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Thermodynamics and Thermochemistry

Laws of Thermodynamics

Internal energy

  • 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 adiabatic work (Wad) required to bring about a change of state is equal to the change in internal energy.

ΔU = U2U1 = Wad

  • Thus, internal energy (U) of the system is a state function.

  • When work is done on the system, Wad = + ve

  • When work is done by the system, Wad = − ve


  • Internal energy of the system can also be changed by transfer of heat from the surroundings to the system or vice versa, without doing any work.

  • This exchange of energy, which is a result of temperature difference, is called heat (q).

  • A system which allows heat transfer through its boundary is shown in the figure.

  • At constant volume, when no work is done, the change in internal energy is, ΔU = q

  • When heat is transferred from the surroundings to the system, q is positive.

  • When heat is transferred from the system to the surroundings, q is negative.

General Case

  • When change in state is brought about both by doing work (W) and by transfer of heat (q):

Change in internal energy, ΔU = q + W

  • If W = 0 and q = 0 (i.e., no transfer of energy as heat or as work), then

ΔU = 0

This means that for an isolated system, ΔU =…

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