Did you know? The H–Cl bond requires 431 kJ mol⁻¹ to dissociate in the SEAB data booklet, while H–Br requires 366 and H–I just 299. The falling bond energies explain why hydrogen iodide is less thermally stable than hydrogen chloride under the Group 17 syllabus comparison.
This original Hougang JC1 H2 Chemistry tutor guide connects atomic size, bond length, orbital overlap, exact SEAB diatomic bond energies, thermal decomposition and the distinction between covalent bond strength and aqueous acid behaviour. It follows 2027 H2 syllabus 9476, Section 5 The Periodic Table.
All data come from the official Chemistry Data Booklet or clearly identified original paper calculations; nothing here requests handling toxic or corrosive hydrogen halide gases. The unchanged eduKateSG Clementi small-group teaching reference is used for structure only.
EDUKATESG · HOUGANG · JC1 H2 CHEMISTRY
14 chapters · Bond energy table · Tutor checklist · FAQs · Sources
Full chapter-by-chapter learning route
- 1. Begin with the changing stability of HCl, HBr and HI
- 2. Read the exact H–X bond energies from the supplied data booklet
- 3. Explain increasing atom size down Group 17
- 4. Separate H–X length from electronegativity alone
- 5. Read the full decomposition equation without confusing it with bond dissociation
- 6. Work a bond-energy comparison of three decomposition processes
- 7. Do not confuse lower bond energy with spontaneous decomposition
- 8. Compare hydrogen halide acids without swapping the meaning of strong
- 9. Separate hydrogen halide stability from halogen oxidising power
- 10. Distinguish intermolecular volatility from intramolecular bond energy
- 11. Use bond length and overlap as the causal chain
- 12. Identify possible syllabus-level mistakes before memorising the order
- 13. Transfer to an unfamiliar Group 17 hydride comparison
1. Begin with the changing stability of HCl, HBr and HI
The hydrogen halides hydrogen chloride, hydrogen bromide and hydrogen iodide contain the same hydrogen element bonded to different Group 17 atoms. The 2027 H2 Chemistry periodicity syllabus explicitly includes explaining the thermal stability of Group 17 hydrides in terms of bond energies, with the Group 17 sequence running from chlorine to iodine.
Within this familiar sequence, hydrogen chloride is more thermally stable than hydrogen bromide, which is more thermally stable than hydrogen iodide. The H–Cl bond is stronger than H–Br, and H–Br is stronger than H–I, making the weakest hydrogen–iodine bond the easiest of the three to dissociate under an energy-input comparison.
Thermal stability is a statement about response to heating and the tendency for a substance to resist thermal decomposition under a specified model. It should not be turned into the claim that HI instantly breaks apart at room temperature or that a measured decomposition rate follows directly from one bond-energy number without considering a reaction mechanism.
A useful opening question gives only the three formulas and asks students to predict a bond-length ordering before the stability ordering. The explanation should derive from atomic size and orbital overlap rather than treat the periodic arrow as an unexplained memory rule.
Student question: Which hydrogen halide is least thermally stable among HCl, HBr and HI?
Tutor answer standard: HI in the standard syllabus comparison.
Back to chapter index · Next chapter
2. Read the exact H–X bond energies from the supplied data booklet
The SEAB Chemistry Data Booklet for use from 2026 tabulates diatomic bond energies in kJ mol⁻¹. It lists H–F 562, H–Cl 431, H–Br 366 and H–I 299. These published data provide an excellent direct source for the direction of bond strength in the hydrogen halide family.
The values are energies required for homolytic bond dissociation in gas-phase diatomic molecules, as described by the data-booklet bond-energy convention. They are not the energy of every possible aqueous acid-base reaction, nor a direct pH measurement, nor a reaction rate constant at a particular temperature.
For the three hydrides in the stated H2 Group 17 period topic, the numerical sequence 431 > 366 > 299 kJ mol⁻¹ demonstrates decreasing bond strength from HCl to HI. A learner should be able to read the table, state which bond is most difficult to break and explain why the stability trend follows.
Students should not rely on a single unsupported textbook value when an official data booklet is available. The calculation can be audited directly against the same resource the syllabus provides for theory questions. This also avoids confusing heteronuclear H–X bond energies with the values for X–X bonds such as Cl–Cl.
Student question: What are the SEAB H–Cl, H–Br and H–I bond energies?
Tutor answer standard: 431, 366 and 299 kJ mol⁻¹ respectively, from the official data booklet.
Back to chapter index · Next chapter
3. Explain increasing atom size down Group 17
Moving down Group 17 from chlorine to bromine to iodine, the halogen atoms have more occupied electron shells. Their atomic radii increase as the relevant outer electron density extends farther from the nucleus, despite increasing proton number. Shielding and shell structure explain the overall direction.
Hydrogen contributes its 1s orbital to the H–X covalent bond. The halogen’s bonding orbital becomes more spatially extended down the group and the H–X bond length increases. The H–I bond is therefore longer than H–Br, which is longer than H–Cl in the familiar structural comparison.
A longer bond in this family generally corresponds to less effective overlap and a weaker covalent interaction. That is the reason the dissociation energies fall from 431 to 366 to 299 kJ mol⁻¹. The correct argument links size to length, length to overlap and overlap to bond strength.
A student who says ‘iodine has more protons, so H–I must be strongest’ has neglected shielding and the large outer electron shells. The nuclear charge of iodine is indeed greater, but bond strength cannot be inferred from proton number alone.
Student question: Why does H–I tend to be a weaker bond than H–Cl?
Tutor answer standard: Iodine is larger; its longer H–I bond has less effective bonding overlap in the stated family.
Back to chapter index · Next chapter
4. Separate H–X length from electronegativity alone
Chlorine is more electronegative than bromine, which is more electronegative than iodine. This trend influences bond polarity, but the thermal stability question is explicitly about bond-energy differences. It is not sufficient to write ‘HI is weakest because iodine is less electronegative’ without discussing the actual covalent bond strength.
In hydrogen halides, multiple factors affect electron distribution, yet the most direct syllabus explanation uses the increasing halogen size and H–X bond length to justify decreasing bond energy. Students should avoid confusing the polarity of a bond with the energy required to dissociate it homolytically.
A polar H–Cl bond is not automatically an ionic bond or a free solution of chloride ions. Hydrogen chloride in the gas phase consists predominantly of neutral covalent molecules. In water it can donate protons strongly, but that is a separate chemical environment with solvent stabilisation.
An H2 answer can therefore distinguish three properties: H–X bond polarity, gas-phase H–X bond dissociation energy and aqueous acid dissociation. They share underlying electronic physics but are not interchangeable numerical measurements.
Student question: Can you explain thermal stability solely by the acid’s aqueous pH?
Tutor answer standard: No. The syllabus asks for the H–X bond-energy trend, not the pH of solutions.
Back to chapter index · Next chapter
5. Read the full decomposition equation without confusing it with bond dissociation
A simple reversible decomposition representation is 2HX(g) ⇌ H₂(g) + X₂(g), with X a halogen. For HI, the balanced formula is 2HI(g) ⇌ H₂(g) + I₂(g), where the physical phase of iodine depends on the temperature and the specified reaction conditions.
This overall decomposition differs from breaking a single H–I bond into separate H· and I· atoms. The latter is the homolytic bond-dissociation process associated with a tabulated H–I bond energy; the former breaks two H–I bonds and forms H–H and I–I bonds.
The difference matters when calculating a reaction enthalpy. The enthalpy of decomposition cannot be identified simply with one H–X dissociation energy. One must account for all bonds broken and formed, using the appropriate physical states and a justified model.
Students often write an unbalanced HI → H₂ + I₂ equation and then read one bond energy as the net energy change. The tutor should require balancing first, identifying the intended process and then applying the bond-energy ledger under the specified gas-phase convention.
Student question: Why is D(H–I) not the complete ΔH for 2HI → H₂ + I₂?
Tutor answer standard: The overall reaction breaks two H–I bonds and forms H–H and I–I bonds.
Back to chapter index · Next chapter
6. Work a bond-energy comparison of three decomposition processes
Using the official gas-phase diatomic bond-energy values and the simplified reaction 2HX(g) → H₂(g)+X₂(g), the model ΔH is approximately 2D(H–X)−D(H–H)−D(X–X). The data booklet gives H–H 436, Cl–Cl 244, Br–Br 193 and I–I 151 kJ mol⁻¹.
For hydrogen chloride, ΔH≈2(431)−436−244=+182 kJ for the balanced decomposition as written. For hydrogen bromide, ΔH≈2(366)−436−193=+103 kJ. For hydrogen iodide, ΔH≈2(299)−436−151=+11 kJ. These are gas-phase bond-energy model calculations, not measured reaction rates.
The pattern illustrates that decomposing HI into the specified gas-phase products requires much less net enthalpy than the corresponding HCl process. The result is consistent with lower thermal stability but does not by itself supply activation energies, equilibrium composition at every temperature or a clock time for decomposition.
This worked exercise is optional deeper quantitative synthesis of data-booklet bond energies. The core Period 3/Group 17 syllabus learning outcome requires describing and explaining the thermal-stability trend in terms of bond energies; do not misrepresent a full kinetic decomposition analysis as compulsory.
Student question: What is the model enthalpy for 2HI(g) → H₂(g)+I₂(g)?
Tutor answer standard: Approximately +11 kJ for the balanced reaction from the SEAB diatomic bond-energy table.
Back to chapter index · Next chapter
7. Do not confuse lower bond energy with spontaneous decomposition
A reaction can be thermodynamically possible under certain conditions yet proceed slowly if its mechanism has a significant activation barrier. The energy needed to reach the transition region is a kinetic issue; the enthalpy difference between fully specified reactants and products is a thermochemical issue.
Hydrogen iodide having the weakest H–X bond among HCl, HBr and HI does not mean all HI molecules break apart immediately at an ordinary room temperature. Relative thermal stability under heating is a tendency derived from energetics, not a guarantee of instantaneous reaction.
Equilibrium considerations also depend on temperature, entropy and concentrations or partial pressures. A bond-energy ledger estimates a standard type of enthalpy change when the states are properly matched, but it does not determine every equilibrium proportion by itself.
To test understanding, ask the learner to distinguish the statement ‘H–I requires less energy to dissociate homolytically’ from ‘a jar of HI will decompose completely in one second’. The first follows from the data; the second invents kinetics and conditions.
Student question: Does lower H–I bond energy prove rapid complete HI decomposition at room temperature?
Tutor answer standard: No. The rate and extent require kinetic and thermodynamic conditions.
Back to chapter index · Next chapter
8. Compare hydrogen halide acids without swapping the meaning of strong
In water, HCl, HBr and HI are conventionally treated as strong acids at ordinary dilute conditions. Acid dissociation in water is a proton-transfer equilibrium affected by solvation of ions and the acid’s structure, not a direct repeat of the gas-phase homolytic bond-energy experiment.
Hydrogen fluoride is often taught as a weak aqueous acid despite having the strongest H–X bond in this family. It also exhibits substantial hydrogen bonding that affects its physical properties. Including HF as an extension helps demonstrate why the entire family should not be described by one simple boiling or pH rule.
The 2027 H2 Periodic Table thermal-stability outcome specifically names Group 17 hydrides down chlorine to iodine. An extension to HF can support understanding but should not replace the precise comparison required for HCl, HBr and HI.
A student saying ‘HI is least stable, so it is the least acidic’ has inverted the aqueous acid trend and conflated properties. The correct explanation defines the environment and property first, then applies the relevant chemical reasoning.
Student question: Is gas-phase H–X thermal stability the same as aqueous acid strength?
Tutor answer standard: No. The properties involve different processes and solvation conditions.
Back to chapter index · Next chapter
9. Separate hydrogen halide stability from halogen oxidising power
Group 17 has another syllabus trend: chlorine is a stronger oxidising agent than bromine, which is stronger than iodine in the standard comparisons, supported by relevant reduction potentials. This describes the tendency of X₂ to gain electrons in a redox reaction, not the strength of the H–X covalent bond.
A chlorine molecule can oxidise bromide ions under suitable conditions in a displacement reaction; this does not automatically prove any specific HCl decomposition rate. Oxidising power compares redox couples, whereas thermal stability of hydrogen halides is explicitly linked to H–X bond energy.
The two trends go in a superficially similar direction down the group but have different physical explanations. Students who merely say ‘reactivity decreases down the group’ without naming the reaction role risk confusing oxidising halogens, reducing halide ions and thermal decomposition of HX.
A tutor can prepare three column headings: X₂ oxidising power, HX bond dissociation energy and aqueous HX acidity. Students should fill each independently with the relevant evidence and avoid copying one arrow into all three columns.
Student question: Can a Group 17 oxidising-power trend substitute for the hydrogen halide stability argument?
Tutor answer standard: No. Oxidising power concerns redox potentials; hydride thermal stability concerns H–X bond energies.
Back to chapter index · Next chapter
10. Distinguish intermolecular volatility from intramolecular bond energy
Down the halogen group, the elements Cl₂, Br₂ and I₂ exhibit increasing boiling and melting tendencies associated with greater polarizability and stronger instantaneous dipole-induced dipole attractions. This is a trend in intermolecular forces between X₂ molecules.
The thermal stability of HCl, HBr and HI instead involves the H–X covalent bond within each molecule. A student who uses only ‘London forces are stronger in iodine’ to explain why HI decomposes more readily has chosen the wrong level of bonding.
Hydrogen halide boiling-point comparisons involve intermolecular interactions and molecular masses, while their gas-phase bond dissociation energies refer to breaking covalent bonds. Although related properties can share electronic origins, an exam answer must identify whether molecules are being separated from one another or atoms are being separated within molecules.
An effective tutor test changes only the question wording: ‘Why does iodine have a higher boiling point than chlorine?’ versus ‘Why is HI less thermally stable than HCl?’ The first asks about X₂ intermolecular attraction; the second about the HX bond.
Student question: Which bond type belongs in a thermal-decomposition explanation for HI?
Tutor answer standard: The intramolecular covalent H–I bond, not just dispersion forces between molecules.
Back to chapter index · Next chapter
11. Use bond length and overlap as the causal chain
The halogen size increases from Cl to Br to I, so the H–X bond length tends to increase. In the simple orbital overlap model, a hydrogen 1s orbital overlaps less effectively with the larger and more diffuse valence orbitals of iodine than with chlorine’s more compact valence orbital.
Less effective overlap contributes to a weaker H–I bond and its smaller tabulated dissociation energy. The chain is halogen atom size → H–X bond length and overlap → bond energy → relative resistance to thermal cleavage.
Students may be tempted to claim that an iodine atom has more electrons, and therefore necessarily forms a stronger H–I bond. The shielding and extended orbital size matter more than a raw electron count. Bond energy is not proportional to total atomic electron number.
This explanation is a qualitative model appropriate for H2. Exact bond energies arise from the full molecular electronic structure; no isolated orbital-overlap sketch can derive 299 kJ mol⁻¹ from first principles with school-level arithmetic.
Student question: Why is the H–I overlap usually weaker than H–Cl?
Tutor answer standard: The iodine bonding orbital is larger and more diffuse, leading to a longer and weaker H–I bond.
Back to chapter index · Next chapter
12. Identify possible syllabus-level mistakes before memorising the order
Error one is reversing the bond-energy table and claiming H–I has the largest energy because iodine has the most electrons. Error two is saying HI has the highest thermal stability because iodine’s atom is large. Both conflict with the observed H–X dissociation trend.
Error three is using Cl₂/Br₂/I₂ boiling points or redox oxidising powers as though they directly measured the H–X bond. Error four is equating aqueous acidity with gas-phase covalent bond dissociation energy. Error five is treating the enthalpy for 2HX decomposition as one D(H–X) value without accounting for bonds formed.
A sixth mistake is giving an absolute kinetic forecast, such as ‘HI instantly decomposes upon heating once the bond energy is below a fixed number’. No such universal threshold follows from a data booklet value. Rates depend on activation barriers and temperature distributions.
The correction sequence is to identify the named process, list H–Cl/H–Br/H–I energies, explain the atomic-size trend and then state the relative stability order. Add any quantitative overall-reaction calculation only when the question asks for it and supplies the required other bond energies.
Student question: What is the shortest evidence-based stability ordering?
Tutor answer standard: HCl more thermally stable than HBr, which is more stable than HI, supported by 431 > 366 > 299 kJ mol⁻¹ H–X energies.
Back to chapter index · Next chapter
13. Transfer to an unfamiliar Group 17 hydride comparison
A paper question gives two fictional Group 17 hydrides HX and HY, where Y lies below X in the group. Under the same structural trend, the lower halogen generally has a larger atom, longer H–Y bond and smaller bond dissociation energy. It is therefore expected to be less thermally stable in the intended simple comparison.
If the problem instead supplies anomalous measured energies, use the supplied data rather than forcing a remembered periodic arrow. Periodicity provides a hypothesis that the data can test, but quantitative evidence takes priority in a scientific inference.
Another transfer task supplies H–X bond energies and asks for a possible overall dissociation enthalpy. The learner should distinguish an individual radical-forming bond cleavage from a balanced equation making H₂ and X₂, then include the appropriate product bond energies.
Parents can ask one sentence: ‘Does HI break more readily because iodine is a weaker acid, or because its H–I covalent bond is weaker?’ The correct answer is the second in the thermal-stability question, with acid strength recognised as a separate property.
Student question: What must be considered when a task gives the whole 2HX → H₂+X₂ reaction?
Tutor answer standard: All bonds broken and formed, not simply D(H–X).
Worked evidence: SEAB bond energies and decomposition comparison
The diatomic bond energies below are taken directly from the SEAB Chemistry Data Booklet for use from 2026. The decomposition enthalpies are calculated original examples applying the ideal gas-phase diatomic bond-energy ledger 2D(H–X)−D(H–H)−D(X–X). They are not measured reaction rates.
| Hydride or reaction | SEAB bond energy / calculation (kJ mol⁻¹) | Partner bond / result | Meaning |
|---|---|---|---|
| HF (extension) | H–F 562 | F–F 158 | Far stronger H–F; outside the main Cl–I hydride trend |
| HCl | H–Cl 431 | Cl–Cl 244 | Most stable of HCl/HBr/HI in this comparison |
| HBr | H–Br 366 | Br–Br 193 | Intermediate H–X bond strength |
| HI | H–I 299 | I–I 151 | Weakest of the three H–X bonds |
| H₂ reference | H–H 436 | — | Product bond energy in 2HX decomposition |
| 2HCl → H₂+Cl₂ | 2(431)−436−244 | ≈+182 kJ | Model gas-phase decomposition enthalpy |
| 2HBr → H₂+Br₂ | 2(366)−436−193 | ≈+103 kJ | Model gas-phase decomposition enthalpy |
| 2HI → H₂+I₂ | 2(299)−436−151 | ≈+11 kJ | Model gas-phase decomposition enthalpy |
The calculations illustrate why the whole decomposition process is not represented by a single HX bond energy. They do not determine a reaction mechanism, exact rate or complete equilibrium composition. The core 9476 learning outcome is the thermal-stability trend explained by bond energy.
Tutor checklist and independent transfer
- Confirm the 2027 H2 Chemistry 9476 Group 17 hydride thermal-stability learning outcome.
- Read H–Cl, H–Br and H–I bond energies from the official SEAB Data Booklet rather than guessing numerical values.
- Explain increasing halogen size and H–X bond length down the group.
- Connect longer, less effectively overlapping covalent bonds to lower dissociation energy.
- Keep gas-phase H–X bond dissociation separate from aqueous acid strength and pH.
- Distinguish thermal stability from Cl₂/Br₂/I₂ oxidising power and volatility.
- Balance 2HX → H₂ + X₂ before calculating an optional overall bond-energy enthalpy.
- Do not claim that a weaker bond guarantees an exact room-temperature decomposition rate.
- Transfer the method to a new hydride using supplied bond-energy data and state its assumptions.
Frequently asked questions
What is the thermal-stability order of HCl, HBr and HI?
HCl > HBr > HI in the standard 2027 H2 Group 17 hydride comparison.
Why is H–I weaker than H–Cl?
Iodine is larger; the H–I bond is longer with less effective orbital overlap, giving lower bond dissociation energy.
What are the SEAB values?
The official data booklet lists H–Cl 431, H–Br 366 and H–I 299 kJ mol⁻¹.
Does lower H–I bond energy mean HI instantly decomposes?
No. Thermal stability is relative and actual rates depend on mechanism, temperature and conditions.
Are acidity and thermal stability the same?
No. Gas-phase bond breaking differs from proton dissociation in aqueous solvent.
Is HF in the syllabus comparison?
The specific 2027 Group 17 periodicity outcome refers to chlorine through iodine; HF may be used as a supplementary contrast.
Does H2 Chemistry require explaining this trend?
Yes. SEAB 9476 Section 5 states that Group 17 hydride thermal stability is explained using bond energies.
Continue with connected Chemistry guides
- Group 17 Halogens, Oxidising Power and Hydrogen Halides | Hougang
- JC1 H2 Chemistry | Mean Bond Energy and Enthalpy Differences
- Hougang Chemistry Tuition and Tutor Library
Parents can bring the student’s H2 syllabus, examination year and the data-booklet bond-energy comparison to discuss learning support. Confirm current teaching group, venue and availability directly. This article does not guarantee results or recommend independent handling of hydrogen halide gases.
Official sources, limitations and model boundaries
- SEAB 2027 H2 Chemistry syllabus 9476, Section 5
- SEAB Chemistry Data Booklet, Table 3(a) diatomic bond energies
- Immutable eduKateSG Clementi small-group tutorial reference
- Hougang Chemistry Tuition and Tutor Library
The official 2027 H2 Chemistry syllabus explicitly requires describing and explaining the thermal stability of Group 17 hydrides in terms of H–X bond energies. The data-booklet values are used as printed. The decomposition enthalpy comparison is a worked ideal gas-phase bond-energy application, not a statement about the actual thermal decomposition mechanism, speed or exact equilibrium at every condition.