Introduction and Terminology used in Thermodynamics

THERMODYNAMICS

Introduction

The term thermodynamics is derived from Greek word, `Thermos' meaning heat and `dynamics' meaning flow. Thermodynamics deals with the inter-relationship between heat and work. It is concerned with the interconversions of one kind of energy into another without actually creating or destroying the energy. Energy is understood to be the capacity to do work. It can exist in many forms like electrical, chemical, thermal, mechanical, gravitational etc. Transformations from one to another energy form and prediction of the feasibility (possibility) of the processes are the important aspects of thermodynamics.

As an illustration, from our common experience steam engines are seen to transform heat energy to mechanical energy, by burning of coal which is a fossil fuel. Actually, the engines use the energy stored in the fuel to perform mechanical work. In chemistry, many reactions are encountered that can be utilised to provide heat and work along with the required products. At present thermodynamics is widely used in physical, chemical

and biological sciences focussing mainly on the aspect of predicting the possibility of the processes connected with each sciences. On the other hand, it fails to provide insight into two aspects: Firstly, the factor of time involved during the initial to final energy transformations and secondly, on the quantitative microscopic properties of matter like atoms and molecules.

Terminology used in Thermodynamics

It is useful to understand few terms that are used to define and explain the basic concepts and laws of thermodynamics.

System

Thermodynamically a system is defined as any portion of matter under consideration which is separated from the rest of the universe by real or imaginary boundaries.

Surroundings

Everything in the universe that is not the part of system and can interact with it is called as surroundings.

Boundary

Anything (fixed or moving) which separates the system from its surroundings is called boundary.

For example, if the reaction between A and B substances are studied, the mixture A and B, forms the system. All the rest, that includes beaker, its walls, air, room etc. form the surroundings. The boundaries may be considered as part of the system or surroundings depending upon convenience. The surroundings can affect the system by the exchange of matter or energy across the boundaries.

Types of systems

In thermodynamics different types of systems are considered, which depends on the different kinds of interactions between the system and surroundings.

Isolated system

A system which can exchange neither energy nor matter with its surroundings is called an isolated system. For example, a sample in a sealed thermos flask with walls made of insulating materials represents an isolated system (Fig.).

Closed system

A system which permits the exchange of energy but not mass, across the boundary with its surroundings is called a closed system.

For example: A liquid in equilibrium with its vapours in a sealed tube represents a closed system since the sealed container may be heated or cooled to add or remove energy from its contents while no matter (liquid or vapour) can be added or removed.

Open system

A system is said to be open if it can exchange both energy and matter with its surroundings.

For eg. a open beaker containing an aqueous salt solution represents open system. Here, matter and heat can be added or removed simultaneously or separately from the system to its surroundings.

All living things (or systems) are open systems because they continuously exchange matter and energy with the surroundings.

Homogeneous and Heterogeneous systems

A system is said to be homogeneous if the physical states of all its matter are uniform. For eg. mixture of gases, completely miscible mixture of liquids etc.

A system is said to be heterogeneous, if its contents does not possess the same physical state. For eg: immiscible liquids, solid in contact with an immiscible liquid, solid in contact with a gas, etc.

Macroscopic properties of system

The properties which are associated with bulk or macroscopic state of the system such as pressure, volume, temperature, concentration, density, viscosity, surface tension, refractive index, colour, etc. are called as macroscopic properties.

Types of macroscopic properties of system

Measurable properties of a system can be divided into two types.

Extensive properties

The properties that depend on the mass or size of the system are called as extensive properties. Examples: volume, number of moles, mass, energy, internal energy etc. The value of the extensive property is equal to the sum of extensive properties of smaller parts into which the system is divided. Suppose x₁ ml, x₂ ml,x₃ ml of 1,2,3 gases are mixed in a system, the total volume of the system equals to (x₁ + x₂ + x₃) ml. Thus volume is an extensive property.

Intensive properties

The properties that are independent of the mass or size of the system are known as intensive properties. For eg. refractive index, surface tension, density, temperature, boiling point, freezing point, etc., of the system. These properties do not depend on the number of moles of the substance in the system.

If any extensive property is expressed per mole or per gram or per ml, it becomes an intensive property. For eg: mass, volume, heat capacity are extensive properties while density, specific volume, specific heat are intensive properties.