R1.3
Energy from fuels
4 SL (SL 1.3.1-1.3.5)
Unverified
← View on the mapParent topic: What drives chemical reactions?
Guiding question
Guiding questionWhat are the challenges of using chemical energy to address our energy needs?
The one big idea
Combustion releases the energy stored in chemical bonds. Fossil fuels, biofuels and fuel cells each convert chemical energy to useful energy, but with different costs to the atmosphere, the climate and the future.
What this rests on
Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.
- EditorialS3.2 Functional groups: Classification of organic compoundsHydrocarbon structure relates to incomplete combustion and the greenhouse effect (linking questions R1.3.3).
- EditorialR1.1 Measuring enthalpy changesFuel evaluation is applied enthalpy.
- EditorialR1.2 Energy cycles in reactionsCombustion energy evaluation applies energy-cycle reasoning.
What rests on this
- EditorialR3.2 Electron transfer reactions
Core concepts that must be mastered
- Combustion reactions(1.3.1)Reactive metals, non-metals and organic compounds combust when heated in oxygen. You must deduce combustion equations including hydrocarbons and alcohols. Complete combustion of a hydrocarbon produces CO₂ and H₂O: C₇H₁₆(l) + 11O₂(g) → 7CO₂(g) + 8H₂O(g). For an alcohol, add the oxygen already in the molecule when balancing: 2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(g). The products of complete combustion are always the fully oxidised species.
- Incomplete combustion(1.3.2)When oxygen is limited, combustion is incomplete and produces carbon monoxide and/or carbon (soot). For methane in limited oxygen: 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g), or 2CH₄(g) + 2O₂(g) → 2C(s) + 4H₂O(g). Carbon monoxide is colourless, odourless and toxic — it binds to haemoglobin more strongly than oxygen, blocking oxygen transport and causing carbon monoxide poisoning. Larger hydrocarbons have a greater tendency to undergo incomplete combustion because they require more oxygen per molecule and burn less readily, which is why poorly maintained boilers and engines produce dangerous CO levels.
- Fossil fuels and the greenhouse effect(1.3.3)Fossil fuels — coal, crude oil and natural gas — form over geological timescales from ancient biological material. They differ in energy released per unit mass, tendency to incomplete combustion, and the CO₂ emitted per unit of energy. Coal produces the most CO₂ per unit energy; natural gas (mostly CH₄) produces the least. You must evaluate the CO₂ added to the atmosphere when different fuels burn and understand the link between atmospheric CO₂ levels and the greenhouse effect: CO₂ absorbs infrared radiation re-emitted from the Earth's surface, trapping heat that would otherwise escape to space. The mechanism is selective: incoming solar radiation is mostly visible and UV, which passes through the atmosphere; the Earth re-emits infrared, which CO₂ (and H₂O, CH₄) absorb because their molecular vibrations change the dipole moment. This is why O₂ and N₂ are not greenhouse gases — their vibrations do not change the dipole moment, so they do not absorb IR. The environmental, economic, ethical and social implications of burning fossil fuels — climate change, ocean acidification (CO₂ dissolving in seawater lowers pH), energy security — are all in scope.
- Biofuels(1.3.4)Biofuels come from biological fixation of carbon over a short period via photosynthesis: CO₂ + H₂O → (CH₂O)n + O₂. They are renewable in the sense that the carbon released on combustion was recently absorbed from the atmosphere, unlike fossil fuels which release carbon that was locked away for millions of years. Bioethanol (from fermentation of sugars) and biodiesel (from vegetable oils) are the main examples. You must consider the advantages (closed carbon cycle, renewable, reduces dependence on fossil fuels) and disadvantages (land use competes with food production, energy input in processing and distillation, lower energy density than fossil fuels). The carbon neutrality claim is not straightforward: the energy used in fertiliser production, harvesting, fermentation and distillation may come from fossil fuels, so the net CO₂ reduction can be significantly less than the theoretical 100%.
- Fuel cells(1.3.5)A fuel cell converts chemical energy from a fuel directly to electrical energy, without combustion. You must deduce half-equations for fuel-cell electrode reactions. For a hydrogen fuel cell (acidic electrolyte): at the anode, H₂(g) → 2H⁺(aq) + 2e⁻; at the cathode, ½O₂(g) + 2H⁺(aq) + 2e⁻ → H₂O(l). Methanol can also serve as a fuel. The overall reaction is the same as combustion but the energy is captured as electricity rather than heat, making fuel cells more efficient — they are not limited by the thermodynamic efficiency losses of heat engines. The only product of a hydrogen fuel cell is water, making it a zero-emission energy source at the point of use (though producing the hydrogen itself may involve fossil fuels). Proton exchange membranes are not assessed.
- How this sub-topic connectsEnergy from fuels is the applied, evaluative companion to R1.1 (measuring enthalpy). Combustion equations require mole-ratio balancing (R2.1) and the oxidising/reducing agents in combustion are a redox question (R3.2). The high activation energy of many fuels explains why they are kinetically stable but thermodynamically unstable — they store energy but do not spontaneously ignite (R2.2). Fuel cells connect directly to electrochemistry (R3.2: voltaic cells, electrode reactions). The environmental evaluation links to the Nature of Science theme (the interplay of science, society and policy) and to the green chemistry concept of atom economy (R2.1).
Quantitative non-negotiables
No quantitative non-negotiables specified.
Common failure modes
No misconceptions recorded for this sub-topic.
What "HL standard" actually looks like
No HL extension. Reactivity 1.3 is entirely SL and shared — it is an applied, evaluative sub-topic, not a calculation-heavy one. The depth is in the chemistry of combustion and the environmental and economic evaluation of fuel choices.
Linking questions
Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.
- Official IB
What properties of a greenhouse gas determine its "global warming potential"?
- Official IB
Why is high activation energy often considered to be a useful property of a fuel?
- Official IB
Which species are the oxidizing and reducing agents in a combustion reaction?
- Official IB
How does limiting the supply of oxygen in combustion affect the products and increase health risks?
- Official IB
Why do larger hydrocarbons have a greater tendency to undergo incomplete combustion?
- Official IB
Why is carbon dioxide described as a greenhouse gas?
- Official IB
What are some of the environmental, economic, ethical and social implications of burning fossil fuels?
- Official IB
What are the main differences between a fuel cell and a primary (voltaic) cell?
- Official IB
Electrical energy can be derived from the combustion of fossil fuels or from electrochemical reactions. What are the similarities and differences in these reactions?
- Official IB
What is the difference between combustion and oxidation of an alcohol?
1 further official linking question target a Tool, Inquiry or Nature of Science strand and are not drawn as edges on the map:
Editorial The decision to surface these off-graph questions here is our editorial judgement — the IB does not prescribe where they should appear.
- Official IB
What might be observed when a fuel such as methane is burned in a limited supply of oxygen?
Bridge: GCSE → IB HL
Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.
No bridge items tagged to this sub-topic.