Thermodynamics deals with the concepts of heat and temperature and the inter-conversion of heat and other forms of energy. The four laws of thermodynamics govern the behaviour of these quantities and provide a quantitative description. William Thomson, in 1749, coined the term thermodynamics.
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What is Thermodynamics?
Thermodynamics in physics is a branch that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter.
To be specific, it explains how thermal energy is converted to or from other forms of energy and how matter is affected by this process. Thermal energy is the energy that comes from heat. This heat is generated by the movement of tiny particles within an object, and the faster these particles move, the more heat is generated.
Thermodynamics is not concerned about how and at what rate these energy transformations are carried out. It is based on the initial and final states undergoing the change. It should also be noted that Thermodynamics is a macroscopic science. This means that it deals with the bulk system and does not deal with the molecular constitution of matter.
Distinction Between Mechanics and Thermodynamics
The distinction between mechanics and thermodynamics is worth noting. In mechanics, we solely concentrate on the motion of particles or bodies under the action of forces and torques. On the other hand, thermodynamics is not concerned with the motion of the system as a whole. It is only concerned with the internal macroscopic state of the body.
Thermodynamics Timeline
Different Branches of Thermodynamics
Thermodynamics is classified into the following four branches:
- Classical Thermodynamics
- Statistical Thermodynamics
- Chemical Thermodynamics
- Equilibrium Thermodynamics
Classical Thermodynamics
In classical thermodynamics, the behaviour of matter is analysed with a macroscopic approach. Units such as temperature and pressure are taken into consideration, which helps the individuals calculate other properties and predict the characteristics of the matter undergoing the process.
Statistical Thermodynamics
In statistical thermodynamics, every molecule is under the spotlight, i.e. the properties of every molecule and how they interact are taken into consideration to characterise the behaviour of a group of molecules.
Chemical Thermodynamics
Chemical thermodynamics is the study of how work and heat relate to each other in chemical reactions and in changes of states.
Equilibrium Thermodynamics
Equilibrium thermodynamics is the study of transformations of energy and matter as they approach the state of equilibrium.
Basic Concepts of Thermodynamics – Thermodynamic Terms
Thermodynamics has its own unique vocabulary associated with it. A good understanding of the basic concepts forms a sound understanding of various topics discussed in thermodynamics preventing possible misunderstandings.
Thermodynamic Systems
System
A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focused. The system boundary may be real or imaginary, fixed or deformable.
There are three types of systems:
- Isolated System – An isolated system cannot exchange energy and mass with its surroundings. The universe is considered an isolated system.
- Closed System – Across the boundary of the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerator, compression of gas in the piston-cylinder assembly are examples of closed systems.
- Open System – In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.
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Surrounding
Everything outside the system that has a direct influence on the behaviour of the system is known as a surrounding.
Thermodynamic Process
A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.
There are four types of thermodynamic processes that have their unique properties, and they are:
- Adiabatic Process – A process where no heat transfer into or out of the system occurs.
- Isochoric Process – A process where no change in volume occurs and the system does no work.
- Isobaric Process – A process in which no change in pressure occurs.
- Isothermal Process – A process in which no change in temperature occurs.
Read More: Thermodynamic Process
A thermodynamic cycle is a process or a combination of processes conducted such that the initial and final states of the system are the same. A thermodynamic cycle is also known as cyclic operation or cyclic processes.
Thermodynamic Equilibrium
At a given state, all properties of a system have fixed values. Thus, if the value of even one property changes, the system’s state changes to a different one. In a system that is in equilibrium, no changes in the value of properties occur when it is isolated from its surroundings.
- When the temperature is the same throughout the entire system, we consider the system to be in thermal equilibrium.
- When there is no change in pressure at any point of the system, we consider the system to be in mechanical equilibrium.
- When the chemical composition of a system does not vary with time, we consider the system to be in chemical equilibrium.
- Phase equilibrium in a two-phase system is when the mass of each phase reaches an equilibrium level.
A thermodynamic system is said to be in thermodynamic equilibrium if it is in chemical equilibrium, mechanical equilibrium and thermal equilibrium and the relevant parameters cease to vary with time.
You may also want to check out these topics given below!
- Kelvin Planck Statement
- Darcy Weisbach Equation Derivation
- Kinetic Theory Of Gases Derivation
- Relation Between Kp And Kc
Thermodynamic Properties
Thermodynamic properties are defined as characteristic features of a system, capable of specifying the system’s state. Thermodynamic properties may be extensive or intensive.
- Intensive properties are properties that do not depend on the quantity of matter. Pressure and temperature are intensive properties.
- In the case of extensive properties, their values depends on the mass of the system. Volume, energy, and enthalpy are extensive properties.
What is Enthalpy?
Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals the total heat content of a system, equivalent to the system’s internal energy plus the product of volume and pressure.
Mathematically, the enthalpy, H, equals the sum of the internal energy, E, and the product of the pressure, P, and volume, V, of the system.
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What is Entropy?
Entropy is a thermodynamic quantity whose value depends on the physical state or condition of a system. In other words, it is a thermodynamic function used to measure the randomness or disorder.
For example, the entropy of a solid, where the particles are not free to move, is less than the entropy of a gas, where the particles will fill the container.
Thermodynamic Potentials
Thermodynamic potentials are quantitative measures of the stored energy in a system. Potentials measure the energy changes in a system as they evolve from the initial state to the final state. Different potentials are used based on the system constraints, such as temperature and pressure.
Different forms of thermodynamic potentials along with their formula are tabulated below:
| Internal Energy | |
| Helmholtz free energy | F = U – TS |
| Enthalpy | H = U + PV |
| Gibbs Free Energy | G = U + PV – TS |
Thermodynamics Solved Problems
Calculate ΔG at 290 K for the following reaction:
Solution:
Given:
ΔH = -120kJ and ΔS = -150JK-1
To make the unit of ΔS the same as ΔH, we have to convert the unit of ΔS as follows:
We know that,
So,
Therefore, ΔG is -77kJ.