Thermodynamical expressions for closed systems

The First Law of Thermodynamics is the law of conservation of energy it simply requires that the total quantity of energy be the same both before and after the conversion. In other words, the total energy of any system and its surroundings is conserved. It does not place any restriction on the conversion of energy from one form to another. The interchange of heat and work is also considered in this first law. 

Suppose any closed system (thus having a constant mass) undergoes a process by which it passes from an initial state to a final state. If the only interaction with its surroundings is in the form of transfers of heat, Q, and work, W, then only the internal energy, U, can be changed, and the First Law of Thermodynamics is expressed mathematically as 

On the other hand, dQ and dW are inexact differentials because they cannot be obtained by differentiation of a function of the system. 

All the state variables and the changes in thermodynamic quantities during a process are measurable in principle. The value of A U is measurable, but the absolute values of U cannot be obtained. Thus, the thermodynamic data are reported with respect to certain internationally agreed standard or reference state values. Normally, a temperature of 25°C and a pressure of 1 bar = 105 pascal (Pa = N/m2) are taken as standard conditions, and for solutions, a molar concentration, c, of 1 mol/dm3 is used as a reference state. 

Fluids (liquid or gas) are known as PVT systems, where the macroscopic properties at internal equilibrium can be expressed as functions of temperature, pressure and composition only. In accepting this model, one assumes that the effects of fields (e.g., electric, magnetic or gravitational) are negligible and that surface and viscous-shear effects are unimportant. The PVT system serves as a satisfactory model in an enormous number of 

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