Carbon Dioxide

Carbon dioxide (CO₂) occurs in nature both free (about 0.03% of air by volume) and combined, as calcium and magnesium carbonate rock.

Production. On an industrial scale, CO₂ is produced by burning coke in excess air: C + O₂ → CO₂, ΔH = -394 kJ mol⁻¹; it is also generated as a by-product of the calcination of lime: CaCO₃ → CaO + CO₂. In the laboratory, it is prepared by the action of dilute hydrochloric acid on a metal carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Properties. Carbon dioxide is a colourless, non-flammable gas, heavier than air, with a critical temperature of 31 °C -- low enough that it can be readily liquefied. It is a very stable compound: even at 3100 K only 76% of it decomposes, into carbon monoxide and oxygen (CO₂ → CO + ½O₂), and only at still higher temperature does it break down fully into carbon and oxygen (CO₂ → C + O₂). At elevated temperatures it can act as a strong oxidising agent, e.g. CO₂ + 2Mg → 2MgO + C. Its equilibrium reaction with hydrogen, CO₂ + H₂ ⇌ CO + H₂O, has many industrial applications and is known as the water gas equilibrium. Its aqueous solution is only slightly acidic, since only a small fraction of dissolved CO₂ forms carbonic acid: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. …