## 11.3 Sulphur Sulphur is a non-metal element located below oxygen in Group 16 (VIA) of the periodic table. At room temperature, elemental sulphur is a bright yellow, crystalline solid. Sulphur atoms typically form stable, cyclic octatomic molecules with the chemical formula $S_8$. ## Electronic Configuration and Stability Sulphur has 6 electrons in its valence shell. According to the , its valence shell configuration is $3s^2 3p^4$. This leads to possible oxidation states of **-2, +2, +4, and +6**. - **Most Stable Oxidation State:** The **+6** oxidation state is the most stable for sulphur. This is due to its ability to achieve a complete octet by sharing electrons with other atoms. - **Examples:** Sulphur in the +6 state is found in important compounds like sulphuric acid ($H_2SO_4$) and sulphate ions ($SO_4^{2-}$). - **Molecular Stability:** The elemental $S_8$ molecule is relatively inert because the single S-S bonds are strong and require significant energy to break. The formation of stable compounds like hydrogen sulphide ($H_2S$) or sulphur dioxide ($SO_2$) involves electron sharing that satisfies the octet rule, enhancing their stability. ## 11.3.1 Properties of Sulphur Sulphur is a versatile element that can function as both an **oxidising agent** and a **reducing agent**, depending on the chemical reaction. - **As an Oxidising Agent:** Sulphur can gain two electrons to achieve a stable octet (like a noble gas), forming the sulphide ion ($S^{2-}$). In this process, it accepts electrons from other substances. - **As a Reducing Agent:** Sulphur can also lose electrons to exist in higher oxidation states like +4 and +6. In this case, it donates electrons to other substances. ## 11.3.2 Sulphur as an Oxidising Agent In these reactions, sulphur's oxidation state is reduced from **0 to -2**. ### 1. Reaction with Hydrogen Sulphur combines directly with hydrogen gas upon heating to form hydrogen sulphide. $$S + H_2 \rightarrow H_2S$$ ### 2. Reaction with Metals Sulphur reacts with active metals like zinc and iron on heating to form metal sulphides. $$Zn + S \rightarrow ZnS$$ $$Fe + S \rightarrow FeS$$ *In these reactions, the oxidation state of sulphur decreases from 0 to -2, while the oxidation state of the metals (Zn and Fe) increases from 0 to +2.* ## 11.3.3 Sulphur as a Reducing Agent In these reactions, sulphur's oxidation state increases from **0 to +4 or +6**. ### 1. Reaction with Oxygen (Combustion) Sulphur burns in air with a characteristic blue flame to produce sulphur dioxide gas. $$S + O_2 \rightarrow SO_2$$ In the presence of moist air, sulphur can be slowly oxidised to sulphuric acid. $$2S + 2H_2O + 3O_2 \rightarrow 2H_2SO_4$$ ### 2. Reaction with Oxidising Agents When treated with strong oxidising agents like concentrated nitric acid ($HNO_3$) or concentrated sulphuric acid ($H_2SO_4$), sulphur is oxidised. - **Reaction with Nitric Acid:** $$S + 6HNO_3 \rightarrow H_2SO_4 + 6NO_2 + 2H_2O$$ *Here, sulphur is oxidised from 0 to +6, while nitrogen in nitric acid is reduced from +5 to +4.* - **Reaction with Sulphuric Acid:** $$S + 2H_2SO_4 \rightarrow 3SO_2 + 2H_2O$$ *In this reaction, elemental sulphur is oxidised from 0 to +4, while the sulphur in sulphuric acid is reduced from +6 to +4.* ## 11.3.4 Uses of Sulphur and its Compounds Sulphur and its compounds have a wide range of important industrial and chemical applications. 1. **Fertiliser Production:** Sulphuric acid and ammonium sulphate are crucial components in fertilisers, providing essential sulphur for the synthesis of amino acids and vitamins in plants. 2. **Gunpowder:** Gunpowder is a mixture of sulphur, charcoal, and potassium nitrate ($KNO_3$). Sulphur acts as a fuel, lowers the ignition temperature, and increases the combustion rate. 3. **Rubber Industry:** Sulphur is used in the **vulcanization** of rubber, a process that cross-links polymer chains to improve the rubber's strength, elasticity, and durability. 4. **Synthetic Organic Chemistry:** Sulphur-containing functional groups are vital in organic synthesis. For example, thiols (-SH) and sulfonyl chlorides are used to create pharmaceuticals, including a class of antibiotics known as sulfonamides. 5. **Dyes and Pigments:** Sulphur compounds are used in the manufacturing of various dyes and pigments. 6. **Food Preservatives:** Sulphur dioxide ($SO_2$) and sulphites (e.g., $Na_2SO_3$) act as reducing agents that inhibit oxidation, preventing spoilage and preserving the color of foods and beverages like wine and dried fruits.