Compare hydrogen halide thermal stability
Use H–X bond strength to explain the decreasing thermal stability of hydrogen halides down the group.
Section 11 · Group 17
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is more thermally stable, HCl or HI?
What you will learn
- Rank HF, HCl, HBr and HI thermal stability
- Relate bond length to bond strength
- Keep acid strength distinct from thermal stability
H–X bond strength and thermal stability decrease down the group
The H–X bond gets longer as the halogen atom becomes larger, and the overlap becomes less effective. Bond strength decreases: H–F > H–Cl > H–Br > H–I.
Thermal stability follows the same order: HF is most stable and HI least stable. The weaker H–X bond is easier to break when hydrogen halide decomposes.
Do not substitute aqueous acid strength for bond stability
Thermal stability concerns decomposition of the hydrogen halide and its H–X bond. Acid strength in water is a different property involving ionisation in solution.
When a question asks why thermal stability changes, name the H–X bond and relate its strength to atomic size and overlap. Do not answer only with an aqueous pH comparison.
Pause and explain
What happens to H–X bond strength from HCl to HI?
Worked example
Explain why HI is less thermally stable than HCl.
Show the worked solution
- Iodine is larger than chlorine, so the H–I bond is longer.
- The longer H–I bond has weaker overlap and lower bond strength.
- Less energy is needed to break H–I, so HI is less thermally stable.
Answer HI is less thermally stable because its longer H–I bond is weaker than H–Cl and is more easily broken.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Rank HCl, HBr and HI in decreasing thermal stability.
Give me a hint
Thermal stability follows the H–X bond-strength order.
Compare my reasoning
- H–Cl is the strongest bond in this set.
- Bond strength falls through H–Br to H–I.
- The stability order follows the same direction.
HCl > HBr > HI in thermal stability.
Look for these in your work
- Correct order
- Bond-strength trend used
- Property named
Explain the HCl-to-HBr bond-strength change using atomic size.
Give me a hint
Relate the larger bromine atom to bond length and overlap.
Compare my reasoning
- Bromine is larger than chlorine.
- The H–Br bond is longer.
- Less effective overlap makes H–Br weaker.
H–Br is weaker than H–Cl because the larger bromine atom gives a longer bond with less effective overlap.
Look for these in your work
- Size comparison correct
- Bond length linked
- Overlap explained
A student uses the pH of hydrochloric and hydrobromic acid solutions to explain hydrogen halide thermal stability. State what evidence would answer the thermal-stability question directly.
Give me a hint
The syllabus asks for bond strength as the explanation.
Compare my reasoning
- Compare H–Cl and H–Br bond strengths or decomposition behavior.
- A weaker bond is more readily broken.
- Aqueous pH alone does not directly measure thermal decomposition stability.
Use the H–X bond strengths or thermal decomposition evidence; aqueous pH alone does not establish thermal stability.
Look for these in your work
- Relevant evidence named
- Bond argument stated
- Properties distinguished
Common mistakes
- Reversing the stability order
- Explaining thermal stability only through acid strength
- Saying the larger halogen forms the shorter H–X bond
- Confusing bond strength within HX with intermolecular forces
What can you explain now?
Which is more thermally stable, HCl or HI?
Compare with the explanation
HCl
H–Cl is stronger than H–I.
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Make it stick
How do H–X bond strength and hydrogen halide thermal stability change from HF to HI?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.