Carbon Dioxide

Carbon dioxide (CO2) appears in more 0620 topics than almost any other compound: combustion, respiration, photosynthesis, carbonate chemistry, the greenhouse effect, and gas testing.

Key facts

Property	Value
Formula	CO2
Mr	12 + 2(16) = 44
Bonding	Simple covalent molecule, O=C=O (two double bonds)
Structure	Simple molecular — weak intermolecular forces, so a gas at room temperature
Key property	Denser than air; does not support combustion; weakly acidic in water
Main uses	Fire extinguishers, carbonated drinks, dry ice (solid CO2)

Where carbon dioxide appears in 0620

• Tests for gases: the limewater test.
• Air quality and climate change: CO2 as a greenhouse gas.
• Rate of reaction: CO2 given off when a carbonate reacts with acid (measured by loss of mass or gas volume).
• Simple molecules and covalent bonds: a model simple molecular substance.
• Oxides: CO2 is an acidic (non-metal) oxide.

Sources of CO2

Source	Equation
Complete combustion of a fuel	CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
Respiration	C6H12O6 + 6O2 -> 6CO2 + 6H2O
Thermal decomposition of carbonates	CaCO3(s) -> CaO(s) + CO2(g)
Carbonate + acid	CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)
Fermentation	C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)

Test for CO2

Bubble the gas through limewater:

Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)

The white precipitate of calcium carbonate makes the limewater turn milky. See calcium hydroxide.

CO2 as an acidic oxide

Being a non-metal oxide, CO2 is acidic. It dissolves slightly in water to form weak carbonic acid, and it reacts with alkalis:

CO2(g) + H2O(l) ⇌ H2CO3(aq)

CO2(g) + 2NaOH(aq) -> Na2CO3(aq) + H2O(l)

CO2 and the greenhouse effect

Burning fossil fuels and deforestation have raised the atmospheric CO2 concentration. CO2 absorbs the infrared radiation re-emitted by the warm Earth and sends some of it back, trapping heat (the enhanced greenhouse effect). Consequences include global warming, climate change and rising sea levels. Photosynthesis removes CO2 (6CO2 + 6H2O -> C6H12O6 + 6O2), so deforestation makes the problem worse. See greenhouse effect chemistry.

Uses

Use	Reason
Fire extinguishers	Denser than air and does not support combustion, so it smothers the flames
Carbonated drinks	Dissolves under pressure to give the fizz
Dry ice (solid CO2)	Sublimes straight to gas, used for cooling and stage effects

Carbon dioxide is described as a greenhouse gas. Explain how carbon dioxide in the atmosphere causes the Earth to warm. (3 marks)

Mark scheme

• The Earth’s surface absorbs energy from the Sun and re-emits it as infrared (thermal) radiation [1]
• Carbon dioxide molecules absorb this infrared radiation instead of letting it escape into space [1]
• and re-emit it in all directions, so some returns to the surface, trapping heat and raising the temperature [1]

Examiner note: award marks for the mechanism, not the consequences. “CO2 traps heat” on its own is too vague — the marks are for infrared radiation being absorbed and re-emitted by CO2.

Common exam mistakes

• Saying limewater “turns blue” or “goes clear”. A positive CO2 test turns limewater milky/cloudy white as calcium carbonate precipitates.
• Blaming CO2 for acid rain. Acid rain is caused by sulfur dioxide and nitrogen oxides; CO2 only makes rain naturally, weakly acidic (carbonic acid).
• Confusing the greenhouse effect with ozone depletion. They are separate problems with different causes.
• Describing CO2 as ionic or giant covalent. It is a simple molecule; its low boiling point is due to weak intermolecular forces, not weak covalent bonds.