Oxides of Carbon: CO and CO2

Carbon forms two important gaseous oxides, carbon monoxide and carbon dioxide, whose structures and chemical behaviour differ sharply.

Carbon monoxide (CO) is produced whenever carbon or a carbon-containing fuel undergoes incomplete combustion — that is, combustion in a limited supply of oxygen, such as in a poorly ventilated stove, an engine, or a blast furnace. Structurally, the CO molecule contains a very short, strong carbon–oxygen triple bond (best represented as $:\!\text{C}\!\equiv\!\text{O}\!:$, with a lone pair on each atom, one bonding interaction being of dative/coordinate character), and the carbon atom is $sp$ hybridised. CO is extremely dangerous because it is highly toxic: it binds to the iron centre of haemoglobin roughly two hundred times more strongly than oxygen does, forming a very stable complex called carboxyhaemoglobin. Because this binding is so much stronger than oxygen's, even a small proportion of CO in inhaled air progressively ties up the blood's haemoglobin, sharply reducing its capacity to transport oxygen to the body's tissues, which is why CO poisoning is so hazardous even at low concentrations and without any warning odour.

Chemically, CO is a powerful reducing agent — it is readily oxidised to $\text{CO}_2$, and this property is exploited on a huge industrial scale in extractive metallurgy. In the blast furnace, CO reduces iron(III) oxide ore to metallic iron:

Carbon dioxide (CO2) is formed by complete combustion of carbon-containing substances, by the action of dilute acids on carbonates, and by thermal decomposition of carbonates. Structurally it is a simple, symmetric linear molecule, $\text{O}=\text{C}=\text{O}$, with a $sp$-hybridised central carbon atom forming two equivalent C=O double bonds. Unlike CO, $\text{CO}_2$ is an acidic oxide: it dissolves in water to form the weak, unstable acid carbonic acid ($\text{H}_2\text{CO}_3$), and it reacts with bases such as sodium hydroxide to form carbonates or (in excess CO2) bicarbonates. Its most familiar characteristic reaction is with lime water (a dilute aqueous suspension/solution of calcium hydroxide), which turns milky white as insoluble calcium carbonate precipitates:

$\text{Ca(OH)}_2 + \text{CO}_2 \longrightarrow \text{CaCO}_3\!\downarrow + \text{H}_2\text{O}$ …