Thermodynamics and Thermochemistry
Laws of Thermodynamics
Internal energy
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Internal energy (U) represents the total energy of a system (i.e., the sum of chemical, electrical, mechanical or any other type of energy).
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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
Work
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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.
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Initial state of the system, (1)
Temperature = T1
Internal energy = U1
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When some mechanical work is done, the new state (2) is obtained.
Temperature at state 2 = T2
Internal energy at state 2 = U2
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It is found that T2 >T1
Change in temperature, ΔT = T2 − T1
Change in internal energy, ΔU = U2 − U1
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The value of internal energy (U) is the characteristic of the state of a system.
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The adiabatic work (Wad) required to bring about a change of state is equal to the change in internal energy.
ΔU = U2 − U1 = Wad
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Thus, internal energy (U) of the system is a state function.
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When work is done on the system, Wad = + ve
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When work is done by the system, Wad = − ve
Heat
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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.
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This exchange of energy, which is a result of temperature difference, is called heat (q).
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A system which allows heat transfer through its boundary is shown in the figure.
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At constant volume, when no work is done, the change in internal energy is, ΔU = q
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When heat is transferred from the surroundings to the system, q is positive.
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When heat is transferred from the system to the surroundings, q is negative.
General Case
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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
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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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