Question

Discuss the formation, reactions and structures of oxides of
sulphur and selenium.

Answer 1

Question #61746, Chemistry / Inorganic chemistry

Discuss the formation, reactions and structures of oxides of sulphur and selenium.

Answer

Both sulfur and selenium can form oxides with formula $\mathrm{EO}_2$ and $\mathrm{EO}_3$ .

Sulfur (IV) oxide can be easily obtained from the reaction between simple elements, burning of the sulfides, by the reduction of S(VI) compounds or by the replacement reaction between sulfides with strong acids:

$$S + O _ {2} = S O _ {2}$$ $$4 \mathrm {F e S} _ {2} + 1 1 \mathrm {O} _ {2} = 2 \mathrm {F e} _ {2} \mathrm {O} _ {3} + 8 \mathrm {S O} _ {2}$$ $$2 \mathrm {C a S O} _ {4} + \mathrm {C} = 2 \mathrm {C a O} + 2 \mathrm {S O} _ {2} + \mathrm {C O} _ {2}$$ $$\mathrm {N a} _ {2} \mathrm {S O} _ {3} + \mathrm {H} _ {2} \mathrm {S O} _ {4} = \mathrm {N a} _ {2} \mathrm {S O} _ {4} + \mathrm {S O} _ {2} + \mathrm {H} _ {2} \mathrm {O}$$

$\mathrm{SO}_2$ is a bent molecule with $\mathrm{C}_{2\mathrm{v}}$ symmetry point group. In terms of electron-counting formalism, the sulfur atom has an oxidation state of $+4$ and a formal charge of $+1$ . A valence bond theory approach considering just s and p orbitals would describe the bonding in terms of resonance between two resonance structures.

Sulfur (IV) oxide shows classical properties of the acidic oxide. It forms weak acid – $\mathrm{H}_2\mathrm{SO}_3$ and reacts with bases forming sulfite and hydrosulfite salts.

$$\mathrm {S O} _ {2} + \mathrm {N a O H} = \mathrm {N a H S O} _ {3}$$ $$\mathrm {S O} _ {2} + 2 \mathrm {N a O H} = \mathrm {N a} _ {2} \mathrm {S O} _ {3} + \mathrm {H} _ {2} \mathrm {O}$$

It is strong reducing agent and weak oxidizing agent.

$$2 \mathrm {S O} _ {2} + \mathrm {O} _ {2} = 2 \mathrm {S O} _ {3}$$ $$\mathrm {S O} _ {2} + \mathrm {C} = \mathrm {S} + \mathrm {C O} _ {2}$$

Sulfur (VI) oxide is usually obtained by oxidizing sulfur (IV) oxide, what was shown above.

Gaseous $\mathrm{SO}_3$ is a trigonal planar molecule of $\mathrm{D}_{3\mathrm{h}}$ symmetry. In terms of electron-counting formalism, the sulfur atom has an oxidation state of $+6$ and a formal charge of $+2$ . The Lewis structure consists of an $\mathrm{S} = \mathrm{O}$ double bond and two $\mathrm{S} - \mathrm{O}$ dative bonds without utilizing d-orbitals. The electrical dipole moment of gaseous sulfur trioxide is zero. This is a consequence of the $120^{\circ}$ angle between the S-O bonds.

It is classical acidic oxide, reacts with vases forming sulfates and hydrosulfates< anhydride to the sulfuric acid. Can react with sulfuric acid with formation of $\mathrm{H}_2\mathrm{S}_2\mathrm{O}_7$ , with hydro halides.

$$\mathrm {S O} _ {3} + \mathrm {N a O H} = \mathrm {N a H S O} _ {4}$$ $$\mathrm{SO_3} + \mathrm{NaOH} = \mathrm{Na_2SO_4} + \mathrm{H_2O}$$ $$\mathrm{SO_3} + \mathrm{HF} = \mathrm{HSO_3F}$$

Selenium (IV) oxide is obtained similar to the $\mathrm{SO_2}$, main method is the reaction of simple elements.

$$\mathrm{Se} + \mathrm{O_2} = \mathrm{SeO_2}$$

In contrast to the $\mathrm{SO_2}$, $\mathrm{SeO_2}$ is solid one-dimensional polymer, the chain consisting of alternating selenium and oxygen atoms. $\mathrm{SeO_2}$ is considered an acidic oxide, with corresponding selenous acid, but it is weaker in comparison to the $\mathrm{SO_2}$. Due to the +4 oxidation it can react as strong reducing agent and weak oxidizing agent.

$$\mathrm{SeO_2} + \mathrm{NaOH} = \mathrm{NaHSeO_3}$$ $$\mathrm{SeO_2} + 2\mathrm{NaOH} = \mathrm{Na_2SeO_3} + \mathrm{H_2O}$$ $$\mathrm{SeO_2} + \mathrm{H_2O_2} = \mathrm{H_2SeO_4}$$ $$\mathrm{SeO_2} + \mathrm{N_2H_4^+H_2O} = \mathrm{Se} + \mathrm{N_2} + 3\mathrm{H_2O}$$

Selenium (VI) oxide, in contrast to the $\mathrm{SO_3}$, can't be obtained by directed oxidation of the selenium. So it's obtained from its salts

$$\mathrm{SO_3} + \mathrm{K_2SeO_4} = \mathrm{K_2SO_4} + \mathrm{SeO_3}$$

Structure of the $\mathrm{SeO_3}$ is similar to the $\mathrm{SO_3}$. It is solid, which can be easily sublimated. $\mathrm{SeO_3}$ is acidic oxide, with corresponding selenic acids and its salts. It easily decomposes on heating to the $\mathrm{SeO_2}$, is strong oxidizing agents.

$$2\mathrm{SeO_3} = 2\mathrm{SeO_2} + \mathrm{O_2}$$ $$\mathrm{SeO_3} + \mathrm{H_2O} = \mathrm{H_2SeO_4}$$ $$\mathrm{SeO_3} + 2\mathrm{HCl} = \mathrm{SeO_2} + \mathrm{Cl_2} + \mathrm{H_2O}$$

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