What is Enthalpy? Definition, Meaning, Equation, Formula, Units, Changes


In this article, we will learn what is enthalpy, along with the definition, meaning, equation, formula, units, and changes to understand the basic concept.

Let’s explore.

What is Enthalpy? Definition & Meaning

Definition of Enthalpy

The sum of internal energy and product of volume and pressure of a thermodynamic system is termed enthalpy. It is a point function (doesn’t depend on the path followed), that’s why enthalpy is a thermodynamic property of a system. In simple words, Enthalpy represents the total heat content of the system. It is represented by “H” and specific heat enthalpy as “h.”

Rather than calculating enthalpy, change in enthalpy is calculated for a system (reason: impossible to know the zero point). Hence, the difference of enthalpy of the two states is measured under constant conditions of pressure.

Meaning of Enthalpy

The quantity of internal energy and pressure-volume output of a thermodynamic system is referred to as enthalpy. It is characterized as having energy-like properties or state-function having energy dimensions.

As a state function, its value is dependent on the only initial and final state of the system. It is an extensive property, meaning dependent upon the mass.

  • Measuring the enthalpy, or heat, of different chemical processes is the aim of the first thermodynamic principle, also the law of energy conservation.
  • All chemical processes are associated with a specific amount of enthalpy received by the system from the surrounding environment.
  • For convenience, enthalpy change characterizes the equation as reactions process at constant pressure. Enthalpy or internal energy values are dependent upon the standard state that is compared with a given quantity. Gases, solids, and liquids are conventionally the most used states. 

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Equation or Furmula & Units of Enthalpy

Enthalpy Units

According to the “law of conservation of energy,” the change in internal energy of the system is equal to heat transmitted to an entity minus its work. A system’s change in internal energy is equal to the transmitted heat to the entity, minus its work.

If the work performed is the change in volume at constant pressure, the change in enthalpy would be equal to the heat transferred to the entity. This amount is called enthalpy, or latent heat of vaporization, and the unit for its expression is joules per mole, denoted as J/mol.

Joule is the standard unit for enthalpy as per SI. At the same time, the other units such as calorie and British Thermal Unit (BTU) can also be used.

Equation or Formula of Enthalpy

The enthalpy equation is stated below:

H= E + PV

As per the enthalpy equation, enthalpy “H” equals the sum of internal energy “E” and product of pressure “P” and volume “V.”

Role of Enthalpy in Thermodynamic System

Enthalpy is a property unique to the thermodynamic system. As per the laws of thermodynamics, energy and enthalpy are related to each other, playing a crucial role in a thermodynamic system. Thermodynamics is a branch of physics that deals with the study of the relationship between heat and its different forms. The measure of heat content in a chemical or physical system is enthalpy in terms of thermodynamics.

Work in thermodynamics is defined as a process of transfer of energy to another system. In terms of the first law of thermodynamics, the energy of a closed system is equivalent to the quantity of supplied heat to the system minus the amount of work done by the system on its surroundings. The equation is represented as :

ΔU= Q – W

Where,

  • U= Total energy of the system
  • Q= heat
  • W= work done

The most common type of work done in a chemical system is P-V work, during which the volume of gas increases. Now, the above equation can be rewritten as :

ΔU= Q – P ΔV

Enthalpy for Chemical Reactions

For chemical reactions, the enthalpy equation is given hereunder:

ΔH = Σp Hp – Σr Hr

“p” and “r” represent products and reactants, respectively.  If Δis negative, then there is a net flow of heat from the chemical system to the surroundings (release of heat during the reaction). Such reactions are known as exothermic reactions.

If ΔH is positive, then the net flow of heat is from surroundings towards the chemical system (means heat is absorbed). Such reactions are called endothermic reactions.

Consider a few reactions:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)ΔH = -890 kJ/mol
(Exothermic Process)  
H2(g) + I2(g) → 2 HI(g)ΔH = 52.2 kJ/mol (Endothermic process)
Enthalpy for Chemical Reactions

Enthalpy; Positive or negative?

Exothermic reactions involve the loss of heat during a chemical reaction. In this case, the change in enthalpy would be negative as heat is leaving the system. However, the scenario is the opposite in the case of endothermic reactions. Endothermic reactions involve the consumption of heat. Therefore, the change in enthalpy would be positive.

Henceforth, we conclude that breaking a bond in a chemical reaction requires a positive change in enthalpy. In contrast, the formation of bonds requires a negative change in enthalpy.

Enthalpy Changes

Enthalpy change or enthalpy transition is the quantity of heat emitted or utilized in a reaction at constant pressure. The representation for enthalpy change is delta H, represented as “∆H.” The equation for the change in internal energy is ∆U = q + w. at constant pressure; it can be written as:

  • ∆U = qP – p∆V

The system absorbs heat at constant pressure, given by qP. Moreover,  – p∆V is the expansion work done because the system absorbed heat. The equation stated above can be rewritten in terms of the initial and final states of the system. It is given as:

  • UF – UI = qP –p(VF – VI)
  • Or qP = (UF + IVF) – (UI + pVI)

Enthalpy (H) is calculated through the formula,  H= U + PV. Now, we put values in the above equation and get:

  • qP = HF – HI = ∆H

Henceforth, change in enthalpy ∆H = qP, showing that the system absorbed heat at a constant pressure. Additionally,  at constant pressure, we have the following formula:

  • ∆H = ∆U + p∆V

Types of Enthalpy Change

Delta H denotes enthalpy change. The quantity represents the amount of heat absorbed or emitted during a chemical reaction. Depending on the nature of the reaction, enthalpy change can be expressed in several different ways. Types of enthalpy change are discussed below in-depth:

Heat of Formation

The heat of formation is the enthalpy change that takes place when one mole of a substance or compound is formed from its elements. The symbolic representation is “ΔHf.” consider the following reaction as an example for the heat of formation:

Fe(s) + S(s) → FeS(s)              ΔHf = -24.0 kcal

Heat of Combustion

When one mole of a substance is burnt in excess of air or oxygen, the change in enthalpy has termed the heat of combustion. We denote the term by  ΔHc. Consider the following reaction between methane and oxygen as an example for the heat of combustion.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)              ΔHc = -21.0 kcal

Heat of Neutralization

When 1g equivalent of an acid is neutralized with 1g equivalent of the base in a diluted solution, the enthalpy change is known as the heat of neutralization. Following is a reaction between nitric acid and sodium hydroxide showing the heat of neutralization.

HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)              ΔH = -13.69 kcal

Heat of Solution

When dissolved in a specific quantity of solvent at a given fixed temperature, one mole of a substance is the heat of the solution. The following equation is an example for the heat of solution:

MgSO4(s) + H2O(l) → MgSO4(aq)              ΔH = -20.28 kcal

Heat of transition

The heat of transition is defined as the enthalpy change when one mole of an element changes from its allotropic form into another. For example, the transition of diamond into graphite, or transition in phosphorus and sulfur.

Latent Heat of Vaporization

Condensation and vaporization are opposite to each other. Latent heat of vaporization is a physical property possessed by an entity. The phenomenon is defined as the amount of heat required to transform one mole of liquid under standard atmospheric pressure at its boiling point.

The quantity is expressed in units like kg/mol or KJ/ kg. In simple words, when a liquid is heated, it absorbs energy during the phenomenon to change its state from liquid to vapor. This procedure is what we call heat of vaporization.

During a phase change, the arrangement of molecules changes, not the temperature. If the new arrangement has immense thermal energy, the substance will ultimately absorb energy from the surroundings to carry out phase change. If the new arrangement’s energy is lower, the entity will release energy to its environment or surrounding.

Molar Heat of sublimation

When solid directly changes to vapor state, the process is called sublimation. However, the molar heat of sublimation is defined as the enthalpy change that occurs when one mole of a substance is converted into a vapor state directly at the sublimation temperature. The following reaction gives the heat of sublimation of iodine:

     I2 (s) → I2 (v)    heat of sublimation= +62.42 kJ

Another typical example is the conversion of solid CO2 at low temperature to its gaseous state.

Measuring Enthalpy Changes

The enthalpy of a system is calculated using the following equation:

H = E + PV ………………….. (1)

 E shows internal energy, P demonstrates pressure, and V is the volume of the system. It is also called heat content. Thus the enthalpy change is given hereunder,

ΔH = Hproducts – Hreactants

ΔH = H– H………………(2)

If ΔV is the change in volume at constant temperature and pressure, ΔE is the sum of the change in internal energy taking volume constant, and ΔH is the change in enthalpy at constant pressure. Then, according to equation (1):

ΔH = ΔE + PΔV ……………………(3)

Now, considering a general equation:

aA + bB → cC + dD

Change in the number of moles = no. of moles of products – no. of moles of reactants.

= (c + d) – (a + b)

= Δn

V = the volume occupied by one mole of gas, then the volume change is given as:

ΔV = change in no. of moles X volume occupied by one mole of gas.

ΔV = Δn X V

PΔV = P(Δn X V)

PΔV = PV X Δn …………………(4)

For one mole of gas, PV = RT. Putting the value in equation (4) we get:

PΔV = RT X Δn

Now, inserting the value of PΔV in equation (4), we obtain:

ΔH = ΔE + Δn RT

It is noteworthy that to calculate Δn, the no. of moles of reactants and products in the only gaseous state are taken. The value of the gas constant is given below:

R = 8.314 J/mol.K or 1.987 cal

Hess’s Law

Statement

Hess’s law states that the change in enthalpy, taking place in a system, accompanying a chemical change, is not dependent on the route of chemical change.

Explanation

The law explains and proves that when reactants are changed into products, overall enthalpy change remains the same whether the process occurs in one step or many steps.

Mathematical form

ΔHnet = Σ ΔHr
Where ΔHnet shows the net change in enthalpy,  

 ΔHr shows the summation of change in enthalpy reactions

Importance of Enthalpy

The importance of enthalpy is stated below in bullet points

  • The heat content of a system while pressure is constant is called enthalpy. The primary significance of enthalpy is that it aids in determining whether a bond is exothermic or endothermic.
  • In chemistry, it also helps in finding the heat of reaction in a chemical reaction. It aids in determining whether the reaction was endothermic or exothermic.
  • The majority of chemical reactions take place at constant pressure. Hence, enthalpy is commonly used in measuring the heat of response as compared to internal energy.

Applications 

The application of enthalpy from our lives are listed below,

  • Enthalpy has various applications in thermal engineering.
  • It can be used in the calculation of the minimum power of a compressor.
  • Enthalpy change is utilized in measuring heat flow in calorimetry.
  • The quantity helps in determining a chemical process’s heat of reaction.
  • The heat of fusion of ice and heat of vaporization of water is utilized in measuring
  • The heat of formation can be calculated using enthalpy.
  • The calorific value of food and fuels is calculated using enthalpy.
  • Change in enthalpy can be associated with hand warmers and refrigerators. Refrigerants like freon are evaporated from refrigerators. Hence the coldness of food equals the enthalpy of vaporization.
  • Chemical hand warmers (heat packs) are used by a few people outside. When the pack is shaken, it heats the hands. It is an example of a change in enthalpy.

Conclusion

The sum of internal energy and product of volume and pressure of a thermodynamic system is termed enthalpy. The symbol used for the representation of the quantity is “H.” It is characterized as having energy-like properties or state-function having energy dimensions. As possessing the property of state function, the value depends on the system’s initial and final state. The quantity is measured in J/mol or British thermal unit (BTU).

 The equation of enthalpy is H= E + PV. As far as enthalpy is concerned, often the confusion arises whether it is positive or negative. Hence, it is explained as breaking a bond in a chemical reaction requires a positive change in enthalpy. In contrast, the formation of bonds requires a negative change in enthalpy.

There are different types of enthalpy, including heat of formation, combustion, neutralization, solution, transition, latent heat of vaporization, and molar heat of sublimation. The enthalpy change and its measurement are discussed in detail along with the mathematical equation above. The blog intends to give you an insight into the concept of Enthalpy. The mathematical derivation, advantages, application, etc., are listed and explained in-depth.

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