Definition of Enthalpy, Types, and Formulas in Chemistry

Definition of Enthalpy – In learning chemistry we will get to know material about chemical thermodynamics which discusses the change of heat energy into other forms of energy. In learning this science, we will also get to know about the enthalpy formula, which is a sub-branch of learning from thermodynamics that discusses the amount of energy, volume, and heat pressure of a substance.

For this reason, Readers who want to know and learn more about the meaning of enthalpy in order to better understand the formula and its application in chemistry lessons, in this discussion we will summarize various information related to the enthalpy formula.

Furthermore, the discussion regarding the meaning of enthalpy can be seen below!

Definition of Enthalpy

Thermodynamics is a branch of physics that deals with the conversion of heat energy into other forms of energy. The first law of thermodynamics and the second law of thermodynamics are references when discussing energy changes. Measurements in thermodynamics are not expressed in microscopic terms but in macroscopic quantities.

Thermodynamics is closely related to statistical mechanics, thus inferring thermodynamic relationships. The origin of the word thermodynamics comes from two Greek words, namely thermos which means heat and dynamic which means change. The discovery of the concept of thermodynamics originated from the efforts of scientists in the 19th century AD to create machines capable of producing energy changes.

The purpose of this energy conversion was initially to facilitate work by converting energy into work with the maximum energy change. The first machine created by scientists was able to convert the energy of motion into potential energy. Its working principle is based on collision events. There were attempts by scientists who at the end of the 20th century succeeded in developing theories related to thermodynamics. The thermodynamic theory applies to states of heat or systems in equilibrium at the beginning or at the end.

After the 19th century AD, the development of thermodynamic theory shifted to quantum physics and phase transitions. Phenomenologically, the development of the thermodynamic theory is aimed at macroscopic systems. In physics, the thermodynamic formula becomes an axiom that includes the three laws of thermodynamics. The key concepts underlying the three laws of thermodynamics are energy and entropy.

Thermochemistry or chemical thermodynamics is a branch of chemistry that studies the energy that accompanies physical changes or chemical reactions. The main goal of thermochemistry is to establish criteria for determining the probability or spontaneity of the required transformation. In this way, thermochemistry is used to predict energy changes that occur in chemical reactions, phase changes, and solution formation. Most properties in thermochemistry evolved from the application of the First Law of Thermodynamics, the law of the “conservation” of energy, to functions of internal energy, enthalpy, entropy, and Gibbs free energy.

Enthalpy is a law of thermodynamics that states the sum of the internal energy, volume, and thermal pressure of a substance. The SI unit for enthalpy is the joule, but the British units for heat and calories are also used. It is not possible to measure the total enthalpy (H) directly. As in classical mechanics, only changes can be evaluated. Enthalpy is a thermodynamic potential, so to measure the enthalpy of a system we must first determine a reference point, then we can measure the change in enthalpy ΔH. . The ΔH change is positive for endothermic reactions and negative for exothermic reactions.

For a process at constant pressure, ΔH equals the change in energy within the system plus the work done by the system on the surroundings. So the enthalpy change under these conditions is the heat absorbed or released by a chemical reaction or external heat transfer.

The enthalpies of ideal gases, solids and liquids are independent of pressure. Real objects at room temperature and pressure often obey this property more or less, which simplifies enthalpy calculations.

Enthalpy Formula

Enthalpy is the amount of energy possessed by the system (U) and work (PV), so that it can be written H = U + PV. While the change in enthalpy is the heat of reaction from the reaction at constant pressure. To calculate enthalpy, it must be measured at a specific temperature and pressure.

According to chemists, a temperature of 25°C and a pressure of 1 atm is a good measure of enthalpy. The enthalpy change measured by standard measurement will be called the standard enthalpy change. The unit is the kilojoule (kJ) in the international system (SI). Writing the enthalpy for the reaction in the reaction equation is done by writing the symbol for the enthalpy change (⧋H) after the reaction equation.

It was also explained that enthalpy is the amount of energy possessed by a system which is denoted by U and work is denoted by PV so that the enthalpy change formula can be written as H = U + PV .

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While the change in enthalpy is the heat of reaction from the reaction at constant pressure. To calculate enthalpy, it must be measured at a specific temperature and pressure. According to chemists, a temperature of 25°C and a pressure of 1 atm is a good measure of enthalpy.

The enthalpy change measured by standard measurement will be called the standard enthalpy change. The unit is the kilojoule (kJ) in the international system (SI). Writing the enthalpy for the reaction in the reaction equation is done by writing the symbol for the enthalpy change (⧋H) after the reaction equation. For example:

A(s) + B(aq) → C(aq) H = + x kJ

Enthalpy Types

When recording thermochemical data, the enthalpy change for a reaction under standard conditions (25∘C, 1 atm) is represented by the symbol H∘ with units of kJ/mol and depends on the type of reaction. is derived into several enthalpy formulas and as an example as follows:

1. Enthalpy of formation

The standard enthalpy of formation in which a compound represents the amount of heat required or released for the formation of 1 mol of the compound from standard state stable elements (STP).

Enthalpy of formation is the change in the amount of heat of formation per 1 mol of a compound from its elements under normal conditions. Standard normal values ​​are determined at a temperature of 298 oK and a pressure of 100 kPa. In free elements, the standard enthalpy of formation is zero. The enthalpy of formation under standard conditions is denoted Δ𝐻𝑓𝑜. The enthalpy combinations form elements or compounds that can be used to calculate the enthalpy change in a chemical reaction. The equation used to determine the enthalpy change for a reaction is: Δ𝐻𝑜(reaction) = ΣΔ𝐻𝑓𝑜(product) – ΣΔ𝐻𝑓𝑜(reactant).

The standard enthalpy change for forming 1 mole of a substance directly from its elements in their standard states (298 K, 1 atm). For example, the enthalpy of formation of standard water is −285 kJ/mol, so the thermochemical equation becomes:

H2(g)+12O2→H2O(l) ΔH=−285 kJ

The thing to note is

  • Considering the shape of the element under standard conditions, choose the most stable allotrope. For example, graphite is the most stable allotrope for carbon
  • In general, the enthalpy of formation of a compound is negative (exothermic), indicating that the compound is more stable than its elements.
  • The enthalpy forming element in its standard form is set to zero

2. Atomization enthalpy

In this concept it is called elemental atomization, which is the enthalpy change if 1 mole of gas consists of elements in the form of matter under standard conditions. The atomization reaction will have a positive ⧋H (endothermic). Indeed, reactions require energy to separate atoms.

Atomization enthalpy is the energy required to produce 1 mole of gaseous atoms in an element. Atomization enthalpy values ​​are determined when an element is at room temperature in its normal phase. Atomized enthalpies are used to measure the formation of metallic compounds by breaking metallic bonds.

The enthalpy change for atomization of 1 mol of a substance into its elements in their standard states (298 K, 1 atm). The standard atomization enthalpy is opposite to the formation reaction, so its value is the same as the formation enthalpy but opposite in sign.

An example of a standard decomposition enthalpy can be seen from the formation reaction above, so the standard decomposition enthalpy is:

H2O(l)→H2(g)+12O2(g)ΔH=+285 kJ

3. Enthalpy of combustion

Standard enthalpy of combustion where a compound represents the amount of heat required or released for the combustion of 1 mol of a compound from its stable element under standard conditions (STP).

The enthalpy of combustion is the difference between the enthalpy of the products under certain conditions and the enthalpy of the reactants under the same conditions. Calculation of enthalpy of combustion is done for complete combustion. The amount of enthalpy change produced in a chemical reaction is called the heat of reaction. Calculation of the heat of reaction can be calculated based on the difference between the heat of formation between the products and reactants.

The enthalpy change for the complete combustion of 1 mole of a substance, measured under standard conditions.

Combustion is said to be complete if:

  • Carbon (C) turns into CO2
  • Hydrogen (H) burns to become H2O
  • Sulfur (S) burns to SO2

Example of Enthalpy Discussion Questions

1. Problem 1

Determination of the enthalpy of combustion of charcoal in carbon dioxide and steam.


The reaction of burning coal can be written as follows.

C(s) + O2(g) → CO(g)

Not all changes that occur in enthalpy can be determined by conducting experiments. For example, in the combustion reaction, carbon (graphite) turns into carbon monoxide. The enthalpy change for reactions that burn carbon to pure CO tends to be more difficult because CO is flammable. And if we react carbon with excess oxygen, CO will immediately burn and turn into CO2. During this time, in a limited amount of oxygen, a mixture of CO and CO2 will be formed.

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However, the enthalpy change that occurs during the formation of CO can be determined on the basis of the enthalpy change that occurs easily. The reaction is easier than burning carbon and turning it into carbon dioxide and burning carbon monoxide turning it into carbon dioxide.

The enthalpy value of this reaction is unknown. The enthalpy value of the coal combustion reaction can be determined using a reaction with a known enthalpy value. We know that the enthalpy of formation of CO2 = –393.5 kJ mol-1 and the enthalpy of combustion of CO = –283 kJ mol-1.

Based on the two enthalpy data, using Hess’s law, the enthalpy of burning carbon converted to carbon monoxide can be calculated as follows.

The thermochemical equation leads to the formation of carbon dioxide (CO2)

(1) C(s) + O2(g) → CO2(g) = -393.5 kJ/mol

The thermochemical equation leading to the burning of carbon monoxide (CO)

(2) CO(g) + 1/2 O2(g) → CO2(g) AH = -283 kJ/mol

To get the equation for the reaction that burns carbon to carbon monoxide, reaction (2) can be reversed and can then be added to reaction (6).

Removing two substances that are equal on both sides, we get the following reaction equation:

C(s) + O2 (g) → CO(g) ∆H = -110.5kJ

Based on the sum of the two reaction stages, the enthalpy change of the results of burning carbon that is converted into carbon monoxide can be determined in a way that tends to be easier, namely by adding up the enthalpy changes of the two reaction phases. . incident. The determination of the magnitude of the enthalpy change in this way was discovered by a Russian chemist, namely GH Hesse (1840). Through a series of experiments he conducted, Hess stated that enthalpy changes depend only on the initial and final state of the reaction, and not on the course of the reaction.

Thus, if a chemical reaction takes place in several reaction steps, the enthalpy change is determined by adding the enthalpy change for each step. This Hess statement became known as Hess’s Law (or better known as the Law of Complement of Heat). Applying Hess’ law we can easily determine the enthalpy change for reactions that are hard to determine when doing experiments.

The thermochemical equation is prepared in such a way that the total yield of the reaction with the change in enthalpy is determined. Usually, several equations must be multiplied by the appropriate coefficients to obtain the required thermochemical equation.

This calculation method follows Hess’s law which states that the enthalpy of a reaction absorbed or released by a reaction does not depend on the course of the reaction. Some principles for calculating thermochemical equations according to Hess’s law that need to be considered are:

  • If you need to reverse the reaction equation, change the sign ∆H. For example,


H2(g) + O2(g) → H2O2(l) ∆H = –187.8 kJ


H2O2(l) → H2(g) + O2(g) ∆H = +187.8 kJ

  • If in an addition reaction a substance appears on both sides of the equation in the same phase, the substance can be removed. The example is:

H2(g) + O2(g) → H2O(g) ∆H = +241.80 kJ

H2O(l) → H2(g) + O2(g) ∆H = –285.85 kJ

H2O(l) → H2O(g) ∆H = –44.05 kJ

Calculations of the ∆H of the reaction can also be made from the basic data of the standard heat of formation reaction (∆Hf°). The standard heat of formation is the heat of formation of compounds based on their elements. Consider the general equilibrium reaction equation below. aA + bB → cC + dD

∆Hreaction = (c × C + d × D) – (a × A + b × B)

= ∆H°f products – ∆H°f reactants

So, in general the reaction ∆H can be determined by the formula:

∆H reaction = H°f products – ∆H°f reactants

Information :

  • ∆H°f gives: is the total standard enthalpy of formation based on the product of the substance.
  • ∆H°f of reaction: is the total standard enthalpy of formation based on the reactants.


So a brief discussion of what is the meaning of enthalpy and an explanation of the formula. Not only knowing the meaning of enthalpy in chemistry, but also discussing the types, formulas, and examples of problems from discussing enthalpy itself.

Knowing what enthalpy is gives us new knowledge about learning chemistry which can be useful in everyday life if the application of the concept of enthalpy is used according to the rules.