Hydrogen fluoride-properties-uses with pH calculation

Hydrogen fluoride details in halogen chemistry

Hydrogen fluoride is a chemical compound of hydrogen and fluorine with molecular formula, HF. It is a polar covalent hydride. Hydrogen fluoride is a colorless, odorless, highly poisonous gas.

Hydrogen fluoride is a mono basic weak acid when dissolved in water. The pKa value of HF acid is 3.4.

The molecular weight of hydrogen fluoride is 20.01. The melting and boiling point of hydrogen fluoride are, –83.6 ᵒC and 19.5 ᵒC respectively.

Although, hydrogen fluoride is a covalent hydride yet it has more ionic character, due smaller size and high electronegativityof fluorine atom.

Due to this ionic character of HF molecule, the bond dissociation energy of HF is higher than that of other halogen acids. The dipole moment of H–F molecule is 1.86 D.

Hydrogen fluoride formula and Lewis dot structure

The chemical formula of hydrogen fluoride is HF. Hydrogen fluoride molecules are associated both in aqueous solution and in solid state through hydrogen bonding. The Lewis dot structure of hydrogen fluoride is shown below.

Hydrogen fluoride preparation

There are different methods for the preparation of hydrogen fluoride gas.

For example, reaction of hydrogen with very reactive fluorine element produce hydrogen fluoride.

Since fluorine is very reactive, hence it reacts with hydrogen explosively, even in the dark, forming hydrogen fluoride. 

Hydrogen fluoride is also prepared industrially and in the laboratory by the 

reaction of calcium fluoride or sodium chloride individually with concentrated sulfuric acid.

Hydrogen fluoride properties details in chemistry

Hydrogen fluoride hydrogen bonding

Hydrogen fluoride is liquid at room temperature. Because, being polar hydrogen fluoride molecules are associated through inter molecular hydrogen bonding.

In solid state, hydrogen fluoride forms a zig-zag type of inter molecular hydrogen bonding in which the bond angle of H – F ……H is 116ᵒ.

Although, the chain is shorter and it consist of five or six HF molecule.

Hydrogen fluoride is liquid at room temperature

Hydrogen fluoride is liquid at room temperature due to formation of inter molecular hydrogen bonding. HF molecules get associated through the formation of inter molecular hydrogen bonding.

Hydrogen fluoride melting point and boiling point

Hydrogen fluoride molecules get associated through the formation of inter molecular hydrogen bonding and hence HF isliquid at room temperature.

But the other halogen acids do not form hydrogen bonding.Now, some amount of heat energy is required to break this hydrogen bonding in HF molecule.

Consequently, the boiling point and melting point of hydrogen fluoride is higher than that of other halogen acids [ HCl, HBr, HI etc ].

Although, the melting point of hydrogen fluoride molecule is than HI .Because of the higher molecular weight and greater inter-molecular Vander waal’s forces of HI molecule.

Dipole moment of hydrogen fluoride

Fluorine is more electronegative than other halogens. Hence H–F bond is more polar than H–Cl, H–Br and H–I bond.

Hence H–F molecule has a greater dipole moment value than other halogen acid. The calculated dipole moment value of hydrogen fluoride is 1.86 D.

Hydrogen fluoride bond dissociation energy

The bond dissociation energy of H–F bond is higher than H–Cl, H–Br and H–I bond. Hence, H – F is more stable compound than, HCl, HBr and HI molecules.

Since the bond dissociation energy of H–F bond is so high than other halogen acid, hence HF acid has a weak reducing properties than other halogen acid.

Hydrogen fluoride bond length and bond angle

Hydrogen fluoride is a diatomic molecule with molecular structure HF in gaseous state and H₂F₂ in aqueous solution.The bond length of hydrogen fluoride molecule is 98.8 Pico meter.

In solid state, hydrogen fluoride forms a zig-zag type of inter molecular hydrogen bonding in which the bond angle of H – F ……H is 116ᵒ.

Hydrogen fluoride pH calculation.

Hydrogen fluoride is highly soluble in water. The concentrated aqueous solution of hydrogen fluoride is known as hydrofluoric acid.

The density of hydrofluoric acid solution is 1.15 g/ml or 1150 g/litre. Now,the molecular weight of HF molecule is 20.01. But in aqueous solution, HF forms dimer structure due to abnormal association.

So, molecular weight of H₂F₂ dimer is 40.02 instead of 20.01. So, the molar strength of HF weak acid is 28.8 mole/liter or 28.8 (N).

Since HF is a weak acid and the pH of HF can’t be calculated directly. The pH of weak HF acid is calculated by determining the dissociation constant(K_a) of weak HF acid.

Now, the experimental K_a value of HF acid is 3.5 x 10 ^–4. So, pKa value of HF is 3.4. Therefore, the pH of weak HF acid is,

Hydrogen fluoride is a weak acid.

Hydrogen fluoride is highly soluble in water. The concentrated aqueous solution of hydrogen fluoride is known as hydrofluoric acid.

However, HF is a weak acid with compare to the other halogen acid, like HCl, HBr, HI etc.

Because, the bond dissociation energy of H – F bond is higher than H–Cl, H–Br and H–I bond. Hence, the degree of ionization of HF in aqueous solution is less than other halogen acids.

     

Besides, F–ion in aqueous solution gets associated with the non ionized HF molecule and form HF₂^–ion. As a result, HF acid produces lesser number of H+ ion in aqueous solution.

Hence hydrogen fluoride acid strength is less than H–Cl, H–Br and H–I acid. This fact has been proved by the measurement of their pKa value. The pKa value of HF,  HCl, HBr and HI acids are , +3.4 , –7 , –9 , –11 respectively.

The lesser the pKa value, higher is the acid strength.That is, HF is a weakest acid and HI is a strongest acid among halogen acids.

Hydrogen fluoride uses in organic and inorganic chemistry

There are many uses of hydrogen fluoride in organic and inorganic chemistry.

Hydrogen fluoride is used to make refrigerants, herbicides, pharmaceuticals, 

gasoline, stainless steel kitchen products, aluminum, plastics, electrical components and incandescent light bulbs etc.

60% of the hydrogen fluoride used in manufacturing of refrigerants, used in refrigeration, freezer and air conditioning systems.

It also is used in manufacturing silicon semiconductor chips. In general, anhydrous HF molecule is common industrially than its aqueous form, hydrofluoric acid.

Hydrogen fluoride is used as a solvent.

Hydrogen fluoride is one of the most important water like solvent. Since it is liquid in at temperatures below 19.5 C upto –89.4 C . 

The dipole moment of HF [1.86D] and dielectric constant [83.6] are also quite close to those of water. Because of its high dielectric constant, it serves as an excellent ionizing solvent.

Although, the use of HF is limited because, it is highly poisonous . HF causes the solubility, conductivity and solubility of compounds.

Organic compounds such as benzene, ethanol, alkanes get protonated when dissolved in HF. Because of the formation of ions, the above said organic compounds give conducting solution in HF.

The aromatic hydrocarbons give colored solution in HF and their solubilities can be increased further by adding fluoride acceptors as BF₃ and SbF₅. 

HF is used to prepare fluorocarbon from chloro carbon.

Hydrogen fluoride is used to prepare fluorocarbons from chloro-carbons. An 

important reaction of hydrogen fluoride to form chloro carbon is the production tetra fluoro ethylene or Teflon .

Again, HF reacts with acetylene in the presence of ‘Hg’ catalyst and form 1,1-difluoroethane.

HF is used to prepare fluorine and metal fluorides.

HF is the pioneer to elemental fluorine, by electrolysis of a solution of HF and anhydrous KHF₂. In this method a large amount of F₂ is prepared per every year.  

HF forms few addition compound with metal fluorides, such as, with KF, it form KHF2, KF.2HF, KF.3HF and NH4F.HF etc.   

HF is used as catalyst.

HF is used as a catalyst in alkylation process in refineries. It is used in the majority of the installed linear alkyl benzene production facilities.

Why is HF less acidic than HI ?

Hydrogen fluoride is highly soluble in water. The concentrated aqueous solution of hydrogen fluoride is known as hydrofluoric acid.

Now, fluorine is more electronegative than iodine. Hence HF bond is more polar than HI bond.

So, HF should be more acidic than HI. But actual order is reversed.That is, HF is a weak acid with compare to HI acid.

Because, the bond length of HF is shorter than HI but the bond dissociation energy of H – F bond is higher than H–I bond. Hence, the degree of ionization of HF in aqueous solution is less than HI acid.

       

Again, F–ion in aqueous solution gets associated with the non ionized HF molecule and form HF₂^– ion. As a result, HF acid produces lesser number of H+ ion in aqueous solution.

Hence HF acid strength is less than H–I acid.This fact has been proved by the measurement of their pKa value.

The pKa value of HF and HI acids are, +3.4 ,–11 respectively. The lesser the pKa value , higher is the acid strength. Besides, the heat of ionization of HI is –57 while that of HF is –12.

The larger the negative value of heat of ionization, the greater is the feasibility of the ionization reaction and hence the greater is the acidic strength of HI acid.

Consequently, HF is a weakest acid and HI is a strongest acid among halogen acids.

• What is hydrogen fluoride in chemistry?
• Properties of hydrogen fluoride.
• How to calculate the pH of HF acid ?
• Is hydrogen fluoride a strong acid ?
• Why is HF a weak acid ?
• Why is HF less acidic than HI ?
• What are uses of HF acid ?   

hydrogen fluoride, HF, hydrogen fluoride properties, hydrogen fluoride uses, hydrogen fluoride pH calculation, 

Read more: Details of hydro-chloric acid in chemistry