Carbon monoxide, carbon dioxide and the carbonates

Two gases with one difference

A closed room with a charcoal fire can fill with a gas that has no colour and no smell, and it can kill. The gas is carbon monoxide, CO. A bottle of lemonade fizzes with another oxide of carbon, carbon dioxide, $\text{CO}_{2}$. The two differ by one oxygen atom, and their behaviour is very different.

Carbon monoxide

Structure. The molecule has 10 valence electrons and a triple bond: $:\text{C}\equiv \text{O}:$. Carbon is at the end of the bond with a lone pair, and so is oxygen. Two of the three bonds are ordinary covalent bonds, and in the third bond oxygen supplies both electrons, as a dative bond. The bond length is $112.8\ \text{pm}$, very short. CO has the same number of electrons as $\text{N}_{2}$, $\text{CN}^{-}$ and $\text{NO}^{+}$. It is a neutral oxide. It does not react with water or alkali under ordinary conditions.

Preparation.

• In the laboratory, warm formic acid with concentrated sulphuric acid, which takes out water, at 373 K: $\text{HCOOH} \to \text{CO} + \text{H}_{2}\text{O}$.
• In industry, coke is burned in a limited supply of air to make producer gas, a mixture of CO and nitrogen: $2\text{C} + \text{O}_{2} + 4\text{N}_{2} \to 2\text{CO} + 4\text{N}_{2}$. Steam over red-hot coke at 1273 K gives water gas, a mixture of CO and hydrogen: $\text{C} + \text{H}_{2}\text{O} \to \text{CO} + \text{H}_{2}$.

Properties.

• It burns with a blue flame: $2\text{CO} + \text{O}_{2} \to 2\text{CO}_{2}$.
• It is a reducing agent. In the blast furnace, $\text{Fe}_{2}\text{O}_{3} + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_{2}$.
• It combines with chlorine to form phosgene, $\text{CO} + \text{Cl}_{2} \to \text{COCl}_{2}$, which is a poison.
• It combines with nickel to form a volatile carbonyl: $\text{Ni} + 4\text{CO} \to \text{Ni}(\text{CO})_{4}$ at about 330 K. The carbon of CO donates a lone pair to the metal. This is used to purify nickel.
• It is poisonous. It binds to the haemoglobin of the blood about 200 times more strongly than oxygen does, forming carboxyhaemoglobin. The blood cannot carry oxygen.

Carbon dioxide

Structure. The molecule is linear, O=C=O, with an angle of $180^\circ$ and a C=O bond length of $116\ \text{pm}$. Carbon is sp hybridised. Each C=O bond is polar, but the two cancel, so the molecule has no dipole moment.

Preparation.

• In the laboratory, marble and dilute hydrochloric acid: $\text{CaCO}_{3} + 2\text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$.
• In industry, heating limestone at about 1100 to 1200 K: $\text{CaCO}_{3} \to \text{CaO} + \text{CO}_{2}$. It is also a product of burning fuels and of fermentation.

Properties.

• It does not burn and does not help things to burn, so it is used in fire extinguishers. It is heavier than air.
• It is an acidic oxide. It dissolves a little in water: $\text{CO}_{2} + \text{H}_{2}\text{O} \rightleftharpoons \text{H}_{2}\text{CO}_{3}$. Carbonic acid is a weak acid.
• It turns limewater milky: $\text{Ca}(\text{OH})_{2} + \text{CO}_{2} \to \text{CaCO}_{3} + \text{H}_{2}\text{O}$. The milkiness goes away with more gas, because soluble calcium hydrogencarbonate forms: $\text{CaCO}_{3} + \text{CO}_{2} + \text{H}_{2}\text{O} \to \text{Ca}(\text{HCO}_{3})_{2}$.
• Solid carbon dioxide is dry ice. It sublimes at 195 K, which is $-78^\circ\text{C}$, and is used as a coolant.
• Green plants take it in during photosynthesis: $6\text{CO}_{2} + 6\text{H}_{2}\text{O} \to \text{C}_{6}\text{H}_{12}\text{O}_{6} + 6\text{O}_{2}$.
• It is a greenhouse gas. It lets sunlight in, but absorbs the infrared radiation that the Earth sends out, so more of it warms the planet.

Set the mass to 1000 g and the temperature to 273 K. The block turns into about 510 L of gas, which would fill a small room. Warm the gas to 300 K, and the volume increases. A closed bottle of dry ice can burst for this reason.

The carbonates

The carbonate ion, $\text{CO}_{3}^{2-}$, is a flat triangle. All three C-O bonds are the same, as a result of resonance.

• Acids and carbonates. Every carbonate and hydrogencarbonate gives carbon dioxide with an acid: $\text{CO}_{3}^{2-} + 2\text{H}^{+} \to \text{CO}_{2} + \text{H}_{2}\text{O}$. This is the test for a carbonate.
• Heating. Calcium carbonate loses carbon dioxide at about 1200 K. Sodium hydrogencarbonate loses it at a much lower temperature, above about 400 K: $2\text{NaHCO}_{3} \to \text{Na}_{2}\text{CO}_{3} + \text{CO}_{2} + \text{H}_{2}\text{O}$. Sodium carbonate does not lose carbon dioxide on heating in the flame of a burner.
• Uses. Washing soda, $\text{Na}_{2}\text{CO}_{3}\cdot 10\text{H}_{2}\text{O}$, softens water. Baking soda, $\text{NaHCO}_{3}$, is used in baking, and in some fire extinguishers. Limestone is the raw material of cement and lime.

Practise with new numbers

Try it in Python

The program compares carbonates, and finds how much carbon dioxide each releases from 1 g on heating.

Common mistakes

• Saying that CO is acidic. It is a neutral oxide. Carbon dioxide is acidic.
• Writing the bonding of CO as a double bond. It is a triple bond with one dative component.
• Thinking that CO$_2$ is polar because its bonds are polar. The two bond dipoles cancel in a linear molecule.
• Thinking that limewater stays milky with excess gas. It clears, because $\text{Ca}(\text{HCO}_{3})_{2}$ dissolves.
• Forgetting that the coefficient 2 of HCl does not apply to CO$_2$.

Remember these

• CO: triple bond, $112.8\ \text{pm}$, neutral, reducing, poisonous, a ligand. Made from HCOOH with hot concentrated $\text{H}_{2}\text{SO}_{4}$, or as producer gas and water gas.
• CO$_2$: linear, $116\ \text{pm}$, no dipole, acidic, a greenhouse gas. Made from a carbonate and an acid, or by heating limestone.
• Limewater: milky with a little CO$_2$ and clear again with excess.
• Dry ice is solid CO$_2$ and sublimes at 195 K.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is carbon monoxide poisonous?

2. Limewater turns milky when carbon dioxide is passed in, and then clear. Write the two equations.

3. How does the formula of producer gas differ from that of water gas?

4. Carbon dioxide has two polar bonds. Why is the molecule non-polar?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals), the section on carbonates.
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), the sections on carbon monoxide and carbon dioxide.
• Atkins and Shriver, Inorganic Chemistry, the chapter on the group 14 elements.