Internal-Energy Vs Enthalpy of Thermodynamic System

The difference between internal energy and enthalpy refers to intercepting energy (heat) in different ways. Internal energy is the heat content of a system, i.e., the sum of all types of specific energies of a system. Whereas, enthalpy is the amount of heat either liberated or engaged in a system.

"For a thermodynamic system, When heat is added to a system at constant volume, we use the term internal energy because no work is done and it is a closed system" 

& 

"When heat is added to a system at constant pressure then we use the term enthalpy because work is done and it is an open system". 

If q is the amount of heat absorbed by the system from surroundings:

Heat (q) at constant volume = Internal energy (U) = No work is done 

Heat (q) at constant pressure = Enthalpy (H) = Work is done 

FACT: 

!!!!We cannot determine the absolute value of internal energy or enthalpy but we can determine the change in internal energy or enthalpy!!!!

Internal Energy

Internal energy is the sum of all kinds of energies in a system. It is the total energy contained in a system. It is due to the translational, vibrational, and rotational motion of atoms in a molecule. It is also due to the interactions between the molecules. 

.....A Part of the total amount of energy within the system is associated with the rearrangement of atoms that occurs in chemical reactions, the energy of interactions between atoms and molecules, and the energy associated with being only temperature. This stored amount of energy is called "Internal Energy". It is represented by E or U......

The first law of thermodynamics states that when heat is absorbed by a system, a part of the absorbed energy (heat) is used to do work while the other part is used to increase the internal energy of that system.

➩ dQ = dU + dW

At constant volume, the change in internal energy is equal to the amount of heat absorbed in a system. From the 1st law of thermodynamics;

➩ dQ = dU + dW

 [ (dQ)v = (dU)v ]

 

Enthalpy

The amount of heat absorbed or released during a chemical reaction is exactly equal to enthalpy (H). Enthalpy is the sum of internal energy and product of pressure-volume work at constant pressure.

[ H = U + PV ]

As U, P and V are state functions, so the enthalpy is a state function. Relationship between enthalpy change and heat transfer at constant pressure;

dH = dU + PdV + VdP

applying the condition of constant pressure where the change in pressure (dP) = 0;

dH = dU + PdV

----------------(1)

From the first law of thermodynamics, we get

dU = dQ + PdV

----------------(2)

If a system is in mechanical equilibrium with its surrounding at pressure P and does expansion

 work then dW = -PdV

(Work done by the system is negative)

dU = dQ – PdV

Put this value in equation (1)

dH = dQ – PdV + PdV

dH = dQ – PdV + PdV

(dH)p = (dQ)p

Thus, energy supplied to a system as heat is equal to the enthalpy change at constant pressure. Because of this relation, enthalpy is often called “Heat Of The Reaction”.

Difference Between Internal Energy And Enthalpy

Definition

Internal Energy It is the sum of kinetic energy and potential energy or it is the total energy contained in a system. 

Enthalpy It is the amount of heat absorbed or released during a chemical reaction or it is the total heat content of a system

Representation

Internal Energy It is represented by E or U

Enthalpy It is represented by H

General Formula

Internal Energy The formula of internal energy is U=q-W

Enthalpy The formula of enthalpy is H=U+PV

Interpretation Of Formula

Internal Energy According to the formula, change in internal energy is equal to heat transfer to the system minus work done.

Enthalpy According to the formula, enthalpy is equal to the sum of internal energy and product of pressure-volume work.

Unit

Internal Energy Its unit is Joule

Enthalpy Its unit is Joule/mole.

Conditions

Internal Energy It is a state of matter 

Enthalpy It is an energy change between two states

Dependence

Internal energy it is a function of temperature only 

Enthalpy it is a function of temperature and pressure

Principle 

Internal energy increases when systems absorb energy and decreases when systems release energy to the surrounding.

Enthalpy endo-thermic reactions have positive enthalpy (H) and exothermic reactions have negative enthalpy (H)

Application 

Internal Energy for closed systems, the term internal energy is used

Enthalpy For open systems, the term enthalpy is used

Conclusion 

Internal Energy it is the total energy in a system.

Enthalpy It is an energy change between the system and the surroundings.

FAQs

What is the relation between enthalpy and internal energy??

Internal energy is the total energy contained in a system where enthalpy is the total heat content of a system.

Enthalpy is equal to the sum of internal energy and product of pressure-volume work.

➩ H = U + PV

Is Enthalpy Just Energy??

Enthalpy is the sum of internal energy and product of pressure-volume work. Energy is a state of matter where enthalpy is an energy change between two states. So, enthalpy is not just energy.

Why Is Enthalpy Used Instead Of Internal Energy??

If a reaction occurs at constant volume, then added heat is equal to a change in internal energy.

If a chemical reaction occurs at constant pressure, then enthalpy is used to measure the heat of the reaction instead of internal energy.

Can Enthalpy Be Greater Than Internal Energy and why??

For an open system, the term enthalpy is used to calculate energy and it is always greater than internal energy. 

Internal energy is a heat of reaction at a constant volume where enthalpy is a heat of reaction plus pressure-volume work at constant pressure. This additional factor of pressure-volume work makes the enthalpy of an ideal gas 40% greater than its internal energy.

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