Sulfuric acid

Sulfuric acid is the most widely used inorganic acid, often called the "King of Chemicals" due to its versatility as a solvent, acid, oxidising, dehydrating, and sulfonating agent. It is a highly viscous, water-soluble liquid with strong hydrogen bonding.

1.0Introduction

Sulfuric acid is a highly corrosive, colourless, and odourless oily liquid with a molecular weight of 98.079 grams per mole. It is known for its acidic solid properties and is a crucial industrial chemical. Initially called "Oil of Vitriol," it is produced mainly through the Contact Process, which involves oxidising sulfur dioxide (SO₂) to sulfur trioxide (SO₃) and then reacting it with water.

Sulfuric acid occurs in both combined and free states and is commonly found in metallic sulfates within minerals. Due to its versatility and effectiveness, it is a strong acid and dehydrating agent that plays a vital role in various industries, including fertiliser production, petroleum refining, and chemical manufacturing.

2.0Oxoacids of sulfur

Oxoacids are acids that contain oxygen, and oxoacids of sulfur are compounds comprising sulfur, oxygen, and hydrogen.

The most significant industrial oxoacid is sulfuric acid (H₂SO₄). In these oxoacids, sulfur typically has a tetrahedral arrangement relative to the oxygen atoms. Sulfuric acid is a common diprotic acid that ionises to release two protons. In sulfuric acid, one sulfur atom is bonded to two hydroxyl groups, while the remaining two oxygen atoms form π-bonds with the sulfur atom.

3.0Structure of Sulfuric Acid

Sulfuric acid (H₂SO₄) has a tetrahedral geometry around the sulfur atom. The molecule consists of one sulfur atom centrally bonded to four oxygen atoms. Two of these oxygen atoms are bonded via double bonds, and the remaining two are bonded as hydroxyl groups (–OH).

The bond length of the sulfur-oxygen double bonds is shorter than that of the sulfur-oxygen single bonds (–OH groups). As a result, the oxygen atoms exert greater repulsion on the –OH groups. This leads to a larger bond angle in the sulfur-oxygen double bonds than in the sulfur-oxygen single bonds.

4.0Manufacture of Sulfuric Acid

Sulfuric acid (H₂SO₄) is produced commercially using two main methods: the Lead Chamber Process and the Contact Process.

1. Lead Chamber Process

The Lead Chamber Process is the traditional method used to manufacture sulfuric acid. The process involves the following steps:

• Formation of Sulfur Dioxide: 

Sulfur dioxide (SO₂​) is produced by burning sulfur in air: S+O₂→SO₂

• Formation of Sulfur Trioxide:

Sulfur dioxide reacts with oxygen in the presence of a catalyst to form sulfur trioxide (SO₃​): 2SO₂+O₂→2SO₃

• Formation of Sulfuric Acid: Sulfur trioxide is then absorbed in water to produce sulfuric acid. This reaction is highly exothermic and can be dangerous.

In practice, sulfur trioxide is absorbed in a solution of existing sulfuric acid to form oleum, which is then mixed with water to produce concentrated sulfuric acid: 

    SO₃+H₂O→H₂SO₄

The Lead Chamber Process uses large lead-lined chambers where the reactions occur. It was historically significant but primarily replaced by the more efficient Contact Process.

2. Contact Process

The Contact Process is the more modern and widely used method for manufacturing sulfuric acid.

5.0Properties of Sulfuric Acid (H₂SO₄)

Physical properties of Sulfuric acid:

Chemical properties of Sulfuric acid:

The chemical reactions of sulfuric acid are influenced by its key characteristics: 

• low volatility
• strong acidic nature,
• strong affinity for water
• ability to act as an oxidising agent.

Sulfuric acid (H₂SO₄) has a molecular weight of 98.079 grams per mole. In its structure, the sulfur atom forms double bonds with two oxygen atoms and single bonds with two hydroxyl groups (OH). Sulfuric acid is a diprotic acid that can donate two protons (hydrogen ions, H⁺) in aqueous solutions.

6.0Common Sulphuric Acid Reactions

1. Oxidizing Agent: Hot concentrated Sulfuric acid acts as an oxidising agent, oxidising metals and non-metals while itself being reduced to sulfur dioxide:

• Oxidation of Copper: Cu(s) + 2H₂SO₄(hot) → CuSO₄(aq) + SO₂(g) + 2H₂O(l)
• Oxidation of Carbon: C(s) + 2H₂SO₄(hot, conc.) → CO₂(g) + 2SO₂(g) + 2H₂O(l)

2. Reactions with Alkalis: Sulfuric acid neutralises alkalis to form bisulfates and sulfates:

• Formation of Sodium Bisulfate: H₂SO₄(conc.) + 2NaOH(aq)→2NaHSO₄(aq)+H₂O(l)
• Formation of Sodium Sulfate: H₂SO₄(conc.) + 2NaOH(aq)→Na2SO₄(aq)+2H₂O(l)

3. Preparation of Volatile Acids: Sulfuric acid is used to liberate more volatile acids from their salts:

• From sodium chloride, concentrated sulfuric acid produces hydrogen chloride: NaCl + H₂SO₄(conc) → NaHSO₄ + HCl
• From calcium fluoride, concentrated sulfuric acid produces hydrogen fluoride: CaF₂ + H₂SO₄(conc) → CaSO₄ + 2HF

4. Dehydrating Agent: Sulfuric acid is a potent dehydrating agent that dries gases and removes water from organic compounds. Sulfuric acid converts glucose, sugar, and starch to carbon.

• Dehydration of Sugars: C₁₂H₂₂O₁₁  +  H₂SO₄(conc.) →12C+11H₂O

7.0Uses of sulphuric acid

Sulfuric acid, often called the "King of Chemicals" due to its broad range of applications, is crucial in various industries and processes. Sulfuric acid's versatility and effectiveness make it indispensable across many fields, reinforcing its title as a cornerstone of industrial chemistry.

Its uses include:

1. Fertilizer Production:

• Ammonium Sulfate: Essential for soil enrichment.
• Superphosphate: A key component in phosphate fertilisers.

2. Manufacture of Dyes, Pigments, and Paints:

• Integral in the production of colourants and coatings.

3. Explosives:

• They are used in the synthesis of explosives like TNT (Trinitrotoluene).

4. Chemical Synthesis:

• Hydrochloric Acid: Produced using sulfuric acid.
• Phosphoric Acid: Essential for various applications, including fertilisers.
• Nitric Acid: Utilized in the manufacture of explosives and fertilisers.
• Sodium Carbonate: Important in glassmaking and other industrial processes.
• Petroleum Refining: Employed in the refining process to remove impurities and enhance the quality of petroleum products.
• Pickling Agent: Used in metal processing to clean and prepare metals for further treatment or coating.
• Laboratory Use: Serves as a vital reagent and is used in various chemical reactions.
• Oxidising and Dehydrating Agent: Acts as a powerful oxidiser and dehydrator in chemical processes.