Sulpuric Acid (H2SO4) : Structure, Properties, & Uses

The H₂SO₄ acid name is sulphuric acid. It is a highly corrosive diprotic because it has two acid protons. In other words, it contains a sulphate molecule bound to the acidic H protons. This acid requires the utmost attention of those who handle it as it is destructive to the skin, teeth, eyes, and lungs. Prolonged exposures can even lead to death. Often its harmful effects are observed in workers who are exposed to sulfuric acid. However, the level of exposure, duration and type significantly impact the extent of harm. The following article explores sulphuric acid’s properties, structure, and various uses. 

H₂SO₄ Compound Name

H₂SO₄ has a strong acidic nature. It serves as an oxidising agent and a dehydrating agent at higher concentrations. It produces heat when dissolved in water. The acid has the following names:

• Sulphuric acid 
• Sulphuric acid
• Mattling acid
• Oil of vitriol
• Dihydrogen sulphate
• Battery acid
• Dipping acid
• Electrolyte acid
• Vitriol Brown Oil

Structure of Sulphuric Acid

It comprises two hydrogen atoms firmly bonded to two oxygen atoms, forming two OH groups. Two oxygen atoms are also bonded to the central sulphur via double bonds. The acid is a covalent molecule with a tetrahedral structure.

This strong acid ionises completely to form the following two ions:

• hydronium ions (H₃O⁺)
• hydrogen sulphate ions (HSO⁴⁻)

Lewis Structure of Sulphuric Acid

The Lewis structure, showing bonded and non-bonded pairs of electrons, is as follows:

Sulphuric acid contains

• Eight lone pairs of electrons or 16 non-bonded electrons
• Eight bonded electron pairs

Preparation of Sulphuric Acid

It is produced by a contact process. The key steps in the reaction are summarised as follows:

Step 1: Preparation of sulphur dioxide

Burning sulphur in excess of air with abundant oxygen gives sulphur dioxide.  The balanced reaction is as follows:

S(s) + O₂ (g) → SO₂(g)

Here, solid sulphur reacts with gaseous oxygen to give gaseous sulphur dioxide.

Step 2: Conversion of sulphur dioxide to sulphur trioxide

When sulphur dioxide reacts with oxygen in an equal ratio, sulphur trioxide is formed. The temperature required for the reaction is 400 °C – 450°C. Also, a pressure of 1-2 atm should be maintained. Vanadium pentoxide serves as a catalyst for the reaction. The reaction demands special attention because it is reversible in nature.

2 SO₂(g) + O₂ (g) ⇌ 2 SO₃(g)

Step 3: Preparation of concentrated sulphuric acid

Now sulphur trioxide reacts with concentrated sulfuric acid since gaseous sulphur trioxide cannot be directly dissolved in water as it leads to the formation of fog.

Thus, when sulphur trioxide reacts with sulfuric acid, oleum is formed. Oleum is then dissolved in water to produce concentrated sulfuric acid.

H₂SO₄ + SO₃(g) → H₂S₂O₇(l)

H₂S₂O₇(l) + H₂O(l) → 2 H₂SO₄

Properties of Sulphuric Acid

As stated earlier, sulpuric acid is strongly acidic and corrosive. It acts as an oxidising agent and a dehydrating agent. It is odourless and colourless. It is water-soluble and a very polar liquid. It is one of the most important heavy industrial chemicals with large-scale uses in various industries.

It occurs as a colourless oily liquid that is corrosive to metals and tissue. It chars wood and other organic matter on contact. However, it doesn’t catch fire easily. Some of the important physical properties are as follows:

• Hygroscopic liquid
• Colour: Colourless to dark-brown
• Odour: Odourless liquid
• Consistency: Oily 
• Molecular Weight: 98.08
• Hydrogen Bond Donor Count: 2
• Hydrogen Bond Acceptor Count: 4
• Boiling Point: 337 °C
• Melting Point: 10.31 °C
• Density: 1.841
• Vapour Density: 3.4

Chemical Reactions Involving Sulphuric Acid

Sulfuric acid reacts with various substances, giving diversified products. Some important chemical reactions are stated below:

Reaction with Metals On Heating

Con­cen­trat­ed sul­fu­ric acid is a strong oxidising agent. It can ox­i­dise many met­als, with the ex­cep­tion of Gold, Iridium, Plat­inum, Rhodi­um, and Tan­ta­lum.  

• Hot and con­cen­trat­ed sul­fu­ric acid reacts with metals like aluminium and can re­duce to H₂S, S and SO₂.

8 Al + 15H₂­SO₄ (concentrated)→ 4Al₂ (SO₄)₃ + 12 H₂O + 3H₂S

• Copper reacts with hot and concentrated sulfuric acid to give the following reaction:

Cu+ 2H₂SO₄ →CuSO₄ +SO₂ +2H₂O

• It reacts with metals of medi­um ac­tiv­i­ty like chromium to give the following products:

2Cr + 4 H₂­SO₄ (concentrated)→ Cr₂₍SO₄)₃ + 4 H₂O + S

• With metals of low activity, sulfuric acid gives the following reaction:

2Bi + 6H₂­SO₄ (concentrated.)→ Bi₂(SO₄)₃ + 6H₂O + 3SO₂

• When metals like aluminium and iron react with cold con­cen­trat­ed sul­fu­ric acid, they get cov­ered with an ox­ide film, and the process is termed pas­si­va­tion.

Reaction with water

An exothermic process is observed when H₂­SO₄ is mixed with wa­ter. The reaction gives a large amount of heat to the extent that the so­lu­tion may even boil.

Therefore, it is always better to add sul­fu­ric acid to wa­ter rather than water to the acid to avoid any hazard.

The reaction is as follows:

H₂SO₄ + H₂O → H₃O⁺ + HSO^4-

As a Dehydrating Agent

The acid is a strong de­hy­drat­ing sub­stance. It forces wa­ter out of var­i­ous com­pounds and thus, it is often used as a dry­ing agent.

When concentrated sulfuric acid reacts with sugar or glucose, it acts as a dehydrating agent. It removes water from sugar, and the process is extremely exothermic.

C₁₂H₂₂O₁₁​ + H₂​SO₄ → 12C + 11H₂O + (mixture of acid and water)

Re­ac­tion of Dilute Sul­fu­ric Acid

A popular reaction is an action of sulfuric acid on zinc to obtain hy­dro­gen. When you add zinc gran­ules to di­lut­e sul­fu­ric acid, the hydrogen gas evolves while the met­al dis­solves. The balanced reaction can be given as follows:

Zn + H₂­SO₄ → Zn­SO₄ + H₂

Di­lut­e sul­fu­ric acid reacts with metals on the left of hy­dro­gen in the reactivity series, and the reaction proceeds as follows:

Ме + H₂­SO₄(di­lut­ed) → salt + H₂↑

Re­ac­tion of sul­fu­ric acid and bar­i­um

For the qual­i­ta­tive analysis, sul­fu­ric acid and its salt are made to react with bar­i­um ions.

H₂­SO₄ + Ba­Cl₂ → Ba­SO₄ + 2HCl

Zn­SO₄ + Ba­Cl₂ → Ba­SO₄ + Zn­Cl₂

Sulphuric Acid Uses

Sulfuric acid is used in many industries. Some of the most popular uses are as follows:

• In the manufacturing of fertilisers
• In the treatment of wastewater
• In lead-acid batteries 
• It is used in manufacturing fertilisers. 
• It is widely used in steel and iron production. 
• As a catalyst in nylon manufacturing
• In petroleum refining
• For phosphoric acid production
• As a cleaning agent to get rid of the rust from iron and steel
• As an electrolyte
• It is useful for making ammonium sulphate. 
• In storage batteries
• In producing other chemicals
• For manufacturing explosives and glue
• To cure metals
• In lead-based car batteries

Risks Associated With Sulphuric Acid

The corrosive acid can cause serious damage to the skin and tissues when it comes into direct contact. Moreover, sulfuric acid vapours are also harmful when inhaled. 

The reactions to exposure are as follows:

• It irritates and burns human skin.
• It causes irritation to the eyes and can even lead to blindness. 
• It can cause a build-up of fluid in the lungs. The condition is termed pulmonary oedema. 
• Sulfuric acid exposure can also cause headaches, vomiting, and nausea. 
• It corrodes teeth if sipped accidentally. 
• Contact with the skin can lead to necrosis. 
• It is extremely irritating to the upper respiratory tract. 
• Sulfuric acid can lead to serious lung damage and emergency conditions.
• It can even lead to death.  

Some examples of workers who are exposed to sulfuric acid include 

• Outdoor workers who work in areas where oil or coal is burned.
• Mechanics who participate in the development or maintenance of batteries.
• Plumbers who use toilet bowl cleaners and water mixture. 
• Those who are working in publishing, photography, or printing shops.
• Firefighters 
• Steelworkers exposed to sulfuric acid mists

Conclusion

Sulfuric acid is a harmful yet useful chemical with a high value in the chemical industry. It causes harm to humans, and therefore healthcare organisations recommend that employers control, prevent or reduce worker exposure. 

Frequently Asked Questions

1. How do you neutralise sulfuric acid?

A. The simple neutralisation reaction works best for sulfuric acid. When you pour the baking soda into the acid spill, it will neutralise the acid to give salt and water. The process works with light acids like acetic and toxic, strong acids like sulfuric acids. 

2. Is sulfuric acid ionic?

A. Sulfuric acid is a covalent compound as it contains covalent bonds. However, it ionises to give hydronium and sulphate ions in solution.

3. Is it possible to neutralise sulfuric acid with water?

A. No, sulfuric acid can’t be neutralised using water. However, you can dilute the acid with water. Add a few drops of acid to the water carefully. It would be best to stir the mixture because the process is exothermic. 

You will require strong bases such as sodium hydroxide to neutralise sulfuric acid. 

4. In which reactions are sulfuric acid used as a catalyst?

A. Sulfuric acid is a direct participant in several reactions. The following reactions use sulfuric acid as a catalyst: 

• The manufacturing process of nylon
• Manheim process for manufacturing of HCl
• Petroleum refining