Sulfuric acid

Sulfuric acid or sulphuric acid, known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, and hydrogen, with the molecular formula. It is a colorless, odorless, and viscous liquid that is miscible with water.
Pure sulfuric acid does not occur naturally due to its strong affinity to water vapor; it is hygroscopic and readily absorbs water vapor from the air. Concentrated sulfuric acid is a strong oxidant with powerful dehydrating properties, making it highly corrosive towards other materials, from rocks to metals. Phosphorus pentoxide is a notable exception in that it is not dehydrated by sulfuric acid but, to the contrary, dehydrates sulfuric acid to sulfur trioxide. Upon addition of sulfuric acid to water, a considerable amount of heat is released; thus, the reverse procedure of adding water to the acid is generally avoided since the heat released may boil the solution, spraying droplets of hot acid during the process. Upon contact with body tissue, sulfuric acid can cause severe acidic chemical burns and secondary thermal burns due to dehydration. Dilute sulfuric acid is substantially less hazardous without the oxidative and dehydrating properties; though, it is handled with care for its acidity.
Many methods for its production are known, including the contact process, the wet sulfuric acid process, and the lead chamber process. Sulfuric acid is also a key substance in the chemical industry. It is most commonly used in fertilizer manufacture but is also important in mineral processing, oil refining, wastewater treating, and chemical synthesis. It has a wide range of end applications, including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, as a dehydrating compound, and in various cleaning agents.
Sulfuric acid can be obtained by dissolving sulfur trioxide in water.

Physical properties

Grades of sulfuric acid

Although nearly 100% sulfuric acid solutions can be made, the subsequent loss of sulfur trioxide| at the boiling point brings the concentration to 98.3% acid. The 98.3% grade, which is more stable in storage, is the usual form of what is described as "concentrated sulfuric acid". Other concentrations are used for different purposes. Some common concentrations are:

Mass fraction	Density	Concentration	Common name
<29%	1.00–1.25	<4.2	diluted sulfuric acid
29–32%	1.25–1.28	4.2–5.0	battery acid
62–70%	1.52–1.60	9.6–11.5
78–80%	1.70–1.73	13.5–14.0
93.2%	1.83	17.4	66 °Bé acid
98.3%	1.84	18.4	concentrated sulfuric acid
100%.	1.84	18.8

"Chamber acid" and "tower acid" were the two concentrations of sulfuric acid produced by the lead chamber process, chamber acid being the acid produced in the lead chamber itself and tower acid being the acid recovered from the bottom of the Glover tower. They are now obsolete as commercial concentrations of sulfuric acid, although they may be prepared in the laboratory from concentrated sulfuric acid if needed. In particular, "10 M" sulfuric acid is prepared by slowly adding 98% sulfuric acid to an equal volume of water, with good stirring: the temperature of the mixture can rise to 80 °C or higher.

Sulfuric acid

Sulfuric acid contains not only molecules, but is actually an equilibrium of many other chemical species, as it is shown in the table below.
Sulfuric acid is a colorless oily liquid, and has a vapor pressure of <0.001 mmHg at 25 °C and 1 mmHg at 145.8 °C, and 98% sulfuric acid has a vapor pressure of <1 mmHg at 40 °C.
In the solid state, sulfuric acid is a molecular solid that forms monoclinic crystals with nearly trigonal lattice parameters. The structure consists of layers parallel to the plane, in which each molecule is connected by hydrogen bonds to two others. Hydrates are known for n = 1, 2, 3, 4, 6.5, and 8, although most intermediate hydrates are stable against disproportionation.

Polarity and conductivity

is a very polar liquid, having a dielectric constant of around 100. It has a high electrical conductivity, a consequence of autoprotolysis, i.e. self-protonation:
The equilibrium constant for autoprotolysis is:
The corresponding equilibrium constant for water, K_w is 10⁻¹⁴, a factor of 10¹⁰ smaller.
In spite of the viscosity of the acid, the effective conductivities of the and ions are high due to an intramolecular proton-switch mechanism, making sulfuric acid a good conductor of electricity. It is also an excellent solvent for many reactions.

Chemical properties

Acidity

The hydration reaction of sulfuric acid is highly exothermic.
As indicated by its acid dissociation constant, sulfuric acid is a strong acid:
The product of this ionization is, the bisulfate anion. Bisulfate is a far weaker acid:
The product of this second dissociation is, the sulfate anion.

Dehydration

Concentrated sulfuric acid has a powerful dehydrating property, removing water from other chemical compounds such as table sugar and other carbohydrates, to produce carbon, steam, and heat. Dehydration of table sugar is a common laboratory demonstration. The sugar darkens as carbon is formed, and a rigid column of black, porous carbon called a carbon snake may emerge.
Similarly, mixing starch into concentrated sulfuric acid gives elemental carbon and water. The effect of this can also be seen when concentrated sulfuric acid is spilled on paper. Paper is composed of cellulose, a polysaccharide related to starch. The cellulose reacts to give a burnt appearance in which the carbon appears much like soot that results from fire.
Although less dramatic, the action of the acid on cotton, even in diluted form, destroys the fabric.
The reaction with copper sulfate can also demonstrate the dehydration property of sulfuric acid. The blue crystals change into white powder as water is removed.

Reactions with salts

Sulfuric acid reacts with most bases to give the corresponding sulfate or bisulfate.
Aluminium sulfate, also known as paper maker's alum, is made by treating bauxite with sulfuric acid:
Sulfuric acid can also be used to displace weaker acids from their salts. Reaction with sodium acetate, for example, displaces acetic acid,, and forms sodium bisulfate:
Similarly, treating potassium nitrate with sulfuric acid produces nitric acid. Sulfuric acid reacts with sodium chloride, and gives hydrogen chloride gas and sodium bisulfate:
When combined with nitric acid, sulfuric acid acts both as an acid and a dehydrating agent, forming the nitronium ion, which is important in nitration reactions involving electrophilic aromatic substitution. This type of reaction, where protonation occurs on an oxygen atom, is important in many organic chemistry reactions, such as Fischer esterification and dehydration of alcohols.
When allowed to react with superacids, sulfuric acid can act as a base and can be protonated, forming the ion. Salts of have been prepared using the following reaction in liquid HF:
The above reaction is thermodynamically favored due to the high bond enthalpy of the Si–F bond in the side product. Protonation using simply fluoroantimonic acid, however, has met with failure, as pure sulfuric acid undergoes self-ionization to give ions:
which prevents the conversion of to by the HF/ system.

Reactions with metals

Even diluted sulfuric acid reacts with many metals via a single displacement reaction, like other typical acids, producing hydrogen gas and salts. It attacks reactive metals such as iron, aluminium, zinc, manganese, magnesium, and nickel.
Concentrated sulfuric acid can serve as an oxidizing agent, releasing sulfur dioxide:
Lead and tungsten, however, are resistant to sulfuric acid.

Reactions with carbon and sulfur

Hot concentrated sulfuric acid oxidizes carbon and sulfur:

Electrophilic aromatic substitution

Benzene and many derivatives undergo electrophilic aromatic substitution with sulfuric acid to give the corresponding sulfonic acids:

Sulfur–iodine cycle

Sulfuric acid can be used to produce hydrogen from water:
The compounds of sulfur and iodine are recovered and reused, hence the process is called the sulfur–iodine cycle. This process is endothermic and must occur at high temperatures, so energy in the form of heat has to be supplied. The sulfur–iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy. It is an alternative to electrolysis, and does not require hydrocarbons like current methods of steam reforming. But note that all of the available energy in the hydrogen so produced is supplied by the heat used to make it.

Occurrence

Sulfuric acid is rarely encountered naturally on Earth in anhydrous form, due to its great affinity for water. Dilute sulfuric acid is a constituent of acid rain, which is formed by atmospheric oxidation of sulfur dioxide in the presence of water—i.e. oxidation of sulfurous acid. When sulfur-containing fuels such as coal or oil are burned, sulfur dioxide is the main byproduct.
Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as pyrite:
The resulting highly acidic water is called acid mine drainage or acid rock drainage.
The can be further oxidized to :
The produced can be precipitated as the hydroxide or hydrous iron oxide:
The iron ion can also oxidize pyrite:
When iron oxidation of pyrite occurs, the process can become rapid. pH values below zero have been measured in ARD produced by this process.
ARD can also produce sulfuric acid at a slower rate, so that the acid neutralizing capacity of the aquifer can neutralize the produced acid. In such cases, the total dissolved solids concentration of the water can be increased from the dissolution of minerals from the acid-neutralization reaction with the minerals.
Sulfuric acid is used as a defense by certain marine species, for example, the phaeophyte alga Desmarestia munda concentrates sulfuric acid in cell vacuoles.