Radium sulfate is the most insoluble sulfate known. Exceptions include calcium sulfate, strontium sulfate, lead(II) sulfate, barium sulfate, silver sulfate, and mercury sulfate, which are poorly soluble. There are numerous examples of ionic sulfates, many of which are highly soluble in water. Some metal sulfides can be oxidized to give metal sulfates. Consequently the product sulfates are hydrated, corresponding to zinc sulfate ZnSO 4♷H 2O, copper(II) sulfate CuSO 4♵H 2O, and cadmium sulfate CdSO 4 Typically metal sulfates are prepared by treating metal oxides, metal carbonates, or the metal itself with sulfuric acid: Zn + H 2SO 4 → ZnSO 4 + H 2 Cu(OH) 2 + H 2SO 4 → CuSO 4 + 2 H 2O CdCO 3 + H 2SO 4 → CdSO 4 + H 2O + CO 2Īlthough written with simple anhydrous formulas, these conversions generally are conducted in the presence of water. On the other hand, in the structure with a dipolar bond, the charge is localized as a lone pair on the oxygen. The apparent contradiction can be cleared if one realizes that the covalent double bonds in the Lewis structure in reality represent bonds that are strongly polarized by more than 90% towards the oxygen atom. However, the bonding representation of Pauling for sulfate and other main group compounds with oxygen is still a common way of representing the bonding in many textbooks. The discrepancy between the S−O bond length in the sulfate ion and the S−OH bond length in sulfuric acid is explained by donation of p-orbital electrons from the terminal S=O bonds in sulfuric acid into the antibonding S−OH orbitals, weakening them resulting in the longer bond length of the latter. In this model, the structure obeys the octet rule and the charge distribution is in agreement with the electronegativity of the atoms. Therefore, the representation with four single bonds is the optimal Lewis structure rather than the one with two double bonds (thus the Lewis model, not the Pauling model). For sulfuric acid, computational analysis (with natural bond orbitals) confirms a clear positive charge on sulfur (theoretically +2.45) and a low 3d occupancy. However, in this description, despite there being some π character to the S−O bonds, the bond has significant ionic character. In this model, fully occupied p orbitals on oxygen overlap with empty sulfur d orbitals (principally the d z 2 and d x 2– y 2). Ī widely accepted description involving pπ – dπ bonding was initially proposed by Durward William John Cruickshank. The outcome was a broad consensus that d orbitals play a role, but are not as significant as Pauling had believed. Pauling's use of d orbitals provoked a debate on the relative importance of pi bonding and bond polarity ( electrostatic attraction) in causing the shortening of the S−O bond. The double bonding was taken by Pauling to account for the shortness of the S−O bond. The S−O bond length of 149 pm is shorter than the bond lengths in sulfuric acid of 157 pm for S−OH. His reasoning was that the charge on sulfur was thus reduced, in accordance with his principle of electroneutrality. Later, Linus Pauling used valence bond theory to propose that the most significant resonance canonicals had two pi bonds involving d orbitals. The first description of the bonding in modern terms was by Gilbert Lewis in his groundbreaking paper of 1916 where he described the bonding in terms of electron octets around each atom, that is no double bonds and a formal charge of +2 on the sulfur atom. The tetrahedral molecular geometry of the sulfate ion is as predicted by VSEPR theory.īonding Two models of the sulfate ion.ġ with polar covalent bonds only 2 with an ionic bond Six resonances Organic sulfate esters, such as dimethyl sulfate, are covalent compounds and esters of sulfuric acid. The sulfate ion carries an overall charge of −2 and it is the conjugate base of the bisulfate (or hydrogensulfate) ion, HSO − 4, which is in turn the conjugate base of H 2SO 4, sulfuric acid. The sulfur atom is in the +6 oxidation state while the four oxygen atoms are each in the −2 state. The symmetry is the same as that of methane. The sulfate anion consists of a central sulfur atom surrounded by four equivalent oxygen atoms in a tetrahedral arrangement. "Sulfate" is the spelling recommended by IUPAC, but "sulphate" was traditionally used in British English. Further information: American and British English spelling differences
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