Hydrogen halides all react with ammonia gas to form white ionic salts.NH3 (g) + HCl (g) → NH4Cl (s)Ammonia acts as a base. The nitrogen atom in NH3 has a lone pair of electrons, which accepts a proton (H+) from the hydrogen halide. This forms the ammonium ion, NH4+, and a halide ion. The new N-H bond is a dative covalent (coordinate) bond, because both electrons in it come from the nitrogen.The other hydrogen halides react in the same way to form ammonium halides:NH3 (g) + HBr (g) → NH4Br (s) NH3 (g) + HI (g) → NH4I (s) Because both reactants are gases and the product is a solid, the reaction is seen as dense white smoke where the two gases meet. The smoke is made of tiny particles of the solid ammonium halide.
This reaction is used as a test for hydrogen halides. A glass rod dipped in concentrated ammonia solution is held near the gas. If white smoke forms, a hydrogen halide such as HCl is present. The same reaction works the other way round: a rod dipped in concentrated hydrochloric acid gives white smoke with ammonia gas, so it can be used to test for ammonia.The ammonium ion has four bonding pairs around nitrogen, so it is tetrahedral with bond angles of 109.5°.
This reaction is used as a test for hydrogen halides. A glass rod dipped in concentrated ammonia solution is held near the gas. If white smoke forms, a hydrogen halide such as HCl is present. The same reaction works the other way round: a rod dipped in concentrated hydrochloric acid gives white smoke with ammonia gas, so it can be used to test for ammonia.The ammonium ion has four bonding pairs around nitrogen, so it is tetrahedral with bond angles of 109.5°.