Complete combustion of a hydrocarbon in sufficient oxygen forms carbon dioxide and water. Balance carbon first, hydrogen second and oxygen last, using whole-number coefficients. In insufficient oxygen, carbon monoxide or soot can form, so complete-combustion products cannot be assumed. For the same family of hydrocarbons, larger molecules generally have stronger dispersion attractions and higher boiling points; their low polarity also explains poor solubility in water.
An alkene undergoes addition across C=C: the double bond becomes a single bond as new atoms attach to the two carbons. Ethene plus bromine gives 1,2−dibromoethane, while hydrogen addition gives ethane under suitable catalytic conditions. Bromine decolourisation is a standard classroom distinction under stated conditions. Alkanes do not undergo that same rapid addition reaction in the dark.
Example 1
Balance complete combustion of propane.
C3H8+5O2→3CO2+4H2O. The products contain ten O atoms, requiring five O2.
Example 2
Predict CH2=CH2+Br2.
Addition gives CH2Br–CH2Br, 1,2−dibromoethane. Each double-bond carbon gains one bromine atom.
Watch out: Those are complete-combustion products; insufficient oxygen can also produce carbon monoxide or carbon.