Inorganic Chemistry: Nonmetals and Their Compounds
Core concepts
Know the chemistry of hydrogen, halogens, oxygen and sulfur, nitrogen and phosphorus, carbon and silicon, plus their key compounds. Focus on preparation, characteristic reactions, and industrial or laboratory importance: chlorine with alkali, hydrogen halides and their acid strength, sulfuric acid manufacture and properties, nitric acid as an oxidiser, ammonia synthesis and its basicity, and the allotropes of carbon and phosphorus.
Traps that appear in questions
- Acid strength of hydrogen halides: it increases down the group (HF < HCl < HBr < HI) because the H–X bond weakens, even though HF is the most weakly acidic yet most hydrogen-bonded.
- Oxidising behaviour of nitric and sulfuric acid: concentrated nitric acid and hot concentrated sulfuric acid oxidise metals, often releasing NO₂ or SO₂ rather than hydrogen.
- Halogen displacement: a more reactive halogen higher in the group displaces a less reactive one from its halide salt.
- Ammonia: it is basic, forms ammonium salts, and is a weak base; do not treat it as an alkali metal hydroxide equivalent.
- Allotropes: diamond, graphite and fullerenes differ in structure and properties; white and red phosphorus differ in reactivity and bonding.
Worked example
Which is the strongest acid: HF, HCl, HBr or HI? Acid strength follows the ease of releasing H⁺. The H–I bond is the longest and weakest, so HI is the strongest acid and HF the weakest.
Rule of thumb
For halogens and hydrogen halides, follow the group trend: bond strength down the group explains reactivity and acid strength.