# Sulphuric Acid Sulphuric acid ($\mathrm{H_2SO_4}$) is a colourless, oily liquid. It is a dense substance with a density of $1.83\ \text{g cm}^{-3}$, making it almost twice as heavy as water. It solidifies at $10.37^{\circ}\text{C}$ into colourless crystals. It is often referred to as the "King of Chemicals" due to its vast industrial applications. ## 11.4.1 Contact Process (Manufacture of Sulphuric Acid) The industrial production of sulphuric acid is carried out using the **Contact Process**. This process relies on a few key raw materials and a series of controlled chemical reactions. **Raw Materials:** Sulphur (S), Air (providing Oxygen, $\mathrm{O_2}$), and Water ($\mathrm{H_2O}$). Sulphur extracted from fossil fuel impurities is also a common source. ### Steps of the Contact Process **1. Oxidation/Burning of Sulphur** Sulphur is burned in an excess of air to produce sulphur dioxide gas. $$ \mathrm{S_{(s)} + O_{2(g)} \rightarrow SO_{2(g)}} $$ **2. Purification of Sulphur Dioxide** The sulphur dioxide gas must be purified to remove impurities like arsenic oxide. These impurities can "poison" the catalyst used in the next step, rendering it ineffective. **3. Catalytic Oxidation of Sulphur Dioxide** The purified sulphur dioxide is mixed with more air (oxygen) and passed over a catalyst to form sulphur trioxide. This is a reversible and exothermic reaction. $$ \mathrm{2SO_{2(g)} + O_{2(g)} \rightleftharpoons 2SO_{3(g)}} \quad \Delta H^{\circ} = -198\ \text{kJ} $$ **Optimal Conditions:** According to Le Chatelier's Principle, the following conditions favor the maximum yield of $SO_3$: - **Temperature:** $450 - 500^{\circ}\text{C}$ - **Pressure:** 2-3 atmospheres (atm) - **Catalyst:** Vanadium(V) oxide ($\mathrm{V_2O_5}$) **Note:** Sulphur trioxide ($\mathrm{SO_3}$) is not dissolved directly in water. The reaction is extremely exothermic and would release a large amount of heat, causing the mixture to boil violently and create a hazardous acid mist. **4. Production of Oleum and Dilution** To manage the reaction safely, sulphur trioxide is first dissolved in concentrated sulphuric acid to form a substance called **oleum** (or fuming sulphuric acid). $$ \mathrm{SO_{3(g)} + H_2SO_{4(l)} \rightarrow H_2S_2O_{7(l)}} $$ The oleum is then carefully diluted with water in a controlled manner to produce concentrated sulphuric acid of the desired concentration. ## 11.4.2 Properties of Sulphuric Acid Sulphuric acid exhibits several key chemical properties, making it a versatile industrial chemical. ### 1. Ionization Reaction In an aqueous solution, sulphuric acid ionizes to produce hydrogen ions and sulphate ions. $$ \mathrm{H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}} $$ ### 2. Reaction with Bases (Neutralization) As a strong acid, it reacts with bases to form a salt and water. $$ \mathrm{H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O} $$ ### 3. Reaction with Active Metals Dilute sulphuric acid reacts with active metals (e.g., Zinc) to produce a metal salt and hydrogen gas. $$ \mathrm{Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2} $$ ### 4. Reaction with Metal Carbonates It reacts with metal carbonates to form a salt, water, and carbon dioxide gas. $$ \mathrm{H_2SO_4 + Na_2CO_3 \rightarrow Na_2SO_4 + H_2O + CO_2} $$ ### 5. As a Dehydrating Agent Concentrated sulphuric acid has a strong affinity for water and can remove it from other compounds, often charring organic matter. - **Decomposition of formic acid:** $$ \mathrm{HCOOH \xrightarrow{H_2SO_4} H_2O + CO} $$ - **Dehydration of ethanol to form ethene:** $$ \mathrm{C_2H_5OH \xrightarrow{H_2SO_4} C_2H_4 + H_2O} $$ ### 6. As an Oxidizing Agent **Hot, concentrated sulphuric acid** is a powerful oxidizing agent. - **Reaction with Metals:** It oxidizes metals like copper, liberating sulphur dioxide gas. $$ \mathrm{Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O} $$ In this reaction, copper is oxidized (oxidation state from 0 to +2), and sulphur in $\mathrm{H_2SO_4}$ is reduced (oxidation state from +6 to +4 in $\mathrm{SO_2}$). - **Reaction with other compounds:** $$ \mathrm{2HI + H_2SO_4 \rightarrow I_2 + SO_2 + 2H_2O} $$ ## 11.4.3 Uses of Sulphuric Acid Sulphuric acid is one of the most important and widely produced industrial chemicals due to its versatility. Its major applications include: - Production of fertilizers (e.g., ammonium sulphate) - Manufacturing detergents, paints, pigments, and dyes - Production of plastics and synthetic fibers (e.g., rayon) - As an electrolyte in lead-acid car batteries - Tanning of leather - Cleaning metal surfaces ("pickling") before electroplating or galvanizing