Oxygen, Ozone and Sulphur: Quick Revision

A brief note on Oxygen, Ozone and Sulphur. Read it before a test. Try each question at the end before you open its answer.

The group and oxygen

• Group 16 is $ns^2np^4$: O, S (non-metals), Se, Te (metalloids), Po (metal). States $-2$, $+2$, $+4$, $+6$.
• Oxygen is small, has no d orbitals, so its covalency is at most 2. It forms hydrogen bonds, and $\text{O}_{2}$ is paramagnetic.
• $\text{H}_{2}\text{E}$: angle falls from 104.5° ($\text{H}_{2}\text{O}$) to about 90°. Acidity and reducing power rise, and stability falls, down the group.
• Oxygen: $2\text{KClO}_{3}\ \xrightarrow{\text{MnO}_{2},\ \ \text{heat}}\ 2\text{KCl}\ +\ 3\text{O}_{2}$, and from liquid air in industry.
• Oxides: basic (metals), acidic (non-metals), amphoteric ($\text{Al}_{2}\text{O}_{3}$, $\text{ZnO}$), neutral ($\text{CO}$, $\text{NO}$, $\text{N}_{2}\text{O}$). Peroxide $\text{O}_{2}^{2-}$, superoxide $\text{O}_{2}^{-}$.

Ozone and sulphur

• Ozone: $3\text{O}_{2}\ \rightleftharpoons \ 2\text{O}_{3}$ in a silent discharge, bent at 117°, a strong oxidiser. $\text{PbS}\ +\ 4\text{O}_{3}\ \rightarrow \ \text{PbSO}_{4}\ +\ 4\text{O}_{2}$.
• CFCs give chlorine atoms, which catalyse the loss of ozone in the stratosphere.
• Sulphur: rhombic (stable below 369 K) and monoclinic (above 369 K), both $\text{S}_{8}$ crowns.
• $\text{SO}_{2}$: bent at about 119°, acidic, a reducing agent, a temporary bleach, and a cause of acid rain. It turns acidified dichromate green.

Sulphuric acid

• Contact process: $2\text{SO}_{2}\ +\ \text{O}_{2}\ \rightleftharpoons \ 2\text{SO}_{3}$ over $\text{V}_{2}\text{O}_{5}$, about 720 K and 1 to 2 bar. Absorb $\text{SO}_{3}$ in concentrated acid to give oleum, then dilute.
• Concentrated $\text{H}_{2}\text{SO}_{4}$ is a dehydrating agent, an oxidiser when hot ($\text{Cu}\ +\ 2\text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{CuSO}_{4}\ +\ \text{SO}_{2}\ +\ 2\text{H}_{2}\text{O}$) and non-volatile.
• Oxoacids: $\text{H}_{2}\text{SO}_{3}$ $+4$, $\text{H}_{2}\text{S}_{2}\text{O}_{6}$ $+5$, and $\text{H}_{2}\text{SO}_{4}$, $\text{H}_{2}\text{S}_{2}\text{O}_{7}$, $\text{H}_{2}\text{SO}_{5}$, $\text{H}_{2}\text{S}_{2}\text{O}_{8}$ at $+6$. The last two have O-O links.

A worked example

Find the volume of sulphur dioxide from $8.0\ \text{kg}$ of sulphur at $273\ \text{K}$ and $1\ \text{atm}$.

1. Moles. $\dfrac{8000\ \text{g}}{32\ \text{g/mol}} = 250\ \text{mol}$.
2. Ratio. One mole of sulphur gives one mole of sulphur dioxide.
3. Volume. $250\ \text{mol} \times 22.4\ \text{L/mol} = 5600\ \text{L}$.

Practice with new numbers

A program to try

The program follows sulphur through the contact process to sulphuric acid.

Mistakes to avoid

• Adding water to concentrated sulphuric acid. Add the acid to the water.
• Saying that sulphur dioxide bleaches by oxidation. It bleaches by reduction.
• Absorbing sulphur trioxide in water. It makes a mist. Use concentrated acid.
• Giving sulphur $+8$ in a peroxo acid. The peroxide oxygen atoms are $-1$.

Check yourself

1. Which oxide of the list $\text{CO}$, $\text{Al}_{2}\text{O}_{3}$, $\text{SO}_{2}$ is amphoteric?

2. What colour change shows sulphur dioxide with acidified dichromate?

3. Why is a catalyst used in the contact process?

4. What is the state of sulphur in $\text{H}_{2}\text{S}_{2}\text{O}_{8}$?

Sources

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements.
• OpenStax, Chemistry 2e, Chapter 18.