JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Internal Energy, Heat And Work

(1) Internal energy (E) : “Every system having some quantity of matter is associated with a definite amount of energy. This energy is known as internal energy.”

\[E={{E}_{\text{translational}}}+{{E}_{\text{rotational}}}+{{E}_{\text{vibrational}}}+{{E}_{\text{bonding}}}+{{E}_{\text{electronic}}}+......\]

(i) Characteristics of internal energy

(a) Internal energy of a system is an extensive property.

(b) Internal energy is a state property.

(c) The change in the internal energy does not depend on the path by which the final state is reached.

(d) There is no change in internal energy in a cyclic process.

(e) The internal energy of an ideal gas is a function of temperature only.

(f) Internal energy of a system depends upon the quantity of substance, its chemical nature, temperature, pressure and volume.

(g) The unit of E is ergs in CGS or joules in SI

1 Joule = \[{{10}^{7}}\] ergs.

(ii) Change in internal energy (\[\Delta E\]) : It is neither possible nor necessary to calculate the absolute value of internal energy of a system then,\[\]\[\Delta E={{E}_{f}}-{{E}_{in}}\] ; \[\Delta E\] is positive if \[{{E}_{f}}>{{E}_{in}}\] and negative if \[{{E}_{f}}<{{E}_{in}}\].

(2) Heat (q) and work (w) : The energy of a system may increase or decrease in several ways but two common ways are heat and work.

Heat is a form of energy. It flows from one system to another because of the difference in temperature between them. Heat flows from higher temperature to lower temperature. Therefore, it is regarded as energy on the move.

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 which the force acts).

There are three main types of work which we generally come across. These are, Gravitational work, electrical work and mechanical work.

Mechanical work = Force \[\times \] displacement = F.d

Electrical work = potential difference \[\times \] charge = V.q

Gravitational work = mgh

(i) Units of heat and work : The heat changes are measured in calories (cal), Kilo calories (kcal), joules (J) or kilo joules (kJ). These are related as, 1 cal = 4.184 J; 1kcal = 4.184kJ

The S.I. unit of heat is joule (J) or kilojoule. The Joule (J) is equal to Newton – metre (1 J= 1 Nm).

Work is measured in terms of ergs or joules. The S.I. unit of work is Joule.

1 Joule = \[{{10}^{7}}\] ergs = 0.2390 cal.

1 cal   > 1 joule > 1 erg

(ii) Sign conventions for heat and work

Heat absorbed by the system = q positive

Heat evolved by the system = q negative

Work done on the system    = w positive

Work done by the system    = w negative.