Bond Energies - Chemistry

1. Fundamental Concepts

Bond energy (bond enthalpy) is the average energy required to break 1 mole of a covalent bond in gaseous state into individual gaseous atoms, measured in kJ/mol. It represents the strength of a covalent bond: the higher the bond energy, the stronger and more stable the bond. All bond energy values are positive (endothermic process for bond breaking; bond formation is exothermic, negative energy value).

2. Key Concepts

Average value: Bond energies are average values (e.g., C-H bond energy is the average for C-H bonds in all organic molecules, not a single fixed value).
Bond strength factors: Shorter bond length = higher bond energy; more bond order (single < double < triple) = higher bond energy (e.g., C≡C > C=C > C-C).
Enthalpy change (ΔH) of reaction: Calculated by total energy to break reactant bonds - total energy released from forming product bonds (ΔH = ΣBE(reactants) - ΣBE(products)).
State requirement: Bond energy data only applies to gaseous reactants and products (adjustments needed for other states).

3. Examples

Easy

Calculate the bond energy of H-Cl if breaking 1 mol of H-Cl bonds absorbs 431 kJ of energy, and forming 1 mol of H-Cl bonds releases 431 kJ of energy.

Answer: 431 kJ/mol (bond energy is the magnitude of the energy for bond breaking/formation).

Medium

Use bond energies to estimate ΔH for the reaction: H₂(g) + Cl₂(g) → 2HCl(g)

(Bond energies: H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, H-Cl = 431 kJ/mol)

Calculation: ΔH = (436 + 243) - 2×431 = 679 - 862 = -183 kJ/mol (exothermic).

Hard

Estimate ΔH for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

(Bond energies: C-H = 413, O=O = 498, C=O = 799, O-H = 464 kJ/mol)

Calculation:

ΣBE(reactants) = 4 × 413 + 2 × 498 = 1,652 + 996 = 2,648 kJ/mol

ΣBE(products) = 2 × 799 + 4 × 464 = 1,598 + 1,856 = 3,454 kJ/mol

ΔH = 2,648 - 3,454 = -806 kJ/mol (exothermic).

4. Problem-Solving Techniques

Step 1: Write Lewis structures for all reactants and products to identify all covalent bonds (count bond types and quantities accurately).
Step 2: List bond energies for all involved bonds (use standard US chemistry reference values).
Step 3: Calculate total bond breaking energy (sum of reactants’ bond energies, always positive).
Step 4: Calculate total bond formation energy (sum of products’ bond energies, magnitude of exothermic release).
Step 5: Compute ΔH (ΔH = ΣBE(reactants) - ΣBE(products)); interpret the sign: negative = exothermic, positive = endothermic.
Step 6: Check state consistency (only use gaseous species; if reactants/products are liquid/solid, add phase change enthalpy if required).
Key check: Verify bond counting (e.g., CO₂ has two C=O bonds, H₂O has two O-H bonds per molecule).