The chemical properties of the halogen elements and the hydrogen halides
Cambridge A-Level Chemistry (9701) · Unit 11: Group 17 · 7 flashcards
The chemical properties of the halogen elements and the hydrogen halides is topic 11.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 11 — Group 17 , alongside Some reactions of the halide ions and The reactions of chlorine. In one line: H₂(g) + Cl₂(g) → 2HCl(g). This reaction requires UV light or heat to overcome the high activation energy.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 2 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Write a balanced chemical equation for the reaction between chlorine and hydrogen
H₂(g) + Cl₂(g) → 2HCl(g). This reaction requires UV light or heat to overcome the high activation energy.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe how the halogens compare with one another in strength as oxidising agents.
- Describe how the halogens react with hydrogen, and explain why their reactivity in these reactions differs.
- Describe how thermally stable the hydrogen halides are relative to one another, and use bond strengths to explain the differences.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Describe the trend in oxidising ability of the halogens down Group 17.
Oxidising ability decreases down the group. This is because atomic radius increases, shielding increases, and nuclear attraction decreases, making it harder to attract an electron to form a negative ion.
Write a balanced chemical equation for the reaction between chlorine and hydrogen.
H₂(g) + Cl₂(g) → 2HCl(g). This reaction requires UV light or heat to overcome the high activation energy.
Explain the relative reactivity of halogens with hydrogen down Group 17.
Reactivity with hydrogen decreases down the group. Fluorine reacts explosively even in the cold and dark; chlorine explodes in sunlight or UV light; bromine reacts only on heating (with a catalyst); iodine reacts slowly and only partially (reversibly) on heating. This is because the H-X bond formed gets weaker down the group, so the reaction releases less energy, and the halogens become weaker oxidising agents.
Describe the trend in thermal stability of the hydrogen halides (HX) down Group 17.
Thermal stability decreases down the group. This is because the bond enthalpy of the H-X bond decreases as you go down the group, making it easier to break the bond upon heating.
Explain the relationship between H-X bond strength and the thermal stability of hydrogen halides.
Stronger H-X bonds require more energy to break, resulting in higher thermal stability. Conversely, weaker H-X bonds break more easily, leading to lower thermal stability.
Arrange the hydrogen halides (HF, HCl, HBr, HI) in order of increasing thermal stability.
HI < HBr < HCl < HF. This is because the H-F bond is the strongest and the H-I bond is the weakest.
Why does fluorine react more vigorously with hydrogen than iodine does?
The main reason is that the H-F bond formed is far stronger than the H-I bond (about 562 vs 299 kJ mol⁻¹), so the reaction of fluorine is much more exothermic. The F-F bond is also weak (about 158 kJ mol⁻¹, similar to I-I at 151 kJ mol⁻¹, because of repulsion between lone pairs on the small F atoms), so the activation energy is low. Fluorine is also the strongest oxidising agent in the group.
More Chemistry flashcards
Browse every 9701 flashcard topic by syllabus area.
All Chemistry FlashcardsMore topics in Unit 11 — Group 17
The chemical properties of the halogen elements and the hydrogen halides sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this The chemical properties of the halogen elements and the hydrogen halides deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this The chemical properties of the halogen elements and the hydrogen halides deck
Start in Study Mode, attempt each card before flipping, then rate Hard, Okay or Easy. Cards you rate Hard come back within a day; cards you rate Easy push out to weeks. Your progress is saved in your browser, so come back daily for 5–10 minute reviews until every card reads Mastered.
Study Mode
Rate each card Hard, Okay, or Easy after flipping.