Interaction with non-metals: oxygen and sulfur
The reactions of carbon with non-metals occur at elevated temperatures and are of immense practical importance.
Combustion (reaction with oxygen) The coal burning process proceeds with the release of a large amount of heat (exothermic reaction). It is worth noting that the heat of combustion of coal to carbon dioxide is equivalent to the heat of combustion of graphite. If oxygen is insufficient, incomplete combustion occurs. In this case, the reaction product is carbon(II) oxide, better known as carbon monoxide. This substance is characterized by high toxicity. Equation for incomplete combustion: 2C + O₂ = 2CO (enthalpy change is -221 kJ).
Reaction with sulfur If sulfur vapor is passed over red-hot coal, carbon disulfide (CS₂) is synthesized. This reaction requires energy input (endothermic, enthalpy 89.4 kJ). The resulting carbon disulfide is a volatile, highly flammable, and toxic liquid with a pungent, repulsive odor. In chemical practice, it is used as a solvent.
Reactions with metals and their oxides
When heated with metals, carbon exhibits oxidizing properties, accepting electrons and forming binary compounds known as carbides.
Examples of carbide formation:
- With calcium: Ca + 2C → CaC₂ (upon heating)
- With aluminum: 4Al + 3C → Al₄C₃ (upon heating)
Reduction of metals from oxides One of the most important industrial properties of carbon is its ability to strip oxygen from metal oxides at high temperatures. This process forms the basis of pyrometallurgy. Example of zinc reduction: ZnO + C → Zn + CO
However, there is an exception to this rule: the most thermodynamically stable metal oxides (such as aluminum oxide Al₂O₃) cannot be reduced with ordinary coal—they do not react with it.
Behavior towards oxidizing acids and alkalis
Ordinary acids do not affect carbon, but strong oxidizing acids can convert it into carbon dioxide upon heating.
- Concentrated sulfuric acid: oxidizes carbon with the release of sulfur dioxide.
C + 2H₂SO₄(conc.) → CO₂↑ + 2SO₂↑ + 2H₂O
- Nitric acid: the reaction proceeds with the release of nitrogen(II) oxide.
3C + 4HNO₃ → 3CO₂↑ + 4NO↑ + 2H₂O
Unlike acids, coal does not chemically interact with solutions and melts of alkalis.
Interaction with salts
Carbon is capable of reacting with salts of oxygen-containing acids upon heating, acting as a strong reducing agent (binding oxygen from the salt composition). The nature of these reactions strongly depends on the nature of the salt:
- With nitrates: the reaction proceeds violently, with a massive release of heat (strongly exothermic).
5C + 4KNO₃ = 2K₂CO₃ + 3CO₂ + 2N₂ (enthalpy -1504.1 kJ)
- With sulfates: the reaction requires a continuous supply of heat (endothermic).
4C + BaSO₄ = BaS + 4CO (enthalpy 571.2 kJ)
Briefly: answers to topical questions
In which reactions does elemental carbon act as an oxidizing agent?
The provided sources indicate the interaction of carbon with metals upon heating, resulting in the formation of carbides:
- with calcium, calcium carbide ($CaC_2$) is formed;
- with aluminum, aluminum carbide ($Al_4C_3$) is formed; at a temperature of 2000 °C, it has an orange color;
- with sodium upon heating, sodium acetylide (carbide) ($Na_2C_2$) is formed.
With which non-metals does carbon react upon heating?
Upon heating, carbon enters into chemical reactions with oxygen and sulfur.
- Oxygen — an exothermic combustion process occurs with the formation of carbon dioxide ($CO_2$). In case of oxygen deficiency, incomplete combustion takes place, resulting in the formation of toxic carbon monoxide ($CO$).
- Sulfur — the reaction proceeds by passing sulfur vapor over red-hot coal. The product is carbon disulfide ($CS_2$), which is a flammable, volatile, and toxic liquid with a repulsive odor.
To avoid confusing the role of carbon: with metals it is "greedy" (accepts electrons, forming carbides), while with complex substances (oxides, acids, salts) it is "generous" (donates electrons, stripping away oxygen).
Class and exam questions
Can carbon be dissolved in an alkali?
No, elemental carbon is chemically inert toward alkalis and does not react with them.
Why can stove heating produce poisonous gas?
In case of insufficient oxygen supply, incomplete combustion of coal occurs, resulting in the formation of toxic carbon monoxide (CO) instead of safe carbon dioxide.
Can aluminum be obtained from its oxide using coal?
No, aluminum oxide belongs to the most stable oxides, which are not reduced by carbon upon heating.
What else to study on this topic
- Thermodynamics of complete and incomplete coal combustion
- Properties and toxicity of carbon disulfide
- Pyrometallurgy: limits of applicability of carbon
- Redox processes during interaction with salts
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