Bond Breaking and Bond Forming
Key idea: Explain required bond breaking and making, with numerical bond-energy calculations clearly marked as out-of-syllabus enrichment.
Before you start: Represent exothermic reactions using energy profile diagramsRepresent endothermic reactions using energy profile diagrams
By the end, you can
- Explain overall enthalpy changes using bond-breaking and bond-making energies
The required idea is the energy balance between bonds broken and bonds formed: breaking bonds absorbs energy, while forming bonds releases energy. The numerical Δ H = Ebᵣₑₐₖ - Efₒᵣₘ method later on this page is out-of-syllabus enrichment.
The qualitative explanation—breaking bonds takes in energy, making bonds releases energy, and their balance determines the overall enthalpy change—is required. The numerical bond-energy method and calculation examples are explicitly out of syllabus for K324 / 6092.
1. Definition
A. Breaking vs Forming Bonds
Breaking bonds is endothermic (energy is absorbed). Forming bonds is exothermic (energy is released).
B. Bond Energy
Bond energy is the energy required to break one mole of a specified covalent bond in gaseous molecules (units: kJ mol⁻¹).
2. Key Ideas
- Energy changes in reactions come from bond breaking and bond forming.
- Breaking bonds absorbs energy: endothermic.
- Forming bonds releases energy: exothermic.
- A reaction is exothermic when bond making releases more energy than bond breaking absorbs.
- A reaction is endothermic when bond breaking absorbs more energy than bond making releases.
- Out-of-syllabus enrichment: quantify that comparison with Δ H = Ebᵣₑₐₖ - Efₒᵣₘ. Bond energies are supplied; do not memorise random tables.
Compare both energy transfers: energy is absorbed to break reactant bonds and released when product bonds form. State which transfer is greater to explain the overall reaction.
3. Detailed Explanations
- Breaking bonds absorbs energy (endothermic step); forming bonds releases energy (exothermic step).
- If bond making releases more energy than bond breaking absorbs, the overall reaction is exothermic.
- If bond breaking absorbs more energy than bond making releases, the overall reaction is endothermic.
- Out-of-syllabus calculations use Δ Hᵣₑₐcₜᵢₒₙ = Ebᵣₑₐₖ - Efₒᵣₘ after balancing the equation and counting bonds.
A. What “Bond Energy” Really Means
If a bond energy is 436kJ mol⁻¹, that means:
- 436 kJ of energy is absorbed to break 1 mol of that bond.
- Forming the same bond releases the same amount of energy (but in the opposite direction).
Recall covalent bonding basics in Covalent Bonds. If you cannot count bonds correctly, you cannot do these questions.
B. Keep the Representations Separate
| Level | What to say |
|---|---|
| Macroscopic | The surroundings warm for an exothermic reaction or cool for an endothermic reaction, if heat exchange is controlled. |
| Particle/bond model | Energy is absorbed to break reactant bonds and released when product bonds form. |
| Symbolic calculation | Δ H = sum E(bonds broken) - sum E(bonds formed). |
Do not describe bond breaking itself as releasing energy because “atoms become free”. Breaking an attraction always requires energy; the overall reaction can still be exothermic when forming product bonds releases more energy.
C. Out-of-Syllabus Enrichment: Calculating Δ H Using Bond Energies
- Write a balanced equation.
- Count bonds in reactants (bonds broken).
- Count bonds in products (bonds formed).
- Calculate:
- State the reaction type:
- Δ H < 0 → exothermic
- Δ H > 0 → endothermic
Bond energies are average values (they vary slightly between different molecules). Your calculated Δ H is usually an estimate.
4. Common Mistakes
- Saying that breaking bonds releases energy because separated atoms are “free”. Breaking an attraction requires energy.
- Mentioning only bond breaking or only bond making. The overall energy change depends on the balance of both transfers.
- Calling every bond-breaking step endothermic without distinguishing it from the overall reaction.
For the optional numerical method:
- Subtracting the wrong way round (writing Efₒᵣₘ - Ebᵣₑₐₖ).
- Not balancing the equation first, then counting the wrong number of bonds.
- Counting “atoms” instead of “bonds” (e.g., H₂ has one H-H bond).
- Forgetting to multiply by the number of bonds (e.g., 2 × H-Cl).
- Missing units: bond energies and the final molar enthalpy change are in kJ mol⁻¹. The value refers to one mole of reaction as written.
5. Exam Tips
Write both sides of the comparison: “Energy is absorbed to break bonds in the reactants, while energy is released when bonds form in the products. More energy is released than absorbed, so the reaction is exothermic.” Reverse the final comparison for an endothermic reaction.
Out-of-syllabus numerical method
- Ebᵣₑₐₖ (reactants) 2) Efₒᵣₘ (products) then Δ H = Ebᵣₑₐₖ - Efₒᵣₘ. Finish by writing “Δ H is negative/positive so the reaction is exothermic/endothermic”.
- If the question says “using the bond energies below”, use those values even if you “remember” a different table.
- Always show at least one line that links the sign of Δ H to exothermic/endothermic (that is often a mark).
- Count bonds from the displayed formula or structure rather than guessing from the molecular formula.
6. Worked Examples
Example 1: Calculate Δ H (Formation of Hydrogen Fluoride)Optional
Use bond energies (in kJ mol⁻¹): H-H = 436, F-F = 158, H-F = 568.
Calculate Δ H for:
Show Answer
Bonds broken (reactants): 1 × H-H and 1 × F-F
Bonds formed (products): 2 × H-F
Δ H is negative, so the reaction is exothermic.
Example 2: Calculate Δ H (Decomposition of Ammonia)Optional
Use bond energies (in kJ mol⁻¹): N-H = 391, N#N = 945, H-H = 436.
Calculate Δ H for:
Show Answer
Bonds broken (reactants): 6 × N-H
Bonds formed (products): 1 × N#N and 3 × H-H
Δ H is positive, so the reaction is endothermic.
Example 3: Decide Exothermic or Endothermic (Formation of Hydrogen Chloride)Optional
Use bond energies (in kJ mol⁻¹): H-H = 436, Cl-Cl = 243, H-Cl = 432.
Determine whether this reaction is exothermic or endothermic:
Show Answer
Bonds broken: 1 × H-H and 1 × Cl-Cl
Bonds formed: 2 × H-Cl
Δ H is negative, so the reaction is exothermic.
Example 4: Error Analysis (Fix the Method)Optional
A student writes: “Δ H = Efₒᵣₘ - Ebᵣₑₐₖ”. Identify the mistake and write the correct method.
Show Answer
They reversed the subtraction. The mark-scheme method is:
You must also finish with the reaction type: negative = exothermic, positive = endothermic.
7. Mind Stretchers
Mind stretcher 1: “Why is my answer different from the data book?”Extension
You calculate Δ H using average bond energies and get a value that is different from the standard enthalpy change in a data book. Give one valid reason.
Show Answer
Bond energies are average values that depend on the molecule. Standard enthalpy changes are measured under standard conditions for specific substances, so the bond-energy method gives an estimate.
Mind stretcher 2: Missing State Symbols TrapExtension
A student writes H₂ + Cl₂ → 2HCl with no state symbols and no “as written” wording. What should they do to reduce mark loss?
Show Answer
Write the equation clearly and consistently with state symbols if provided/expected (e.g., H₂(g) + Cl₂(g) → 2HCl(g)). State that Δ H is for the reaction as written (per 1 mol of reaction).
8. Quiz
K324 and 6092 core practice check the required qualitative bond-breaking and bond-making explanation. The numerical questions on the lesson quiz are out-of-syllabus enrichment and do not contribute to course mastery.
G3 Pure / O-Level Core Practice Out-of-Syllabus Bond-Energy Quiz