Sulfur Dioxide (SO₂) — Properties, Uses & Safety

Overview

Sulfur dioxide (SO2) is a colorless gas with a pungent, suffocating odor and a molar mass of 64.066 g/mol. The molecule has a bent geometry with a bond angle of approximately 119 degrees, where the central sulfur atom is bonded to two oxygen atoms with resonance structures that give the S-O bonds a bond order of about 1.5. At standard conditions, SO2 is a gas but can be easily liquefied under moderate pressure. Sulfur dioxide is a major air pollutant produced primarily by burning sulfur-containing fossil fuels such as coal and oil in power plants and industrial facilities, as well as by volcanic eruptions. When SO2 reacts with water vapor in the atmosphere, it forms sulfurous acid (SO2 + H2O -> H2SO3), which contributes to acid rain — a phenomenon that damages buildings, acidifies lakes and soils, and harms ecosystems. GHS hazard codes for SO2 include H280 (gas under pressure), H314 (causes severe skin burns and eye damage), and H331 (toxic if inhaled). Despite its toxicity, sulfur dioxide has beneficial uses. It serves as a preservative in dried fruits and wines (labeled as E220), a bleaching agent for paper and textiles, and a key intermediate in the production of sulfuric acid via the Contact Process (2 SO2 + O2 -> 2 SO3, followed by SO3 + H2O -> H2SO4). For chemistry students, sulfur dioxide connects to numerous curriculum topics. Its bent molecular geometry illustrates VSEPR theory for molecules with lone pairs on the central atom. The resonance structures of SO2 demonstrate delocalized bonding. The formation of acid rain from SO2 is a real-world application of acid-base chemistry and chemical equilibrium. The Contact Process for manufacturing sulfuric acid introduces concepts of industrial chemistry, catalysis using V2O5, and Le Chatelier's principle. Understanding SO2 helps students see how chemistry directly impacts environmental policy and public health.

IUPAC Name

sulfurous anhydride

CAS Number

7446-09-5

PubChem CID

1119

Molecular Formula

SO₂

Molar Mass

64.066 g/mol

State at STP

gas

Appearance

Colorless gas with pungent sulfur odor