Key Difference – Bond Energy vs Bond Enthalpy
Both bond energy and bond enthalpy describe the same chemical concept; the amount of energy required to break apart a mole of molecules into its component atoms. This measures the strength of a chemical bond. Therefore it is also called bond strength. The bond energy is calculated as an average value of bond dissociation energies at 298 K for chemical species in the gaseous phase. There is no considerable difference between the terms bond energy and bond enthalpy, but bond energy is denoted by “E” whereas bond enthalpy is denoted by “H”.
CONTENTS
1. Overview and Key Difference
2. What is Bond Energy
3. What is Bond Enthalpy
4. Side by Side Comparison – Bond Energy vs Bond Enthalpy in Tabular Form
5. Summary
What is Bond Energy?
Bond energy or bond enthalpy is a measure of bond strength. Bond energy is the amount of energy required to break apart a mole of molecules into its component atoms. This means bond energy is the energy required to break a chemical bond. Bond energy is denoted as “E”. The unit of measurement is kJ/mol.
Chemical bonds are formed between atoms in order to obtain a stable state when the individual atoms have high energy which is unstable. This means the chemical bond formation decreases the energy of a system. Therefore, some of the energy is released (usually as heat) when forming chemical bonds. Hence, the bond formation is an exothermic reaction. In order to break this chemical bond, energy should be provided (an equal amount of energy to that of energy released while bond forming). This amount of energy is known as the bond energy or bond enthalpy.
Figure 1: Energy diagram for bond formation (left) and bond dissociation (right).
The bond energy is equal to the difference between the enthalpy of products (atoms) and reactants (starting molecule). Each molecule should have its own bond energy values. But there are exceptions. For example, the bond energy of C-H bond depends on the molecule where the bond occurs. Therefore, the bond energy is calculated as an average value of bond dissociation energies.
The bond energy is the average bond dissociation energies for the same species in gaseous phase (at 298 K temperature). For example, the bond energy of methane molecule (CH4) is the amount of energy required to form a carbon atom and 4 hydrogen radicals. Then the bond energy of C-H bond can be calculated by taking the sum of bond dissociation energies of each C-H bonds and dividing the total value by 4.
Ex: Bond energy of O-H bond in H2O molecule can be calculated as follows.
Amount of energy required to break the H-OH bond = 498.7 kJ/mol
The amount of energy required to break the O-H bond (in the remaining OH radical) = 428 kJ/mol
The average bond dissociation energy = (498.7 + 428) / 2
= 463.35 kJ/mol ≈ 464 kJ/mol
Hence, the bond energy of O-H in H2O molecule is considered as 464 kJ/mol.
What is Bond Enthalpy?
Bond enthalpy or bond energy is the amount of energy required to separate a molecule into its atomic components. It is a measure of the bond strength. The bond enthalpy is denoted as “H”.
What is the Difference Between Bond Energy and Bond Enthalpy?
- Bond energy or bond enthalpy is the amount of energy required to break apart a mole of molecules into its component atoms.
- Bond energy is denoted as “E” while bond enthalpy is denoted as “H”.
Summary – Bond Energy vs Bond Enthalpy
The bond energy or bond enthalpy is the amount of energy required to separate a mole of molecules into its atomic components in gaseous phase. It is calculated using the bond dissociation energy values of chemical bonds. Therefore the bond energy is the average value of bond dissociation energies. It is always a positive value because the bond dissociation is endothermic (bond formation is exothermic). There is no considerable difference between bond energy and bond enthalpy.
Reference:
1. “Bond Energies.” Chemistry LibreTexts, Libretexts, 20 Jan. 2017, Available here.
2. Helmenstine, Anne Marie. “What Is Bond Energy?” ThoughtCo, Available here.
3. “Bond energy.” Wikipedia, Wikimedia Foundation, 13 Mar. 2018, Available here.