Specific Enthalpy Vs Enthalpy: Comparative Analysis and FAQs

To know Specific Enthalpy vs Enthalpy of a system, the total heat content of the system as well as the mass of the system must be known.

Ehthalpy of a system is  defined as the summation of its internal energy and product of the pressure and volume of the system, wheres the specific enthalpy defined as enthalpy per unit mass.

Enthalpy of a system is defined as

H = E + PV,

Where,

H= Enthalpy

E= Internal energy

PV= Pressure volume work done

In the above expression for all practical purposes the PV value is insignificant or fairly small and enthalpy is comprised mostly of the internal energy content.

It is difficult to calculate the internal energy in absolute terms and for engineering calculations, the change of enthalpy or specific enthalpy is of primary importance and same is accounted for.

Enthalpy changes are frequently used in chemistry to calculate heat of reactions. For an exothermic reaction, which occurs with liberation of heat the enthalpy change is considered positive.

For an endothermic reaction, which takes place with absorption of heat, the change is enthalpy is considered negative. The heat of reaction can be calculated by enthalpy change that takes place between the reactants and its products at its standard states ( 25°C and 1 atmosphere pressure).

Enthalpy is an extensive property, which means enthalpy varies with the size or mass of the system, whereas specific enthalpy is an intensive property which means it depends on the type of system or matter and it does not depend upon its size.

Specific Enthalpy vs Enthalpy:

The comparative analysis of Specific Enthalpy vs Enthalpy is shown below:

Specific Enthalpy of Steam Water System

Enthalpy is a property of a system and it is specific for a particular temperature or pressure. For example, specific enthalpy for vaporization of water at atmospheric pressure is 2257 KJ/kg.

Similarly, for saturated water the specific enthalpy is 419 KJ/Kg at atmospheric pressure. Thus for saturated steam at atmospheric pressure, the specific enthalpy is

hs = 2676KJ/kg = hw + he

hw = Sp enthalpy of saturated water

he = Sp enthalpy of evaporation of water

hs = Sp enthalpy of saturated steam

This specific enthalpy value increases with increase in the system pressure.

For super heated steam at a particular degree of superheat the specific enthalpy is given by:

hss = hs + cps (tss – tf), Where,

hss = enthalpy of supersaturated steam

hs = enthalpy of saturated steam

Cps = Sp heat of steam at constant pressure

tss = temp of superheated steam

tf = saturation temperature

Molar Enthalpy vs Specific Enthalpy

Molar enthalpy denotes the enthalpy per moles of a substance and specific enthalpy is denoted as enthalpy per unit mass of the substance.

Different types of Enthalpy Change

An enthalpy change takes place, when a matter undergoes a physical transformation or a chemical change. Various enthalpy changes are described as below:

Enthalpy of reaction: The enthalpy change that place when one mole of a reactant transforms into the product

Enthalpy of combustion: The enthalpy change that occurs when one mole a substance reacts with oxygen and burns completely.

Enthalpy of formation: The enthalpy change that takes place when one mole of a compound is formed from it basic constituent elements.

Enthalpy of neutralization: The enthalpy change that takes place when one mole of water is formed after an acid reacts with a base.

Enthalpy of a solution: The enthalpy change that takes place when one mole of a solute is dissolved completely in a solvent which is in excess to form a solution.

Enthalpy of fusion: The enthalpy change required to completely change one mole of a matter from solid to liquid state.

Enthalpy of sublimation: The enthalpy change required to completely change one mole of a matter from solid to gaseous state.

Enthalpy of vaporization: The enthalpy change required to completely change one mole of a matter from liquid to gaseous state.

Enthalpy of mixing: The change in the enthalpy when two chemical substances are mixed without any chemical reaction taking between them.

What is favorable Enthalpy?

As per the laws of nature, the favorable enthalpy change is negative.

This implies, a reaction is favored if the enthalpy change is negative or the reaction is exothermic. However, spontaneity of a reaction or favorability of a reaction shall also depend upon what is the entropy change.

When a reaction is taking place, it is always favorable if the system losses energy or the enthalpy change is negative. On the other hand, it is nature’s law to increase entropy of a system. Hence, a reaction is favored if leads to increase in entropy of the system.

If a reaction occurs which leads to increase in entropy but increase in enthalpy as well, then the favorability of the reaction depends upon the contribution of each component.

This answer in this case is quantitatively provided by another expression called Gibbs free energy as shown below:

ΔG = ΔH –TΔS

Where,

ΔG = Change in Gibbs free energy

ΔH = Change in Enthalpy

ΔS = Change in Entropy

T= Temperature in Kelvin

In the above equation, a reaction is favored if Gibbs free energy is negative. The reaction in fact is carried out spontaneously. On the other hand, the reaction cannot proceed spontaneously, if gibbs free energy is positive.

Sangeeta Das

I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at https://www.linkedin.com/in/sangeeta-das-57233a203/