| [Visitor (220.202.*.*)]answers [Chinese ] | Time :2023-03-25 | | Fluoride can be selected to prevent dental caries, so what is the principle of fluoride to prevent dental caries? In fact, what we call worm teeth is mainly caused by bacteria, so there will be a film of bacteria on the surface of the teeth, which will be called dental plaque. The formation of caries is directly related to the pathogenic bacteria that make caries, but these pathogenic bacteria are not the direct damage to the teeth, but the occurrence of these bacteria together with food to create caries. |
| [Visitor (220.202.*.*)]answers [Chinese ] | Time :2023-03-25 |  hydrogen fluoride
Physical
Heat of gasification: 30.31kJ·mol;
Melting point (°C): -83.7
Relative density (water = 1):
1.15
Boiling point (°C): 19.5
Relative vapor density (air = 1):
1.27
Molecular Formula: HF
Molecular weight:
20.01
Main ingredients:
Saturated vapor pressure (kPa): 53.32(2.5°C)
Heat of combustion (kJ/mol):
Pointless
Critical temperature (°C): 188
Critical pressure (MPa):
6.48
Chemical properties
△fH:-271kJ·mol ;
△fG:-273kJ·mol ;
Key energy: 568.6kJ·mol;
Hydration heat: -48.14kJ·mol;
pKa:3.2 。
Hydrogen fluoride is chemically thermodynamically stable and hardly decomposes even at 1273K.
Gaseous HF is a mixture of HF monomer and cyclic hexamer (HF)6, and jagged chain HF polymers exist in solid HF.
Liquid HF is a very acidic solvent that protonates sulfuric and nitric acid. But HF is a weak acid in aqueous solution, when the concentration of HF exceeds 5 mol· At L, its acidity increases, but it still cannot be completely ionized .
1. It is highly corrosive, can erode glass and silicate to form gaseous silicon tetrafluoride, but does not corrode polyethylene, lead and platinum;
2. Very volatile, white fog when placed in the air, and heated to form hydrofluoric acid when dissolved in water;
3. React with metal salts, oxides and hydroxides to generate fluoride;
4. It can react with ordinary metals and release hydrogen to form an explosive mixture with air;
5. Hydrogen fluoride is thermally stable and only slightly decomposed when heated to 1000 °C;
6. React with silicon and silicon compounds to form gaseous silicon tetrafluoride: SiO2 (s) 4 HF (aq) → SiF4 (g) 2 H2O (l) to generate SiF4 can continue to interact with excess HF to generate fluorosilicic acid: SiF4 (g) 2HF (aq) = H2 [SiF6] (aq), fluorosilicic acid is a binary strong acid;
7. Hydrofluoric acid has two equilibrium in aqueous solution: 1.HF=H F- k1=7.2*10-4,2. HF F-=HF2- k2=5.2At this time, with the increase of concentration (greater than 5mol), HF is already a fairly strong acid.
8. Hydrofluoric acid can form acid salts, hydrofluoric acid is originally a monoacid, but can prepare a series of acid salts such as NaHF2, KHF2, NH4HF2, etc., this property is not available in the other three hydrohalic acids. |
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