Enthalpy - Definition, Endothermic, Exothermic Reaction, Units, FAQs

Enthalpy in thermodynamics is defined as the addition of the internal energy of the system and product of its pressure and volume. Enthalpy meaning also represents the total heat change under constant pressure and temperature. What is meant by enthalpy for the particular thermodynamics system is that total energy of the system including internal energy and flow energy; the thermodynamics system is the system which is under the thermodynamics observation. The term enthalpy is the state function which means it does not depend on the path followed to complete the process on the contrary it only depends upon the final configurations of the pressure, volume and internal energy. Mathematically enthalpy (H), where symbol of the enthalpy is H can be written as follows

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This Story also Contains

1. SI Unit of the enthalpy is Joule
2. Units of the Enthalpy
3. Specific Enthalpy (h)
4. Relationship between 1st Law of Thermodynamics and Enthalpy
5. Enthalpy of Reaction
6. Entropy and Enthalpy
7. Hess’s Law of Enthalpy

∆H= ∆U+P∆V.....(1)

SI Unit of the enthalpy is Joule

Where,

ΔH is change in enthalpy of the system

P is pressure and

ΔV is volume

∆U is equal to the total internal energy of the system

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The second term in the enthalpy equation is related to the work required to establish the physical dimension of the system. The product of pressure P and volume V in enthalpy H is sometimes also called the flow work. Flow work (PV) is essentially defined as the work required maintaining an uninterrupted flow through a controlled volume. Flow work can be viewed on the control volume system.

The change in enthalpy for the system can be written as

Depend upon the value of ∆H the Process or the reaction can be of two types

1. If ∆H>0 The process is called as the Endothermic Process (Absorbs Heat)

2. If ∆H<0, The Process is called as the Exothermic Process (Expels Heat)

It can be said from the above change in enthalpy equation that when the heat is added to the system it have twofold effect on the system. First is the increase in internal energy of the system, which is nothing but the sum of potential energy and kinetic energy of atoms and molecules and other is to do flow work i.e expansion of the system against the pressure applied by the atmosphere.

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Units of the Enthalpy

As stated above, Enthalpy is nothing but the determination of heat flow at a constant pressure.

In other words:

ΔH=Qp.......(3)

Where, Q is Heat in Joules.

We can derived this by starting from the following expression:

∆H= ∆U+P∆V........(4)

And ∆PV= P∆V+V∆P+∆V∆P..........(5)

So, from first law of thermodynamics,

∆U=Q-W.........(6)
And W=-P∆V......(7)

Hence,

ΔH=Q-PΔV+PΔV+VΔP+ΔPΔV........(8)

But the pressure is assumed to be constant, so:

ΔH=Q......(9)

Hence, as a result; at constant pressure:

ΔH=Qp.......(10)

So, enthalpy must then be in Joules, more often kJ is used.

Other units of the enthalpy are Joule = kg⋅m²s² = N⋅m

Specific Enthalpy (h)

Specific enthalpy is defined as the total enthalpy of the system per unit mass of the system.

h=H/m (J/kg)........(11)

h=u+pv (J/kg)......(12)

Relationship between 1st Law of Thermodynamics and Enthalpy

First law of thermodynamics expresses that, total heat provided to the system is used to increase the internal energy of the system and to do the work done or in other word between two systems the change in internal energy is equal to the difference between heat transfer into the system and work done by the system.

Mathematically,

E2-E1=Q-W.......(13)

For the constant pressure process

Substitute equation 14 in equation 13 we get

Solving it for Q we get

E2+PV2-(E1+PV1)=Q........(16)
H2-H1=Q

We know that, for a constant pressure the heat transfer is given by

Q=Cp(T2-T1).......(17)

Hence

Specific Enthalpy is given by

Specific Enthalpy of Wet Steam

Enthalpy of Reaction

One way to report the heat absorbed or released is to compile a large number of reference tables listing all possible enthalpy changes of chemical reactions, which will require a lot of effort. Luckily, since enthalpy is a state function, we only need to know the original and finishing state of the reaction. This allows us to use a relatively small set of tabular data to calculate the enthalpy change of almost any possible chemical reaction, for example:

• Enthalpy of combustion (ΔH_comb): The enthalpy change that occurs during a combustion reaction. Enthalpy change is measured for the combustion of almost all substances burned in oxygen; these values are usually expressed as the enthalpy of combustion per mole of substance.

• The enthalpy of fusion (ΔH_fus): changes with the enthalpy of the fusion of 1 mole of matter. The enthalpy change that accompanies the fusion or fusion of 1 mole of matter; these values are measured for almost all elements and most simple compounds.

• Enthalpy of Vaporization (ΔH_vap): The change in enthalpy associated with the vaporization of 1 mole of substance. The enthalpy change that accompanies the evaporation of 1 mole of substance; these values are also measured for almost all elements and most volatile compounds.

• Enthalpy of Dissolution (ΔH_soln): The change in enthalpy that occurs when a certain amount of solute is dissolved in a given amount of solvent. When a certain amount of solute is dissolved in a certain amount of solvent, the enthalpy

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Entropy and Enthalpy

Like internal energy, entropy and enthalpy are thermodynamic properties. The entropy is displayed in the symbols s and the entropy changes Δs in kJ / kgK units. Entropy is in a chaotic state. Entropy is the theme of the Second Law of Thermodynamics, which describes changes in the entropy of the cosmic system and its surroundings. Entropy is defined by the absolute temperature heat transfer coefficient of the system for a reversible thermodynamic pathway.

Where qrev is the heat transfer along the path which is reversible.

Enthalpy (h) is a property of a state and is defined as shown in equation (12)

Where h is the specific enthalpy, u is the specific internal energy v is the specific volume, and p is the pressure.

But from Equation (6)

Therefore,

Tds=du+vdp.......(20)

If Equation (18) is differentiated and substituted into the above equation, above equations are all about the change in entropy of the reversible which is given by

Hess’s Law of Enthalpy

According to Hess's law, the enthalpy change of a chemical reaction (i.e. heat of reaction at constant pressure) is independent of the path between the initial state and the final state. That is, if a chemical change occurs in several different pathways, the overall enthalpy change is the same regardless of the pathway in which the chemical change occurs (provided the original and last conditions are the same).

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