Example 1
Balance complete combustion of ethanol.
C2H5OH+3O2→2CO2+3H2O. Ethanol already contains one O atom; the products contain seven, so six more arrive as 3O2.
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Year 12 · SACE Stage 2
Combustion equations and fuel energy
Balance fuel reactions and calculate energy per mole, gram and litre.
Method and worked examples
Complete combustion of a fuel containing carbon, hydrogen and possibly oxygen produces CO2 and H2O. Balance carbon, then hydrogen, then oxygen. Limited oxygen or poor mixing can cause incomplete combustion, producing toxic CO or soot. Long-chain fuels can be harder to mix efficiently with air, so their combustion conditions matter.
For water calorimetry use q= mcΔT, with c=4.18Jg−1∘C−1 when stated. The water gains heat while an exothermic fuel reaction has negative ΔH. Divide the heat released by moles burned for molar combustion enthalpy, or by mass for energy per gram. Convert liquid volume using density. Heat loss usually makes the measured released-energy magnitude too small.
Example 1
Example 2
Burning 0.0100 mol fuel heats 200g water by 15.0∘C. Use c=4.18Jg−1∘C−1 and ignore other heat capacities. Find ΔHcomb.
qwater =200×4.18×15.0=12540J=12.54 kJ. Fuel releases this heat, so ΔHcomb =0.0100−12.54=−1254 kJ mol−1.
Watch out: Bond breaking requires energy; forming product bonds releases energy. Combustion releases energy overall when product-bond formation releases more.