Rhodium

Rhodium, symbol Rh, is the rarest of all non-radioactive metals on Earth, and therefore an expensive metal. On the 22nd July 2011, 1 gram of rhodium cost $(AUD)38 compared to 1 gram of gold which cost $(AUD)31 or 1g of silver for only 78 cents !
Rhodium is a transition metal with a density of 12.41 gcm^-3 and is found in nature as the free metal, or alloyed with similar metals such as platinum or nickel, but not as a chemical compound.
Naturally occurring rhodium is composed of only one isotope, rhodium-103.
Only about 3 tonnes of rhodium are produced in the world each year, and most of this is used for catalyzing chemical reactions.
Approximately 80% of the rhodium produced is used as a reduction catalyst in the three-way catalytic converters of cars.
In a three-way catalytic converter three processes occur simultaneously:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO_x → xO₂ + N₂
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water

Other uses of rhodium include :

• plating white gold to make it appear more silvery (white gold is actually an alloy of gold with atleast one other metal such as nickel, manganese, palladium)
• plating sterling silver to make it appear more silvery (sterling silver is an alloy of silver containing 92.5% by mass silver and 7.5% by mass of other metals such as copper)

Queen's University chemists have just discovered that rhodium that is modified using carbon, nitrogen or hydrogen-based complexes changes colour to yellow in the presence of nitrogen, deep blue in the presence of oxygen, and brown in the presence of carbon monoxide. Modified metals, such as modified rhodium, that change colour in the presence of particular gases could warn consumers if packaged food has been exposed to air or if there's a carbon monoxide leak at home. This finding could potentially influence the production of both industrial and commercial air quality sensors.

Reference
Queen's University (2011, July 21). Modified metals change color in the presence of particular gases. ScienceDaily. Retrieved July 23, 2011, from http://www.sciencedaily.com­ /releases/2011/07/110721131159.htm

Further Reading
Periodic Table
Definitions of a Mole
Mass-Mole Calculations
Density
Isotopes
Relative Atomic Mass
Metals & Non-metals
Percentage Composition

Study Questions

1. Locate rhodium in the Periodic Table and give its
   • atomic number
   • relative atomic mass
2. Using the prices per gram of metal given in the story above, calculate
   • the price of the 3 tonnes of rhodium produced in the world each year
     
   • the price of 1 mole of rhodium
   • the price of 10 cubic centimeters of rhodium
   • the mass of $57,000 worth of rhodium
   • the volume of $57,000 worth of rhodium
3. Naturally occurring rhodium has only 1 isotope, rhodium-103. For this isotope give:
   • the number of protons in the nucleus of a rhodium atom
   • the number of neutrons in the nucleus of a rhodium atom
   • the mass number of this isotope of rhodium
   • the atomic number for this isotope of rhodium
4. If naturally occurring rhodium only has 1 isotope why is its relative atomic mass 102.9?
5. List the physical properties you would expect rhodium to have based on its position within the Periodic Table.
6. Why would coating white gold in rhodium make it appear more silvery?
7. A 25 kg sample of sterling silver contains only silver and copper.
   • What mass of silver is present in the sample?
   • What mass of copper is present in the sample?
8. A sample of white gold is found to contain only 1.39 g gold and 0.09g of nickel. Calculate the percent by mass of each element present in the sample.
   
   
   
   
   

Organic Aqua Regia

While noble metals such as platinum and palladium are becoming increasingly important, the world has limited supplies of these metals so that it is vitally important that industry can recycle these metals efficiently.

Many of the transition metals have negative standard reduction potentials, indicating that these metals will dissolve in dilute acid, eg, clean chromium will dissolve in dilute hydrochloric acid

2 x [Cr(s) ----> Cr³⁺ + 3e] E^o = 0.74V
3 x [2e + 2H⁺ ----> H₂(g)] E^o = 0.00
_____________________________________________
2Cr(s) + 6H⁺ -----> 2Cr³⁺ + 3H₂(g) E^o = 0.74V

Some transition metals have positive reduction potentials, so they are poorer reducing agents than hydrogen, and are difficult to dissolve in acid. These metals are referred to as the noble metals and include silver, gold and platinum as well as ruthenium, rhodium, palladium, osmium, and iridium. Dissolving the noble metals requires the use of an oxidizing agent and sometimes a complexing agent. The most common reagent used to dissolve noble metals is aqua regia.

Aqua regia is a highly corrosive, fuming yellow or red solution formed when 1 part of concentrated nitric acid is added to 3 parts of concentrated hydrochloric acid. The nitric acid part is a powerful oxidizer, it oxidizers the noble metal atoms to cations. The hydrochloric acid provides a supply of chloride anions which react with the noble metal cations. For example, gold can be dissolved in aqua regia:

• gold reacts with nitric acid to form gold (III) ions:

Au(s) + 3NO₃^-(aq) + 6H⁺(aq) -----> Au³⁺(aq) + 3NO₂(g) + 3H₂O(l)

• gold (III) ions react with chloride ions to form chloroaurate anions:

Au³⁺(aq) + 4Cl^-(aq) -----> AuCl₄^-(aq)

However, aqua regia will dissolve all the metals together which introduces impurities into the recycling process. Georgia Institute of Technology scientists have developed a new organic solvent process that may solve this problem since the concentration of each component of the solvent can be adjusted to preferentially dissolve gold or palladium, but will not dissolve platinum. This solvent has been dubbed organic aqua regia.

Reference
Wei Lin, Rong-Wei Zhang, Seung-Soon Jang, Ching-Ping Wong, Jung-Il Hong. 'Organic Aqua Regia'-Powerful Liquids for Dissolving Noble Metals. Angewandte Chemie, 2010; 122 (43): 8101 DOI: 10.1002/ange.201001244

Further Reading
http://www.ausetute.com.au/acidbase.html
http://www.ausetute.com.au/trmetals.html
http://www.ausetute.com.au/ligands.html
http://www.ausetute.com.au/redox.html
http://www.ausetute.com.au/calcelemf.html

Study Questions

1. What is meant by the term transition metal?
2. What are the typical properties of a transition metal?
3. Refer to the equation above for the reaction between chromium and hydrochloric acid. Is this an example of a redox reaction? Explain your answer.
4. Refer to the equation above for the reaction between gold and nitric acid. Has the gold been oxidised or reduced? Explain your answer.
5. Refer to the equation above for the reaction between gold and nitric acid. Has nitrogen been oxidised or reduced? Explain your answer.
6. Could the reaction between gold and nitric acid be described as a redox reaction? Explain your answer.
7. Refer to the equation for the reaction between gold (III) ions and chloride ions. Is this a redox reaction? Explain your answer.
8. Write suitable equations for dissolving the following metals in aqua regia:

• iron
• tin
• copper
  
• silver
• platinum
• palladium