CO — Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas that is highly poisonous to humans and animals. It is composed of one carbon atom and one oxygen atom connected by a triple covalent bond. Carbon monoxide is formed primarily due to the incomplete combustion of carbon-containing fuels such as coal, wood, petrol, and natural gas.

Although carbon monoxide occurs naturally in small quantities in the atmosphere, its levels rise due to industrial emissions, vehicle exhaust, and residential heating systems. Inhalation of CO interferes with oxygen transport in the blood by forming carboxyhemoglobin with hemoglobin, which prevents oxygen from binding to red blood cells, leading to hypoxia or suffocation.

Carbon monoxide occurs in the atmosphere in trace amounts but is primarily generated by human activities. The major sources include:

• Incomplete Combustion: When fuels like coal, wood, or gasoline burn without sufficient oxygen, CO is formed: \( 2C + O_2 \rightarrow 2CO \).
• Automobile Exhaust: Motor vehicles are a major source of urban CO pollution.
• Industrial Processes: Steel manufacturing, metal smelting, and chemical industries produce CO as a byproduct.
• Natural Sources: Volcanic eruptions, wildfires, and the oxidation of methane in the atmosphere contribute to natural CO formation.

Despite being toxic, carbon monoxide plays a role in atmospheric chemistry by reacting with hydroxyl radicals, influencing the balance of greenhouse gases.

Laboratory Preparation:

Carbon monoxide is commonly prepared in the laboratory by heating formic acid or oxalic acid with concentrated sulfuric acid, which acts as a dehydrating agent.

From formic acid:

From oxalic acid:

The mixture of CO and CO₂ can be purified by passing it through a solution of potassium hydroxide, which absorbs CO₂, leaving pure CO.

Industrial Preparation:

Industrially, carbon monoxide is produced by:

• Partial Oxidation of Carbon: \( 2C + O_2 \rightarrow 2CO \)
• Water-Gas Reaction: When steam is passed over red-hot carbon (coke), it produces a mixture of CO and hydrogen, known as water gas:

  \( C + H_2O \xrightarrow{1000°C} CO + H_2 \)

This gas mixture is an important industrial fuel and a source of hydrogen for the Haber process.

Physical Properties:

• Carbon monoxide is a colorless, odorless, and tasteless gas.
• It is slightly less dense than air and slightly soluble in water.
• It burns with a blue flame to form carbon dioxide: \( 2CO + O_2 \rightarrow 2CO_2 \).
• It is highly toxic even at low concentrations.

Chemical Properties:

• Combustion: Carbon monoxide burns in air with a blue flame, forming carbon dioxide:

  \( 2CO + O_2 \rightarrow 2CO_2 \)

• Reducing Agent: It acts as a powerful reducing agent and reduces metal oxides to metals:

  \( Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2 \)

  This property is utilized in metallurgical processes like iron extraction from ores.

• Reaction with Chlorine: Forms carbonyl chloride (phosgene), a poisonous compound:

  \( CO + Cl_2 \xrightarrow{hv} COCl_2 \)

• Formation of Metal Carbonyls: CO reacts with transition metals to form metal carbonyl complexes such as nickel carbonyl:

  \( Ni + 4CO \rightarrow Ni(CO)_4 \)

• Metallurgy: Carbon monoxide acts as a reducing agent for metal oxides during extraction of metals like iron and nickel.
• Chemical Industry: Used in the production of methanol, acetic acid, and phosgene.
• Fuel Gas Mixtures: Forms part of water gas and producer gas used as industrial fuels.
• Metal Refining: Utilized in the Mond process for the purification of nickel using nickel carbonyl.
• Laboratory Use: Serves as a reducing agent and as a carbonyl source in organometallic chemistry.

Despite its industrial utility, carbon monoxide must be handled with extreme care due to its toxicity and potential for lethal poisoning.

Carbon monoxide is extremely toxic because it binds to hemoglobin about 200 times more strongly than oxygen, forming carboxyhemoglobin (\( HbCO \)). This prevents the transport of oxygen to body tissues, leading to oxygen deprivation, dizziness, unconsciousness, and even death.

At low concentrations, CO exposure causes headaches and fatigue, while at high levels it can be fatal within minutes. Therefore, adequate ventilation is crucial in enclosed spaces with combustion sources.

Environmentally, carbon monoxide contributes to the formation of ground-level ozone and smog. It reacts with hydroxyl radicals, reducing the atmosphere’s ability to clean itself. Controlling CO emissions from vehicles and industries is vital for maintaining air quality.