Carbon Dioxide

Occurrence

Free, 20% in air, and 0.04% dissolved in water

Preparation

Lab method

1.        By heating a metal carbonate

CaCO₃ Δ→ CaO + CO_2↑

ZnCO₃ Δ→ ZnO + CO_2↑

CuCO₃ Δ→ CuO + CO_2↑ (bluish-green to black)

Sodium and potassium carbonates do not decompose on heating. Sodium carbonate may give off water vapor.

               Na₂CO₃ · 10H₂O → Na₂CO₃ + 10H₂O

2.        By adding hydrochloric acid to calcium carbonate

CaCO₃ + 2HCl →  CaCl₂ + H₂O + CO_{2 ↑}

(Other acids, like sulphuric acid are not preferred because calcium carbonate gets coated by another calcium salt, rendering it passive)

The gas is collected under air.

               NaHCO₃ + HCl →  NaCl + H₂O + CO_{2 ↑}

               2NaHCO₃ Δ→ Na₂CO3 + H₂O + CO_2↑

               Ca(HCO₃)₂ Δ→ Ca₂CO3 + H₂O + CO_2↑

Industrial method

Steam is passed over heated coke. The reaction produces hydrogen and carbon monoxide. This water gas, if passed with excess steam over a catalyst, iron (III) oxide, forms carbon dioxide and hydrogen.

C + H₂O → CO + H₂

CO + H₂ + H₂O → CO₂ + 2H₂

Properties

Physical

1.        It is a colorless, odorless, tasteless acidic gas.

2.        It is slightly soluble in water. (It can be dried using P₂O₅)

Chemical

1.        It does not support combustion, and it does not burn. However, magnesium burns in CO₂ to form its oxide and carbon.

2Mg + CO₂ → 2MgO + C

               Sodium and potassium also burn, but later form carbonates.

4Na + CO₂ → 2Na₂O + C

4K + CO₂ → 2K₂O + C

Na₂O + CO₂ → Na₂CO₃

K₂O + CO₂ → K₂CO₃

2.        It neutralizes alkalies.

CO₂ + 2KOH → K₂CO₃ + H₂O

CO₂ + 2NaOH → Na₂CO₃ + H₂O

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Uses

1.        In fire extinguishers – formed by the action of sulphuric acid on sodium hydrogen carbonate.

2.        Used as a refrigerant – dry ice.

3.         Is a leavening agent – baking soda, sodium bicarbonate and an acid in dry form react.

4.        In soft drinks, dissolved and under pressure.