combustion reactions
Summary
A combustion reaction is the exothermic reaction of a fuel with O₂; complete burning of a hydrocarbon gives CO₂ and H₂O. The most testable points are balancing it (O last) and that ΔH°comb is negative, while limited O₂ gives CO or soot instead.
Detail
Meaning: Combustion is the rapid reaction of a fuel with O₂ that releases heat. Complete combustion of a hydrocarbon or other C, H, O compound (alcohol, glucose) gives CO₂ and H₂O. With limited O₂, incomplete combustion also gives CO or soot (C) and releases less heat. It is a redox reaction: the fuel is oxidized and O₂ is reduced. Some metals, such as Mg, also burn in O₂ to form oxides.
Equation: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O, where x and y are the numbers of C and H atoms per molecule of fuel (unitless coefficients); ΔH°comb < 0, in J/mol.
On the MCAT: Balance the equation (C first, H next, O last; double everything if a fraction appears), then use stoichiometry for moles of O₂ consumed or CO₂ produced. In bomb calorimetry, q = CΔT (C is the heat capacity of the calorimeter in J/K, ΔT in K) gives the heat released, and dividing by moles burned gives the magnitude of ΔH°comb.
Common confusion: Students give combustion a positive ΔH or assume CO₂ and H₂O form regardless of O₂ supply. ΔH°comb is negative, and limited O₂ gives CO or soot. A more oxidized fuel, such as ethanol, tends to release less heat per mole than the comparable alkane.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Ebbing and Gammon)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.