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Nikita Parmar

Updated on 30th June, 2023 , 8 min read

Enthalpy Formula: Definition, History, Chemical and Physical Properties, Steps to Calculate, and Sample Questions

Enthalpy Formula Overview 

Enthalpy is the measurement of energy released in thermodynamic systems during particular chemical processes. Enthalpy is determined by the changes in the reactions as well as the heat content of the materials (H). Enthalpy H represents the change in response (also called the heat of reaction). The amount of enthalpy equals the entire amount of heat in a system. The internal energy required to produce a system is described by enthalpy.

What is Enthalpy?

Enthalpy is a thermodynamic system's measurement of energy. Enthalpy is an energy-dimensional state function or energy-like quality (and is thus calculated in joules or erg units). Enthalpy serves as a proxy for energy in chemical systems;bonds, lattice, solvation, and other "energies" in chemistry represent enthalpy differences. Enthalpy, as a state function, is determined only by the ultimate configuration of internal energy, pressure, and volume, not by the path taken to get there.

Chemical enthalpies are often provided for 1 bar (100 kPa) pressure as a standard condition. While enthalpies and enthalpy changes for reactions vary with temperature, lists normally provide the standard temperature for the production of compounds at 25 °C (298 K). The change H is positive for endothermic (heat-absorbing) processes and negative for exothermic (heat-releasing) processes.

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What is the Enthalpy Formula?

Enthalpy change is defined as the sum of internal energy (U) and the product of volume and pressure (PV), expressed below-


Enthalpy may alternatively be characterized as a state function that is entirely reliant on the state functions P, T, and U. It is often represented as the difference in enthalpy (H) of a process between its initial and end states. It is expressed below-


If the pressure and temperature do not vary during the operation and the job is confined to pressure and volume, the enthalpy change is given by,


According to the following equation, the flow of heat (q) at constant pressure in a process equals the change in enthalpy. The following is expressed below-


Knowing whether q is endothermic or exothermic allows us to characterize the connection between q and H. An endothermic reaction absorbs heat and demonstrates that heat is absorbed in the reaction from the surroundings, implying that q > 0 (positive). With constant pressure and temperature for the preceding equation, if q is positive, then H is likewise positive. Similarly, if heat is released as a result of an exothermic process, the heat is transferred to the surroundings. As a result, q = 0 (negative). As a result, if q is negative, H will be negative.

History of Enthalpy Formula

The term "enthalpy" was coined very late in thermodynamic history, in the early twentieth century. Thomas Young created the term "energy" in its contemporary meaning in 1802, whereas Rudolf Clausius coined the term "entropy" in 1865. Enthalpy was first used in print in 1909. It is ascribed to Heike Kamerlingh Onnes, who most likely first mentioned it verbally the year before at the Institute of Refrigeration's inaugural conference in Paris. It only became popular in the 1920s, most notably with the publication of the Mollier Steam Tables and Diagrams in 1927.

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Petroleum Engineering Salary in India

Chemical and Physical Properties of Enthalpy Formula

The following are some of the chemical and physical properties of enthalpy-

Chemical Properties of Enthalpy Formula

  1. The enthalpy of atomization is the enthalpy change necessary to thoroughly divide one mole of a material into its component atoms.
  2. The enthalpy of combustion is the change in enthalpy that occurs in a thermodynamic system's component when one mole of a material burns entirely in the presence of oxygen.
  3. The enthalpy change needed to denature one mole of a substance is known as the standard enthalpy of denaturation (biochemistry).
  4. The enthalpy of formation is the change in enthalpy that occurs in a thermodynamic system's component as one mole of a compound is created from its fundamental constituents.
  5. The enthalpy of hydration is the change in enthalpy that results from the full dissolution of one mole of gaseous ions in one mole of water, creating one mole of aqueous ions.
  6. The enthalpy of hydrogenation is the change in enthalpy that results when one mole of an unsaturated chemical fully interacts with an excess of hydrogen to generate a saturated molecule.
  7. The enthalpy of neutralization is the change in enthalpy in a thermodynamic system that occurs when an acid and a base combine to generate one mole of water.
  8. The enthalpy of reaction is the change in enthalpy that occurs in a thermodynamic system's component when one mole of material fully reacts.
  9. Standard Enthalpy of Solution is the change in enthalpy that occurs in a component of a thermodynamic system when one mole of a solute entirely dissolves in an excess solvent, resulting in an infinite dilution of the solution.

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Physical Properties of Enthalpy Formula

  1. The enthalpy of fusion is the amount of energy needed to totally transform one mole of a material from a solid into a liquid.
  2. The energy necessary to create separated gaseous ions from one mole of an ionic compound at an infinite distance apart is known as lattice enthalpy (meaning no force of attraction).
  3. The enthalpy of mixing is the change in enthalpy that occurs when two (non-reacting) chemical compounds are combined.
  4. Theenthalpy of sublimation is the amount of energy needed to transform one mole of a material from a solid into a gas.
  5. The enthalpy of vaporization is the amount of energy needed to transform one mole of a material from a liquid to a gas in its entirety.

Steps to Calculate Enthalpy

A scientist can use H to assess whether a reaction emits heat (or "is exothermic") or absorbs heat (or "is endothermic").

H = m x s x T 


m = mass of the reactants

s = specific heat of the product

T = temperature change caused by the reaction

Enthalpy formula ΔH may be computed in several ways, including-

Step 1: If the work done by or on a system is zero, the volume of the container remains constant. Theheat transfer (q) will be equal to the enthalpy change.

q = m ✕s ✕ΔT

In this equation, the mass is m, the specific heat is s, and the temperature change is ΔT.

Step 2: If the reaction is already known, a table of heat change values ΔHf can be used to compute it. Theheat of formation is denoted by the symbol ΔHf. It is the heat that is utilized to produce material from its fundamental parts. As a result, 

ΔHreaction = ΣΔHf(products) - ΣΔHf (reactants)

Step 3: Use Hess's Law to compute the enthalpy of a reaction.

Step 4: The ΔHreaction may be estimated using the bond energies of the reactants and products.

ΔHreaction= ΣΔH bondsbroken – ΣΔHf (bondsformed)

Read Related Articles: Ionization Enthalpy

Points to Remember

  1. Enthalpy is a thermodynamic property
  2. It is the sum of the system's total internal energy plus the product of pressure and volume.
  3. The following equation may be used to calculate enthalpy: H = E + PV.
  4. The heat of the device is lost to the surrounding environment during exothermic processes. H is negative for such reactions
  5. During endothermic processes, the system absorbs heat from the surrounding environment. H is positive for such responses.

Sample Questions for Enthalpy Formula

Sample Question 1- Compute the enthalpy (H) of the process that converts 45.0 grams of water from liquid at 10° C to vapor at 25° C.

Solution: a) Heating water from 10.0 to 25.0 °C

ΔkJ = 45.0g H20 x (4.184J/gH2O °C) x (25.0 - 10.0) °C x 1kJ/1000J = 2.82 kJ

b) Vaporizing water at 25.0 °C

ΔkJ = 45.0 g H2O x 1 mol H2O/18.02 g H2O x 44.0 kJ/1 mol H2O = 110 kJ

c) Total Enthalpy Change

ΔH = 2.82 kJ + 110kJ

Sample Question 2- What is the practical use of the enthalpy formula?

Solution: Hand warmers and freezers may both benefit from the enthalpy shift. For example, freon is a refrigerant that evaporates in a refrigerator. In this process, the enthalpy of vaporization equals the coolness of the meal.

Sample Question 3- What are the often-used enthalpy formula conditions?

Solution: The standard enthalpy of formation is the change in enthalpy that occurs when one mole of material in its standard state (1 atm of pressure and 298.15 K) is created from its pure ingredients under the same circumstances.

Sample Question 4- What does the enthalpy formula change tell you?

Solution: An enthalpy formula change is a difference between the energy expended to break bonds in a chemical reaction and the energy gained by the process of producing new chemical bonds. It describes the energy change of a system under continuous pressure. H is an abbreviation for enthalpy change.

Sample Question 5- What happens when the enthalpy formula is raised?

Solution: A system's enthalpy determines whether it may undergo a reaction by growing (i.e. when energy is added) or decreasing (i.e. when energy is released). The amount of energy lost or gained during a reaction is roughly equivalent to its enthalpy change.

Sample Question 6- What is the difference between enthalpy and energy?

Solution: The words energy and enthalpy are not interchangeable since the latter is the amount of heat emitted or absorbed during a change. The entire kinetic and potential energy of a system is referred to as its energy.

Sample Question 7- What factors have an impact on the enthalpy formula?

Solution: Enthalpy is a broad notion impacted by the amount of material used. The condition of reactants and products influences the enthalpy value of a system (solid, liquid, or gas). The direction of the reaction influences the enthalpy value.

Sample Question 8- Extensive or intensive: what is the property of the enthalpy formula?

Solution: The term "enthalpy" refers to several different qualities. The value of enthalpy is determined by the mass of the material.

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Frequently Asked Questions

Is it true that enthalpy is always bigger than internal energy?

Ans. In thermodynamics, the heat energy produced is frequently employed to enhance the energy of the system or to do beneficial work. Enthalpy is the energy associated with an open system, and it is frequently larger than or equal to internal energy.

What exactly is the enthalpy of a chemical reaction?

Ans. During chemical processes, atom bonds can break, reform, or both to receive or release energy. Enthalpy is the amount of heat absorbed or released by a device under constant pressure, while reaction enthalpy is the change in enthalpy caused by a chemical reaction.

What is the significance of enthalpy?

Ans. Enthalpy is significant because it tells us how much heat is in a system (energy). Heat is crucial because it allows us to generate productive work. An enthalpy shift indicates how much enthalpy was lost or gained during a chemical reaction, with enthalpy referring to the heat energy of the system.

When enthalpy is positive, what happens?

Ans. Heat absorption is an endothermic process. Its enthalpy will be positive, and the environment will cool. This is an exothermic process (negative enthalpy, heat release). As the reaction occurs, the temperature of the atmosphere may rise due to the increase in heat emitted by the device.

What exactly is the distinction between enthalpy and internal energy?

Ans. Internal energy is the total amount of energy stored in the gadget. It is the quantity of potential and kinetic energy stored by the mechanism. Enthalpy is defined as the sum of the system’s internal energy and the product of the system’s gas pressure and length.

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