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First Law of Thermodynamics:
Energy is in the form of potential or kinetic energy. The water stored in the tank is in the form of potential energy. When it falls it is in the form of kinetic energy. The energy can be converted from one form to another.
But energy can neither be created nor be destroyed.. During chemical reaction the energy is absorbed (endothermic) or energy is released (exothermic)
If a system absorbs "q" amount of heat Its internal energy (E1) increases and its value becomes E1+q'
If "w" is the work done on the system
The new internal energy E2 becomes
E2 = E1 +q +w
E2 -E1 = q+w
Change in internal energy = heat added to the system + work done on the system If a system has done the work 'w' to the surroundings then its internal energy will decrease. So work is taken as negative _w.
If " q" is the amount of heat added to the system and "w" is the work done by the system
The change in internal energy = q + (-w) = q-w
Change in internal energy = heat added to the system- work done on the system
At constant pressure let the change in volume be DV then work is PDVIf a gas expand work is done by the system on the surroundings .
First law of thermodynamics says
DE = q+w
DE = q - PDV
since the work is done by the system the value of PDV is negative If the process is done at constant volume PDV= 0
DE = q
At constant volume and at constant temperature the change in internal energy is equal to heat absorbed or heat evolved
If the work is done on the system
q = DE + PDV
= (E2-E1) + P (V2 - V1)
= E2-E1 +PV2- PV1
= E2 + PV2) - (E1+PV1)
= H2 - H1
= DH
q = DH
The change in enthalpy is heat absorbed or heat released at constant pressure and constant temperature.
The first law of thermodynamics tells that energy can be changed from one form to another. But can neither be created nor be destroyed.
Limitations
It indicates that there is transformation of energy between the various forms of energy. But it does not provide any information regarding the feasibility of such transformations
It does not provide any information regarding the direction a process will take whether it is spontaneous or non spontaneous.
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