Welcome to Chem Zipper.com......: FIRST LAW OF THERMODYNAMICS (FLOT)

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Wednesday, September 26, 2018


(1) It is also known as law of energy conservation.
(2)The first law of thermodynamics states that energy can neither be created nor destroyed,      although it can be transformed from one form to another.
(3) Another Statement is “total energy of universe is remain constant”
(4) In FLOT we introduce the term ENTHALY and INTERNAL ENERGY.
Let UA be the energy of a system in its state A and UB be the energy in its state B. Suppose the system while undergoing change from state A to state B absorbs heat q from the surroundings and also  performs some work (mechanical or electrical), equal to w. The absorption of heat by the system tends to raise the energy of the system. The performance of work by the system, on the other hand, tends to lower the energy of the system because performance of work requires expenditure of energy. Hence the change of internal energy, DU, accompanying the above process will be given by
                                  dU =UB  -UA = q + w
In general, if in a given process the quantity of heat transferred from the surrounding to the system is q and work done in the process is w, then the change in internal energy,
                                  dU = q + w
This is the mathematical statement of the first law of thermodynamics.


(1) If work is done by the surroundings on the system (compression of a gas), w is taken as positive so that dU = q + w.
(2) If however work is done by the system on the surroundings ( expansion of a gas), w is taken as negative so that dU = q – w.
(3) q+w it will be independent of the way the change is carried out, it only depend on initial and final state.
(4) If their is no transfer of energy as heat or work (Isolated System) 
      ie, if w=0 and q=0.    then dU=0 

EXAMPLE: A system gives out 25 J of heat and also does 35 J of work. What is the internal energy change ?
SOLUTION:   According to FLOT dU = q + w
                                                          dU =-25 J+(-35 J)
                                                          dU = -60 J 
(1) dU = q + w is invalid for open system.
(2) 1st law of T.D. is applicable for closed system in which system is at rest or moving with   constant velocity and in absence of external fields.
(3) The macroscopic energy changes with velocity and elevation of the system are not considered in internal energy change of system.

EXAMPLE: The pressure of a fluid is a linear function of volume P=a+bV and internal energy of fluid is U=34+3PV in SI unit. find  a ,b, W ,dU for change in state (100 Pascal ,3m cube ) to (400 Pascal ,6m cube). (given Pascal =1J) .
SOLUTION:  We know by FLOT DE=q+W

A major limitation of the first law of thermodynamics is that it’s merely indicates that in any process there is an exact equivalence between the various forms of energies involved, but it provides no information concerning the spontaneity or feasibility of the process.
For example, the first law does not indicate whether heat can flow from a cold end to a hot end or not.

The answers to the above questions are provided by the second law of thermodynamics. 

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