Difference Between Sulfate and Sulfite

Main Difference – Sulfate vs Sulfite

When a neutral atom or a molecule gain electrons from outside, it becomes a negatively charged species since electrons are negatively charged and there aren’t sufficient positive charges to neutralize the negative charge. When a neutrally charged atom or a molecule gain electrons, it becomes a negatively charged species called an anion. Sulfate and sulfite are such anions. The main difference between sulfate and sulfite is that sulfate is composed of four oxygen atoms bonded to a sulfur atom whereas sulfite is composed of three oxygen atoms bonded to a sulfur atom.

Key Areas Covered

1. What is Sulfate
      – Definition, Properties, Examples
2. What is Sulfite
      – Definition, Properties, Examples
3. What are the Similarities Between Sulfate and Sulfite
      – Outline of Common Features
4. What is the Difference Between Sulfate and Sulfite
      – Comparison of Key Differences

Key Terms: Anion, Oxygen, Sulfate, Sulfite, Sulfur

What is Sulfate

Sulfate is an anion composed of a sulfur atom bonded to four oxygen atoms around it. The charge of the sulfate anion is -2. The molecular formula of sulfate is SO₄^-2. Sulfate anion is the conjugated base of the sulfuric acid. When sulfuric acid is dissociated into its ions, sulfate anion and a proton (H⁺) are given.

When considering the bonding between sulfur atom and oxygen atoms, two oxygen atoms are bonded through double bonds and other two oxygen atoms are bonded through single bonds. This is because sulfur atom can have a maximum of 6 bonds around it. Therefore the two negative charges are seen on the oxygen atoms that are attached to the single bond. The oxidation state of sulfur atom is +6 and the oxidation state of each oxygen atom is -2. But when experimentally determined, the bond lengths between sulfur and oxygen atoms are the same. This is due to the phenomena called resonance. Due to the overlapping of orbitals of sulfur and oxygen atoms, the electrons around these atoms are delocalized. Therefore, the bond length between a sulfur atom and an oxygen atom is a length between that of S-O single bond and S=O double bond. The actual bond length has been found as 149 pm.

Figure 1: The resonance in sulfate

The molar mass of sulfate anion is about 96 g/mol. Normally, sulfate anion is soluble in water. But compounds like calcium sulfate are poorly soluble in water. The geometry around the sulfur atom is tetrahedral and the bonds around the sulfur atom are considered to be the same due to resonance. The sulfite anion cannot undergo oxidation because the sulfur atom is in its highest possible oxidation state.

Examples of Some Common Sulfates

Baryte (BaSO₄)
Anglesite (PbSO₄)
Anhydrite (CaSO₄)
Gypsm (CaSO₄.2H₂O)
Epsomite (MgSO₄.7H₂O)

What is Sulfite

Sulfite is an anion composed of sulfur and oxygen atoms. Sulfite anion has one sulfur atom bonded to three oxygen atoms. The charge of Sulfite anion is -2. In sulfite anion, one oxygen atom is bonded to sulfur through a double bond and other two oxygen atom are bonded to sulfur atom through single bonds. However, the bond lengths around the sulfur atom are the same and the value of bond length is between S-O single bond and the S=O double bond. That is due to the resonance of the structure. Therefore, all the bonds are considered as the same.

Figure 2: The resonance structure of sulfite

In addition, sulfite anion has a lone pair of electrons on sulfur atom. The oxidation state of sulfur in sulfite is +4 and the oxidation state of each oxygen atom is -2. The molar mass of sulfite anion is about 80 g/mol. The geometry around the sulfur atom in sulfite is trigonal pyramidal geometry.

Sulfites of Na⁺, K⁺ and NH₄⁺ are soluble in water. But most other sulfites are insoluble in water. Sulfite can undergo oxidation reactions because the sulfur atom in sulfite is in +4 oxidation state ad it can be oxidized up to +6 oxidation state.

Examples of Some Common Sulfites

Copper sulfite (CuSO₃)
Zinc Sulfite (ZnSO₃)
Magnesium Sulfite (MgSO₃)
Potassium Sulfite (K₂SO₃)

Similarities Between Sulfate and Sulfite

Both are anions bearing negative charges
The overall charge of the anion is -2 for both anions.
Both anions are composed of a sulfur atom and oxygen atoms bonded to the sulfur atom.
Both anions show resonance in their chemical structures
The oxidation state of oxygen in both anions is -2.
The sulfur in both species can undergo reduction reactions.

Definition

Sulfate: Sulfate is an anion composed of a sulfur atom bonded to four oxygen atoms around it.

Sulfite: Sulfite is an anion composed of sulfur and oxygen atoms.

Molar Mass

Sulfate: The molar mass of sulfate is about 96 g/mol.

Sulfite: The molar mass of sulfite is about 80 g/mol.

Solubility

Sulfate: Most sulfates are soluble in water.

Sulfite: Most sulfites are insoluble in water.

Geometry

Sulfate: The geometry around the sulfur atom is tetrahedral in sulfates.

Sulfite: The geometry around the sulfur atom is trigonal pyramidal in sulfites.

Oxidation State of Sulfur

Sulfate: The oxidation state of sulfur in sulfate is +6.

Sulfite: The oxidation state of sulfur in sulfite is +4.

Oxidation Reactions

Sulfate: Sulfate cannot undergo oxidation reactions.

Sulfite: Sulfites can undergo oxidation reactions.

Conclusion

Sulfates and sulfites share several similarities as well as differences. However, both of these species are commonly used in laboratory practices as well as in industries. The main difference between sulfate and sulfite is that, sulfate is composed of four oxygen atoms bonded to a sulfur atom whereas sulfite is composed of three oxygen atoms bonded to a sulfur atom.

References:

1. “Sulfates.” Study.com, n.d. Web. Available here. 10 July 2017.
2. “Sulfite reactions.” Sulfite. N.p., n.d. Web. Available here. 10 July 2017.

Image Courtesy:

1. “Sulfate-resonance-2D” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Sulfite-ion-2D-dimensions” (Public Domain) via Commons Wikimedia