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Chemical Thermodynamics and Energetic

Thermodynamic Terms : System and Surroundings, Types of System

The System and the Surroundings

  • System − Part of the universe in which observations are made

  • Surroundings − Part of universe excluding system

  • Universe = System + Surroundings

Types of the System

  • Open system − There is exchange of energy and matter between system and surroundings. (shown in figure)

Example − Presence of reactants in an open beaker

  • Closed system − There is no exchange of matter, but exchange of energy is possible between the system and the surroundings. (shown in figure)

Example − Presence of reactants in closed vessel made of conducting material

  • Isolated system − There is no exchange of energy or matter between the system and the surroundings. (shown in figure)

Example − Presence of reactants in a thermos flask or any other closed insulated vessel


The State of System

  • State of a thermodynamic system can be described by properties such as its pressure (p), temperature (T), volume (V), composition of the system, etc.

  • Variables such as p, V, T are called state variables or state functions.

  • The values of state functions or state variables depend only on the state of the system and not on how it is reached.

  • To define the state of a system, it is not necessary to define all the properties of the system.

  • Internal energy (U) represents the total energy of a system (i.e., the sum of chemical, electrical, mechanical or any other type of energy).

  • Internal energy of a system may change when:

    • Heat passes into or out of the system
    • Work is done on or by the system
    • Matter enters or leaves the system


  • For an adiabatic system which does not permit the transfer of heat through its boundary (shown in the figure), a change in its internal energy can be brought by doing some work on it.

  • Initial state of the system, (1)

Temperature = T1

Internal energy = U1

  • When some mechanical work is done, the new state (2) is obtained.

Temperature at state 2 = T2

Internal energy at state 2 = U2

  • It is found that T2 >T1

Change in temperature, ΔT = T2T1

Change in internal energy, ΔU = U2U1

  • The value of internal energy (U) is the characteristic of the state of a system.

  • The adiabat…

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