physical and chemical changes are generally  accompanied by the  energy changes. These energy changes take place because of the rearrangement of atoms in reactants to form products. Sometimes, the energy changes associated with the chemical reactions are more significant than the products of the reaction e.g., burning of fuel, coal or natural gas. These processes satisfy our energy needs. One of the most common form in which the energy appears that is heat. There are other forms of energy also such as mechanical energy, electrical energy, radiant energy, chemical energy and nuclear energy. These different forms of energy are interrelated and under certain conditions, these may be transformed from one into the other. For example, heat can be converted into mechanical energy to operate a machine or propel a vehicle. The chemical energy may also be converted into electrical energy through a galvanic cell like drycell.

 The transformation of heat into work is achieved with the help of devices called engines. The branch of science which deals with the theory of different forms of energy and their inter-conversion is called thermodynamics. There are three  laws of thermodynamics

SOME BASIC TERMS AND CONCEPTS

Let us learn some terms which are commonly used of thermodynamics.

 

System and Surroundings

LAW OF CONSERVATION OF ENERGY: FIRST LAW OF THERMODYNAMICS

 

The first law of thermodynamics is also known as law of conservation of energy. During a physical or chemical change, the energy may be converted from one form into another but the total energy remains constant. It states that energy can neither be created nor destroyed although it can be converted from one form into another.

 

This law holds good in chemical reactions also. Energy may be absorbed or evolved in chemical reaction but the total energy of the reacting system and the surroundings remains constant. This law has been stated in several other ways but the basic idea remains the same i.e., conservation of energy. For Example, the law can also be stated as

 

(i)                  the total energy of the universe remains constant.

MODES OF TRANSFERENCE OF ENERGY BETWEEN SYSTEM AND SURROUNDINGS

 

Every system has a definite amount of energy. It can be exchange energy (lose or gain) with the help of surroundings in a variety of ways. The two important modes of transference of energy between the system and the surroundings are heat and work as

1. Heat (g)

Energy is exchanged between the system and the surroundings as heat if they are at different temperatures. For example, if a system is at a higher temperature than the surroundings, then energy (or heat) is lost to the surroundings causing a fall in temperature of the system and rise in temperature of the surroundings [Fig. 5 (a)). This process continues till the temper…

2. Work (w)

It is another mode of transference of energy. Work is said to be performed if the point of application of force is displaced in the direction of the force. It is equal to the force multiplied by the displacement (distance through displacement of the point of application in the direction in which force acts, then work done is given by acts). If f is the force and dl is the et state be called

w=fx dl

There are two main types of work which we generally come across. These are (i) electrical work, and (ΓΌ) mechanical work. Electrical work is important in systems where reaction takes place between jobs whereas mechanical work is performed when a system changes its volume in the presence of external pressure.

State Variables

For example, temperature, pressure, volume, composition, etc. are the state variables. It may be noted that for describing a system, it is not necessary to specify the values of all state variables.