Rhodium is a transition, noble and precious metal. When heated with nitric acid in powder form, its nitrate salt is formed. The chemical reaction is not complete because of its nobility and oxide layer formation on its surface.
Rh + HNO3 → Rh(NO3)2 + NO + H2O
When it is heated strongly in the presence of excess of oxygen, it forms RhO.
2Rh + O2 → 2RhO
Similarly its nitrate salt by heating also gives RhO.
Rh(NO3)2 → RhO + NO + O2
The common oxidation state for rhodium is +3, so previous equations are not correct. As I know Rh will not react with nitric acid, so the only way to get pure Rh2O3 is oxidizing under the high temperature (600-700 C). But rhodium should be powdered. But if you need to get rhodium(III) salt, it will be better to obtain RhCl3 by chlorination under high temperature (500 C):
2Rh + 3Cl2 → 2RhCl3
You can also convert rhodium to Rh2(SO4)3 reacting powdered rhodium with molten KHSO4, but this reaction is very slow.
Anyway from RhCl3 or Rh2(SO4)3 you can get rhodium(III) oxide hydrate (Rh2O3 x 5H2O) or rhodium(III0 hydroxide by the reaction with NaOH and then get rhodium(III) nitrate.
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