3.4 Bond Energy (Bond Enthalpy)

This topic explains bond energy — what it is, what factors affect it, and how it relates to bond length and reactivity.

Definition

Bond energy is the energy required to break one mole of bonds in a gaseous substance.

Factors Affecting Bond Energy

Examples

Bond Energy and Reactivity

• Higher bond energy → less reactive.
• Lower bond energy → more reactive.
• Example: Alkyl fluorides are less reactive than alkyl chlorides.

Main Idea

• Electronegativity difference between bonded atoms
• Size of atom
• Bond order
• Bond length

Core Concept

When a bond is formed, energy is released. When a bond is broken, energy is absorbed. The bond energy is the energy required to break one mole of bonds in a gaseous substance. Greater the bond energy, stronger the bond, more stable the molecule, less reactive it is.

Detailed Explanation

What is Bond Energy?

• Energy which is required to break the bond of the same type in one mole of a substance is known as bond energy.
• Its unit is kJ mol⁻¹.

Factors Affecting Bond Energy

Bond energy depends upon the following factors:

• Electronegativity difference between two bonded atoms
• Size of atom
• Bond order
• Bond length

1. Electronegativity Difference

- The greater the electronegativity difference between two bonded atoms, the more polar will be the bond and the greater will be the bond energy.

Example: Hydrogen Halides (HX)

Conclusion: Greater the electronegativity difference, higher the value of bond energy. For example, HF has the greatest value of bond energy.

2. Size of Atom

• Smaller the size of atoms, the greater the bond energy due to shorter bond length.
• Larger the size of atoms, the lower the bond energy due to longer bond length.

• Bond energy of H−H is 436 kJ mol⁻¹.
• Bond energy of H−Br is 366 kJ mol⁻¹.
• This is due to the shorter bond length of hydrogen molecule compared to HBr (size of Br is greater than hydrogen).

3. Bond Length

• Bond length is the distance between the nuclei of two covalently bonded atoms.
• It is measured in Å (angstrom) or picometer (pm).
• 1 pm = 10^-12 m
• Bond length depends upon the size of the atom.
• Smaller the size of atoms, greater the bond energy due to shorter bond length.
• Larger the size of atoms, lower the bond energy due to longer bond length.

Example: Halogens

• The atomic radius increases from fluorine to iodine.
• Bond energy decreases from chlorine to iodine.
• Exception: Fluorine molecule has lower bond energy than chlorine.
• Reason: Due to the smaller size of fluorine atoms, their lone pairs repel each other, hence less amount of energy is required to break the bond between fluorine atoms.

4. Bond Order

• Higher the bond order, greater the bond energy.
• Bond order: Single bond (1) < Double bond (2) < Triple bond (3)

• C−C (single bond): 347 kJ mol⁻¹
• C=C (double bond): 611 kJ mol⁻¹
• C≡C (triple bond): 837 kJ mol⁻¹

Bond Energy and Reactivity

• Greater the bond energy, stronger the bond, the molecule is stable and less reactive.
• Lower the bond energy, weaker the bond, the molecule is less stable and more reactive.

• In alkyl halides, alkyl fluorides are less reactive than alkyl chlorides.
• Reason: Bond energy of C−F is greater than C−Cl.

Trend of Bond Length and Bond Energy of Halogens

• Bond length: Increases from F₂ to I₂.
• Bond energy: Decreases from Cl₂ to I₂ (F₂ is an exception).

Important Points

• 1. Bond energy = energy required to break one mole of bonds.
• 2. Unit: kJ mol⁻¹.
• 3. Factors: Electronegativity difference, size, bond order, bond length.
• 4. Greater electronegativity difference → higher bond energy.
• 5. Smaller atoms → shorter bond → higher bond energy.
• 6. Higher bond order → higher bond energy.
• 7. Shorter bond length → higher bond energy.
• 8. F−F has lower bond energy than Cl−Cl due to lone pair repulsion.
• 9. Higher bond energy → less reactive.

Remember

• - Bond energy: Energy to break a bond
• - Unit: kJ mol⁻¹
• - Factors: EN difference, size, bond order, bond length
• - H−F > H−Cl > H−Br > H−I
• - Cl−Cl > Br−Br > I−I (F−F is exception)
• - Higher bond energy → less reactive

Common Mistakes

• 1. Thinking F−F has the highest bond energy among halogens: It is an exception due to lone pair repulsion.
• 2. Confusing bond energy with bond dissociation energy: Bond energy is an average; bond dissociation energy is for a specific bond.
• 3. Forgetting the role of bond length: Shorter bond → higher bond energy.
• 4. Ignoring lone pair repulsion: It can weaken bonds (e.g., F−F).

Quick Revision

• Bond energy = energy required to break one mole of bonds.
• Unit: kJ mol⁻¹.
• Factors: Electronegativity difference, size, bond order, bond length.
• Greater electronegativity difference → higher bond energy.
• Smaller atoms → shorter bond → higher bond energy.
• Higher bond order → higher bond energy.
• Shorter bond length → higher bond energy.
• F−F has lower bond energy than Cl−Cl due to lone pair repulsion.
• Higher bond energy → less reactive.

60-Second Revision

• Bond energy: Energy to break a bond
• Unit: kJ mol⁻¹
• Factors: EN difference, size, bond order, bond length
• H−F > H−Cl > H−Br > H−I
• Cl−Cl > Br−Br > I−I (F−F is exception)
• Higher bond energy → less reactive

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