The first law of thermodynamics enthalpy

Let us suppose that a system of unit mass takes in a certain quantity of heat energy q (measured in joules). As a result, the system may do a certain amount of external work w (also measured in joules). The excess energy supplied to the system, over and above the external work done by the system, is q-w. Therefore, if there is no change in the macroscopic kinetic and potential energy of the system, it follows from the principle of conservation of energy that the internal energy of the system must increase by q — w. That is. 

Equations (2.3) and (2.4) are quite general and represent the work done by any substance or system when its volume changes. 

If we are dealing with a unit mass of a substance, the volume V is replaced by the specific volume a (i.e., volume per unit mass) and the work dw that is done by a unit mass of the substance when its specific volume increases by da is 

If heat is added to a system at constant pressure, so that the specific volume of the system increases from a to 2, Ihe work done by a unit 

If the quantity of heat (in joules) required to raise a unit mass of the system by 1°C at constant pressure (called the specific heat at constant pressure) is Cp, then 

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The first law of thermodynamics (enthalpy) expresses the equivalence and interchangeability of the different forms of energy (heat, work, etc.), so that a molar mass of polysaccharide, for example, undergoing transformation from A to B, absorbs or evolves an increment of energy (AE) expressed (Glasstone and Lewis, 1960 Knight, 1970) as... 

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