HF acid-properties-strength-pH calculation

Important properties of HF acid in chemistry

Hydrofluoric acid is also known as hydrogen fluoride gas which is a chemical compound of hydrogen and fluorine with molecular formula, HF. 

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

Density of hydrofluoric acid 1.15 g/c.c.It is a polar covalent compound of hydrogen and fluorine elements. The dipole moment of H–F acid is 1.86 D.

Hydrofluoric acid is a colorless, odorless and highly poisonous chemical of fluorine element.It is a mono basic weak acid.The pKa value of HF acid is 3.4.

The melting and boiling point of hydrofluoric acid are, –83.6 ᵒC and 19.5 ᵒC respectively.

Although, hydrofluoric acid is a covalent hydride of hydrogen yet it has more ionic character, due smaller size and high electronegativity of fluorine atom.

Due to this ionic character, the bond dissociation energy of HF acid is higher than that of other halogen acids.

Hydrofluoric acid preparation

A small amount of anhydrous hydrogen fluoride is obtained by heating dry KHF2 in a copper retort which connected with a copper condenser.

Hydrogen fluoride is also prepared industrially and in the laboratory by heating of fluorspar and sodium chloride individually with concentrated sulfuric acid in a lead vessel.

The producing hydrogen fluoride distils over and the vapors are condensed in water taken in lead container. 

Aqueous hydrogen fluoride, that is, hydrofluoric acid thus obtained is stored in gutta parcha or wax bottles.

 Properties of HF acid in chemistry

Physical state

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

In solid crystalline, hydrogen forms a zig-zag type of inter molecular hydrogen bonding. Although, the chain is short and it consist of five or six HF molecule.

Melting and boiling point of HF acid

Due to formation of inter molecular hydrogen bonding, HF molecules get associated.

As a result, the boiling point and melting point of hydrogen fluoride acid 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 this higher molecular weight and greater inter-molecular Vander waal’s force of HI molecule.

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 acid has a greater dipole moment value than other halogen acid. The calculated dipole moment value of hydrogen fluoride is 1.86 D.

Bond dissociation energy and stability

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

Reducing properties

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 acids.

 HF acid strength

The bond dissociation energy of H–F bond is higher than that of 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, hydrofluoric acid is a weak acid among all the halogen hydracids. The calculated pKa value of hydrofluoric acid is 3.4 .

HF acid pH calculation in chemistry

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/liter.

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 determined 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 0.97.

Uses of hydrofluoric acid.

In general, anhydrous HF molecule is common industrially than its aqueous form, hydrofluoric acid. 

There are many application of hydrofluoric acid in chemistry. Some uses of hydrofluoric acid is discussed below.

 HF is used as a solvent.

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

Because of its high dielectric constant[ 83.6], 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 different chemical compounds

HF is used to prepare fluorocarbons from chloro-carbons. For example, HF is react with 1,1,2-trichloroethene to form 1,1,1,2-tetrafluoroethane.

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

HF is used as catalyst.

HF is used as a catalyst in Friedel craft alkylation of benzene to prepare ethyl benzene and cyclohexene to cyclohexylbenzene.

Is HF more stronger 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 HIacid.

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

Important properties of HF acid in chemistry

HF acid strength in halogen chemistry

Is HF more stronger than HI ?

Calculation of pH of HF acid in chemistry

Uses of hydrofluoric acid

Read more: Uses of hydrochloric acid