Complete combustion

Complete combustion is burning in a plentiful supply of oxygen so that the fuel is fully oxidised, giving carbon dioxide and water for a hydrocarbon or alcohol. Malay: pembakaran lengkap · Chinese: 完全燃烧.

When oxygen is in excess, every carbon atom in the fuel becomes carbon dioxide and every hydrogen atom becomes water, and the reaction releases the maximum heat the fuel can give. This is the condition assumed in the definition of the heat of combustion, which is why “in excess oxygen” appears in that definition.

Example. Ethanol burning completely follows C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. In a heat-of-combustion experiment you want this clean, complete burning under the copper can, because it gives the largest and most reliable temperature rise for the mass of fuel used.

Confusion to avoid. Complete combustion is the opposite of incomplete combustion, which happens in a limited supply of oxygen and produces carbon monoxide or soot (carbon) instead of, or as well as, carbon dioxide. Incomplete combustion releases less heat, so it lowers an experimental heat of combustion and is one of the reasons the measured value falls below the accepted value.

Recognising the products of complete combustion lets you write the correct balanced equation and count moles properly, which is the first step before Q = mcθ in the heat-of-combustion calculations that SPM Chemistry sets. You can recognise complete combustion in the laboratory from a clean blue flame and the absence of soot on the base of the can, while a yellow, sooty flame signals incomplete combustion and a limited oxygen supply. Because complete combustion releases the maximum heat, it gives the most reliable temperature rise, and providing good ventilation during the experiment is a simple way to make sure the fuel burns fully.