Combustion reactions

Combustion reactions (informally known as simply burning) are chemical reactions in which a substance is reacted exothermically with molecular oxygen (O₂) at a high temperature. Combustion reactions can either be complete or incomplete.

In ideal circumstances, all reactions would be complete; or in other words, oxygen would fully oxidize the other reactants. For example, a combustion reaction involving a hydrocarbon (any molecule consisting only of carbon and hydrogen) would result in only carbon dioxide (CO₂) and and water (H₂O) as products (possibly with excess oxygen). The general form of complete saturated hydrocarbon combustion is as follows:

$C_{n}H_{2n+2}+({\frac {3}{2}}n+{\frac {1}{2}})O_{2}\rightarrow nCO_{2}+(n+1)H_{2}O$

In the real world, however, combustion reactions rarely go to completion. This is due to imperfect conditions, such as insufficient oxygen or low ignition temperature. Incomplete combustion results in additional products which are not fully oxidized. For example, an incomplete combustion reaction involving a hydrocarbon would include carbon monoxide (CO) as a product, along with hydrogen (H₂), carbon dioxide and water.

$C_{n}H_{2n+2}+mO_{2}\rightarrow XnCO_{2}+(1-X)nCO+Y(n+1)H_{2}O+(Y-1)H_{2}$, where

• X is the fraction of carbon completely combusted
• Y is the fraction of hydrogen completely combusted
• m is the molar ratio of oxygen fed to the system, equal to $n({\frac {3}{2}}X+{\frac {1}{2}}Y+{\frac {1}{2}})$

See also

• Activation Energy