Intermediate Topic 661 of 1102 Inorganic Chemistry
Hydrogen Halides
HF, HCl, HBr and HI dissolve in water to give acids of increasing strength down the group.
In-depth explanation of Hydrogen Halides
The hydrogen halides, HF, HCl, HBr and HI, are colourless gases that dissolve in water to give acidic solutions. Acid strength rises from HF to HI because the H-X bond weakens and the halide ion becomes larger and more stable. Hydrogen fluoride has an unusually high boiling point because of hydrogen bonding between its molecules. Hydrogen chloride is produced in large quantities and is the main component of gastric acid. Hydrogen halides are made from metal halides and sulfuric acid only for HF and HCl, because sulfuric acid oxidizes HBr and HI.
Going deeper: Hydrogen Halides
Key ideas: Acid strength rises HF less than HCl less than HBr less than HI because the H-X bond weakens down the group, despite electronegativity falling. Boiling points: HF is unusually high (19.5 C) via hydrogen bonding, then HCl -85, HBr -67, HI -35 rising with London forces. HBr and HI are made by NaBr or NaI with phosphoric acid, not conc H2SO4, which oxidizes them to Br2 and I2.
Common mistakes and connections: Students link acid strength to electronegativity, which would predict HF strongest; the correct driver is bond enthalpy and solvation. HF is a weak acid, pKa 3.2, in water. Preparation by H2SO4 works for HF and HCl but not HBr or HI because of redox. Links to hydrofluoric acid, trends in bond strength and acidity.
Visualization of Hydrogen Halides
Acid strength rises from HF (pKa 3.2) to HI (about -10) as the H-X bond weakens down the group. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Hydrogen chloride gas dissolves in water to give hydrochloric acid, a strong acid.
Frequently asked questions about Hydrogen Halides
What is Hydrogen Halides?
HF, HCl, HBr and HI dissolve in water to give acids of increasing strength down the group.
Is Hydrogen Halides a beginner, intermediate or advanced topic?
Hydrogen Halides is a intermediate topic in Inorganic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Hydrogen Halides?
Study these earlier topics in Inorganic Chemistry first: Sulfuric Acid, Halogens, Halogen Reactivity Trend.
What comes after Hydrogen Halides?
The next topics in Inorganic Chemistry are Hydrofluoric Acid and Interhalogen Compounds.
Can I practise Hydrogen Halides with quizzes?
Yes. The interactive lesson includes a self-check question on Hydrogen Halides, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Hydrogen Halides fits in Inorganic Chemistry
Hydrogen Halides is topic 46 of 113 in Inorganic Chemistry.