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What is sulfuric acid?

The sulfuric acid , also called hydrogen sulfate and oil of vitriol, is an acid oxyacid of sulfur formed by the reaction of sulfur trioxide (SO 3 ) and water. Its molecular formula is H 2 SO 4 and it is by far the most widely produced and used mineral acid in the world.

It is a strong and highly corrosive mineral acid that is miscible with water so it can be prepared in the form of solutions of almost any concentration between 0 and around 18 molar.

The high volumes of production and consumption of this compound are due to its multiple applications in industries as varied as agriculture, the petrochemical industry, in organic and inorganic synthesis and in wastewater treatment, to name just a few.

Structure of sulfuric acid

Sulfuric acid is formed by a central sulfur atom with valence VI which is surrounded by 4 oxygen atoms, two of which are bonded to hydrogen atoms. Its Lewis structure is presented below:

Being surrounded by 4 atoms, the valence electron pair repulsion theory (TRePEV) predicts that this compound should assume an approximately tetrahedral geometry, which is shown in the following image:

Due to the high electronegativity of oxygen and the different resonance structures that sulfuric acid can have, the bonds between oxygens and hydrogens are strongly polarized, which makes them highly acidic and easily dissociated hydrogens.

Sulfuric acid properties

As a consequence of its structure, sulfuric acid has the following physical and chemical properties:

Physical properties

  • Pure sulfuric acid is a colorless and odorless liquid that has a density of 1.8302 g / mL.
  • Its melting point is 10.31 ° C and its boiling point is 337 ° C, although at this temperature it tends to decompose to produce water and sulfuric anhydride (also called sulfur trioxide).
  • It is completely miscible with water, so solutions of any concentration can be prepared. However, solutions with more than 98.3% by mass of this acid are unstable and the acid decomposes until its concentration drops to 98.3%. This solution is stable and is what is known as concentrated sulfuric acid.
  • In its pure or concentrated state, it is a more viscous liquid than water.

Chemical properties

  • Pure sulfuric acid is a strongly oxidizing and corrosive substance. It has the ability to oxidize, dehydrate and sulfonate a wide variety of organic compounds and to carbonize others.
  • It is a strong diprotic acid that completely loses its first proton when it dissolves in water, thus becoming its conjugate base, the bisulfate ion (HSO  ). This ion is also an acid, although weaker than the original acid.
  • Aqueous solutions of sulfuric acid are strongly acidic and are capable of dissolving many metals, while generating hydrogen gas. They are also capable of completely neutralizing strong bases.
  • It reacts violently with many alcohols and with water, releasing large amounts of heat in the process.

How is sulfuric acid obtained?

The three best known industrial processes for the production of sulfuric acid are: the contact process, the wet sulfuric acid process and the lead chamber process.

1. Synthesis of sulfuric acid by the contact process

This is the most widely used method today to produce sulfuric acid. The process consists of 5 steps in which elemental sulfur is converted to sulfur dioxide (SO 2 ), then sulfur trioxide (SO 3 ), disulfuric acid (H 2 S 2 O 7 ) and finally sulfuric acid. The reactions involved are:

The second step in the process is simply the purification of sulfur dioxide before it is converted to sulfur trioxide or sulfuric anhydride. The third step reaction occurs when gaseous sulfur dioxide comes into contact with a solid vanadium pentaoxide (V 2 O 5 ) catalyst , and this is why the method is called the contact process.

2. Synthesis of sulfuric acid by the wet process

This process was designed to purify emission gases from a wide variety of industries and chemical plants . The by-product of this purification is commercial grade sulfuric acid in addition to hot water vapor, which can be reused in the original industrial process.

In the case of the wet process, the reactions are mostly carried out in the gaseous state and consist of a combustion stage, an oxidation stage, then a hydration stage and, finally, a condensation stage of the final product. The important reactions that occur at each stage are:

3. Synthesis of sulfuric acid by the lead chamber process

This is an ancient method of producing sulfuric acid that is rarely used today. The process consisted of burning elemental sulfur in the presence of oxygen from the air in lead-coated wooden chambers, hence its name.

Following this combustion, the sulfur dioxide was then reacted with nitrogen oxides produced by the decomposition of sodium nitrate or by other means. In the end, after a series of additional reactions, sulfuric acid was obtained.

Common applications

  • The main application of sulfuric acid is as a source of sulfur in the form of sulfates in the manufacture of fertilizers. In fact, around 60% of the sulfuric acid produced in the world is destined for this purpose.
  • In the chemical industry it is used for the synthesis of detergents, pigments, catalysts for the petrochemical industry and in the processing of minerals to obtain different metals.
  • Finally, sulfuric acid is the electrolyte used in lead batteries used in most automobiles.

Risks associated with sulfuric acid

Due to its chemical properties, sulfuric acid can be very dangerous for humans, animals and the environment in general. These are some of the risks associated with this chemical:

  • Its corrosive and dehydrating characteristics mean that pure acid can cause great damage if it comes into contact with the skin.
  • On the other hand, the reaction of aqueous solutions of sulfuric acid with metals generates hydrogen gas, which is a combustible and explosive substance.
  • Sulfuric acid itself does not undergo a combustion reaction, but, being a strong oxidant, it stimulates the combustion of other substances, acting as a kind of catalyst, thus representing a considerable fire risk.
  • In addition, when exposed to fire, it can decompose into toxic gases and generate sulfuric acid fumes that can easily destroy the respiratory tract.

For these reasons and more, sulfuric acid is a substance that must be handled with care.

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