IB DP Chemistry Reactivity 1.3: Energy from fuels – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

This set practises Reactivity 1.3, Energy from fuels, from the IB Chemistry guide for the Diploma Programme (first assessment 2025): questions 1 and 2 cover 1.3.1, 3 and 4 cover 1.3.2, 5 to 7 cover 1.3.3, 8 and 9 cover 1.3.4, and 10 and 11 cover 1.3.5. Nothing in Reactivity 1.3 is marked additional higher level, so every question suits SL and HL.

If the content is new, start with the study guide; for a quick refresh, use the revision notes. Other units are on the IB DP chemistry hub; track them with the printable checklist.

Where needed, take Ar(C) = 12.01, Ar(H) = 1.01 and Ar(O) = 16.00.

Questions

1. Write a balanced equation for the complete combustion of each substance.

(a) sodium, forming sodium oxide [1] (b) aluminium [1] (c) heptane, C₇H₁₆ [1]

2. Pentan-2-ol, CH₃CH(OH)CH₂CH₂CH₃, is being tested as a fuel additive.

(a) Write a balanced equation, with state symbols, for its complete combustion. [2] (b) Calculate the mass of oxygen needed to burn 10.0 g of pentan-2-ol completely. [3] (c) Pentan-2-ol does not catch fire when stored in air at room temperature. Suggest why. [1]

3. Dodecane, C₁₂H₂₆, is a component of kerosene.

(a) Write an equation for the incomplete combustion of dodecane forming carbon monoxide and water. [2] (b) Write an equation for the incomplete combustion of dodecane forming carbon and water. [2] (c) State one observation you would expect when kerosene burns in a limited supply of air. [1]

4. In a poorly ventilated room, a spirit stove burns ethanol to give equal amounts, in mol, of carbon monoxide and carbon dioxide, plus water.

(a) Deduce the balanced equation for this reaction. [2] (b) Explain why this situation is a health risk. [2]

5. Methane, CH₄, and icosane, C₂₀H₄₂, are both alkanes. Using calculations where helpful, explain why icosane has a greater tendency than methane to undergo incomplete combustion. [3]

6. Use these values for the enthalpy of combustion.

Fuel	Formula	ΔHc⦵ / kJ mol⁻¹
propane	C₃H₈	−2219
hexadecane	C₁₆H₃₄	−10 700
ethanol	C₂H₅OH	−1367

(a) Calculate the energy released per gram of each fuel. [3] (b) Calculate the mass of CO₂ released per MJ of energy for each fuel. [3] (c) Ethanol and hexadecane release similar masses of CO₂ per MJ. Explain why bioethanol may still add less CO₂ to the atmosphere than hexadecane from crude oil. [2] (d) State one advantage and one disadvantage of natural gas compared with coal as a fuel for power stations. [2]

7. Explain how an increase in the concentration of carbon dioxide in the atmosphere leads to a rise in the Earth’s average surface temperature. [4]

8. Biodiesel is made from plant oils; diesel is made from crude oil.

(a) Write the equation for photosynthesis. [2] (b) Explain why biodiesel is described as renewable but diesel is not. [2]

9. A city council must choose a fuel for its bus fleet. Option A is biogas made from food waste and sewage. Option B is biodiesel made from palm oil grown on land that was cleared tropical forest. Evaluate the two options and recommend one. [6]

10. A hydrogen fuel cell has an alkaline electrolyte.

(a) Write the half-equations at the anode and the cathode. [2] (b) State which electrode is the negative electrode. [1] (c) State two differences between this fuel cell and a primary cell. [2]

11. A portable generator uses a methanol fuel cell with an acidic electrolyte.

(a) Deduce the half-equation at the anode. [2] (b) State the cathode half-equation. [1] (c) Combine the half-equations to give the overall equation. [1] (d) The cell uses 64.1 g of methanol. Calculate the amount, in mol, of electrons transferred and the mass of CO₂ released. [3] (e) Give one advantage and one disadvantage of methanol compared with hydrogen as a fuel-cell fuel. [2]

Answers

1. (a) 4Na + O₂ → 2Na₂O [1] (b) 4Al + 3O₂ → 2Al₂O₃ [1] (c) C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O [1] Examiner insight: In (a), an equation with NaO or Na₂O₂ as the product is wrong however well it is balanced; work out the oxide formula from the ion charges first.

2. (a) Correct products CO₂ and H₂O with C₅H₁₁OH [1]; 2C₅H₁₁OH(l) + 15O₂(g) → 10CO₂(g) + 12H₂O(l) with state symbols [1] (b) M = 88.17 g mol⁻¹, so n = 10.0 ÷ 88.17 = 0.1134 mol [1]; n(O₂) = 0.1134 × 15/2 = 0.851 mol [1]; mass = 0.851 × 32.00 = 27.2 g [1] (c) The reaction has a high activation energy, so heating or a spark is needed to start it [1] Examiner insight: In (b), the key step is using the 15 : 2 mole ratio from your equation; assuming 1 : 1 makes every later number wrong.

3. (a) CO and H₂O as the only products [1]; 2C₁₂H₂₆ + 25O₂ → 24CO + 26H₂O [1] (b) C and H₂O as the only products [1]; 2C₁₂H₂₆ + 13O₂ → 24C + 26H₂O [1] (c) A yellow, smoky flame or black soot deposits [1] Examiner insight: An equation that shows CO₂ as well as CO does not answer (a), which asks for one specific carbon product.

4. (a) Products CO, CO₂ and H₂O in a 1 : 1 ratio of CO to CO₂ [1]; 2C₂H₅OH + 5O₂ → 2CO + 2CO₂ + 6H₂O [1] (b) Carbon monoxide binds to haemoglobin more strongly than oxygen [1], so the blood delivers less oxygen to tissues; with no colour or smell, a build-up goes undetected [1] Examiner insight: “CO is poisonous” restates the risk; credit comes from the mechanism, its binding to haemoglobin.

5. Methane needs 2 mol O₂ per mol for complete combustion; icosane needs 20 + 42/4 = 30½ mol O₂ per mol, so far more oxygen must reach each molecule [1]. Icosane is 85.0% carbon by mass compared with 74.8% for methane, so there is more carbon to oxidise per gram [1]. Icosane has stronger London forces, so it is a solid that does not vaporise easily and mixes poorly with air [1] Examiner insight: Give three separate reasons; repeating “it is a bigger molecule” in different words adds no new point.

6. (a) Propane: 2219 ÷ 44.11 = 50.3 kJ g⁻¹ [1]; hexadecane: 10 700 ÷ 226.50 = 47.2 kJ g⁻¹ [1]; ethanol: 1367 ÷ 46.08 = 29.7 kJ g⁻¹ [1] (b) Propane: 3 × 44.01 × 1000 ÷ 2219 = 59.5 g [1]; hexadecane: 16 × 44.01 × 1000 ÷ 10 700 = 65.8 g [1]; ethanol: 2 × 44.01 × 1000 ÷ 1367 = 64.4 g [1] (c) The carbon in bioethanol was removed from the air by photosynthesis recently, so burning it returns that carbon [1]; the carbon in hexadecane was locked underground for millions of years and is new to the atmosphere [1] (d) Advantage: less CO₂ per unit energy, or more energy per gram, or less soot and SO₂ [1]. Disadvantage: needs pipelines or pressurised storage, or methane leaks are explosive and a greenhouse gas [1] Examiner insight: In (b), forgetting to multiply by the number of carbon atoms in each fuel gives answers that are too small for propane and hexadecane; show the carbon count in your working so the method is visible.

7. The Earth’s surface absorbs sunlight and emits infrared radiation [1]. CO₂ molecules absorb infrared, causing their bonds to vibrate [1]. The energy is re-emitted in all directions, including back towards the surface [1]. More CO₂ means more infrared is absorbed and returned, so less energy escapes to space and the average temperature rises (an enhanced greenhouse effect) [1] Examiner insight: Saying CO₂ “reflects” heat, or confusing this with ozone depletion, loses the absorption and re-emission marks.

8. (a) Correct formulas of reactants and products [1]; 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ balanced [1] (b) Biodiesel crops can be regrown each season, so the source is replaced as fast as it is used [1]; crude oil takes millions of years to form, so it is used much faster than it is replaced [1] Examiner insight: “Renewable means it will not run out” is too vague; tie the answer to the rate of replacement compared with the rate of use.

9. Any six points, at least one for each side, with a justified recommendation:

• A uses waste that would otherwise rot and release methane, so it reduces emissions [1]
• A needs no farmland, so it does not compete with food crops [1]
• A may be limited by how much waste the city produces, so it may not fuel the whole fleet [1]
• Biogas needs pressurised tanks and new refuelling equipment [1]
• B can be blended with diesel and used in existing engines [1]
• B’s land was cleared forest, releasing stored carbon and destroying habitats, so its net CO₂ may exceed that of diesel [1]

Recommendation: A, because it gives lower net emissions without land-use harm, even though it costs more to set up (B is also acceptable if justified). Examiner insight: Weigh at least one strength and one weakness of each option, then make the recommendation follow from that weighing; a one-sided list is description, not evaluation.

10. (a) Anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻ [1]; cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ [1] (b) The anode [1] (c) Any two: a fuel cell is supplied continuously with reactants while a primary cell contains a fixed amount [1]; reactants for a fuel cell are stored outside the cell, while a primary cell’s are sealed inside it [1]; or a fuel cell keeps working while fuel is supplied, but a primary cell stops when its reactants are used Examiner insight: Half-equations with H⁺ describe an acidic cell, so they do not answer (a) even if they are correctly balanced.

11. (a) CH₃OH and H₂O on the left, CO₂ on the right [1]; CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻ [1] (b) O₂ + 4H⁺ + 4e⁻ → 2H₂O (or 3/2 O₂ + 6H⁺ + 6e⁻ → 3H₂O) [1] (c) CH₃OH + 3/2 O₂ → CO₂ + 2H₂O [1] (d) n(CH₃OH) = 64.1 ÷ 32.05 = 2.00 mol [1]; electrons = 6 × 2.00 = 12.0 mol [1]; CO₂ = 2.00 × 44.01 = 88.0 g [1] (e) Advantage: methanol is a liquid, so it is easier and safer to store and transport [1]. Disadvantage: it releases CO₂, while hydrogen gives only water [1] Examiner insight: Before adding half-equations, scale them so the electrons match; an overall equation with electrons left in it is not balanced.

Where marks are usually lost

• Leaving out the oxygen atom in an alcohol when balancing O₂.
• Including CO₂ in an equation that asks for carbon monoxide or carbon only.
• Giving wrong oxide formulas for metals, such as NaO or AlO.
• Forgetting to multiply by the number of carbon atoms when finding moles of CO₂.
• Writing “CO₂ traps heat” with no mention of infrared absorption and re-emission.
• Calling biofuels carbon neutral with no qualification.
• Using H⁺ in alkaline half-equations, or putting electrons on the wrong side.

Next steps

• Reactivity 1.3 revision notes for a final read-through
• Reactivity 1.3 study guide for full explanations
• Course hub for IB DP chemistry
• Printable topic checklist
• Take all free 10-minute diagnostics
• Book a free trial class

Official syllabus

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