Law of Thermodynamics, Chemistry tutorial

Thermodynamic terminology:

In this part, a number of generally employed terms in thermodynamics are stated and explained. These terms must be understood clearly before you carry on further.


Any part of the universe which is beneath study is known as a system. This can be as simple as a gas contained in the closed vessel or as complex as a rocket shooting towards the moon. A system might be homogeneous or heterogeneous based on its contents and conditions. A system is stated to be homogeneous if physical properties and chemical composition are similar all through the system. Such a system is as well termed as a single phase system.

A heterogeneous system comprises of two or more than two stages separated via mechanical boundaries.


The rest of universe around the system is taken as its surroundings. A system and its surroundings are for all time separated via boundaries across which matter and energy might be exchanged. The boundaries can be real (that is, fixed or moveable) or imaginary.

Based on the exchange of energy and matter between the system and the surroundings, a system can be categorized into the following three kinds:

1) Isolated system:

The Isolated system is one which exchanges neither energy nor matter by its surroundings. There is no perfectly isolated system; however, a system that is thermally well insulated (that is, doesn't allow heat flow) and is sealed to inflow or outflow of matter can be taken as an isolated system. A sealed thermos flask having-some matter therefore approximates to art isolated system.

2) Closed system:

Closed system lets the exchange of energy (heat or work) with the surroundings however, matter is not permitted to enter or leave it. A properly scaled system (that is, to prevent the passage of matter across its boundary) can be considered as the closed system.

3) Open system:

Open system lets exchange of both matter and energy by its surroundings. This the most general kind oil system encountered in our everyday life. All living things are the illustrations of open system since these are capable of freely exchanging energy and matter by their surroundings. Reaction vessels having permeable membranes are the illustrations of open system.

State Variables:

The thermodynamic system has to be macroscopic (that is, of adequately large size); this facilitates measurement of its properties like volume, pressure, temperature, composition and density. These properties are thus termed as macroscopic or bulk s. These are as well known as state or thermodynamic variables. These don't based on the past history of the system. The state variable which based on other variables is termed as a dependent variable; others, on which it is dependent, are termed as independent variables.

For illustration, an ideal gas equation is represented as:

V = nRT/P

Then, 'V' is the dependent variable, while n, T and p are the independent variables. We are familiar that 'R' is the gas constant. On contrary, if you write this equation as:

V = nRT/P

Then, 'p' is the dependent variable, while n, T and V are independent variables. The choice of dependent and independent variables is the matter of convenience.

State of a System:

The state of a system is stated whenever the state variables encompass definite values. It is essential to state all the state variables as these are interdependent. For exam if the system is the ideal gas, then its pressure, volume, temperature and the amount of the gas (that is, number of moles) are associated by the gas equation. Therefore, if we specify three of these; the fourth variable is automatically fixed. Likewise most of its other properties such as density, heat capacity and so on are as well fixed however via more complex relations.

The Zeroth law of Thermodynamics:

The Zeroth law of thermodynamics is mainly based on the theory of thermal equilibrium. It assists us in defining the temperature. If two closed systems are brought together so that these are in thermal contact, changes take place in the properties of both the systems, but, eventually a state is reached when there is no further change in any of the systems. This is the state of thermal equilibrium. Both the systems are at the same temperature. In order to find out whether two systems are at similar temperature, the two can be brought into thermal contact; then the changes in the properties of either of these are to be examined. If no change takes place, then they are at similar temperature.

The Zeroth law of thermodynamics defines that if a system 'A' is in thermal equilibrium by system 'C' and, system 'B' is as well in thermal equilibrium with 'C', then A and B are as well in the thermal equilibrium by each other. This is the experimental fact. This might be illustrated by supposing that the systems A and B are two vessels having different liquids and C is an ordinary mercury thermometer. If A is in thermal equilibrium with C, then the mercury level in thermometer will exhibit a constant reading.  This point out the temperature of system A and also of C. Now if A is as well in thermal equilibrium with B, then the height of mercury level in the thermometer (in contact with B) is similar as before; B as well consists of the similar temperature as A.  There is thermal equilibrium in both A and B or these are at similar temperature.

Extensive and Intensive Variables:

The extensive property of a homogeneous system is one which is dependent on the amount of a phase in the system. For a heterogeneous system made up of some phases, the net value of an extensive property is equivalent to the sum of the contributions from different phases. Mass, volume and energy are the illustrations of extensive properties. Therefore, if a system, at equilibrium comprises of 0.100 kg of ice and 0.100 kg of liquid water at 273.15 K, the net volume of the system is the sum of two volumes, each of which is directly proportional to the mass.

Volume of 0.100 kg of ice = Mass of ice/Density of ice = 0.100kg/917 kgm-3

                                        = 1.09 × 10-4 m3

Likewise, the volume 0.100 kg of water = Mass of water/Density of water

                                                               = 0.100 kg/(1.00 x 103 Kgm-3)

                                                               = 1.00 x 10-4 m3

Net volume = (1.09 + 1.00) 10-4 m3

                   = 2.09 x 10-4

The intensive property of a state or phase is independent of the amount of phase. Therefore refractive index, density and pressure are intensive properties. Though if a system comprises of some phases, then some of the intensive properties might be dissimilar. For illustration, density is an intensive property however its value is different for ice and liquid water in the equilibrium at 273.15 K. For thermal equilibrium, the intensive property, temperature, has to be some all through the system. Or else heat will flow from one point of the system to the other. Likewise, for mechanical equilibrium, the intensive property, pressure, has to be similar all through the system. The extensive property whenever divided via mass or molar mass of the system becomes the intensive property.

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