Carbon monoxide

Carbon monoxide ($\text{CO}$) is a colourless, odourless gas. It is not the same substance as the more familiar carbon dioxide ($\text{CO}_2$), even though both come from burning carbon — the difference is purely a matter of how much oxygen was available during combustion. When carbon (or a carbon fuel) burns with a limited supply of oxygen, combustion is incomplete and carbon monoxide is the product:

where: $\text{C}$ is carbon, $\text{O}_2$ is the limited oxygen available, and $\text{CO}$ is carbon monoxide, the incomplete-combustion product. When the same carbon burns in excess oxygen, combustion goes to completion and carbon dioxide is the product instead:

where: here $\text{O}_2$ is in plentiful supply, so every carbon atom picks up two oxygen atoms rather than one. This single contrast — limited oxygen gives $\text{CO}$, excess oxygen gives $\text{CO}_2$ — is the idea examiners return to again and again, and it is the reason a charcoal stove burning in a shut room behaves so differently from the same stove burning outdoors.

Two properties of carbon monoxide matter most at this level. First, it is a reducing agent in the extraction of iron. In a blast furnace, carbon monoxide gas takes oxygen away from iron(III) oxide ore, leaving behind metallic iron, while the carbon monoxide itself becomes carbon dioxide:

where: $\text{Fe}_2\text{O}_3$ is iron(III) oxide ore, $\text{CO}$ removes its oxygen, $\text{Fe}$ is the metallic iron produced, and $\text{CO}_2$ is the carbon monoxide after it has picked up that oxygen. Using the classical, oxygen-based definition of reduction — reduction is loss of oxygen — the iron ore is reduced and carbon monoxide is the reducing agent. You need only the outline of this role, not the full staged blast-furnace process. Second, carbon monoxide is toxic, causing asphyxia (suffocation-like oxygen starvation of the body) when inhaled, because it interferes with the blood's ability to carry oxygen to the cells. Since it has no colour and no smell, a person breathing it in gets no warning sign — which is exactly why real precautions matter: never sit behind a vehicle with its engine idling in an enclosed space such as a closed garage, and never burn coal or charcoal stoves in a room without ventilation.

Question: A charcoal stove burns inside a small room with the door and windows shut. (a) Explain, with an equation, why this is dangerous and name the harmful effect. (b) If the same charcoal stove were instead used to help reduce iron(III) oxide to iron in a furnace with a good air supply of carbon monoxide, write the balanced equation for that reaction and state which substance is reduced.

Step 1. In the closed room, fresh air cannot enter, so the oxygen available to the burning charcoal is limited. Limited oxygen means incomplete combustion of the carbon: $2\text{C} + \text{O}_2 \rightarrow 2\text{CO}$.

Step 2. This incomplete combustion produces carbon monoxide, a gas with no colour and no smell, so nobody in the room gets a warning sign of its build-up.

Step 3. If inhaled, carbon monoxide stops the blood from carrying oxygen properly, causing asphyxia, which can be fatal — this is why the room must be well ventilated whenever such fuels are burned.

Step 4. For part (b), the furnace reaction is the balanced equation derived above, $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$, and by the classical (oxygen-loss) definition of reduction, iron(III) oxide is reduced, with carbon monoxide acting as the reducing agent.

Final answer: in the closed room, incomplete combustion produces poisonous carbon monoxide, causing asphyxia; in the furnace, $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$, with iron(III) oxide being reduced.

• Mixing up carbon monoxide ($\text{CO}$, from limited oxygen, toxic) with carbon dioxide ($\text{CO}_2$, from excess oxygen, not acutely poisonous) — always link the gas to the oxygen supply that produced it.
• Assuming carbon monoxide would announce itself with a smell or colour — it has neither, which is precisely why it is so dangerous in an enclosed space.
• Using the electron-transfer definition of reduction on the iron-ore equation — at this level, only the classical oxygen-loss/gain definition is used, so state it as "loses oxygen," not "gains electrons."
• Forgetting to balance oxygen and carbon together — placing the coefficient $3$ on $\text{CO}$ balances carbon automatically, so re-checking oxygen alone can leave carbon silently wrong if the coefficient is misplaced.

Ask the tutor: if the classical oxygen-loss definition of reduction or the atom-by-atom balancing of the iron-ore equation still feels shaky, ask your AI tutor to walk through a few more balancing practice equations with you.