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Bond Enthalpy & Bond Energy

bond enthalpy bond energy

Core Concepts

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

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Vocabulary

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 or energy required to break a molecule

Bond enthalpy – same as bond energy, or bond dissociation energy

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

So what is bond energy and bond enthalpy? 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. Another term for bond energy is bond enthalpy, or bond dissociation energy. 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. You can find a nice table of bond energies here.

Bond energy is the amount of energy required to break one mole of bonds of a particular type between two atoms in gaseous state. If you are talking about separating solid crystalline compounds into gaseous ions, you will want to look at the lattice energy.

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

The definition of 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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