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

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

Edited By Vishal kumar | Updated on Jul 02, 2025 04:58 PM IST

We know what thermodynamics is and how important it is, so therefore one should learn about one of the concepts in thermodynamics. The enthalpy is used to determine the total heat energy present in a system; it tells whether a reaction absorbs energy or releases it. If you watch closely, then you will find out that enthalpy changes are very much related to the phenomena seen in our daily lives, like the warmth caused when burning wood, the cooling effect while melting ice, etc. In the following article, this amazing topic is defined by the types of reactions, units, and commonly asked questions.

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

Enthalpy Definition

Enthalpy in thermodynamics is defined as the addition of the internal energy of the system and the 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 the 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 the symbol of the enthalpy is H can be written as follows

Background wave

ΔH=ΔU+PΔV

Where,
ΔH is the change in enthalpy of the system
P is pressure and
ΔV is volume
ΔU is equal to the total internal energy of the system

SI Unit of the enthalpy is Joule.

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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

change in enthalpy

Depending upon the value of ΔH the Process or the reaction can be of two types
1. If ΔH>0 The process is called the Endothermic Process (Absorbs Heat)
2. If ΔH<O, The Process is called the Exothermic Process (Expels Heat)

enthalpy changes

Fig. 1

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.

Units of the Enthalpy

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

In other words:

In other words:
ΔH=Qp
Where Q is Heat in Joules.
We can derive this by starting from the following expression:

ΔH=ΔU+PΔV.

And ΔPV=PΔV+VΔP+ΔVΔP.

So, from the first law of thermodynamics,
ΔU=QW.
And W=PΔV.
H= is enthalpy,
Hence,
U= internal energy,
Q= heat flow,

ΔH=QPΔV+PΔV+VΔP+ΔPΔV


w=work,
But the pressure is assumed to be constant, so:
P= pressure and V is volume

ΔH=Q.


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 =kgm2 s2=Nm

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)
h=u+pv( J/kg)

Enthalpy Change

From the first of thermodynamics, we have

ΔQ=ΔU+ΔW

or, ΔU=ΔQΔW
Clearly, we know that Work can also be expressed as, ΔW=PΔV
Substituting this in the above equation we obtained,

ΔU=ΔQPΔV(1)
Now, from the second law of thermodynamics, we have,

ΔQ=TΔS( expression of entropy) 
On substituting the value of Q from equation (1), we obtain,

ΔU+PΔV=TΔS

or, ΔU=TΔSPΔV(2)
Now, adding VAP on LHS and RHS of Equation (2), we obtained

ΔU+VΔP=TΔSPΔV+VΔPΔU+PΔV+VΔP=TΔS+VΔP

ΔU+Δ(PV)=TΔS+VΔPΔ(U+PV)=TΔS+VΔP(3)
Clearly, H=U+PV (definition of enthalpy) -(4)
From equation (3) and (4) we obtained,

ΔH=TΔS+VΔP
This is the required expression for the change in enthalpy of the system and is a function of entropy and Pressure.

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 ( ΔHcomb  ): 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 a substance.
  • The enthalpy of fusion (ΔHfus ): 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 (ΔHvap ): The change in enthalpy associated with the vaporization of 1 mole of a 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 (ΔHsoln ): 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.

Specific Enthalpy

The specific heat enthalpy of the system is defined as the enthalpy per unit mass. It can be mathematically expressed as,

h=H/m
Here,
h= specific enthalpy of the system
H= enthalpy of the system

m= Mass 
Are Enthalpy and specific enthalpy the same or is there some difference between them?
Please note that it is the measure of the total heat content of the system, whereas, specific enthalpy is equivalent to the sum of specific Internal energy and the product of pressure and specific volume

h=u+pv
Here,
h= specific enthalpy
u= specific Internal energy
v= specific volume

p= pressure
S.I .unit: Joules per Kg

Dimensional Formula: [M0L2T2]

Entropy and Enthalpy

Entropy and enthalpy are regarded as fundamental concepts in thermodynamics, explaining fundamentally different energy properties in a system.

Enthalpy describes the total heat content of a system while keeping attention on the heat exchanged during chemical reactions or physical processes under constant pressure explains whether a process absorbs or releases energy as it does in endothermic and exothermic reactions.

Entropy, on the other hand, is a measure of a degree of disorder or randomness in a system. It, instead, reflects the innate tendency of systems to progress from relatively less random to more random-properly; for example from ice to water and then to the arrangement of random gases.

Frequently Asked Questions (FAQs)

1. What is enthalpy?

In Thermodynamics, the enthalpy is defined as the sum of the total energy of the system. Its symbol is H. Enthalpy is also represented by the sum of the internal energy and the flow energy.

2. What is a thermodynamic system?

The thermodynamic system is the system which is under observation. The observer can be anyone who is paying attention to the system being working.

3. What is the unit of the Enthalpy?

The enthalpy is nothing but the some form of energy so it has the same units as energy that is Joule or KJ

4. Is enthalpy a point function or path function?

The enthalpy is the path function so it depends on the path followed by the system to achieve the equilibrium.

5. Write down the formula for enthalpy?

Conderding delta H as the enthalpy and delta u as the total internal energy P pressure and  V as the volume then the enthalpy of the system can be written as

∆H= ∆U+P∆V

6. What is enthalpy and why is it important in thermodynamics?
Enthalpy is a measure of the total heat content of a system. It's important in thermodynamics because it helps us understand energy changes during chemical reactions and physical processes, especially those occurring at constant pressure. Enthalpy is crucial for predicting whether a reaction will absorb or release heat, which has practical applications in fields like chemistry and engineering.
7. How does enthalpy differ from internal energy?
Enthalpy (H) and internal energy (U) are related but distinct concepts. Enthalpy includes both the internal energy of a system and the product of pressure and volume (H = U + PV). While internal energy accounts for the kinetic and potential energy of particles within a system, enthalpy also considers the energy required to displace the surrounding atmosphere, making it particularly useful for processes occurring at constant pressure.
8. What is the difference between endothermic and exothermic reactions in terms of enthalpy?
Endothermic reactions absorb heat from the surroundings, resulting in a positive change in enthalpy (ΔH > 0). Exothermic reactions release heat to the surroundings, resulting in a negative change in enthalpy (ΔH < 0). In simpler terms, endothermic reactions feel cold to the touch, while exothermic reactions feel warm.
9. Can you explain the concept of standard enthalpy of formation?
The standard enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states at 1 atm pressure and a specified temperature (usually 25°C). This value is crucial for calculating enthalpy changes in chemical reactions and serves as a reference point for comparing the stability of different compounds.
10. What are the units of enthalpy?
Enthalpy is typically measured in units of energy per amount of substance. The SI unit for enthalpy is joules per mole (J/mol). Other common units include kilojoules per mole (kJ/mol) and calories per mole (cal/mol). The choice of unit often depends on the scale of the energy changes being considered and the field of study.
11. How is enthalpy related to Hess's Law?
Hess's Law states that the total enthalpy change in a chemical process is independent of the pathway taken and depends only on the initial and final states. This principle allows us to calculate enthalpy changes for complex reactions by breaking them down into simpler steps with known enthalpy values, then summing these values to find the overall enthalpy change.
12. How does bond breaking and bond formation affect enthalpy changes in chemical reactions?
Bond breaking is an endothermic process (requires energy), while bond formation is exothermic (releases energy). The overall enthalpy change in a chemical reaction is the difference between the energy required to break bonds in the reactants and the energy released when new bonds form in the products. If more energy is released in bond formation than is required for bond breaking, the reaction is exothermic, and vice versa.
13. How can you determine if a reaction is spontaneous using enthalpy?
While enthalpy is an important factor, it alone cannot determine if a reaction is spontaneous. A spontaneous reaction generally has a negative change in Gibbs free energy (ΔG < 0). Enthalpy contributes to this through the equation ΔG = ΔH - TΔS, where T is temperature and ΔS is the change in entropy. An exothermic reaction (negative ΔH) is more likely to be spontaneous, but entropy changes must also be considered.
14. What is the significance of the standard state in enthalpy calculations?
The standard state provides a consistent reference point for comparing enthalpy values across different substances and reactions. It typically refers to a pressure of 1 atm and a specified temperature (usually 25°C or 298.15 K). Using standard state values allows scientists to tabulate and share data that can be universally applied in calculations, making it easier to predict and compare enthalpy changes for various processes.
15. How does the strength of chemical bonds relate to enthalpy changes?
Stronger chemical bonds require more energy to break and release more energy when formed. In a chemical reaction, if the bonds formed in the products are stronger than those broken in the reactants, the reaction will be exothermic (negative ΔH). Conversely, if the bonds in the products are weaker, the reaction will be endothermic (positive ΔH). Understanding bond strengths helps predict and explain enthalpy changes in reactions.
16. What is the significance of Kirchhoff's law in relation to enthalpy?
Kirchhoff's law states that the change in the heat of reaction with temperature is equal to the difference in heat capacities of products and reactants. This law is important because it allows us to calculate enthalpy changes at different temperatures, which is crucial when dealing with reactions or processes that occur at non-standard temperatures.
17. How does enthalpy relate to the concept of heat capacity?
Heat capacity is the amount of heat required to raise the temperature of a substance by one degree. It's related to enthalpy through the equation ΔH = CpΔT, where Cp is the heat capacity at constant pressure, and ΔT is the change in temperature. This relationship is important for understanding how substances store and transfer thermal energy, which is crucial in many applications, from designing heating systems to predicting climate changes.
18. How does the concept of enthalpy apply to chemical equilibrium?
Enthalpy plays a crucial role in chemical equilibrium through Le Chatelier's principle. For an exothermic reaction (negative ΔH), increasing temperature shifts the equilibrium towards the reactants, while for an endothermic reaction (positive ΔH), increasing temperature favors the products. Understanding this relationship helps predict how temperature changes affect equilibrium positions, which is vital in optimizing industrial processes and understanding natural systems.
19. What is the relationship between enthalpy and activation energy?
While enthalpy describes the overall energy change of a reaction, activation energy is the minimum energy required for a reaction to occur. The enthalpy change of a reaction can be visualized on an energy diagram, where the activation energy is represented by the "hill" that reactants must overcome. Exothermic reactions (negative ΔH) have products at a lower energy state than reactants, while endothermic reactions (positive ΔH) have products at a higher energy state.
20. How does enthalpy contribute to the concept of chemical potential?
Chemical potential, a key concept in thermodynamics, is partially determined by enthalpy. It represents the change in Gibbs free energy when the amount of a substance in a system changes. The relationship is given by μ = H - TS + PV, where μ is chemical potential, H is enthalpy, T is temperature, S is entropy, P is pressure, and V is volume. Understanding this relationship is crucial for predicting the direction of chemical reactions and phase transitions.
21. What is the significance of the Born-Haber cycle in enthalpy calculations?
The Born-Haber cycle is a theoretical approach used to calculate lattice energies of ionic compounds using enthalpy values. It breaks down the formation of an ionic compound into several steps, each with its own enthalpy change. By applying Hess's Law to these steps, we can determine difficult-to-measure values like lattice energy. This cycle is crucial for understanding the stability of ionic compounds and predicting their properties.
22. How does enthalpy relate to bond dissociation energy?
Bond dissociation energy is the enthalpy change when a particular chemical bond is broken. It's directly related to the strength of the bond and contributes to the overall enthalpy change in chemical reactions. When calculating the enthalpy of a reaction, we consider the sum of bond dissociation energies for bonds broken in reactants and formed in products. This relationship is key to predicting and understanding reaction energetics.
23. What is the importance of enthalpy in biological systems?
In biological systems, enthalpy plays a crucial role in understanding metabolic processes, enzyme kinetics, and the energetics of biochemical reactions. For instance, the enthalpy changes in ATP hydrolysis drive many cellular processes. Understanding enthalpy helps explain how organisms maintain body temperature, how certain biochemical reactions are favored over others, and how energy is stored and transferred within living systems.
24. How does enthalpy contribute to the greenhouse effect?
Enthalpy is relevant to the greenhouse effect through the concept of heat capacity and energy absorption. Greenhouse gases like CO2 can absorb and re-emit infrared radiation, increasing the atmosphere's enthalpy. This increased heat content leads to a rise in global temperatures. Understanding the enthalpy changes involved in atmospheric processes is crucial for climate science and predicting the impacts of human activities on global climate.
25. What is the relationship between enthalpy and entropy in determining spontaneity?
The spontaneity of a process is determined by the Gibbs free energy change (ΔG), which is related to enthalpy and entropy by the equation ΔG = ΔH - TΔS. A process is spontaneous when ΔG < 0. This means that a reaction can be spontaneous even if it's endothermic (ΔH > 0), provided the entropy increase (ΔS > 0) is large enough. Understanding this relationship is crucial for predicting the direction of chemical reactions and physical processes.
26. How does pressure affect the enthalpy of phase transitions?
Pressure can significantly affect the enthalpy of phase transitions, especially for transitions involving gases. For example, the enthalpy of vaporization typically decreases with increasing pressure because less energy is required to overcome intermolecular forces at higher pressures. This relationship is described by the Clausius-Clapeyron equation and is crucial in understanding phenomena like the boiling of liquids at different altitudes.
27. What is the role of enthalpy in determining reaction rates?
While enthalpy doesn't directly determine reaction rates, it plays a crucial role through its relationship with activation energy. The Arrhenius equation, which describes how reaction rates depend on temperature, includes an exponential term related to activation energy. Exothermic reactions (negative ΔH) often have lower activation energies and thus tend to proceed faster than endothermic reactions. Understanding this relationship is key to predicting and controlling reaction kinetics.
28. How does the concept of enthalpy apply to fuel cells and batteries?
In fuel cells and batteries, enthalpy changes are crucial for understanding energy conversion and storage. The enthalpy of reaction in these devices determines the maximum theoretical voltage and energy output. For fuel cells, the enthalpy of combustion of the fuel is directly related to the electrical energy produced. In batteries, the enthalpy changes during discharge reactions influence the cell potential and energy density. Understanding these enthalpy relationships is vital for designing more efficient energy storage and conversion devices.
29. What is the significance of enthalpy in food science and nutrition?
In food science and nutrition, enthalpy is important for understanding the energy content of foods and metabolic processes. The enthalpy of combustion of food components (carbohydrates, fats, proteins) relates directly to their caloric value. Additionally, enthalpy changes are crucial in food processing techniques like cooking, freezing, and dehydration. Understanding these enthalpy relationships helps in developing nutritional guidelines, designing food preservation methods, and optimizing cooking processes.
30. How does enthalpy relate to the concept of chemical affinity?
Chemical affinity, which describes the tendency of chemical species to combine with others, is closely related to enthalpy. Generally, reactions with a large negative enthalpy change (highly exothermic) indicate a strong chemical affinity between reactants. This concept is crucial in predicting the likelihood and extent of chemical reactions, which is important in fields ranging from materials science to drug design.
31. How does enthalpy contribute to the concept of chemical potential in solutions?
In solutions, enthalpy contributes to the chemical potential through enthalpy of mixing and enthalpy of dilution. These enthalpy changes affect how the chemical potential of a component varies with concentration. Understanding this relationship is crucial for predicting solubility, osmotic pressure, and other colligative properties of solutions. It's particularly important in fields like pharmaceutical science for drug formulation and delivery.
32. What is the significance of enthalpy in geochemical processes?
In geochemistry, enthalpy is crucial for understanding mineral formation, metamorphism, and magmatic processes. The enthalpy of formation of minerals influences their stability under different geological conditions. Enthalpy changes in reactions between minerals and fluids drive many geochemical processes, including weathering and ore deposit formation. Understanding these enthalpy relationships is essential for interpreting geological history and predicting future geological events.
33. How does pressure affect enthalpy?
Pressure directly affects enthalpy because enthalpy includes the PV term (pressure × volume). At constant temperature, increasing pressure generally increases enthalpy slightly for most substances. However, the effect is often small for solids and liquids and more significant for gases. This relationship is important in processes like gas compression or expansion.
34. What is enthalpy of vaporization and why is it important?
The enthalpy of vaporization is the energy required to transform a given quantity of a substance from a liquid to a gas at constant pressure. It's important because it helps us understand phase changes, predict boiling points, and calculate energy requirements in processes involving evaporation or condensation, which are crucial in many industrial and natural processes.
35. What is the relationship between enthalpy and the First Law of Thermodynamics?
The First Law of Thermodynamics states that energy is conserved in any process. Enthalpy is related to this law through the equation ΔH = ΔU + PΔV, where ΔH is the change in enthalpy, ΔU is the change in internal energy, P is pressure, and ΔV is the change in volume. This relationship shows how enthalpy accounts for both the change in a system's internal energy and the work done against the atmosphere in constant pressure processes.
36. How does the concept of enthalpy apply to phase changes?
Enthalpy is crucial in understanding phase changes because each transition (melting, freezing, vaporization, condensation, sublimation) involves an enthalpy change. For example, the enthalpy of fusion is the energy required to melt a substance, while the enthalpy of vaporization is the energy needed to vaporize it. These enthalpy values help predict the energy requirements for processes involving phase changes, which is vital in many industrial and natural phenomena.
37. What is the enthalpy of combustion and why is it important?
The enthalpy of combustion is the heat released when one mole of a substance completely combusts with oxygen under standard conditions. It's important because it helps determine the energy content of fuels, which is crucial for assessing their efficiency and environmental impact. This concept is widely used in fields like energy production, engine design, and environmental science.
38. What is the enthalpy of solution and how does it affect dissolution processes?
The enthalpy of solution is the heat absorbed or released when a substance dissolves in a solvent. It's the sum of the enthalpy of hydration (energy released when solute particles interact with solvent molecules) and the lattice energy (energy required to break the solute's crystal structure). If the enthalpy of solution is negative (exothermic), dissolution is favored energetically. This concept is crucial in understanding solubility and designing processes involving solutions.
39. What is the significance of standard molar enthalpy in thermochemistry?
Standard molar enthalpy refers to the enthalpy of a substance in its standard state (usually 1 atm pressure and 25°C). It serves as a reference point for calculating enthalpy changes in reactions. By using standard molar enthalpies, chemists can predict the enthalpy changes for a wide range of reactions without having to measure each one individually. This concept is fundamental to thermochemical calculations and the creation of enthalpy tables.
40. How does enthalpy relate to the concept of work in thermodynamics?
Enthalpy is related to work through the First Law of Thermodynamics. In a constant pressure process, the change in enthalpy (ΔH) equals the heat transferred to the system (q) plus the work done by the system (w). Specifically, ΔH = q + w, where w = -PΔV. This relationship is crucial for understanding energy transfers in various thermodynamic processes, especially those occurring at constant pressure.
41. What is the role of enthalpy in understanding phase diagrams?
Enthalpy plays a crucial role in phase diagrams by influencing the position of phase boundaries. The slopes of these boundaries are related to the enthalpy changes associated with phase transitions. For example, the Clausius-Clapeyron equation, which describes the slope of the liquid-vapor boundary, directly involves the enthalpy of vaporization. Understanding these enthalpy relationships is essential for predicting material behavior under various temperature and pressure conditions.
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