The rhodium It is a transition metal that belongs to the palladium group and whose chemical symbol is Rh. It is noble, inert under normal conditions, while it is rare and expensive, since it is the second least abundant metal in the earth's crust. Also, there are no minerals that represent a profitable method of obtaining this metal..
Although its appearance is that of a typical silvery-white metal, most of its compounds share a reddish coloration in common, in addition to the fact that their solutions appear pinkish tones. That is why this metal was given the name 'rhodon', which is Greek for pink..
However, its alloys are silver, as well as expensive, since it is mixed with platinum, palladium and iridium. Its high noble character makes it a metal almost immune to oxidation, as well as totally resistant to attack by strong acids and bases; therefore, their coatings help protect metal objects, such as jewelry.
In addition to its ornamental use, rhodium can also protect tools used at high temperatures and in electrical devices..
It is popularly known best for helping to break down toxic car gases (NOx) inside the catalytic converters. It also catalyzes the production of organic compounds, such as menthol and acetic acid..
Interestingly, it only exists in nature as the isotope 103Rh, and its compounds are easy to reduce to metal due to its noble character. Of all its oxidation numbers the +3 (Rh3+) is the most stable and abundant, followed by +1 and, in the presence of fluoride, +6 (Rh6+).
In its metallic state it is harmless to our health, unless we breathe its dispersed particles in the air. However, its colored compounds or salts are considered carcinogens, in addition to being strongly attached to the skin.
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The discovery of rhodium was accompanied by that of palladium, both metals were discovered by the same scientist: the English chemist William H. Wollaston, who by 1803 was examining a platinum mineral, supposedly from Peru..
I knew from Hippolyte-Victor Collet-Descotils, a French chemist, that there were reddish salts in platinum minerals, the color of which was probably due to an unknown metallic element. Thus, Wollaston digested his platinum ore in aqua regia, then neutralized the acidity of the resulting mixture with NaOH..
From this mixture Wollaston had to, by means of precipitation reactions, gradually separate the metallic compounds; separated platinum as (NH4)two[PtCl6], after adding NH4Cl, and other metals reduced them with metallic zinc. He tried to dissolve these spongy metals with HNO3, leaving two metals and two new chemical elements: palladium and rhodium.
However, when he added aqua regia, he noticed that a metal was hardly dissolving, at the same time that it formed a red precipitate with the NaCl: Na3[RhCl6] NHtwoO. This is where its name came from: the red color of its compounds, designated with the Greek word 'rhodon'.
This salt was reduced with metallic zinc, again, thus obtaining spongy rhodium. And since then the obtaining techniques improved, as did the demand and technological applications, finally appearing shiny rhodium pieces..
Hard, silvery white metal with virtually no oxide layer at room temperature. However, it is not a very malleable metal, which means that when struck it will crack.
102.905 g / mol
1964 ° C. This value is higher than that of cobalt (1495 ºC), which reflects an increase in the strength of the strongest metallic bond as it descends through the group..
3695 ° C. It is one of the metals with the highest melting points.
-12.41 g / mL at room temperature
-10.7 g / mL at the melting point, that is, just when it melts or melts
26.59 kJ / mol
493 kJ / mol
24.98 J / (mol K)
2.28 on Pauling scale
-First: 719.7 kJ / mol (Rh+ gaseous)
-Second: 1740 kJ / mol (Rhtwo+ gaseous)
-Third: 2997 kJ / mol (Rh3+ gaseous)
150 W / (m K)
43.3 nΩ · m at 0 ºC
6
Paramagnetic
Rhodium, although it is a noble metal, does not mean that it is an inert element. It hardly rusts under normal conditions; but when it is heated above 600 ºC, its surface begins to react with oxygen:
Rh (s) + Otwo(g) → RhtwoOR3(s)
And the result is that the metal loses its characteristic silver shine..
It can also react with fluorine gas:
Rh (s) + Ftwo(g) → RhF6(s)
The RhF6 it is black. If it heats up, it can transform into RhF5, releasing fluoride to the environment. When the fluorination reaction takes place in dry conditions, the formation of RhF is favored3 (red solid) above that of RhF6. Other halides: RhCl3, RhBr3 and RhI3 are formed in a similar way.
Perhaps the most surprising thing about metallic rhodium is its extreme resistance to attack by corrosive substances: strong acids and strong bases. Aqua regia, a concentrated mixture of hydrochloric and nitric acids, HCl-HNO3, may be difficult to dissolve, resulting in a pinkish solution.
Molten salts, such as KHSO4, are more effective in dissolving it, as they lead to the formation of water-soluble rhodium complexes.
The rhodium atoms crystallize into the face-centered cubic structure, fcc. The Rh atoms remain united thanks to their metallic bond, a force responsible on a macro scale for the measurable physical properties of the metal. In this bond the valence electrons intervene, which are given according to the electronic configuration:
[Kr] 4d8 5s1
It is therefore an anomaly or exception, since it would be expected that it would have two electrons in its 5s orbital, and seven in the 4d orbital (obeying the Moeller diagram).
There are a total of nine valence electrons that, together with the atomic radii, define the fcc crystal; structure that appears to be very stable, since little information is found of other possible allotropic forms under different pressures or temperatures.
These Rh atoms, or rather their crystalline grains, can interact in such a way that they create nanoparticles with different morphologies..
When these Rh nanoparticles grow on top of a template (a polymeric aggregate, for example), they acquire the shapes and dimensions of its surface; thus, mesoporous rhodium spheres have been designed to supplant the metal in certain catalytic applications (which accelerate chemical reactions without being consumed in the process).
Since there are nine valence electrons, it is normal to suppose that rhodium can “lose them all” in its interactions within a compound; that is, assuming the existence of the Rh cation9+, with an oxidation number or state of 9+ or (IX).
The positive oxidation numbers found for rhodium in its compounds vary from +1 (Rh+) to +6 (Rh6+). Of all of them, +1 and +3 are the most common, along with +2 and 0 (metallic rhodium, Rh0).
For example, in the RhtwoOR3 the oxidation number of rhodium is +3, since if it assumes the existence of Rh3+ and a 100% ionic character, the sum of the charges will be equal to zero (Rhtwo3+OR3two-).
Another example is represented by the RhF6, in which now its oxidation number is +6. Again, only the total charge of the compound will remain neutral if the existence of Rh is assumed.6+ (Rh6+F6-).
The more electronegative the atom with which rhodium interacts, the greater its tendency to show more positive oxidation numbers; such is the case of RhF6.
In the case of Rh0, corresponds to its atoms of the crystal fcc coordinated with neutral molecules; for example, CO, Rh4(CO)12.
Unlike other metals, there is no mineral available that is rich enough in rhodium to make it economical to obtain from it. That is why it is rather a by-product of the industrial production of other metals; specifically the noble or their congeners (the elements of the platinum group), and nickel.
Most of the minerals used as raw materials come from South Africa, Canada and Russia.
The obtaining process is complex because, even though it is inert, rhodium is found in the company of other noble metals, in addition to having impurities that are difficult to remove. Therefore, several chemical reactions must be carried out to separate it from the initial mineralogical matrix..
Its low chemical reactivity keeps it unchanged while the first metals are being extracted; until only the nobles remain (the gold among them). These noble metals are then treated and melted in the presence of salts, such as NaHSO.4, to have them in a liquid mixture of sulfates; in this case, the Rhtwo(SW4)3.
To this mixture of sulfates, from which each metal is precipitated separately through different chemical reactions, NaOH is added to form rhodium hydroxide, Rh (OH)x.
The Rh (OH)x it redissolves adding HCl and thus form H3RhCl6, which is still dissolved and shows a pink color. After, H3RhCl6 reacts with NH4Cl and NaNOtwo to precipitate as (NH4)3[Rh (NOtwo)6].
Again, the new solid is redissolved in more HCl and the medium is heated until a sponge of metallic rhodium precipitates while the impurities are combusted..
Its noble character is used to cover metallic pieces with a coating of the same. In this way, silver objects are plated with rhodium to protect it from oxidizing and darkening (forming a black layer of AgO and AgtwoS), plus they become more reflective (shiny).
Such coatings are used in jewelry garments, reflectors, optical instruments, electrical contacts, and X-ray filters in breast cancer diagnostics..
It is not only a noble metal but also a hard one. This hardness can be contributed to the alloys it makes up, especially when it comes to palladium, platinum and iridium; of which, those of Rh-Pt are the best known. Also, rhodium improves the resistance of these alloys to high temperatures.
For example, rhodium-platinum alloys are used as a material to make glasses that can shape molten glass; in the manufacture of thermocouples, capable of measuring high temperatures (more than 1000 ºC); crucibles, bushings for cleaning glass fibers, induction furnace coils, aircraft turbine engines, spark plugs, etc..
Rhodium can catalyze reactions either as a pure metal or coordinated with organic ligands (organorodiums). The type of catalyst depends on the specific reaction to be accelerated, as well as other factors..
For example, in its metallic form it can catalyze the reduction of nitrogen oxides, NOx, to ambient gases oxygen and nitrogen:
2 NOx → x Otwo + Ntwo
This reaction occurs constantly on a daily basis: in the catalytic converters of vehicles and motorcycles. Thanks to this reduction, the gases NOx they do not pollute cities to a worse degree. For this purpose, mesoporous rhodium nanoparticles have been used, which further improve the decomposition of NO gases.x.
The compound [RhCl (PPh3)3], known as a Wilkinson catalyst, is used to hydrogenate (add Htwo) and hydroformylate (add CO and Htwo) alkenes, to form alkanes and aldehydes, respectively.
Rhodium catalysts are briefly used to hydrogenate, carbonylate (add CO), and hydroformylate. The result is that many products are dependent on them, such as menthol, an essential chemical compound in chewing gum; in addition to nitric acid, cyclohexane, acetic acid, organosilicon, among others.
Rhodium, being a noble metal, even if it seeped into our body, its Rh atoms could not (as far as it knows) be metabolized. Therefore, they do not pose any health risk; unless there are too many Rh atoms scattered in the air, which could end up accumulating in the lungs and bones.
In fact, in the processes of rhodium plating on jewelery or silver jewelery jewelers are exposed to these “puffs” of atoms; reason for which they have suffered from discomfort in their respiratory system. Regarding the risk of its finely divided solid, it is not even flammable; except when burning in the presence of OFtwo.
Rhodium compounds are classified as toxic and carcinogenic, whose colors deeply stain the skin. Here is another clear difference in how the properties of a metallic cation vary compared to that of the metal from.
And finally, in ecological matters, the scarce abundance of rhodium and its lack of assimilation by plants makes it a harmless element in case of spills or waste; as long as it is metallic rhodium.
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