Carbon and its Compounds: Oxides of Carbon

Oxides of Carbon

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Carbon forms two important gaseous oxides. These are:  carbon monoxide or carbon(II) oxide (CO) and carbon dioxide or carbon(IV) oxide (CO₂). They differ sharply in structure, properties, and toxicity because of the different bonding arrangements between carbon and oxygen.

 

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Carbon(II) oxide (CO)

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Structure of CO(g)

Carbon(II) oxide exists as a gas at room temperature. It has a triple bond between carbon and oxygen (C≡O) consisting of two ordinary covalent bonds and one coordinate (or dative) covalent bond.; making it a neutral oxide overall. The lone pair on carbon (coordinate covalent bond) allows CO to act as a ligand toward metal ions (e.g. haemoglobin and transition metals). This bonding is central to its toxicity and its use in some industrial chemistry.

The structure of carbon(II) oxide (CO) is shown below:

You can read on acids, basicity and types of acid here.

 

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Physical properties of CO(g)

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• Carbon(II) oxide is a colorless, odorless and tasteless gas.

• It is slightly less dense than air. This is so because it has a molecular weight of 28.01g/mol, relative to dry air (dry air = 28.96g/mol), and vapor density of 0.97 relative to air (air = 1.0)

• It is almost insoluble in water, but dissolves in a solution of ammoniacal copper(I) chloride (also known as ammoniacal cuprous chloride) by forming a soluble addition complex.

• It has a melting point of −205°C (−337°F), and boiling point of −191.5°C (−313°F)

• It is a neutral oxide (does not turn litmus red or blue, and does not react with acids or bases under normal conditions).

Please read the introduction to carbon, its structure and allotropes here.

 

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Chemical Properties of CO(g)

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• Carbon(II) oxide is highly flammable. It burns readily in air/oxygen with a characteristic blue flame to form carbon(IV) oxide. It is shown via the equation below:

 2CO_(g) + O_2(g) → 2CO_2(g)

• It is a good reducing agent at high temperature. In this regard, it can remove oxygen from metal oxides. See the equations below:

 CO_(g) + CuO_(s) → Cu_(s) + CO₂_(g)

 3CO_(g) + Fe₂O₃_(s) → 2Fe_(s) + 3CO₂_(g).  (Important in the blast furnace)

• Carbon(II) oxide combines with haemoglobin in red blood cells to form a stable compound called carboxyhaemoglobin (COHB). The formation of COHb in the red blood cells make it a poisonous gas to humans. This is shown via the reaction below:

CO + Hb → COHb

Please read on the physical and chemical properties of acids, preparation, tests and uses of acids here.

 

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Preparation of Carbon(II) oxide (CO)

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Laboratory preparation of CO(g)

Carbon(II) oxide can be prepared in the laboratory by the dehydration of methanoic (formic) acid using concentrated sulfuric acid (as a dehydrating agent) with gentle heating:

HCOOH_(aq) --(conc. H₂SO₄, heat) → CO_(g) + H₂O_(l)

Precaution: This reaction must be done in a fume cupboard because the gas (CO) is poisonous.

Alternatively, it is also prepared by the dehydration of ethanedioic (oxalic) acid with conc. H₂SO₄, giving a mixture of CO and CO₂. The CO₂ can be removed by passing the gas mixture through caustic soda (NaOH) solution, leaving CO.

H₂C₂O_4(aq)  --(conc. H₂SO₄) → CO_(g) + CO_2(g) + H₂O_(l)

Please read on bases and alkalis, their definitions, classifications, physical and chemical properties here.

 

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Industrial preparation of CO(g)

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Carbon(II) oxide is produced industrially (or in a large-scale) by:

• Incomplete combustion of carbon (coke) in a limited supply of air/oxygen:

 2C_(s) + O₂_(g) → 2CO_(g)

• Passing steam over red-hot coke (water gas)

C_(s) + H₂O_(g) → CO_(g) + H₂_(g)

• Passing air over red-hot coke (producer gas, a mixture mainly of CO and N₂):

 2C_(s) + O₂_(g) + 4N₂_(g) [from air] → 2CO_(g) + 4N₂_(g)

Please read more on the amorphous allotropes of carbon (coal and coke) here.

 

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Test for Carbon(II) oxide (CO)

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Main confirmatory test for CO(g)

Carbon(II) oxide burns with a blue flame, producing CO₂ gas in the process. This gas produced (when passed into limewater) turns limewater milky, (which confirms the burning product is CO₂, indirectly confirming the original gas was CO).

2CO_(g) + O_2(g) → 2CO_2(g).

The CO₂ gas turns limewater -Ca(OH)₂ milky.

Note: CO does not turn limewater milky. (This distinguishes it from CO₂).

 

Other test for CO(g) in the Lab.

CO can also be identified by passing it over heated palladium(II) chloride solution. It turns the solution black through the deposits a black precipitate of palladium metal. (This is a specific lab test).

Note: The method of  burning of CO in air with the product passed through imewater remains the standard test expected at this level.

Please read on pH indicators, types and importance of pH and pOH

 

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Processes that introduce Carbon(II) oxide (CO) into the Atmosphere

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Carbon(II) oxide (CO) is introduced into the atmosphere through the following:

• Incomplete combustion of fuels (petrol/diesel engines, especially with insufficient air supply).

• Exhaust fumes from motor vehicles.

• Incomplete combustion in industrial processes and poorly ventilated fires/stoves.

• Cigarette smoke.

• Use of generators in closed environments.

• Some natural processes (e.g. volcanic activity and forest fires) in small amounts.

 

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Processes that remove Carbon(II) oxide (CO) from the Atmosphere

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Carbon(II) oxide (CO) is removed from the atmosphere through the following:

• Oxidation in the atmosphere (where it slowly reacts with oxygen/atmospheric radicals over time) to form CO₂.

• Uptake and conversion by certain soil microorganisms.

• Natural atmospheric mixing/dilution and gradual chemical breakdown.

Note: Carbon(II) oxide does not have a large or well-defined "natural cycle" as carbon(IV) oxide (CO₂). In this regard, iit is mainly regarded as a pollutant that is slowly oxidized away.)

You can read a brief note on dynamic equilibrium in chemistry here.

 

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Toxicity of Carbon(II) oxide (CO)

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Carbon(II) oxide (CO) is highly poisonous, even in small concentrations, and is especially dangerous because it is colorless and odorless (cannot be detected by human senses).

 

Mechanism of toxicity

Carbon(II) oxide (CO) binds to haemoglobin in red blood cells far more strongly than oxygen does (forming carboxyhaemoglobin), with an affinity roughly 200+ times greater than that of oxygen. This prevents haemoglobin from carrying oxygen around the body, starving tissues and organs (especially the brain and heart) of oxygen. This condition is called hypoxia.

Since the binding is very strong and only slowly reversible, even relatively low concentrations of CO over time can cause serious harm.

 

The symptoms of CO poisoning include headache, dizziness, nausea, confusion, weakness. At higher exposure, CO poisoning leads to loss of consciousness and death.

 

Sources of CO danger: include the following:

• Faulty gas heaters/boilers

• Blocked chimneys/flues

• Car engines running in enclosed spaces (e.g. closed garages)

• charcoal stoves used indoors without ventilation.

• Generators used indoors without ventilation.

Please read on the adverse effects of chemistry here.

 

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Uses of Carbon(II) oxide (CO)

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• Carbon(II) oxide is used as a fuel (component of water gas and producer gas), applied in heating; and in various industries.

• As a reducing agent, it is used in metallurgy, especially in the extraction of iron in the blast furnace.

• It is used in the manufacture of certain chemicals, e.g. in some industrial synthesis routes (such as production of methanol) as a source of carbon.

• It can react with transition metals (like nickel or iron) to form metal carbonyls. These metal carbonyls are used in refining high-purity metals.

• It serves as a reagent in carbonylation reactions to produce organic compounds such as aldehydes, carboxylic acids, and polymers.

Please read on the uses of chemistry here.

 

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Summary Comparison Table for CO vs CO₂

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Property	CO	CO₂
Bonding	Triple bond (C≡O)	Double bonds (O=C=O), linear
Nature of oxide	Neutral oxide	Acidic oxide
Color/odor	Colorless, odorless	Colorless, odorless
Density vs air	Slightly less dense	About 1.5× denser
Solubility in water	Almost insoluble	Fairly soluble (forms weak acid)
Combustion	Burns with blue flame	Does not burn/support combustion
Test	Burns blue; product turns limewater milky	Turns limewater milky, then clear in excess
Main hazard	Highly toxic. Binds haemoglobin, causes hypoxia / Asphyxiant	Displaces oxygen, not directly poisonous
Key source	Incomplete combustion	Respiration, complete combustion, decay and volcanic activity
Key removal process	Slow atmospheric oxidation to CO₂	Photosynthesis, dissolving in oceans, carbonate rock formation

 

Please read on the next topic (Oxides of carbon: carbon(IV) oxide) here.