away from state of equilibrium can exact path of process can not be defined as different part of the system are under different conditions. |
Properties of a thermodynamic system
State of the system
(a) State function or State variable: State Functions or State Variables are the physical quantity having a definite value at a particular (present state) state and value is independent from the fact how the system achieved that state, e.g., Pressure, volume, temperature, Gibbs’s free energy, internal energy, entropy.
Mathematical Condition for a function to be a state function:
There are three conditions that must be satisfied simultaneously for a function to be state function.
(i) If ∆φ is a state function
It means change in ∮ depends only on end states and not on the path which it followed during the process.
(ii) If ∆φ is a state function
It implies, in cyclic integral as the end states are same, so ∆φ value will be zero.
(iii) If ∆φ = f(x, y) is a state function, Euler’s reciprocity theorem must be satisfied.
If ∮dz=0 then, are we sure that z = 0 state function?
“Change in state function (z) is fixed in between two states so ∆z is also a state function example ∆P, ∆T, ∆V, ∆H = state function is a wrong statement”
(b) Path function: Functions which depend on the path means how the process is carried out to reach a state from another state depends on path e.g. work, heat, loss of energy due to friction.
Note : S, U, H, V, T etc are state function but ∆S, ∆U, ∆H, ∆V, ∆T, etc.are not state function. Since, ∆ terms are not function itself and it is very misleading and frequently asked in the exams.
Properties of the system: Intensive and Extensive property
The state of a system is defined by a particular set of its measurable parameters called properties, by which a system can be described for example, Temperature (T), Pressure (P) and volume (V) defines the thermodynamics state of the system.
Intensive property: After specifying the parameter of the system, when system is divided in parts the parameter whose value remains unchanged due to division is known as Intensive parameter or properties. the value of intensive is independent of the mass (size or quantity) of the system,
e.g., Refractive index, Surface tension, Viscosity, Molar Mass, Density, Free energy per mole, Specific heat capacity, Molar heat capacity, Free energy per mole, Pressure (P), Temperature (T), Boiling point, Freezing point, Molar enthalpy, Molar conductivity, Equivalent conductivity, Molarity, Normality, Mole fraction, %w/w, %V/V, EMF of cell.
Extensive property: the parameter whose value change on division known as extensive properties and these are depends on the mass (size, quantity) of the system,
e.g., Volume, Number of moles, Mass, Mole, Entropy (S), Enthalpy (H), Internal energy (E&U), Heat capacity, K.E., P.E., Gibbs free energy (G), Resistance, Conductance
1. Extensive properties are additive but intensive properties are non additive.
2. Ratio of two extensive property gives an intensive property.
3. An extensive property can be converted into intensive property by defining it per mole/per gram/per liter.
Read More: What is Substitution Reaction and its types?