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πŸ§ͺ Bond Energy Calculations

Spec 5.5.1.3 πŸ“™ Higher
πŸ“– In-Depth Theory

Bond Breaking and Bond Making

ALL chemical reactions involve two processes:
BOND BREAKING (in reactants):
Requires energy INPUT β€” ENDOTHERMIC.
Breaking covalent bonds requires energy to pull atoms apart against their attraction.
BOND MAKING (in products):
Releases energy β€” EXOTHERMIC.
Forming new covalent bonds releases energy as atoms are attracted to each other.
OVERALL ENERGY CHANGE:
Ξ”H = energy required to break bonds βˆ’ energy released from forming bonds
Ξ”H = (energy in) βˆ’ (energy out)
EXOTHERMIC overall: more energy released (bond making) than required (bond breaking) β†’ Ξ”H is NEGATIVE.
ENDOTHERMIC overall: more energy required (bond breaking) than released (bond making) β†’ Ξ”H is POSITIVE.
A useful analogy: breaking bonds is like paying rent (costs energy); making bonds is like earning salary (releases energy). If you earn more than you pay, you have a net gain (exothermic).

Using Bond Energies

BOND ENERGIES (also called bond dissociation enthalpies) are the energy needed to break ONE MOLE of a specific bond in the gas phase, in kJ/mol.
Common bond energies (approximate):
H-H: 436 kJ/mol
H-Cl: 432 kJ/mol
Cl-Cl: 243 kJ/mol
C-H: 413 kJ/mol
C-C: 347 kJ/mol
C=C: 614 kJ/mol
C-O: 358 kJ/mol
C=O: 805 kJ/mol
O-H: 464 kJ/mol
O=O: 498 kJ/mol
N≑N: 945 kJ/mol
N-H: 391 kJ/mol
METHOD:
1. Draw the displayed formulae for ALL reactants and products.
2. LIST all bonds broken (in reactants) and bonds formed (in products).
3. Calculate total energy in (breaking) and total energy out (forming).
4. Ξ”H = energy in βˆ’ energy out.

Worked Example β€” Hydrogen Burning

EXAMPLE: Hβ‚‚ + Clβ‚‚ β†’ 2HCl
BONDS BROKEN (reactants):
1 Γ— H-H bond: 1 Γ— 436 = 436 kJ
1 Γ— Cl-Cl bond: 1 Γ— 243 = 243 kJ
Total energy IN = 436 + 243 = 679 kJ
BONDS FORMED (products):
2 Γ— H-Cl bonds: 2 Γ— 432 = 864 kJ
Total energy OUT = 864 kJ
Ξ”H = 679 βˆ’ 864 = βˆ’185 kJ/mol
Negative β†’ EXOTHERMIC βœ“
EXAMPLE 2: CHβ‚„ + 2Oβ‚‚ β†’ COβ‚‚ + 2Hβ‚‚O
BONDS BROKEN: 4Γ—C-H (413Γ—4=1652) + 2Γ—O=O (498Γ—2=996) = 2648 kJ
BONDS FORMED: 2Γ—C=O (805Γ—2=1610) + 4Γ—O-H (464Γ—4=1856) = 3466 kJ
Ξ”H = 2648 βˆ’ 3466 = βˆ’818 kJ/mol (exothermic β€” combustion of methane)
Note: bond energy values are AVERAGE values β€” actual values vary by molecular environment, so calculated Ξ”H values are approximate.
⚠️ Common Mistake

Ξ”H = energy IN (breaking) MINUS energy OUT (forming). Not the other way round. If you subtract the wrong way, you'll get the wrong sign. Remember: breaking bonds costs energy (positive contribution to Ξ”H); forming bonds releases energy (negative contribution). Ξ”H negative = exothermic.

πŸ“ Variables
Ξ”HEnthalpy change (Ξ”H) is measured in kJ/mol (kJ/mol)
πŸ“ Key Equations
Ξ”H = energy in (bonds broken) βˆ’ energy out (bonds formed)
Exothermic: energy out > energy in β†’ Ξ”H negative
Endothermic: energy in > energy out β†’ Ξ”H positive
πŸ“Œ Key Note

Bond breaking: endothermic (energy in). Bond forming: exothermic (energy out). Ξ”H = energy in βˆ’ energy out. Negative Ξ”H = exothermic. Positive Ξ”H = endothermic. Use displayed formulae to count every bond broken and formed. Bond energies are averages β€” calculated values are approximate.

🎯 Matching Activity β€” Bond Energy Concepts

Match each concept to its correct description. β€” drag the symbols on the right to match the component names on the left.

Bond breaking
Drop here
Bond forming
Drop here
Ξ”H negative
Drop here
Ξ”H positive
Drop here
Bond energy
Drop here
Always releases energy β€” exothermic process
Always requires energy input β€” endothermic process
Endothermic β€” more energy absorbed in bond breaking than released in forming
Energy needed to break 1 mole of a specific bond β€” in kJ/mol
Exothermic β€” more energy released from bond forming than absorbed in breaking
⚽ FIFA Worked Examples
Bond Energy Calculation

Calculate Ξ”H for Hβ‚‚ + Clβ‚‚ β†’ 2HCl. Bond energies: H-H=436, Cl-Cl=243, H-Cl=432 kJ/mol.

F

Ξ”H = energy in (bonds broken) βˆ’ energy out (bonds formed)

I

Broken: 1Γ—H-H(436) + 1Γ—Cl-Cl(243) = 679 kJ. Formed: 2Γ—H-Cl(432) = 864 kJ

F

Ξ”H = 679 βˆ’ 864 = βˆ’185

A

Ξ”H = βˆ’185 kJ/mol (exothermic)

🎯 Test Yourself
Question 1 of 2
1. A reaction has bond breaking energy of 800 kJ/mol and bond forming energy of 1050 kJ/mol. What is Ξ”H and is the reaction exothermic or endothermic?
2. Why are bond energy calculations only approximate?
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