Selenic acid
Encyclopedia
Selenic acid is the chemical compound
with the formula
. It is an oxoacid
of selenium
, and its structure is more accurately described as .
As predicted by VSEPR theory
, the selenium center is tetrahedral, with a Se–O bond length of 161 pm
. In the solid state, it crystallizes in an orthorhombic
structure.
, it is impractical to synthesize selenic acid by dissolving selenium trioxide in water, unlike sulfuric acid
, which can be synthesized by dissolving sulfur trioxide
in water. Instead, it must be prepared by oxidising selenium compounds in lower oxidation states.
One method of preparing selenic acid is by the oxidation of selenium dioxide
with hydrogen peroxide
:
To obtain the anhydrous
acid as a crystalline solid, the resulting solution is evaporated at temperatures below 140 °C (413.2 K; 284 °F) in a vacuum.
Selenic acid may also be prepared by the oxidation of selenous acid
with halogens, such as chlorine
or bromine
, or with potassium permanganate
. However, using chlorine or bromine as the oxidising agent also produces hydrochloric or hydrobromic acid as a side-product, which needs to be removed from the solution since they can reduce the selenic acid to selenous acid.
Another method of preparing selenic acid is by the oxidation of elemental selenium in water suspension by chlorine:
, selenic acid is a strong acid that is hygroscopic and extremely soluble in water. Concentrated solutions are viscous. Crystalline mono- and di-hydrates
are known. The monohydrate melts at 26°C, and the dihydrate melts at −51.7°C.
Selenic acid is a stronger oxidiser than sulfuric acid, capable of liberating chlorine from chloride ions, being reduced to selenous acid in the process:
It decomposes above 200°C, liberating oxygen gas and being reduced to selenous acid:
Selenic acid reacts with barium salts to precipitate , analogous to the sulfate. In general, selenate salts resemble sulfate salts, but are more soluble. Many selenate salts have the same crystal structure as the corresponding sulfate salts.
Treatment of selenic acid with fluorosulfuric acid
gives the dioxydifluoride
(b.p. −8.4 °C):
Hot, concentrated selenic acid is capable of dissolving gold, forming a reddish-yellow solution of gold(III) selenate:
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
. It is an oxoacid
Oxoacid
An oxoacid is an acid that contains oxygen. To be more specific, it is an acid that:#contains oxygen#contains at least one other element#has at least one hydrogen atom bound to oxygen#forms an ion by the loss of one or more protons....
of selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, and its structure is more accurately described as .
As predicted by VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
, the selenium center is tetrahedral, with a Se–O bond length of 161 pm
Picometre
A picometre is a unit of length in the metric system, equal to one trillionth, i.e. of a metre, which is the current SI base unit of length...
. In the solid state, it crystallizes in an orthorhombic
Orthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
structure.
Preparation
Due to the instability of selenium trioxideSelenium trioxide
Selenium trioxide is a chemical compound used in the manufacture of photoelectric cells and solar energy devices.A laboratory preparation is to react anhydrous K2SeO4 and SO3. The compound is thermodynamically unstable with respect to selenium dioxide. The solid consists of cyclic tetramers,...
, it is impractical to synthesize selenic acid by dissolving selenium trioxide in water, unlike sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, which can be synthesized by dissolving sulfur trioxide
Sulfur trioxide
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...
in water. Instead, it must be prepared by oxidising selenium compounds in lower oxidation states.
One method of preparing selenic acid is by the oxidation of selenium dioxide
Selenium dioxide
Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium.-Properties:...
with hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
:
- + →
To obtain the anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
acid as a crystalline solid, the resulting solution is evaporated at temperatures below 140 °C (413.2 K; 284 °F) in a vacuum.
Selenic acid may also be prepared by the oxidation of selenous acid
Selenous acid
Selenous acid is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by 2SeO. It is the principal oxoacid of selenium; the other being selenic acid.-Formation and properties:...
with halogens, such as chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
or bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
, or with potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
. However, using chlorine or bromine as the oxidising agent also produces hydrochloric or hydrobromic acid as a side-product, which needs to be removed from the solution since they can reduce the selenic acid to selenous acid.
Another method of preparing selenic acid is by the oxidation of elemental selenium in water suspension by chlorine:
- Se + 4 + 3 → + 6 HCl
Chemistry
Like sulfuric acidSulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, selenic acid is a strong acid that is hygroscopic and extremely soluble in water. Concentrated solutions are viscous. Crystalline mono- and di-hydrates
Water of crystallization
In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...
are known. The monohydrate melts at 26°C, and the dihydrate melts at −51.7°C.
Selenic acid is a stronger oxidiser than sulfuric acid, capable of liberating chlorine from chloride ions, being reduced to selenous acid in the process:
- + 2 + 2 → + +
It decomposes above 200°C, liberating oxygen gas and being reduced to selenous acid:
- 2 → 2 +
Selenic acid reacts with barium salts to precipitate , analogous to the sulfate. In general, selenate salts resemble sulfate salts, but are more soluble. Many selenate salts have the same crystal structure as the corresponding sulfate salts.
Treatment of selenic acid with fluorosulfuric acid
Fluorosulfuric acid
Fluorosulfuric acid is the inorganic compound with the formula HSO3F. It is one of the strongest acids commercially available and is a superacid. The formula HFSO3 emphasizes its relationship to sulfuric acid, H2SO4; HSO3F is a tetrahedral molecule.-Chemical properties:Fluorosulfuric acid is a...
gives the dioxydifluoride
Selenoyl fluoride
Selenoyl fluoride, selenoyl difluoride, selenium oxyfluoride, or selenium dioxydifluoride is a chemical compound with the formula SeO2F2.-Structure:...
(b.p. −8.4 °C):
- + 2 → + 2
Hot, concentrated selenic acid is capable of dissolving gold, forming a reddish-yellow solution of gold(III) selenate:
- 2 Au + 6 → + 3 + 3