The Parameters Describing Thermodynamics

Entropy It is considered in the seeond law of thermodynamies. It is thermodynamic property that ean be used to determine the energy available for useM work in a thermodynamic process, such as in energy eonversion devices, engines, or maehines. In thermodynamics, the concept of entropy is defined by the second law of thermodynamics, which states that the entropy of an isolated system always inereases or remains constant. Thus, entropy is also a measure of the tendeney of a process, such as a chemical reaction. Thermodynamic entropy has the dimension of energy divided by temperature, and a unit of joules per kelvin (J/K) in the International System of Units. 

Enthalpy It is a thermodynamic potential. Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy. The total enthalpy, H, of a system cannot be measured directly. Thus, change in enthalpy. Ml, is a more useful quantity than its absolute value. The change A/f is positive in endothermic reactions, and negative in exothermic processes. The unit of measurement for enthalpy in the International System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie. 

Assume, for example, that we start with the following reaction imder standard-state conditiorrs, as shown below. 

The value of AG at that moment in time will be equal to the standard-state 

As the reaction gradually shifts to the right, converting N2 and H2 into NH3, the value of AG for the reaction will decrease. If we could find some way to harness the tendency of this reaction to come to equilibrium, we could get the reaction to do work. The free energy of a reaction at any moment in time is therefore said to be a measure of the energy available to do work. 

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