Enthalpy of: Bond Dissociation, Combustion, Formation, Atomisation, Sublimation, Phase Transition, Ionisation, Solution, and Dilution

What is Enthalpy?

In a thermodynamic system, enthalpy is the measurement of energy. It is a thermodynamic property of a system. 

The product of volume and pressure, plus the system’s internal energy, determines the quantity of enthalpy; it is equal to the total heat content of a system. The SI unit of enthalpy is kJ mol-1. 

H = U + PV

Where H is enthalpy, U is the internal energy of a system, and PV is the product of the pressure and volume of the system. 

The change in enthalpy between initial and final states. 

△H = △U + △PV 

When a system is at constant pressure, the change in enthalpy is equal to the heat flow (q) for the process. In equation form, we can define the above statement as: 

△H = qp

H is positive if the heat flow is positive during an endothermic reaction. Similarly, during an exothermic reaction, the △H is negative when the heat flow is negative.

Use of Enthalpy

The concept of enthalpy is important to determine the temperature and pressure required during a chemical reaction. It helps determine the total amount of heating and cooling a substance may require during commercial production. 

Types of Enthalpy

There are several different types of enthalpy of reaction, which are enthalpy of 

• • Bond Dissociation 
  • Combustion
  • Formation
  • Atomisation

• Sublimation

• • Phase Transition 

• Ionisation

• Solution And Dilution 

This article will discuss these different types in detail.

Enthalpy of Bond Dissociation 

Enthalpy of bond dissociation refers to the heat absorbed or heat evolved during the complete dissociation of one mole of a substance into its ions. It is nothing but the energy required to break the bond of one mole to give separated atoms. 

The concept of enthalpy of bond dissociation is useful to determine the strength of a chemical bond. The strongest enthalpy of bond dissociation is between silicone and fluorine. 

Enthalpy of Sublimation 

Sublimation refers to the chemical process in which a solid transforms into a gas without passing through the liquid phase. 

At a constant temperature, the energy required for converting one mole of a solid into vapour is called the enthalpy of sublimation. The symbol for enthalpy of sublimation is:

 △Hsub

Enthalpy of Ionisation 

You can measure the tendency of an element to lose its electrons by its ionisation enthalpy. It refers to the removal of an electron, in its ground state, from an isolated gaseous atom. The SI unit of ionisation enthalpy is kJ mol-1. 

Enthalpy of ionisation is always positive because energy is required to remove electrons from an atom. It is harder to remove an electron from a positively charged ion than a neutral atom; therefore, the second ionisation enthalpy will be higher than the first ionisation enthalpy. 

Enthalpy of Atomisation 

The change in enthalpy accompanying the total separation of all atoms in a chemical compound is known as the enthalpy of atomisation. 

Enthalpy of atomisation can be defined as the energy or heat absorbed or evolved to break 1 mole of a bond into atoms. You can symbolise it as:

△Ha

Enthalpy of Solution and Dilution 

Enthalpy of solution (△Hsol) refers to the quantity of heat evolved or absorbed when one mole of a substance is dissolved in a solvent so that no further dilution of solution produces any heat. Enthalpy of solution at infinite dilution refers to a scenario where the addition of a solvent doesn’t produce any more change in heat. 

Enthalpy of dilution (△Hdil) refers to the heat evolved or absorbed to dilute one mole of a solution from one concentration to another. 

Enthalpy of Phase Transition 

The transformation of a substance/element from one state to another under specific circumstances is known as a phase transition. 

There are three major types of enthalpy changes during phase transitions. These are enthalpy of fusion, enthalpy of vaporisation, and enthalpy of sublimation. 

Enthalpy of fusion refers to the transformation of 1 mole of a solid substance into a liquid state at its melting point. Enthalpy of vaporisation refers to the amount of heat absorbed or evolved to convert 1 mole of a liquid into its vapour state at its boiling point. 

Enthalpy of Combustion 

The heat change or absorbed when 1 mole of a substance is completely oxidised and burnt in oxygen is known as the enthalpy of combustion (△Hc). The concept of enthalpy of combustion is useful while elucidating the structure of various organic compounds. 

Standard enthalpy of combustion refers to the amount of heat absorbed when 1 mole of a substance is completely burnt under normal conditions, i.e. under 298 K and 1 bar pressure. 

Enthalpy of Formation 

Under given conditions and pressures, 1 mole of a substance is formed due to the heat evolved, absorbed, or changed. Enthalpy of formation is defined as △Hf. 

Standard heat of formation refers to the value of heat formation at 298 K and 1 atmospheric pressure. When elements are in a free state, the standard heat of formation is zero. 

Conclusion 

With this, we conclude our article on the enthalpy of different chemical reactions. Enthalpy refers to the measurement of energy determined by the system’s internal energy, pressure, and volume of the product. 

We also looked at the enthalpy of elements under different circumstances like combustion, solution, dilution, ionisation, and atomisation.