7.5 Strengths of Ionic and Covalent Bonds

Learning Objectives

• Describe the energetics of covalent and ionic bond formation and breakage.
• Use the Born-Haber cycle to compute lattice energies for ionic compounds.
• Use average covalent bond energies to estimate enthalpies of reaction.

Introduction

• Bond strength indicates how strongly atoms are bonded and the energy needed to break the bond.
• This section explores the bond strength in covalent and ionic bonds.

Covalent Bond Strength

• Covalent bonds hold atoms together in stable molecules.
• Bond strength is measured by bond dissociation energy, the energy needed to break a bond.
• DXY is defined as the standard enthalpy change for the endothermic reaction of breaking a bond.
  • Example: Bond energy of HH is 436 kJ/mol.
• For molecules with multiple bonds, the sum of all bond energies equals the enthalpy change for breaking all bonds.
  • Example: CH4 has a total bond energy of 1660 kJ.
• Bond strength increases with the number of electron pairs; bond length decreases as bond strength increases.
  • Triple bonds > double bonds > single bonds.
  • CF bond (439 kJ/mol) is stronger than CCl (330 kJ/mol).

Calculating Enthalpy Changes

• Bond energies help estimate enthalpy changes in reactions.
• Exothermic reactions have stronger product bonds than reactant bonds.
• Enthalpy change (ΔH) = Σ(bonds broken) - Σ(bonds formed).
  • Example: The formation of HCl from H2 and Cl2 releases 185 kJ.

Ionic Bond Strength and Lattice Energy

• Ionic compounds are stable due to electrostatic attraction.
• Lattice energy is the energy required to separate one mole of solid into gaseous ions.
  • Example: NaCl has a lattice energy of 769 kJ/mol.
• Lattice energy increases with higher ion charges and smaller ionic size.
  • Example: LiF has lower lattice energy (1023 kJ/mol) than MgO (3900 kJ/mol).

The Born-Haber Cycle

• Used to calculate lattice energy indirectly using Hess's law.
• Steps involve ionization energy, electron affinity, enthalpy changes, etc.
• Example: The cycle for CsF calculates its lattice energy as 756.9 kJ/mol.

Comparison of Lattice and Bond Energies

• Lattice energies are typically higher than covalent bond dissociation energies.
• Ionic lattice energies range 600-4000 kJ/mol, while covalent bond energies range 150-400 kJ/mol.

Summary

• Bond strength is crucial in chemical reactions and compounds' stability.
• Covalent and ionic bonds have different characteristics and energy profiles.
• Understanding these concepts is key for predicting reaction energetics.