Rubidium fluoride (RbF) is the
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...
salt of
rubidium
Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...
. It is a
cubic
Cubic may refer to:
Science and mathematics
* Cube (algebra), "cubic" measurement
* Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex
** Cubic crystal system, a crystal system w ...
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
with
rock-salt structure.
Synthesis
There are several methods for synthesising rubidium fluoride. One involves reacting
rubidium hydroxide with
hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling p ...
:
:RbOH + HF → RbF + H
2O
Another method is to neutralize
rubidium carbonate with hydrofluoric acid:
:Rb
2CO
3 + 2HF → 2RbF + H
2O + CO
2
Another possible method is to react rubidium hydroxide with
ammonium fluoride:
:RbOH + NH
4F → RbF + H
2O + NH
3
The least used method due to expense of rubidium metal is to react it directly with fluorine gas, as rubidium reacts violently with
halogen
The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...
s:
:2Rb + F
2 → 2RbF
Properties
Rubidium fluoride is a white crystalline substance with a cubic crystal structure that looks very similar to common
salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
(NaCl). The crystals belong to the
space group
In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...
Fm3m (space group no. 225) with the lattice parameter ''a'' = 565 pm and four formula units per
unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector
In mathematics, a unit vector i ...
.
The
refractive index
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of the crystals is nD = 1.398.
Rubidium fluoride colors a flame (Bunsen burner flame) purple or magenta red (spectral analysis).
Rubidium fluoride forms two different hydrates, a sesquihydrate with the stoichiometric composition 2RbF·3H
2O and a third hydrate with the composition 3RbF·H
2O.
In addition to simple rubidium fluoride, an acidic rubidium fluoride with the molecular formula HRbF
2 is also known,
which can be produced by reacting rubidium fluoride and hydrogen fluoride.
The compounds H
2RbF
3 and H
3RbF
4 were also synthesized.
The solubility in acetone is 0.0036 g/kg at 18 °C and 0.0039 g/kg at 37 °C.
The standard enthalpy of formation of rubidium fluoride is Δ
fH
0298 = −552.2 kJ mol−1,
the standard free enthalpy of formation ΔG
0298 = −520.4 kJ mol−1,
and the standard molar entropy S
0298 = 113.9 J K −1 ·mol−1.
The enthalpy of solution of rubidium fluoride was determined to be −24.28 kJ/mol.
References
*
{{DEFAULTSORT:Rubidium Fluoride
Rubidium compounds
Fluorides
Alkali metal fluorides
Rock salt crystal structure