IB DP Chemistry Reactivity 1.3: Energy from fuels – Revision Notes

These notes condense Reactivity 1.3, Energy from fuels (understandings 1.3.1 to 1.3.5) from the IB Chemistry guide for the Diploma Programme (first assessment 2025). Nothing in Reactivity 1.3 is marked additional higher level, so all of it applies at SL and HL. The study guide has the full teaching and worked examples.

Next, work through the practice questions. For the wider course, use the IB DP chemistry hub; to track progress, use the printable checklist; and the free diagnostics point to your weakest topics.

Key definitions

Term	Meaning
Combustion	Reaction with oxygen, started by heating, that releases energy
Complete combustion	Organic fuel burns in excess oxygen; products are CO₂ and H₂O only
Incomplete combustion	Organic fuel burns in limited oxygen; carbon ends up as CO and/or C
Fossil fuel	Coal, crude oil or natural gas, formed from organisms over millions of years
Renewable source	Replaced naturally at least as fast as it is used
Non-renewable source	Used much faster than it forms, so supplies are finite
Biofuel	Fuel made from recently living matter; its carbon was fixed by photosynthesis
Fuel cell	Converts a fuel’s chemical energy directly into electrical energy, with a continuous supply of fuel

Reactivity 1.3.1 Complete combustion

• Metals → metal oxide: 2Ca + O₂ → 2CaO
• Non-metals → non-metal oxide: C + O₂ → CO₂; S + O₂ → SO₂
• Hydrocarbons and alcohols → CO₂ + H₂O
• O₂ is the oxidizing agent; the fuel is the reducing agent.
• A high activation energy lets a fuel be stored in air without igniting.

Method in steps (organic fuel)

1. Balance C with CO₂.
2. Balance H with H₂O.
3. Count O atoms on the right.
4. Subtract the O atoms in the fuel (one for a simple alcohol).
5. Halve to get O₂. If you get a half, keep it or double everything.

Reminder: propan-2-ol, C₃H₈O. 3CO₂ + 4H₂O has 10 O atoms; minus 1 leaves 9, so 4½ O₂: 2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O.

Reactivity 1.3.2 Incomplete combustion

Same method, but the carbon product changes:

Carbon product	Oxygen needed, compared with complete combustion	Example (butane, C₄H₁₀)
CO₂	Most	2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
CO	Less	2C₄H₁₀ + 9O₂ → 8CO + 10H₂O
C (soot)	Least	2C₄H₁₀ + 5O₂ → 8C + 10H₂O

Reminder for an alcohol: methanol forming carbon monoxide. CO + 2H₂O holds 3 O atoms; the fuel supplies 1, so 2 O atoms (one O₂) are needed: CH₃OH + O₂ → CO + 2H₂O.

Observations with limited oxygen: yellow, sooty flame; black deposit. With plenty of oxygen: blue, clean flame.

Health and other effects:

• CO attaches to haemoglobin more tightly than O₂, so blood carries less oxygen. It has no colour or smell.
• Soot particles damage the lungs and blacken surfaces.
• Less energy is released per mole of fuel.

Reactivity 1.3.3 Fossil fuels

	Coal	Crude oil	Natural gas
Main content	Mostly carbon	Mixture of hydrocarbons	Mostly methane
Energy per gram	Lowest	High	Highest
CO₂ per unit energy	Highest	Middle	Lowest
Plus points	Large reserves, cheap, easy to stockpile	Liquids easy to transport; feedstock for chemicals	Cleanest burning
Minus points	SO₂ from sulfur; soot; dangerous mining	Spills; larger fractions burn sootily	Pipelines and pressurised storage; leaks of CH₄ (a greenhouse gas)

Why larger hydrocarbons burn incompletely more often: more O₂ needed per molecule, higher percentage of carbon by mass, lower volatility so poorer mixing with air.

Comparing CO₂ from fuels in steps

1. Energy per gram = |ΔHc⦵| ÷ M.
2. Moles of CO₂ per mole of fuel = number of C atoms.
3. Mass of CO₂ per unit energy = (C atoms × 44.01) ÷ |ΔHc⦵|.
4. Mass of CO₂ per gram of fuel = (C atoms × 44.01) ÷ M.

Small reminder, using ΔHc⦵(ethane) = −1560 kJ mol⁻¹: M = 30.08 g mol⁻¹, so 51.9 kJ g⁻¹; 2 × 44.01 ÷ 1560 = 0.0564 g of CO₂ per kJ, which is 56.4 g per MJ.

Greenhouse effect in four steps

1. Visible light from the Sun is absorbed by the ground, which warms up.
2. The surface emits infrared radiation.
3. CO₂, H₂O and CH₄ absorb infrared; their bonds vibrate.
4. They give the energy out again randomly, so part of it heads back down to the ground.

More CO₂ from fossil fuels enhances this effect. NOAA gives about 280 ppm before the Industrial Revolution and about 423 ppm as the 2024 global average.

Reactivity 1.3.4 Biofuels

Photosynthesis (learn the reactants and products):

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

• Biofuels: bioethanol, biodiesel, biogas.
• Their carbon was taken from the air over a short time, so burning them returns recently fixed carbon.
• Not truly carbon neutral: farming, fertiliser, processing and transport usually burn fossil fuels.

Carbon bookkeeping reminder for bioethanol: making 1 glucose fixes 6 CO₂. Fermenting it gives 2 ethanol and 2 CO₂, and burning the 2 ethanol gives 4 CO₂. Released: 2 + 4 = 6, equal to the 6 fixed, so the cycle balances on paper only.

For biofuels	Against biofuels
Renewable	Compete with food crops for land
Lower net CO₂	Deforestation risk and biodiversity loss
Usable in blends in existing engines	High water and fertiliser use
Can use waste (biogas)	Ethanol gives less energy per gram than petrol

Reactivity 1.3.5 Fuel cells

Fuel and electrolyte	Anode (negative, oxidation)	Cathode (positive, reduction)
H₂, acidic	H₂ → 2H⁺ + 2e⁻	O₂ + 4H⁺ + 4e⁻ → 2H₂O
H₂, alkaline	H₂ + 2OH⁻ → 2H₂O + 2e⁻	O₂ + 2H₂O + 4e⁻ → 4OH⁻
CH₃OH, acidic	CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻	3/2 O₂ + 6H⁺ + 6e⁻ → 3H₂O

Half-equation method (acidic): balance the key atom, then O with H₂O, then H with H⁺, then charge with e⁻.

Combining half-equations: match the electrons first. For the acidic hydrogen cell, double the anode equation (2H₂ → 4H⁺ + 4e⁻), add the cathode equation and cancel 4H⁺ and 4e⁻ to get 2H₂ + O₂ → 2H₂O. No electrons or spare ions should remain.

Must-know distinctions

• Fuel cell vs primary cell: a fuel cell is supplied with reactants continuously; a primary cell holds a fixed amount and runs down.
• Fuel cell vs combustion: the same overall reaction, but the energy comes out as electricity, not mainly as heat.
• Hydrogen cell vs methanol cell: hydrogen gives only water; methanol also gives CO₂ but is a liquid and easier to store.
• Membrane details are not required.
• Greenhouse effect vs enhanced greenhouse effect: the natural one makes the planet warm enough for life, while the enhanced one is additional warming caused by human emissions of CO₂ and similar gases.
• Renewable vs carbon neutral: renewable is about the rate of replacement; carbon neutral is about net CO₂. A biofuel is renewable but is rarely fully carbon neutral.
• Complete vs incomplete: the hydrogen always becomes water; only the carbon product changes.

Quick self-test

1. Write the equation for the complete combustion of cyclohexane, C₆H₁₂.
2. Write the equation for ethanol burning to give carbon and water.
3. Write the equation for hexane burning to give carbon monoxide and water.
4. Write the equation for calcium burning in oxygen.
5. Calculate the mass of CO₂ released by burning 1.00 kg of butane completely (take Ar(C) = 12.01, Ar(H) = 1.01, Ar(O) = 16.00).
6. Which fossil fuel adds least CO₂ per unit of energy released?
7. Explain why carbon monoxide is dangerous.
8. State the reactants and products of photosynthesis.
9. Define a renewable energy source.
10. Write the anode half-equation for a hydrogen fuel cell with an alkaline electrolyte.
11. How many electrons are released per molecule of methanol at the anode of a methanol fuel cell?

Answers

1. C₆H₁₂ + 9O₂ → 6CO₂ + 6H₂O
2. C₂H₅OH + O₂ → 2C + 3H₂O
3. 2C₆H₁₄ + 13O₂ → 12CO + 14H₂O
4. 2Ca + O₂ → 2CaO
5. M(C₄H₁₀) = 58.14 g mol⁻¹. Each mole gives 4 mol CO₂, so 4 × 44.01 ÷ 58.14 = 3.03 g of CO₂ per gram; 3.03 kg of CO₂.
6. Natural gas (methane).
7. It attaches to haemoglobin in place of oxygen, so less oxygen reaches the tissues; you cannot detect it by sight or smell.
8. Reactants: carbon dioxide and water. Products: glucose and oxygen.
9. One that is replaced by natural processes at least as fast as it is used.
10. H₂ + 2OH⁻ → 2H₂O + 2e⁻
11. Six.

Where marks are usually lost

• Not subtracting the oxygen atom already in an alcohol, so the O₂ coefficient is too high.
• Giving CO₂ alongside CO when the question asks for the equation producing carbon monoxide only.
• Missing out (s), (l), (g) or (aq) where an equation must show physical states.
• Comparing fuels per mole when the question asks per gram or per unit of energy.
• Explaining the greenhouse effect with “trapping heat” instead of absorption and re-emission of infrared radiation.
• Confusing the greenhouse effect with ozone depletion.
• Describing biofuels as carbon neutral with no qualification.
• Swapping anode and cathode, or putting electrons on the wrong side.
• Using H⁺ in an alkaline half-equation.

Official syllabus

Source: Chemistry guide, International Baccalaureate Diploma Programme, February 2023 (first assessment 2025). Condensed here: Reactivity 1.3, Energy from fuels, understandings 1.3.1 to 1.3.5, common to SL and HL.