The iron (II) hydroxide, also called ferrous hydroxide, it is an inorganic compound with the chemical formula Fe (OH)two. If your purity is high, your solid consists only of Fe ionstwo+ and OH- in a 2: 1 ratio; however, it can contain water molecules and different ionic species, changing the position.
It represents the “reduced” form of the famous rust, a reddish surface composed of Fe ions.3+; while in greenish rust, Fe predominatestwo+, next to the OH- and other amount of anions: CO3two-, SW4two-, NOT3- and halides (F-, Cl-,… ), for example. The result, although the base of this green rust is Fe (OH)two, is that various solids are obtained.
In daily life this hydroxide can be seen in common places. For example, the boat in the picture above has its surface covered in green rust (not patina). There is the Faith (OH)two, but accompanied by numerous ions from the waves of the sea.
Chemically speaking, the properties and uses of this material depend on the Fe cation.two+ and its tendency to oxidize to become the Fe3+. It is a reducing agent, which reacts rapidly with oxygen under basic conditions. Therefore it must be used in inert atmospheres and in acid solutions..
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Considering only Fe (OH)two pure, has nothing but Fe ionstwo+ and OH-.
The interactions between them must, in theory, be ionic in nature; each Fe cationtwo+ attracts two OH anions-, whose non-directional attractions end up establishing a structural and repetitive order that defines a crystal; which in turn gives rise to green rust dust.
The problem is that there is a certain covalent character between both ions, and therefore, an ionic Fe-OH bond cannot be ignored. Considering a partially ionic Fe-OH bond, it can be understood that they are grouped in a kind of polymeric layer.
The structure of pure ferrous hydroxide is shown above with a ball-and-stick model. The Fe cationstwo+ are represented by the bright green spheres, while the OH anions- by the red and white spheres. Notice how the Fe ions aligntwo+ thanks to the interactions (ionic-covalent) with the OH-.
Why is this structure called double hydroxide layers? The image itself offers the answer: there are two rows or layers of OH- for each one of Faithtwo+; that is, the 2: 1 ratio of Fe (OH) is maintainedtwo mentioned at the beginning.
The structural units for Fe (OH)two are then these sandwiches, where the layers of OH- they would come to represent the loaves; negatively charged breads that therefore fail to set properly to define a striking brown crystal.
But on the other hand, the H atoms also line up and overshadow each other. Remembering the concept of polarity and electronegativity, these hydrogens have a slight positive partial charge, δ +; which, although weak, can have a positive zone where other anions or molecules are housed.
When the Fe (OH)two dissolves in water, its molecules coordinate with the metal center of Fetwo+ to form a complex aqueous: [Fe (HtwoOR)4(OH)two]. This complex has an octahedral geometry because it has six ligands: four water molecules and two OH molecules.-. However, in hydrated ferrous hydroxide the picture is a little different..
In the hydrate, of the hypothetical formula Fe (OH)twoNHtwoOr, the water molecules are housed exactly between the two layers of hydroxide; that is, they interact through dipole-ion forces with the hydrogens or white spheres. It is as if two OHFeOH sandwiches were grabbed and water was inserted to group them: OHFeOH (HtwoO) OHFeOH.
The water molecules are very dynamic, preventing the hydrate particles from acquiring considerable sizes and, consequently, they form a colloidal or gelatinous solid..
In hydrates, the hydroxide layers have interspersed water molecules; however, they can have other anions (already mentioned), causing a wide variety of green rusts. Likewise, they can even "trap" molecules as large as DNA, or drugs.
Not only can Fe ions be presenttwo+, but also Faith3+, product of internal oxidation caused by intercalated oxygen. It would visibly be observed that the green rust (or oxide) begins to turn reddish as the concentration of Fe increases.3+.
Fe (OH) shown abovetwo precipitated in a test tube. Being in abundant water, complex aqueous should predominate over the structure just mentioned. Observe that the surface shows an orange color, a product of the oxidation of Fetwo+ to Faith3+ by the oxygen in the air; that is, this hydroxide is a reducing agent:
Faithtwo+ <=> Faith3+ + and-
The physical appearance of this compound in its pure state is that of a brown solid:
Which, depending on its humidity level, can appear as a jelly-like green solid. It is quite insoluble in water (Ksp= 8 10-16 and solubility = 0.72 g / 100mL at 25ºC), and has a molar mass of 89.86 g / mol and a density of 3.4 g / mL.
No boiling or melting points are reported for this compound; probably due to the fact that, before a heat source, it dehydrates and converts into ferrous oxide, FeO:
Fe (OH)two => FeO + HtwoOR
Its reducing power is used to determine the presence of nitro compounds, RNOtwo. The reaction for which a positive test is obtained is represented by the following chemical equation:
RNOtwo + 6Fe (OH)two + 4HtwoO => RNHtwo + 6Fe (OH)3
The Fe (OH)3 precipitates as a reddish-brown solid, which certifies the presence of the nitro group, -NOtwo.
The reducing power of Fe (OH)two it has also been used to reduce SeO anions3two- (selenite) and SeO4two- (selenate) to elemental selenium, Se. Practically, it allows the removal of such anions, harmful to health, in the form of an insoluble and easily extractable selenium solid.
Its pure structure, and the green rust, is a source of inspiration for the design of new materials with mineralogical characteristics..
It is estimated that their function is to allow the transport of a specific species between its layers, in such a way that its release to the active site (soils, cells, metallic surface, etc.) can be controlled or reduced..
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