IB Chemistry SLTopic 4 — Energy from FuelsPaper 1 & 2Materials~13 min read
Biofuels
A biofuel burns to carbon dioxide just like petrol does. The difference is where the carbon came from: not from underground, but out of the air, last summer. That single fact is the whole argument — and the whole argument about it.
📚 What you need to know
Renewable resources are replaced as fast as they are used; non-renewable ones are not. Biofuels are renewable, fossil fuels are not.
Bioethanol comes from the fermentation of sugars produced by photosynthesis.
Biodiesel comes from vegetable oils by transesterification with methanol, catalysed by NaOH or KOH.
Biogas comes from the anaerobic decay of organic waste and is mainly methane and carbon dioxide.
Carbon neutral means the CO2 released on burning equals the CO2 absorbed while growing — true in principle, not in practice.
Biofuels have lower specific energy than the fossil fuels they replace.
Renewable and non-renewable
Renewable resource
one that can be replaced at least as fast as it is used, so it will not run out
Fossil fuels took hundreds of millions of years to form and are being burnt in centuries, which makes them finite. A crop grown, harvested and replanted in a season is sustainable in a way that a coal seam simply is not. The three biofuels you need are bioethanol, biodiesel and biogas, and all three trace back to biological carbon fixation — carbon that a plant took from the air.
Bioethanol
The route has two steps, and both are ones you can write equations for. First, plants build glucose from atmospheric carbon dioxide:
Then yeast converts the glucose to ethanol, anaerobically:
Fermentation
C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
“Bioethanol” is not a different substance from ethanol — the molecule made by fermentation is identical to the one made industrially from crude oil. The prefix records the source, not the chemistry.
Carbon goes round rather than up. Fossil carbon, by contrast, only ever travels in one direction.
WORKED EXAMPLE
Use the equations for photosynthesis, fermentation and the combustion of ethanol to show that bioethanol is, in principle, carbon neutral.
Step 1 — carbon taken IN6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ → 6 CO₂ absorbedStep 2 — carbon OUT during fermentationC₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ → 2 CO₂ releasedStep 3 — carbon OUT on burning2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O → 4 CO₂ releasedStep 4 — balance the booksout = 2 + 4 = 6 in = 6every carbon released was absorbed firstSix carbons in, six carbons out. Note that the fermentation CO₂ counts too — it is easy to forget the 2 mol given off before the fuel is even used.
In principle is doing real work in that sentence. Tractors, fertiliser manufacture, irrigation, distillation and transport all consume energy, usually from fossil fuels. Bioethanol is lower carbon, not zero carbon, and saying so is often worth a mark.
WORKED EXAMPLE
Calculate the maximum mass of ethanol obtainable from 1.00 kg of glucose by fermentation. (Mr: glucose = 180.18, ethanol = 46.08)
Step 1 — moles of glucosen = 1000 ÷ 180.18 = 5.550 molStep 2 — ratio 1 : 2n(ethanol) = 2 × 5.550 = 11.10 molStep 3 — mass11.10 × 46.08 = 511.4511 g of ethanolOnly about half the mass of the glucose ends up as fuel — the rest leaves as carbon dioxide and water. That is the theoretical maximum; real yields are lower still.
Biodiesel
Vegetable oils are triglycerides: three long fatty acid chains attached to one glycerol molecule. They will burn, but they are far too viscous to run through a modern injector. The fix is to detach the chains from glycerol and re-attach them to something much smaller — methanol.
Three ester links are broken and three are made. The products are far less viscous because each molecule is now one chain instead of three.
The reaction is called transesterification, and the details worth remembering are these:
The products are fatty acid methyl esters (FAME) — the biodiesel itself — plus glycerol, which is sold as a by-product.
It can be catalysed by acid (which protonates the carbonyl group) or by alkali (which deprotonates the methanol). Alkaline catalysts such as NaOH or KOH are the usual industrial choice.
The reaction is reversible, so excess methanol is used to shift the position of equilibrium towards the products.
Under good conditions yields reach around 98%.
Recognise the trick being played here. Nothing is being oxidised or reduced — the energy content barely changes. The whole point of the reaction is to improve a physical property, viscosity, so that an existing engine can burn it.
Biogas
Biogas is made when microorganisms break organic matter down in the absence of oxygen — anaerobic digestion, in a sealed digester. The gas produced is mainly methane and carbon dioxide, with traces of others such as hydrogen sulfide.
What is in the mixture depends on what was fed in. Carbohydrates give an even split:
Digesting a carbohydrate
C6H12O6(s) → 3CO2(g) + 3CH4(g)
Three molecules of each, so 50% methane. Fatty acids are more reduced to begin with — they contain far less oxygen per carbon — so they yield a richer gas, around 69% methane. Since only the methane burns, a higher methane fraction means a better fuel.
Biogas is a neat double win: it turns waste that would otherwise rot in landfill and release methane straight into the atmosphere into a fuel that is burnt to the much weaker greenhouse gas CO2.
Weighing them up
Every biofuel loses this comparison, mainly because the molecules already contain oxygen — partly oxidised carbon has less energy left to give.
WORKED EXAMPLE
ΔHc for ethanol is –1367 kJ mol–1 (Mr = 46.08). Octane has a specific energy of 47.9 kJ g–1. Compare the two and comment on what it means for a car running on ethanol.
Step 1 — specific energy of ethanol1367 ÷ 46.08 = 29.7 kJ g⁻¹Step 2 — compare29.7 ÷ 47.9 = 0.62ethanol delivers about 62% as much energy per gramSo roughly 1.6 times the mass of fuel is needed for the same journey: a bigger tank, or more frequent refuelling. Blending ethanol into petrol raises the energy content back up and makes the flame more visible, which matters because ethanol burns almost invisibly.
In favour of biofuels
Against
Renewable and sustainable if crops are replanted
Lower specific energy than the fuels they replace
Close to carbon neutral, so lower net greenhouse emissions
Not truly neutral once farming, processing and transport are counted
Made from waste in the case of biogas and some biodiesel, reducing landfill
Land use conflict: fuel crops compete with food crops and can drive deforestation
Low sulfur, so little acid rain
Fertilisers and pesticides cause their own pollution; crops deplete soil
Can generate rural income and employment
Costly to convert engines and infrastructure
💡 Exam tip
Learn the three equations — photosynthesis, fermentation, anaerobic digestion of glucose. They are the ones asked for.
Asked to justify “carbon neutral”? Count the carbon in and the carbon out, then add the caveat about farming and processing.
For transesterification, name the products as methyl esters and glycerol and state the catalyst.
Evaluation questions want both sides plus a judgement. A list of advantages alone will not reach the top band.
Use the right comparison: biofuels lose on specific energy but win on net CO2. Say which measure you mean.
⚠️ Common mix-up
Saying biofuels release no CO2. They release just as much on burning — the point is where it came from.
Forgetting the CO2 given off during fermentation when doing the carbon accounting.
Confusing biodiesel with bioethanol. Different feedstock, different reaction, different product.
Calling transesterification a combustion or a redox reaction. It is an ester exchange.
Treating “renewable” as identical to “carbon neutral”. A crop can be renewable and still be grown using fossil fuels.
Up next: Fuel Cells — getting energy out of a fuel without burning it at all, and why that turns out to be so much more efficient.
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