Carbon and its Compounds · Lesson 8 of 13
Chemical Properties Of Carbon Compounds
“Give carbon enough oxygen for a clean burn; starve it and soot takes over.”
• Explain combustion of carbon and carbon compounds as oxidation reactions. • Balance combustion equations for methane and ethanol. • Distinguish clean and sooty flames using oxygen supply and saturation. • Explain why cooking vessels blacken when fuel burns incompletely. • Explain why some substances burn with a flame while others glow.
Carbon and many carbon compounds are used as fuels because their combustion releases heat and often light. Combustion is also an oxidation process because oxygen is added to the burning substance.
Combustion
Carbon in its allotropes burns in oxygen to form carbon dioxide. Carbon compounds also burn, and complete combustion of hydrocarbons or alcohols produces carbon dioxide and water.
Balancing Methane Combustion
Problem
Balance CH₄ + O₂ → CO₂ + H₂O.
- 1.Carbon is already balanced with one carbon atom on each side.
- 2.Hydrogen is four on the left, so place 2 before H₂O to give four hydrogen atoms on the right.
- 3.The products now contain four oxygen atoms in total: two in CO₂ and two in 2H₂O.
- 4.Place 2 before O₂.
- 5.The balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O.
Balancing Ethanol Combustion
Problem
Balance C₂H₅OH + O₂ → CO₂ + H₂O.
- 1.Ethanol contains two carbon atoms, so place 2 before CO₂.
- 2.It contains six hydrogen atoms, so place 3 before H₂O.
- 3.The products now contain seven oxygen atoms: four in 2CO₂ and three in 3H₂O.
- 4.Ethanol already provides one oxygen atom, so six more oxygen atoms are needed from O₂.
- 5.Place 3 before O₂.
- 6.The balanced equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Clean And Sooty Flames
Saturated hydrocarbons generally burn with a cleaner flame when enough oxygen is supplied. Unsaturated carbon compounds tend to produce a yellow flame with black smoke and soot. Even saturated fuels can produce soot if the oxygen supply is restricted.
Why Cooking Vessels Become Black
A gas or kerosene stove is designed with air inlets so fuel can mix with enough oxygen. If these air holes are blocked, combustion becomes incomplete. Sooty carbon can then deposit on the bottom of the vessel, and some fuel energy is wasted.
Why Some Substances Burn With A Flame
A flame is produced when gaseous substances burn. LPG burns as a gas and therefore produces a flame. Wood initially gives volatile vapours that burn with a flame. Coal or charcoal may later glow red and release heat without a visible flame because the solid itself is not producing enough burning vapour.
Luminous Flame And Soot
A luminous flame appears when hot particles or atoms glow. Incomplete combustion produces soot, which is carbon. This helps explain the yellow luminous appearance of a candle flame.
Fossil Fuels
Coal and petroleum formed from ancient biomass altered by biological and geological processes over very long periods. Coal formed mainly from buried plant material, while oil and gas formed from tiny marine organisms whose remains were buried, transformed and trapped in porous rock. Their biological origin explains the term fossil fuels.
Pollutants From Fuel Combustion
Coal and petroleum can contain nitrogen and sulphur. During combustion, oxides of nitrogen and sulphur can form and act as important environmental pollutants.
Quiz
What are the main products of complete combustion of methane?
Why does a yellow sooty flame form when oxygen is limited?
What does blackening of the bottom of a cooking vessel usually indicate?
Why can coal glow without a visible flame?
Which equation is balanced for methane combustion?
Practice Problems
- Balance the combustion equation of methane and explain every coefficient.
- Balance the combustion equation of ethanol and verify carbon, hydrogen and oxygen atoms.
- Explain why a saturated hydrocarbon can still give a sooty flame when the air supply is restricted.
- Explain why blocked stove air holes can blacken cooking vessels and waste fuel.
- Compare burning with a flame and glowing without a flame using LPG, wood and charcoal.
- Explain briefly how coal and petroleum formed from biomass and why they are called fossil fuels.
Key Takeaways
• Combustion of carbon and carbon compounds is an oxidation process that releases heat and often light. • Complete combustion of carbon compounds produces carbon dioxide and water. • Methane and ethanol combustion equations must be balanced by conserving carbon, hydrogen and oxygen atoms. • Limited oxygen causes incomplete combustion and soot, producing a yellow smoky flame. • A visible flame is associated mainly with burning gaseous substances or vapours.