It is quantity of heat necessary to be added or taken to increase or decrease the temperature of a unit mass of a substance by one degree.
Specific heat is a property of material and its value is tabulated.
ΔH= m Cp (To-Tin)
Where; m: is the mass flowrate and Cp is specific heat under constant pressure. Since the specific heat is a function of temperature for more accurate calculations tables are available to find the changes in enthalpy as well as Internal energy rather than calculate them from the above equations. For pure substance such as wet steam or steam not highly superheated, Ammonia vapor, Freon vapor Change in Enthalpy and internal energy are determined from tables and chart rather than being calculated.
Second Law of Thermodynamics
The first law of thermodynamics does not impose limitations on the fraction of available energy that can be used in a desired way. The second law of thermodynamics has the effect of imposing such limitation. The second law has several statements. The second law highlights the concept of availability; which can be explained in a way that for a machine operating on a thermodynamic cycle it is impossible to convert all the heat available into work; part of the heat received should be rejected to a lower temperature level. This rejected heat will be at minimum, if all the processes are reversible.
The second law highlights the concept of irreversibility which can be explained in way that heat cannot flow by itself from a low temperature source to a higher temperature level unless work is paid. Other sources of irreversibility in addition to heat transfer are both surface friction and internal friction such as that found in gases and liquid due to their flow.