Rhodium: reactions of elements
Reaction of rhodium with air
Rhodium is largely immune to atmospheric attack. On heating with oxygen at 600°C, rhodium metal gives rhodium (III) oxide, Rh2O3.
4Rh(s) + 3O2(g) → 2Rh2O3(s) [dark grey]
Usually second and third row d-block elements show similar chemistries, but in this case, iridium (immediately below rhodium in the periodic table) burns to give iridium (IV) oxide, IrO2. There are reports of RhO2, but it does not appear to be well-characterized.
Reaction of rhodium with water
Rhodium does not react with water under normal conditions.
Reaction of rhodium with the halogens
Metallic rhodium reacts directly with fluorine gas to form the highly corrosive rhodium(VI) fluoride, RhF6. This material, with care, can be heated to form rhodium(V) fluoride, which has the dark red tetrameric structure [RhF5]4.
Rh(s) + 3F2(g) → RhF6(s) [black]
The trihalides rhodium(III) fluoride, IrCl3, rhodium(III) chloride, IrCl3, and rhodium(III) bromide, IrBr3, can be formed through the direct reaction of the metal with the halogen under anhydrous (dry) conditions.
2Rh(s) + 3F2(g) → 2RhF3(s) [red]
2Rh(s) + 3Cl2(g) → 2RhCl3(s) [red]
2Rh(s) + 3Br2(g) → 2RhBr3(s) [red-brown]
Reaction of rhodium with acids
Rhodium metal is notably inert to reaction with acids, including aqua regia (a mixture of hydrochloric acid, HCl, and nitric acid, HNO3, known for its ability to dissolve gold metal).