Limitations of First law of thermodynamics PDF

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Limitations of first law of thermodynamics...

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Limitations of First law of thermodynamics First law of thermodynamics summarizes the interconversion of various forms of energies into one another. But the law has some limitations. 1. It fails to tell us the extent to which one form of energy can be transformed into another. First law states that energy of one form can be converted into an equivalent amount of energy of another form. But it does not tell that heat energy cannot be completely converted into an equivalent amount of work. 2. It does not tell us anything about the direction of a process If two bodies A & B are brought in contact with each other, the first law will simply tell us that if one of them loses some amount of heat , the other will simultaneously gain same amount of heat. But the first law will be unable to tell us whether the flow of heat will occur from A to B or B to A. If temperature of A & B were known we express the direction of flow of heat. PCl5 ↔ PCl3 + Cl2; ∆H = -x kJ/mol The first law tells us only that if decomposition of one mole of PCl5 occurs, x, kJ of energy is produced and conversely the combination of PCl3 & Cl2 to give one mole of PCl5 requires , x kJ of energy. Now if we have a mixture of PCl5, PCl3 & Cl2, enclosed in a vessel , the first law of thermodynamics will not be able to make any prediction about the direction of chemical change under the conditions. In other words, we cannot know from the first law whether the reaction would involve dissociation of PCl5 or the combination of PCl3 and Cl2. 3. It cannot predict the feasibility of a process According to first law, the energy of an isolated system remains constant during a specified change of state. But it does not tell whether a specified change or a process including a chemical reaction can occur spontaneously. Ie. Whether it is feasible. 4. It does not rule out the existence of a 100% efficient heat engine which is in fact impossible. Eventhough various forms of energy can be completely transformed into one another, heat is a typical form of energy which cannot be completely transformed into work....