Alkenes

• Alkenes are unsaturated because they have a double carbon bond (C=C), they're also hydrocarbons
• Alkenes are hydrocarbons which have a double bond between 2 of the carbon atoms in their chain
• The C=C double bond means that alkenes have 2 fewer hydrogens compared with alkanes containing the same number of carbon atoms. This makes then unsaturated
• The C=C double bond can open up to make a single bond, allowing the 2 carbon atoms to bond with other atoms. This makes alkenes reactive - far more than alkanes
• The first 4 alkanes are ethene, propene, butene and pentene
• Straight-chain alkenes have twice as many hydrogen atoms as carbon
• General formula for alkenes = CnH2n
• Ethene:C2H4

  
• Propene:C3H6

  
• Butene: C4H8

  
• Pentene: C5H10

  
• There are 2 different structures for butene (C4H8) and pentene (C5H10) as the double bond can be in 2 different places
• Alkenes burn with a smokey flame
• In a large amount of oxygen, alkenes combust completely to produce only water and carbon dioxide
• However if there isn't enough oxygen in the air for complete combustion, so when you burn them they tend to undergo incomplete combustion
• When alkenes undergo incomplete combustion, carbon dioxide and water are still produced, but you can also get carbon and carbon monoxide (CO) which is a poisonous gas
• alkene + oxygen -> carbon + carbon monoxide + carbon dioxide + water (+ energy)
• Incomplete combustion results in a smokey yellow flame, and less energy being released compared to complete combustion of the same compound
• Incomplete combustion equation e.g: C4H8 + 5O2 -> 2CO + 2CO2 + 4H2O

  This is just one possibility. The products depend on how much oxygen is present