Tests for fluoride ion, Fluorine, HF

Fluorine and fluoride differs from other halogens and halides respectively. Hydrogen fluoride is a weak acid and other hydrogen halides are strong acids. Fluorine's oxidation power is higher than other halogens. Salts of fluorides are almost neutral in aqueous solutions. The F- ion is colourless.

Hydrofluoric acid, HF

  1. Volatile liquid
  2. Boiling temperature is 19.50C
  3. HF vapour has a pungent smell and is highly poisonous
  4. A weak acid
  5. F- ions cannot be oxidized by chemical reagents to free F2 because F2 is the most powerful oxidizing agent known.

Qualitative tests for fluoride ion

Fluoride ion has some difference behavior respect to other halide ions. Therefore we cannot do same qualitative tests for fluoride ion like other halide ions.

Qualitative analysis of halide ions, chloride, bromide, iodide

Experiments to identify fluoride (F-) ion

Calcium fluoride and concentrated sulphuric acid | CaF2 + H2SO4

It will give HF(g)

Reaction of calcium fluoride and concentrated sulphuric acid

Formation of SiF4

A characteristic peculiarity of HF is its reaction with SiO2 and silicates to form gaseous silicon fluoride, SiF4.

Solubility of fluoride ion

  • AgF is readily soluble in water.
  • Fluorides of alkali metals, ammonium, aluminium, tin and mercury are soluble in water.
  • Fluorides of alkaline earth metals are insoluble in water.
  • Potassium fluoride (KF) is soluble in HF.

concentrated sulphuric acid and solid fluoride compound reaction.

HF vapor is released. That vapor can turn glass bristle into gelatinize state.

Differences of fluorine and fluoride comparing with other halogens and halide ions

There are very different characteristics of fluorine(F2) and other halogens(Cl2, Br2, I2). Also F- ion also shows different characteristics than other halides (Cl-, Br-, I-).

differences of fluorine and other halogens

  • Fluorine only shows -1 and 0 oxidation states. But other halogens show positive oxidation states too.
  • Oxidation power of F2 is much higher than other halogens.
  • F2 forms compounds with inert gases such as Kr, Xe. But other halogens doesn't.
  • F's ionization enthalpy is greater than other halogens.
  • Electronegativity of F is much higher than other halogens.
  • F makes strong bonds with non metals.
  • F2 can oxidize water.

Water and fluorine reaction | H2O + F2

Reaction of H2O and F2 are occurred in two ways.

Send fluorine slowly through water

If F2 is sent slowly to the water, HF and O2 are given as products.

F2 + H2O = O2 + HF

Send excess fluorine quickly through water

When F2 is sent quickly and in excess amount to water, HF and O3 are given as products.

excess F2 + H2O

Hydrolysis of fluorine

F is a non metal. Therefore F2 hydrolyses. Due to F's higher electronegativity F2O is formed.

F2 hydrolysis

When producing F2O by bubbling F2 in lower temperatures, solubility is less. Therefore F2 is bubbled through 2% dilute NaOH to prepare F2O efficiently.

differences of fluoride and other halides

  • HF is a weak acid in water. Other hydrogen halide acids are strong acids.
  • AgF and PbF2 are soluble in water. But other halides of Ag and Pb are insoluble in water.
  • LiF is insoluble in water due to its high lattice energy because of small cation and small anion size.
  • Fluorides of alkali earth metals are sparingly soluble in water.

More to learn about fluoride

  • Hydrogen fluoride formed acts on the silicates of the wall of the test tube. It forms gaseous silicon fluoride, SiF4.
  • BaCl2 forms a bulky white precipitate of BaF2 which is soluble in excess mineral acid and in ammonium salts, with the F- ion

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