Basic Concepts of Thermodynamics: Definition, Scope, & Limitations

Basic Concepts of Thermodynamics: Thermodynamics is the study of how heat or any other form of energy flows into and out of a system as it goes through a physical or chemical transformation. While examining and evaluating the flow of energy into and out of the system, it will be important to evaluate changes in specific system parts. These parameters include the system’s temperature, pressure, volume, and concentration. Measuring changes in these qualities from the start to the end state can reveal information about energy and related quantities like heat and work.

Laws of Thermodynamics

The study of thermodynamics is founded on three major generalisations obtained from well-known experimental results. The First, second, and third laws of Thermodynamics are the names given to these generalisations. These rules have endured the test of time and are unaffected by any atomic or molecule structure theories. 

Scope of Thermodynamics

(1) The rules of thermodynamics can be used to generate the most essential physical chemistry laws, such as the Van’t Hoff law of reducing vapour pressure, the phase rule, and the distribution law.

(2) It indicates whether a specific physical or chemical change is possible under a given set of temperature, pressure, and concentration conditions.

(3) It also aids in anticipating how far a physical or chemical change can progress before reaching equilibrium circumstances.

Limitations of Thermodynamics

(1) Thermodynamics applies to macroscopic systems made up of bulk matter rather than tiny ones made up of individual atoms or molecules. It does not take into account the internal structure of atoms and molecules.

(2) Thermodynamics is unconcerned with the passage of time. That is, it provides no information on the rate at which a physical change or chemical reaction occurs. It is only concerned with the system’s beginning and final states.

System, Boundary, Surroundings

A system is that part of the universe that is under thermodynamic study, and the rest of the universe is its surroundings. The real or imaginary surface separating the system from the surroundings is called the boundary.

Homogeneous and Heterogeneous Systems

The term “homogeneous system” refers to a system that is uniform throughout. Pure single solids, liquids, and gases, as well as mixtures of gases and real solid-liquid solutions, are examples. Only one phase makes for a homogenous system. A phase is a homogenous, physically distinct, and mechanically separable component of a system.

The term “heterogeneous system” refers to a system that has two or more stages. To put it another way, it is not consistent throughout. Ice in contact with water, ice in contact with vapour, etc., are examples of heterogeneous systems.

Types of Thermodynamic Systems

Depending on the nature of the border, there are three types of thermodynamic systems. 

(1) Isolated System

No interaction with the environment is possible when the barrier is both sealed and insulated. As a result, an isolated system is one that cannot exchange matter or energy with its surroundings.

Let’s take a look at an example of a system. In an insulated closed vessel, \(100 \,\text {mL}\) of water is in touch with its vapour. No water vapour (matter) may escape because the vessel is sealed. Furthermore, because the vessel is insulated, no heat (energy) may be transferred to the environment.

A substance enclosed in a thermos flask, such as boiling water, is another example of an isolated system.

(2) Closed System

The boundary is sealed but not insulated in this case. As a result, a closed system is one that cannot transmit matter but can transfer energy to and from its surroundings in the form of heat, work, and radiation.

For example, A closed system is one that contains a fixed amount of hot water in a sealed tube. While no water vapour may escape from this arrangement, heat can be transferred to the environment through the tube’s walls.

Another example, A closed system consists of a gas enclosed in a cylinder with a piston. The gas expands as the piston is raised, transferring heat (energy) in the form of work to the surroundings.

(3) Open System

The border in such a system is open and uninsulated. As a result, an open system is one that may exchange energy and matter with its surroundings.

For example, An open system contains hot water in a beaker on a laboratory table. Through the imaginary boundary, water vapour (matter) and heat (energy) are transmitted to the surroundings.

Another open system is zinc granules reacting with weak hydrochloric acid to make hydrogen gas in a beaker. The hydrogen gas escapes and the reaction’s heat is transferred to the surroundings.

Intensive and Extensive Properties

The macroscopic or bulk properties of a system (volume, pressure, mass, etc.) can be divided into two classes: (a) Intensive properties (b) Extensive properties.

Intensive Properties

The intensive property is a property that is independent of the amount of matter present in the system.

Pressure, temperature, density, and concentration are examples of intense properties. If the overall temperature of a glass of water (our system) is \(20\,^\circ {\rm{C}}\), then any drop of water in that glass also has a temperature of \(20\,^\circ {\rm{C}}\). Similarly, if the salt concentration in a glass of water is \(0.1\) mol/litre, any drop of water from the glass has a salt content of \(0.1\) mol/litre as well.

Extensive Properties

An extensive property is a property that is dependent on the amount of matter present in the system. Volume, number of moles, enthalpy, entropy, and Gibbs’ energy are all examples of extensive properties. You may be unfamiliar with some of these qualities, but they will be clarified and shown later.

Extensive properties are additive by definition, whereas intensive properties are not. Consider the system to be a glass of water. When the mass of water is made twice, the volume, the number of moles, and the internal energy of the system are also doubled.

State of a System

When all of the attributes of a thermodynamic system are fixed, it is considered to be in a given state. Pressure \((\text {P})\), temperature \((\text {T})\), volume \((\text {V})\), mass, and composition are the essential properties that determine the state of a system. These are referred to as State variables, State functions, or Thermodynamic parameters since a change in the magnitude of such attributes changes the state of the system. It, therefore, stands to reason that a change in the state variables will accompany a change in the system from the initial state to the final state (\(2\)nd state).

To completely define a system, it is not required to state all of the properties (state variables). The composition of a pure gas is fixed automatically because it is \(100\) percent pure. The equation of state is an algebraic connection that connects the remaining state variables \(\text {P}, \text {V}\), and \(\text {T}\). The equation of state for one mole of a pure ideal gas is \(\text {PV} = \text {RT}\), where \(\text {R}\) is the gas constant. If just \(\text {P}\) and \(\text {T}\) of the three state variables \((\text {P}, \text {V}, \text {T})\) are supplied, the value of the third \((\text {V})\) is automatically fixed and can be determined from the state equation. Independent state variables are the variables (\(\text {P}\) and \(\text {T}\)) that must be given in order to define the state of a system. The dependent state variable refers to the remaining state variable \((\text {V})\) that is dependent on the values of \(\text {P}\) and \(\text {T}\). 

A state variable (or state function) has the property that when the state of a system is changed, the change in the variable is dependent on the system’s starting and final states. If we heat a sample of water from \(0\) to \(25\) degrees Celsius, for example, the temperature change is equal to the difference between the initial and final temperatures.

\(\Delta {\rm{T}} = {{\rm{T}}_{{\rm{final }}}} – {{\rm{T}}_{{\rm{initial }}}} = 25\,^\circ {\rm{C}}\)

FAQs on Basic Concepts of Thermodynamics

Q.1. Briefly discuss the scope of thermodynamics.
Ans: The scope of thermodynamics is given below:
(i) The rules of thermodynamics can be used to generate most essential physical chemistry laws, such as the van’t Hoff law of reducing vapour pressure, the Phase Rule, and the Distribution Law.
(ii) It indicates whether a specific physical or chemical change is possible under a given set of temperature, pressure, and concentration conditions.
(iii) It also aids in anticipating how far a physical or chemical change can progress before reaching equilibrium circumstances.

Q.2. What are the limitations of thermodynamics?
Ans: The limitations of thermodynamics are given as:
(i) Thermodynamics applies to macroscopic systems made up of bulk matter rather than tiny ones made up of individual atoms or molecules. It does not take into account the internal structure of atoms and molecules.
(ii) Thermodynamics is unconcerned with the passage of time. That is, it provides no information on the rate at which a physical change or chemical reaction occurs. It is only concerned with the system’s beginning and final states.

Q.3. Give an example of an isolated system.
Ans: A substance enclosed in a thermos flask, such as boiling water, is an example of an isolated system.

Q.4. What are intensive and extensive properties?
Ans: The Intensive Property is a property that is independent of the amount of matter present in the system. Pressure, temperature, density, and concentration are examples of intense properties.
An Extensive Property is a property that is dependent on the amount of matter present in the system. Volume, number of moles, enthalpy, entropy, and Gibbs’ energy are all examples of extensive properties.

Q.5. What are state variables?
Ans: When all of the attributes of a thermodynamic system are fixed, it is considered to be in a given state. Pressure \((\text {P})\), temperature \((\text {T})\), volume \((\text {V})\), mass, and composition are the essential properties that determine the state of a system. These are referred to as state variables, state functions, or thermodynamic parameters since a change in the magnitude of such attributes changes the state of the system.

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