Hydrogen chloride, interhalogen compounds and oxoacids of the halogens

The acid in your stomach

Your stomach makes hydrochloric acid, and so does a factory. The acid is a solution of hydrogen chloride gas in water. Let us see how the gas is made and how it compares with the other hydrogen halides.

Hydrogen chloride

In the laboratory, hydrogen chloride comes from common salt and concentrated sulphuric acid. At about 420 K the reaction stops at the hydrogen sulphate:

$\text{NaCl}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{NaHSO}_{4}\ +\ \text{HCl}$

At about 823 K the hydrogen sulphate reacts with more salt:

$\text{NaHSO}_{4}\ +\ \text{NaCl}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ \text{HCl}$

The gas is a colourless gas with a sharp smell. It fumes in moist air, and it is extremely soluble in water. Nearly every molecule gives up its hydrogen to water, so the solution is a strong acid: $\text{HCl}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{3}\text{O}^{+}\ +\ \text{Cl}^{-}$. The gas meets ammonia and forms white smoke: $\text{NH}_{3}\ +\ \text{HCl}\ \rightarrow \ \text{NH}_{4}\text{Cl}$.

Aqua regia. Gold and platinum do not dissolve in hydrochloric acid alone or in nitric acid alone. A mixture of 3 parts of concentrated hydrochloric acid and 1 part of concentrated nitric acid by volume does dissolve them. It is called aqua regia, royal water. The nitric acid oxidises the metal, and the chloride ions hold the metal ion as a complex:

$\text{Au}\ +\ 4\text{H}^{+}\ +\ \text{NO}_{3}^{-}\ +\ 4\text{Cl}^{-}\ \rightarrow \ \text{AuCl}_{4}^{-}\ +\ \text{NO}\ +\ 2\text{H}_{2}\text{O}$

$3\text{Pt}\ +\ 16\text{H}^{+}\ +\ 4\text{NO}_{3}^{-}\ +\ 18\text{Cl}^{-}\ \rightarrow \ 3\text{PtCl}_{6}^{2-}\ +\ 4\text{NO}\ +\ 8\text{H}_{2}\text{O}$

The hydrogen halides

The hydrogen halides HF, HCl, HBr and HI are colourless gases. We compare them.

• H-X bond enthalpy in kJ/mol: HF 574, HCl 428, HBr 363, HI 295. Books differ by a few kJ/mol. The bond gets weaker as the halogen gets larger.
• Boiling point in K: HF 293, HCl 188, HBr 206, HI 238. HF is highest because of hydrogen bonds. The others rise with the size of the molecule.
• Acid strength in water rises from HF to HI. The pKa values are about 3.2 for HF, $-7$ for HCl, $-9$ for HBr and $-10$ for HI. The weaker the H-X bond, the easier it is for the acid to give the proton.
• Reducing power also rises from HF to HI. The iodide ion gives up its electron the most easily.

Interhalogen compounds

Two different halogens can bond together. The result is an interhalogen compound. The formula is $\text{XY}_n$, with $n$ = 1, 3, 5 or 7. The larger halogen X is in the middle and the smaller, more electronegative halogen Y is round it.

• XY (n = 1): ClF, BrF, BrCl, ICl, IBr.
• XY3: ClF3, BrF3, IF3.
• XY5: IF5, BrF5, ClF5.
• XY7: IF7. It is the only one, because only the large iodine atom can hold seven fluorine atoms.

The number $n$ is odd, because the central atom uses its seven outer electrons in pairs: some in bonds with the atoms of Y and the rest in lone pairs.

The shapes follow from counting. The centre has 7 outer electrons. Each bond uses one. The remaining electrons make lone pairs: $\dfrac{7 - n}{2}$. The steric number is the number of bonded atoms plus the lone pairs. It gives the arrangement of the regions of electron density round the centre. Then we place the lone pairs in the positions with the most room.

• ICl: 1 bond and 3 lone pairs. Two atoms always lie on a straight line, so ICl is linear.
• ClF3: 3 bonds and 2 lone pairs, steric number 5 (trigonal bipyramid). The lone pairs go in the middle plane. The shape of the atoms is T-shaped.
• BrF5: 5 bonds and 1 lone pair, steric number 6 (octahedron). The shape of the atoms is a square pyramid.
• IF7: 7 bonds and no lone pair, steric number 7. The shape is a pentagonal bipyramid.

The hybridisation is $sp^3d$ for ClF3 and BrF3, $sp^3d^2$ for the XY5 compounds and $sp^3d^3$ for IF7. IB books speak of electron domains, not regions. They mean the same count.

Properties. Most are volatile solids or liquids. ClF is a gas. They are very reactive. NCERT explains it by saying that the X-X' bond is weaker than the X-X bond, except for F-F. The measured bond enthalpies do not all follow this rule, and the real cause is mostly that fluorine is such a strong oxidising agent. Chlorine trifluoride, $\text{ClF}_{3}$, is used to make uranium hexafluoride for nuclear fuel: $\text{U}\ +\ 3\text{ClF}_{3}\ \rightarrow \ \text{UF}_{6}\ +\ 3\text{ClF}$. They hydrolyse to give the halide ion of the more electronegative halogen and an oxoacid of the other:

$\text{ICl}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{HCl}\ +\ \text{HOI}$

$\text{IF}_{5}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ \text{HIO}_{3}\ +\ 5\text{HF}$

$\text{IF}_{7}\ +\ 6\text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{5}\text{IO}_{6}\ +\ 7\text{HF}$

Set $n = 3$ and read the bar: 3 bonded atoms and 2 lone pairs, so 5 regions. The sim says T-shaped. Now move to 5 and then to 7 and watch the red bar vanish.

Practise with new numbers

The oxoacids of chlorine

Chlorine forms four oxoacids. Fluorine forms only one, HOF. It is unstable.

• HOCl, hypochlorous acid: chlorine is $+1$.
• HClO2, chlorous acid: chlorine is $+3$.
• HClO3, chloric acid: chlorine is $+5$.
• HClO4, perchloric acid: chlorine is $+7$.

The acid strength rises in the order HOCl, HClO2, HClO3, HClO4. Two things explain it. The oxygen atoms without a hydrogen pull electron density from the O-H bond and make it easier to lose the proton. After the proton has gone, the negative charge of the anion is spread over more oxygen atoms, so the anion is more stable. A rough rule of Pauling gives the first pKa as $8 - 5n$, where $n$ is the number of oxygen atoms with no hydrogen. The measured values are about 7.5, 2, $-1$ and $-8$ for the four acids.

The acid strength of the hypohalous acids falls in the order HOCl > HOBr > HOI, because the halogen gets less electronegative.

The anions have shapes that follow VSEPR. The hypochlorite ion $\text{ClO}^-$ is linear. The chlorite ion $\text{ClO}_2^-$ is bent. The chlorate ion $\text{ClO}_3^-$ is a trigonal pyramid, with a lone pair on chlorine. The perchlorate ion $\text{ClO}_4^-$ is a tetrahedron. Sodium hypochlorite, $\text{NaOCl}$, is household bleach. Potassium chlorate loses oxygen when heated: $2\text{KClO}_{3}\ \rightarrow \ 2\text{KCl}\ +\ 3\text{O}_{2}$, and heating it more gently gives the perchlorate: $4\text{KClO}_{3}\ \rightarrow \ 3\text{KClO}_{4}\ +\ \text{KCl}$.

Try it in Python

The program counts the lone pairs of each interhalogen compound from the seven outer electrons of the centre and names its shape.

Common mistakes

• Counting HF as a strong acid because the H-F bond is polar. The bond is the strongest of the four, so HF is the weakest.
• Forgetting the lone pairs on the centre. ClF3 is T-shaped, not trigonal planar.
• Putting the smaller halogen in the middle. The larger halogen is the centre.
• Using aqua regia in the wrong ratio. It is 3 parts of hydrochloric acid to 1 part of nitric acid.

Remember these

• Hydrogen chloride: $\text{NaCl}\ +\ \text{H}_{2}\text{SO}_{4}$ at about 420 K, then $\text{NaHSO}_{4}\ +\ \text{NaCl}$ at about 823 K.
• Acid strength: HF < HCl < HBr < HI. Aqua regia is 3 : 1 hydrochloric to nitric acid.
• Interhalogens: linear (XY), T-shaped (XY3), square pyramidal (XY5), pentagonal bipyramidal (XY7).
• Acid strength of the oxoacids: HOCl < HClO2 < HClO3 < HClO4. The perchlorate ion is a tetrahedron.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is HF a weaker acid than HCl in water?

2. How many lone pairs are on bromine in BrF5, and what is the shape?

3. Why is perchloric acid stronger than hypochlorous acid?

4. What is the oxidation state of chlorine in $\text{ClO}_{3}^{-}$ and what is the shape of the ion?

References

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements, on hydrogen chloride, interhalogen compounds and the oxoacids of the halogens.
• OpenStax, Chemistry 2e, Chapter 7 (molecular geometry), and Chapter 18 (the halogens).