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What is Enthalpy?

Energy can neither be created nor be destroyed. It can only be transformed into various forms which are interchangeable. For instance, tides are harnessed to convert Tidal Energy into Electrical Energy. Similarly, we rotate a turbine to convert the Mechanical Energy into Electrical Energy. Also, this Electrical Energy is further converted into Light or Heat Energy by lighting a bulb or an appliance. 


Simultaneously, Enthalpy is one such term that describes the amount of change in energy during the process of conversion. Students have to understand Enthalpy definition and derivation for a comprehensive study of the same. Therefore, to make it easier and more straightforward, here is a brief explanation along with relevant diagrams and examples along with Mathematical equations. 


Enthalpy - Definition 

Enthalpy is the quantity of heat in a system. This heat is utilized in the occurrence of a process. Any heat-related system is called a thermodynamic system, so Enthalpy is a thermodynamic quantity.


Besides, it should be noted that any system comprises multiple participants. Each of these participants has its pressure and volume. As we already know, the product of pressure and volume of a specific system is constant. 


This Enthalpy is equal to the sum of internal energy of such a system with the constant. 

You need to understand that the forms of energy get changed, but Enthalpy remains constant. For example, when water freezes into ice, some amount of energy is spent in doing the work, which is called Enthalpy.


As a result, scientists are often found to be calculating Enthalpy instead of energy. You should be cautious about learning both the terms. Although they seem to be equal, they are actually not. 


Enthalpy - Mathematical Equation

Similar to any other scientific theory, Enthalpy also has a Mathematical formula. Refer to the image below. It shows the equation of Enthalpy definition and derivation with appropriate labels.


You should note that Δ (delta) is a symbol that represents a change in some quantity. Herein the change in internal energy is expressed through ΔU. 


Also, consider another essential aspect; that of energy loss due to it being is used up for work. This amount of energy shall be deducted from the amount of heat added to the system to find the actual amount of energy change.


These are basic terms and expressions, which will help in understanding the overall concept with clarity. 


It is often observed that students confuse both these terms - entropy and Enthalpy. Also, they often use it interchangeably. However, the ground reality is entirely different. Both these terms are hugely different. 


Refer to the table below for a clear comparison:


Enthalpy and Entropy

Sl. No. 




It is a property that helps in the measurement of the movement of molecules.

It is a type of energy that is composed of both internal energy and energy flow.


It is considered only under limitations or controlled conditions.

It can be measured or is applicable at normal or regular conditions.


The unit of measurement is Joule per Kelvin.

The measuring unit is Joule per mole.


A thermodynamic system is likely to favor a maximum amount of entropy in it.

Any thermodynamic system will favor only a minimum amount of Enthalpy.


Now that you know the difference, it is quite clear that Enthalpy definition and derivation is a form of energy. So, when the internal energy changes, it is called Enthalpy during constant pressure.


As mentioned above, there are other properties, such as pressure and volume. Let us dig deeper into those terminologies to understand Enthalpy. 


Enthalpy - Relation with Volume and Pressure

Herein, let us consider a situation where there is no work done. In terms of Chemistry, we are considering a chemical reaction as a system which is capable of performing a certain action. However, for the sake of understanding, we are acknowledging that no work is being done.


Hence, if any amount of energy is emitted or absorbed from or by the system, there would be an equal amount of change in internal energy. It is observed that this kind of reaction is possible only when the volume and pressure are constant. 


Nevertheless, the experiments are conducted in open flasks, resulting in exchange of heat within and outside the surrounding. This further leads to the concept of work done and Enthalpy.


You should note that work done implies both ways; it is not just surrounding work on the system or vice versa.  Since a specific amount of energy has been spent on doing work, the heat absorbed or emitted is not equal to internal energy.


Thus, internal energy equals the sum of work done and the product of volume and pressure. The Enthalpy definition and derivation is represented as:

  • H =  E + PV



  • H = Enthalpy

  • E = internal energy

  • P = pressure 

  • V = volume


Therefore, now if we want to represent the changes in energy levels or Enthalpy, we can write it as :

ΔH =  ΔE + Δ(PV)


With this, you should note the following two points:

  • At constant volume, the heat emitted or absorbed during a reaction equals the internal energy of a system.

  • At constant pressure, heat emitted or absorbed during a chemical reaction equals the Enthalpy of the system.

Both these points again go back to the original equation. Make sure you understand the individual concepts so that you can follow the comparison well. 


Preparing for a subject requires you to be clear on every topic included in it. You should be able to relate Enthalpy definition and derivation with the subject matter at hand. Being familiar with the concepts and numerical in that specific chapter is the key to ace the Exam.


Endothermic and Exothermic Reactions

If you look carefully at both the terms, you can decipher their meanings. Thermic means heat. Similarly, ‘exo’ means outside, and ‘endo’ refers to inside. Therefore, when a system pulls in heat from the outside surroundings, it is termed an endothermic reaction. 


On the other hand, if a system gives out heat into the surrounding, it is called an exothermic reaction. Now you can understand that it is neither hot, nor cold. It is only the absence of heat that defines coldness. 


Similarly, when there is a presence of heat, an object is said to be hot. So, when heat leaves an object, that specific place becomes cold as a fall in temperature. Now, the takeaway points in this regard are as follows:

  • Endothermic reactions contain more energy within them, whereas exothermic reactions do not have much energy within them.

  • Endothermic reactions make their surroundings cooler, but exothermic reactions make their surroundings hotter.

Now that you have developed an idea over the Enthalpy definition and derivation, it is time that you test your understanding. Below is an exercise for you to check your learning.


Test Your Understanding

Fill in the blanks

1. During constant volume, change in internal energy is equal to ………..heat.

2. Enthalpy is an …………… of a system.

3. …………...of an ideal gas depends on the temperature.

4. Heat  ………… constant …………, increases the Enthalpy of a system.

5. Enthalpy and internal energy are properties of the temperature of an ……..gas.

6. Heat is supplied at constant pressure, there is a change in ………..

7. Enthalpy changes can be found using ………, which cannot be found by a calorimeter.

8. Endothermic changes are represented by………. values.

9. Which of the following reactions is endothermic?

  • Melting copper

  • Melting ice

  • Freezing water

  • Butane combustion

10. When Enthalpy of a substance directly changes from a solid to vapor, it is called ……….

11. Which is an intensive property?

  • Temperature

  • Viscosity

  • Surface tension

  • All of these.

12. There is a ……….. in temperature in case of adiabatic expansion.

13. During the formation of a chemical bond, ……….is released. 


Transferred, intensive, Enthalpy, transferred, pressure, ideal, Enthalpy, Hess’s law, positive, melting copper, all of these, decrease, energy.


For more information on the Enthalpy definition and derivation chapter and its counterparts, you can go through our online learning programs. This will help you in understanding the other advanced concepts as well. 


Our subject experts compile these, so you can get a holistic approach towards understanding the same. So, without any further delay, hurry up and grab your learning kit now. You can also download our Vedantu app for a comprehensive learning experience.

Change in Reaction of Enthalpy

Thermodynamics is the investigation of the connection between hotness (and energy) and work. Enthalpy is a focal element in thermodynamics. It is the hotness content of a framework. The hotness that passes into or out of the framework during a response is the Enthalpy change. Regardless of whether the Enthalpy of the framework increments (for example at the point when energy is added) or diminishes (on the grounds that energy is radiated) is an essential component that decides if a response can occur.

Here and there, we call the energy of the particles going through change the "inside Enthalpy". In some cases, we consider it the "Enthalpy of the framework." These two expressions allude to exactly the same thing. Additionally, the energy of the atoms that don't participate in the response is known as the "outer Enthalpy" or the "Enthalpy of the environmental elements".

Generally speaking, the energy changes that we checked out in the prolog to thermodynamics were changes in Enthalpy. We will find in the following area that there is another lively element, entropy that we likewise need to consider in responses. For the time being, we will simply take a gander at Enthalpy.

  • Enthalpy is the hotness content of a framework.

  • The Enthalpy change of a response is generally identical to how much energy lost or acquired during the response.

  • A response is leaned toward assuming the Enthalpy of the framework diminishes over the response.

That last assertion is a great deal like the depiction of energetics on the past page. In the event that a framework goes through a response and emits energy, its own energy content abatements. It has less energy left finished assuming it parted with a few. For what reason does the energy of a bunch of atoms change when a response happens? To respond to that, we want to contemplate what occurs in a compound response.

In a response, there is an adjustment of compound holding. A portion of the bonds in the reactants are broken, and new bonds are made to shape the items. It costs energy to break bonds, yet energy is delivered when new bonds are made.

In fact, Enthalpy depicts the inner energy that is needed to produce a framework and how much energy that is needed to account for it by building up its strain and volume and dislodging its current circumstance.

At the point when an interaction starts at steady tension, the developed hotness (either assimilated or delivered) rises to the adjustment of Enthalpy. Enthalpy change is the amount of interior energy signified by U and result of volume and Pressure, meant by PV, communicated in the accompanying way.


Enthalpy is likewise depicted as a state work totally dependent on state capacities P, T and U. It is ordinarily shown by the adjustment of Enthalpy (ΔH) of an interaction between the start and last states.


On the off chance that the strain and temperature don't change all through the cycle and the errand is restricted to tension and volume, the adjustment of Enthalpy is given by,


The progression of hotness (q) at steady strain in a cycle rises to the adjustment of Enthalpy dependent on the accompanying condition,


A connection among q and ΔH can be characterized knowing whether q is endothermic or exothermic. An endothermic response is the one that assimilates heat and uncovers that hotness is burned-through in the response from the environmental elements, subsequently q>0 (positive). Assuming that q is positive, then, at that point, ΔH is likewise certain, at steady tension and temperature for the above condition. Likewise, assuming the hotness is delivered as an exothermic response, the hotness is given to the environmental factors. Subsequently, q<0 (negative). Hence, ΔH will be negative assuming q is negative.

FAQs on Enthalpy

1. For what reason is Enthalpy valuable?

Enthalpy is significant in light of the fact that it illuminates us how much hotness is in a framework (energy). Heat is significant, since from it, we can determine important work. An Enthalpy shift shows us how much Enthalpy was lost or acquired as far as a substance response, Enthalpy meaning the framework's hotness energy.

2. What is the distinction between Enthalpy and inward energy?

The combined energy that is put away in the gadget is inward energy. It is the sum held by the component of potential and active energy. Enthalpy is determined as how much the framework's interior energy is in addition to the blend of the framework's gas tension and its length.

3. What happens when Enthalpy is positive?

A hotness ingestion response is endothermic. Its Enthalpy will be positive, and its environmental factors will chill off. This response (negative Enthalpy, heat discharge) is exothermic. At the point when the response occurs, because of the addition in heat the gadget produces, the environment might ascend in temperature.

4. What is the Enthalpy of synthetic response?

The connections between molecules can break up, change or both during substance responses to either retain or deliver energy. The hotness ingested or transmitted under steady tension from a gadget is alluded to as Enthalpy, and the response Enthalpy is the adjustment of Enthalpy emerging from a substance response.

5. Is Enthalpy consistently more noteworthy than inward energy?

The hotness energy created is regularly utilized in thermodynamics to work on the framework's energy or to accomplish some helpful work. The energy related with an open framework is called Enthalpy, which is regularly more prominent than or equivalent to inside energy.

6. What is Enthalpy?

Enthalpy is the amount of heat absorbed or emitted by a substance during a change at constant pressure.

7. How are Enthalpy and energy not the same?

Energy and Enthalpy are not the same, since the latter is the amount of heat released or absorbed during a change. Here, energy is the overall kinetic and potential energy in a system.

8. What property does Enthalpy display, extensive or intensive?

Enthalpy is an extensive property. The value of Enthalpy depends on the mass of a substance.

9. Why is Enthalpy necessary?

Enthalpy is vital in calculating the amount of heat exchange during a change in a substance. It helps in assessing if the reaction was endothermic or exothermic.