Rhodium: the essentials
Rhodium metal is silvery white. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. Upon heating it turns to the oxide when red and at higher temperatures turns back to the element. It is a major component of industrial catalytic systems such as the BP-Monsanto process.
Ruthenium, rhodium, palladium, osmium, iridium, and platinum together make up a group of elements referred to as the platinum group metals (PGM).
Rhodium: historical information
William Hyde Wollaston discovered rhodium in 1803-4 in crude platinum ore from South America rather soon after his discovery of another element, palladium. He dissolved the ore in aqua regia (a mixture of hydrochloric and nitric acids), neutralised the acid with sodium hydroxide (NaOH), and precipitated the platinum by treatment with ammonium chloride, NH4Cl, as ammonium chloroplatinate. Palladium was then removed as palladium cyanide by treatment with mercuric cyanide. The remaining material was a red material containing rhodium chloride salts from which rhodium metal was obtained by reduction with hydrogen gas.
Rhodium around us Read more »
Rhodium has no biological role.
Rhodium is found as the free metal in North America, as well as with nickel and copper deposits (from which it is recovered commercially) in Canada and South Africa. There are very few rhodium minerals.
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Physical properties Read more »
Heat properties Read more »
- Melting point: 2237 [1964 °C (3567 °F)] K
- Boiling point: 3968 [3695 °C (6683 °F)] K
- Enthalpy of fusion: 21.7 kJ mol-1
Crystal structure Read more »
The solid state structure of rhodium is: ccp (cubic close-packed).
Rhodium: orbital properties Read more »
Rhodium atoms have 45 electrons and the shell structure is 18.104.22.168.1. The ground state electronic configuration of neutral Rhodium is [Kr].4d85s1 and the term symbol of Rhodium is 4F9/2.
- Pauling electronegativity: 2.28 (Pauling units)
- First ionisation energy: 719.7 kJ mol‑1
- Second ionisation energy: 1740 kJ mol‑1
Isolation: it would not normally be necessary to make a sample of rhodium in the laboratory as the metal is available, at a price, commercially. The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. Sometimes extraction of the precious metals such as rhodium, platinum and palladium is the main focus of a partiular industrial operation while in other cases it is a byproduct. The extraction is complex because of the other metals present and only worthwhile since rhodium is the basis of very important catalysts in industry.
Preliminary treatment of the ore or base metal byproduct is required to remove silver, gold, palladium, and platinum. The resulting residue is melted with sodium bisulphate (NaHSO4) and the resulting mixture extracted water to give a solution containing rhodium sulphate, Rh2(SO4)3. The rhodium is precipitated out as the hydroxide by addition of sodium hydroxide, NaOH, and redissolved in hydrochloric acid, HCl, to give H3RhCl6. This is treated with NaNO2 and NH4Cl to form a precipitate of the rhodium complex (NH4)3[Rh(NO2)6]. Dissolution of the precipitate in HCl gives a solution of pure (NH4)3RhCl6. Evaporation to dryness and burning under hydrogen gas gives pure rhodium.
Rhodium isotopes Read more »