Bond Energy

Core Concepts

In this tutorial, you will learn about bond energies as well as its relationship to bond order. You will also learn about formation and breakdown of a bond, along with exothermic and endothermic reactions.

Topics Covered in Other Articles


Enthalpy – a measure of heat in a chemical system; measured under conditions of constant pressure

Bond energy – a measure of bond strength; determined by measuring the enthalpy required to break a molecule

Exothermic reaction – a chemical reaction that releases heat (energy) to its surroundings

Endothermic reaction – a chemical reaction that absorbs heat (energy) from its surroundings

Introduction to Bond Energy

Bond energy (E) measures the strength of a chemical bond. Essentially, it is the amount of energy required to break down a molecule into its atoms. When a chemical reaction occurs, molecular bonds are broken and new bonds are formed to make different molecules. Breaking a bond requires energy and forming a bond releases energy.

Bond Order and Bond Energy

Bond order is the number of chemical bonds present between atoms in a molecule. The more bonds there are between a pair of atoms, the shorter the bond length, and the stronger the bonds. Therefore, triple bonds are shorter than double bonds, and double bonds are shorter than single bonds. Hence, triple bonds are stronger than double bonds, and double bonds are stronger than single bonds.

Consequently, the higher the bond energy, the stronger the bond between a pair of atoms, and the shorter the bond length.

Bond Formation and Breakdown

When a chemical reaction occurs, the atoms in the reactants rearrange to form products. The newly arranged bonds do not have the same energy as the bonds in the reactants. Therefore, when a chemical reaction occurs, there will always be a change in energy.


Enthalpy (H) is a measure of the total heat energy in a thermodynamic system. When the change in enthalpy (ΔH) is positive, the reaction is endothermic; whereas when ΔH is negative, the reaction is exothermic.

Generally, a positive change in enthalpy is required to break a bond, while a negative change in enthalpy is accompanied to form a bond. Therefore, formation of a bond is an endothermic process and breakdown of a bond is an exothermic process.

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