Thermodynamics: Introduction


Thermodynamics is the study of energy transfer. “Where the energy comes from and where it goes?” is what we are going study in this chapter. It is one of the most important subject as it is applicable to any process, anything that we associate energy and mass transfer with. It is a broad term which encompasses flow of energy and matter both.
We know from our childhood lessons that energy can neither be created nor destroyed; it can just be transformed from one form to the other. In this chapter we will study all these transformations through various functions and variables. We have a general perception that the word Thermodynamics just includes changes in heat which is not true as it is much broader. The knowledge gained in this chapter will be extensively used in further study of chemical processes and also in Physics. Before we begin our study we need to know certain basic terms which will used throughout the chapter.


System, Surroundings and the Universe

When we study the thermodynamic changes in the Universe we focus our study to a certain region. This is called “system”. That is anything under the thermodynamic scanner is called system. For example if we want to study the heat and matter inflow and outflow of the room we are in the room becomes our system.
Everything outside the system is called “surroundings”. If there is a boundary which separates system and surroundings it is called “system boundary”. We can also say that the Universe consists of the system and the surroundings and heat and matter can flow between the two. On the basis of this flow we can define three types of systems.

1. Open system:

“A system which can exchange both energy and matter with the surroundings is called an Open system.” For example: an open room, a cup of hot water etc.

2. Closed system:

“As system which can only exchange energy and not matter is called closed system.”
For example: a closed steel container having hot water, a closed room etc.

3. Isolated system:

“A system which can neither exchange energy nor matter with the surroundings is called isolated system.” For example: an insulated and closed container like thermos flask etc.
It is very important to identify a system correctly before proceeding with the thermodynamic analysis.

State of a system

It is the physical condition of a system in terms of state variables like Pressure, Volume, Temperature, number of moles etc. and in general our analysis will be centered on gases so we will be using the ideal gas equation extensively to relate these state variables. A set of state variables define a particular state of a system. If any of the variable changes that means the state also changes.
Change of state: P1 V1 T1 ---à P2 V2 T2 (for given number of moles)

Thermodynamic Process

An event which tends to change the state of a system by changing its state variables and other functions is called “Thermodynamic Process”. It may be supplying energy to a system, compressing a system by increasing pressure etc.

Extensive and Intensive Properties of a System

Any property which depends on the net matter content that is number of molecules or mass in a system is called Extensive Property. Example: Total Energy, Volume etc
Any property which is independent of net matter content and depends only on the concentration of molecules is called “Intensive Property”. Example: Pressure, Density, Refractive Index, Viscosity, Temperature etc.
For example, we say density of water is 1 g/mL independent of the mass of water taken. Pressure of gas depends on concentration (P = CRT). Both are intensive properties.

State Functions and Path Functions

Those functions of a system which are only dependent on the state of the system and not on how the state has been achieved or the path are called “state functions”.
Those functions which are path dependent are called “path functions”.
For example, if we compare displacement and distance travelled then displacement depends only on the initial and final positions or locations but distance travelled depends on the path taken. Therefore, displacement is a state function whereas distance travelled is a path function. Similarly when we talk about potential energy change in moving an object from initial position to a final position it depends only on the initial and final height of the object from the ground and that is why it is a state function.
Whereas work done, in general, is a path function. Only when work is done by conservative forces it becomes a state function. Whenever there is an energy loss or dissipation work done becomes a path function.
Many functions that we will study in Thermodynamics like Internal Energy, Enthalpy,
Entropy, Gibb’s Free energy are all state functions whereas functions like heat transferred and work done are path functions.

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