Photosynthesis stores absorbed light energy: 6CO2+6H2O→C6H12O6+6O2. Aerobic respiration has the reverse overall reactant–product relationship and releases energy. Fermentation converts glucose to ethanol and CO2:C6H12O6→2C2H5OH+2CO2. Distillation can concentrate ethanol after fermentation because mixture components have different volatilities.
Biodiesel forms by transesterification: a triglyceride reacts with three methanol molecules to give glycerol and three fatty-acid methyl esters. Biofuels use recently fixed carbon, while fossil fuels add carbon from long-term geological stores. Renewable does not mean impact-free: farming, processing, transport, land-use change and competition for food can matter. A valid comparison uses the same delivered-energy basis and includes its stated life-cycle boundary.
Example 1
Find theoretical ethanol moles from 0.250 mol glucose by C6H12O6→2C2H5OH+2CO2.
The ratio glucose:ethanol is 1:2, so ethanol =2×0.250=0.500 mol, assuming complete theoretical conversion.
Example 2
How much methanol is needed for 0.0300 mol triglyceride in complete transesterification?
Each triglyceride needs 3 mol methanol. Required amount =3×0.0300=0.0900 mol. Products include 0.0300 mol glycerol and 0.0900 mol methyl esters.
Watch out:Carbon-containing biofuels emit CO2; their potential advantage concerns recent carbon uptake and the complete production life cycle.
Photosynthesis, fermentation and biodiesel practice | Year 12 Chemistry | Ascent Arcade