Combustion

Alkenes burn in oxygen. In complete combustion (plenty of oxygen) all the carbon is converted into carbon dioxide and all the hydrogen into water:C₂H₄ + 3O₂ → 2CO₂ + 2H₂OBecause the formula is C_nH_2n, every alkene gives equal moles of CO₂ and H₂O, and needs 1.5n mol of O₂ per mole:C_nH_2n + 1.5nO₂ → nCO₂ + nH₂OWorked example: propene, n = 3. It needs 1.5 × 3 = 4.5 mol O₂: C₃H₆ + 4.5O₂ → 3CO₂ + 3H₂O, or doubled to whole numbers, 2C₃H₆ + 9O₂ → 6CO₂ + 6H₂O.In incomplete combustion (limited oxygen) the products include carbon monoxide and carbon (soot):C₂H₄ + 2O₂ → 2CO + 2H₂O    C₂H₄ + O₂ → 2C + 2H₂OCarbon monoxide is toxic: it binds to haemoglobin and stops the blood carrying oxygen.Alkenes burn with a smokier flame than alkanes with the same number of carbons, because they have a higher percentage of carbon by mass (ethene is 86% C, ethane is 80% C), so more carbon is left unburnt. Alkenes are rarely used as fuels: they are far more valuable as a feedstock for making polymers and other chemicals.