Bond Enthalpy

Bond enthalpy describes the amount of energy stored in a bond between atoms in a molecule (also known as bond-dissociation enthalpy, or bond strength). Anionic or homolytic bonds can be cleaved with the energy required through homolytic or symmetrical cleavage in the gas phase. When a bond is broken through homolytic or symmetrical cleavage, every original participant gets an electron and becomes a radical rather than an ion.

What is Bond Enthalpy?

Bond dissociation energy, or bond enthalpy, is the amount of energy necessary to break one mole of a specific type of bond and split them into gaseous atoms.

The greater the bond enthalpy, the stronger the bond.

Bond enthalpy is expressed in kJmol-¹.

Types of Bond Enthalpies

Enthalpy of Atomisation 

Each spontaneous process tends to result in the production of novel products. Some processes, out of all the ones we know about, tend to absorb energy, while others result in energy evolution. As a result, we usually experience enthalpy or heat change when we complete these activities. When bonds of a compound tend to break and elements of that complex reduce to singular atoms, the enthalpy of atomisation refers to the amount of change in heat.

The heat of atomisation is always a positive number and can never be negative. The symbol for the enthalpy of atomisation is ΔaH. 

The post discusses the enthalpy of atomisation, what different enthalpy changes are, and how to calculate the enthalpy change of atomisation. Let us start with a discussion of the various changes in atomisation heat. The amount of energy required when bonds dissociate and the constituent atoms reduce to individual atoms is known as enthalpy of atomisation. The transition of enthalpy occurs when one mole of a substance is entirely dissociated through its atoms under normal conditions (298.15 K, 1 bar).

Cl2 (g) → 2Cl (g)           ………………ΔaH

Enthalpy of Sublimation

Converting a solid to a gas without it passing through the liquid phase is sublimation. We need a specific quantity of energy to sublimate a substance, which must be supplied to the substance in the form of heat or work. The enthalpy of sublimation is the energy required to convert a mole of solid into a gas at constant pressure. 

C (s) → C (g)           ………………ΔsubH

Enthalpy Change During Phase Transition: 

When a substance’s phase changes from one form to another, some energy is released or absorbed. For example, when ice melts into water, energy is required to melt it.

Standard Enthalpy of Vaporisation:

At constant temperature and pressure, the enthalpy of vaporisation is the amount of heat it takes to vaporise one mole of a liquid.

H2O (l)  →    H2O (g) ………………..ΔvapH0

Standard Sublimation Enthalpy:

The drop in enthalpy that happens when one mole of solid substance sublimes at constant pressure and temperature is known as the standard enthalpy of sublimation (at 1 bar).

C (s) → C (g)           ………………ΔsubH0

Factors Affecting Bond Enthalpy

Certain factors play a significant role and affect Bond Enthalpy:

1. Bond enthalpy is affected by atoms, such as the more significant the atom’s size, the greater is the bond length. 

Similarly, the lesser the bond enthalpy, the lesser the bond strength.

1. Bond enthalpy is also affected by the presence of multiple bonds; the greater the number of bonds, the greater the bond enthalpy.
2. The number of lone pairs of electrons present on the bonded atoms also affects the enthalpy. The repulsion is great between the atoms, and the bond dissociation energy is less. This is also one of the factors that affect bond enthalpy.
3. When atomic bonds overlap each other, a bond is formed. The direction of overlap determines the direction of bonding.
4. The bond angle is formed by the lines representing the bond’s direction and affects bond enthalpy, i.e. the orbitals holding the bonding electrons.
5. Order of bond affects bond enthalpy as a three-electron bond is comparable to a half-covalent bond in an odd-electron molecule; the bond order can be fractional.
6. The higher the order of bond, the more stable the bond is, i.e. the higher the bond enthalpy. The higher the bond order, the shorter the bond length.
7. Bond energy decreases in the following order: sp>sp²>sp³.

Conclusion

Bond enthalpy and reaction enthalpy help us understand how energy is used in a chemical system during reactions. Bond enthalpy describes the amount of energy required to break or form a bond and serves as a measure of bond strength. We can determine whether a reaction will be endothermic or exothermic based on whether the enthalpy of the reaction is positive or negative. The bond enthalpy measures bond strength and describes how much energy is required to break or build a bond.