Rubidium fluoride
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| Template:Longitem | RbF |
| Molar mass | 104.4662 g/mol |
| Appearance | white crystalline solid |
| Density | 3.557 g/cm3 |
| Melting point | Template:Chembox CalcTemperatures |
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| Template:Longitem | −31.9·10−6 cm3/mol |
| NFPA 704 (fire diamond) | Template:NFPA 704 diamond |
| Flash point | Template:Chembox CalcTemperatures |
| Template:Longitem | Rubidium chloride Rubidium bromide Rubidium iodide Rubidium astatide |
| Template:Longitem | Lithium fluoride Sodium fluoride Potassium fluoride Caesium fluoride Francium fluoride |
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Rubidium fluoride (RbF) is the fluoride salt of rubidium. It is a cubic crystal with rock-salt structure.
Synthesis
There are several methods for synthesising rubidium fluoride. One involves reacting rubidium hydroxide with hydrofluoric acid:[1]
- RbOH + HF → RbF + H2O
Another method is to neutralize rubidium carbonate with hydrofluoric acid:[1]
- Rb2CO3 + 2HF → 2RbF + H2O + CO2
Another possible method is to react rubidium hydroxide with ammonium fluoride:
- RbOH + NH4F → RbF + H2O + NH3
The least used method due to expense of rubidium metal is to react it directly with fluorine gas, as rubidium reacts violently with halogens:[1]
- 2Rb + F2 → 2RbF
Properties
Rubidium fluoride is a white crystalline substance with a cubic crystal structure that looks very similar to common salt (NaCl). The crystals belong to the space group Fm3m (space group no. 225) with the lattice parameter a = 565 pm and four formula units per unit cell.[2] The refractive index of the crystals is nD = 1.398.[2] 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·3H2O and a third hydrate with the composition 3RbF·H2O.[3]
In addition to simple rubidium fluoride, an acidic rubidium fluoride with the molecular formula HRbF2 is also known,[4] which can be produced by reacting rubidium fluoride and hydrogen fluoride.[4] The compounds H2RbF3 and H3RbF4 were also synthesized.[5][4]
The solubility in acetone is 0.0036 g/kg at 18 °C and 0.0039 g/kg at 37 °C.[6]
The standard enthalpy of formation of rubidium fluoride is ΔfH0298 = −552.2 kJ mol−1,[7] the standard free enthalpy of formation ΔG0298 = −520.4 kJ mol−1,[7] and the standard molar entropy S0298 = 113.9 J K −1 ·mol−1.[7] The enthalpy of solution of rubidium fluoride was determined to be −24.28 kJ/mol.[8]
References
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