Quick Simpler Start
In one sentence
You learn how fuels burn, how energy changes during reactions, and how to calculate heat changes in a clear way.
Exam habit
When you revise this chapter, ask yourself: "Can I explain this to a friend in one easy paragraph?" If yes, you are in a good place.
Interactive Simulator
This small tool gives you a quick visual check before you move deeper into the chapter notes.
Simulator
Heat Energy Calculator
Use the heating equation to see how much energy a reaction or warming step needs.
Enter the values to calculate q.
Chapter 5: Fuels and Heats of Reaction (Thermochemistry)
1. Objectives
- Describe crude oil as a source of hydrocarbons and explain fractional distillation.
- Name and draw simple alkanes, alkenes and alkynes.
- Recognise structural isomerism in hydrocarbons.
- Explain octane rating, knocking, cracking, reforming, isomerisation and oxygenates.
- Distinguish between exothermic and endothermic reactions.
- Calculate heat changes using Q = mcΔT.
- Understand heats of reaction, combustion and formation.
- HL: Apply Hess's Law using energy cycles.
2. Key Definitions
| Term | Exam-ready meaning |
|---|---|
| Hydrocarbon | A compound containing hydrogen and carbon only. |
| Fractional distillation | Separation of crude oil into fractions according to boiling point range. |
| Homologous series | A family of organic compounds with the same functional group and similar chemical properties. |
| Isomers | Compounds with the same molecular formula but different structural formulae. |
| Octane number | A measure of how resistant a fuel is to knocking in an engine. |
| Exothermic reaction | A reaction that releases heat energy to the surroundings. |
| Endothermic reaction | A reaction that absorbs heat energy from the surroundings. |
| Heat of combustion | The heat change when one mole of a substance is completely burned in oxygen. |
| Hess's Law | The total enthalpy change for a reaction is independent of the route taken, provided the initial and final states are the same. |
3. Visual Learning Zone
A. Fractional distillation of crude oil
Short chains rise higher because they have lower boiling points. Long chains condense lower down.
B. Hydrocarbon families
Examiner focus: recognise the bond type first, then use the general formula.
C. Energy profile diagrams
In exothermic reactions products have lower energy. In endothermic reactions products have higher energy.
D. HL Hess's Law cycle
Hess's Law works because enthalpy is a state function: only start and finish matter.
4. Core Theory
5.1 Sources of hydrocarbons
Crude oil is a mixture of many hydrocarbons. It is separated by fractional distillation because different fractions have different boiling point ranges. Smaller molecules have lower boiling points, are more volatile and usually burn more easily. Larger molecules have higher boiling points and are more viscous.
| Fraction | Typical use | Property trend |
|---|---|---|
| Refinery gases | Bottled gas, heating, cooking | Very low boiling point |
| Petrol | Car fuel | Volatile and easy to ignite |
| Kerosene | Jet fuel | Intermediate boiling point |
| Diesel | Diesel engines | Less volatile than petrol |
| Lubricating oil | Lubricants, waxes | High boiling point |
| Bitumen | Road surfaces, roofing | Very viscous residue |
5.2 Structure and nomenclature
| Family | Bonding | General formula | Example |
|---|---|---|---|
| Alkane | Single bonds only | CnH2n+2 | Ethane, C2H6 |
| Alkene | Contains C=C | CnH2n | Ethene, C2H4 |
| Alkyne | Contains C≡C | CnH2n-2 | Ethyne, C2H2 |
Naming tip: meth-, eth-, prop-, but-, pent-, hex- show the number of carbon atoms. The ending tells the family: -ane, -ene or -yne.
5.3 Octane rating
Knocking is premature ignition of the petrol-air mixture in an engine. A higher octane number means the fuel is more resistant to knocking. Branched hydrocarbons usually have higher octane ratings than straight-chain hydrocarbons.
Methods to improve octane rating
- Isomerisation: straight chains → branched chains
- Catalytic cracking: long chains → shorter useful molecules
- Reforming: changes molecular structure to improve fuel quality
- Oxygenates: oxygen-containing additives improve burning
Examiner trap
Do not write “octane number means percentage octane in petrol.” It is a rating scale comparing knocking resistance.
5.4 Thermochemistry
Thermochemistry studies heat changes in chemical reactions. Energy may be released to the surroundings or absorbed from them. The sign of ΔH is important: exothermic reactions have negative ΔH; endothermic reactions have positive ΔH.
Q = mcΔT
ΔH = heat change per mole
Heat of combustion: one mole burned completely in oxygen
Heat of formation: one mole formed from elements in standard states
ΔH = heat change per mole
Heat of combustion: one mole burned completely in oxygen
Heat of formation: one mole formed from elements in standard states
5. Worked Examples
Example 1: Heat released by a fuel
A spirit burner heats 200 g of water from 20°C to 45°C. Calculate the heat gained by the water. Take c = 4.2 J g⁻¹ °C⁻¹.
Step 1: Q = mcΔT
Step 2: ΔT = 45 - 20 = 25°C
Step 3: Q = 200 × 4.2 × 25 = 21,000 J
Answer: 21.0 kJ
A spirit burner heats 200 g of water from 20°C to 45°C. Calculate the heat gained by the water. Take c = 4.2 J g⁻¹ °C⁻¹.
Step 1: Q = mcΔT
Step 2: ΔT = 45 - 20 = 25°C
Step 3: Q = 200 × 4.2 × 25 = 21,000 J
Answer: 21.0 kJ
Example 2: Structural isomers
Draw two structural isomers of C4H10.
Answer: butane: CH3CH2CH2CH3; methylpropane: CH3CH(CH3)CH3.
Exam note: Same molecular formula, different arrangement of atoms.
Draw two structural isomers of C4H10.
Answer: butane: CH3CH2CH2CH3; methylpropane: CH3CH(CH3)CH3.
Exam note: Same molecular formula, different arrangement of atoms.
HL Example 3: Hess's Law using formation enthalpies
Calculate ΔH for: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Given: ΔHf CO2 = -394 kJ mol⁻¹, ΔHf H2O(l) = -286 kJ mol⁻¹, ΔHf CH4 = -75 kJ mol⁻¹, ΔHf O2 = 0.
Formula: ΔH = ΣΔHf(products) - ΣΔHf(reactants)
Products: -394 + 2(-286) = -966 kJ
Reactants: -75 + 2(0) = -75 kJ
Answer: ΔH = -966 - (-75) = -891 kJ mol⁻¹
Calculate ΔH for: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Given: ΔHf CO2 = -394 kJ mol⁻¹, ΔHf H2O(l) = -286 kJ mol⁻¹, ΔHf CH4 = -75 kJ mol⁻¹, ΔHf O2 = 0.
Formula: ΔH = ΣΔHf(products) - ΣΔHf(reactants)
Products: -394 + 2(-286) = -966 kJ
Reactants: -75 + 2(0) = -75 kJ
Answer: ΔH = -966 - (-75) = -891 kJ mol⁻¹
6. Practical Skills
Measuring heat of combustion using a spirit burner
- Measure mass of burner before and after burning.
- Measure mass of water in the calorimeter.
- Record initial and final temperature of the water.
- Calculate heat gained by water using Q = mcΔT.
- Calculate moles of fuel burned.
- Heat of combustion = heat released / moles burned.
7. Examiner Tips
- For fractional distillation, always link separation to different boiling points.
- For fuels, mention complete combustion when defining heat of combustion.
- For energy diagrams, products lower than reactants means exothermic.
- For Hess's Law, keep signs carefully. Reversing a reaction changes the sign of ΔH.
8. Common Mistakes
- Confusing crude oil fractions with pure substances.
- Using CₙH₂ₙ for alkanes instead of alkenes.
- Forgetting that oxygen has zero enthalpy of formation in its standard state.
- Writing ΔT as initial - final instead of final - initial.
- Forgetting to convert J to kJ when needed.
9. Examiner Traps
"Higher octane" does not mean the fuel contains more octane. It means the fuel is more resistant to knocking.
"Heat of combustion" requires one mole of fuel and complete combustion in oxygen.
"Heat of combustion" requires one mole of fuel and complete combustion in oxygen.
10. Exam Practice Questions
- Explain how fractional distillation separates crude oil into useful fractions. [4]
- Name the compound C3H8 and state whether it is an alkane, alkene or alkyne. [2]
- Define structural isomerism. [2]
- A fuel heats 250 g of water from 18°C to 58°C. Calculate the heat gained by the water. Take c = 4.2 J g⁻¹ °C⁻¹. [3]
- Explain what is meant by knocking and how octane rating relates to it. [3]
- HL: Use Hess's Law to calculate the enthalpy change for a reaction from given formation enthalpies. [6]
11. MCQs with Explanations
| Question | Answer & explanation |
|---|---|
| 1. Which family has the general formula CnH2n? A Alkanes B Alkenes C Alkynes D Alcohols | B. Alkenes contain a C=C bond and follow CnH2n. |
| 2. Which crude oil fraction has the lowest boiling point? A Bitumen B Diesel C Petrol D Refinery gases | D. Refinery gases are small molecules and leave near the top of the column. |
| 3. In an exothermic reaction, products have: A higher energy than reactants B lower energy than reactants C same energy as reactants D no energy | B. Energy is released, so products are lower in energy. |
| 4. What does a high octane number indicate? A Higher boiling point B More carbon atoms C Greater knocking resistance D More carbon dioxide | C. Octane rating measures resistance to knocking. |
12. HL Extension: Hess's Law Checklist
- Write the target equation.
- Identify the data route: combustion values or formation values.
- If using formation values: ΔH = products - reactants.
- If using combustion values: draw a cycle to avoid sign errors.
- Multiply ΔH values when equations are multiplied.
- Reverse sign when an equation is reversed.
13. Last-Minute Revision Sheet
- Crude oil is a mixture of hydrocarbons.
- Fractional distillation separates by boiling point.
- Alkanes: CnH2n+2; Alkenes: CnH2n; Alkynes: CnH2n-2.
- Isomers have same molecular formula but different structures.
- Higher octane number = less knocking.
- Exothermic ΔH is negative. Endothermic ΔH is positive.
- Q = mcΔT.
- HL: Hess's Law depends only on initial and final states.
14. Self-Assessment Checklist
- I can explain fractional distillation of crude oil.
- I can identify alkanes, alkenes and alkynes from formulae or structures.
- I can draw simple structural isomers.
- I can explain octane rating and knocking.
- I can distinguish exothermic and endothermic reactions.
- I can calculate heat changes using Q = mcΔT.
- I can define heat of combustion and heat of formation.
- I can apply Hess's Law in HL calculations.
15. Answers / Mark Scheme
Q1 [4] crude oil is heated/vaporised [1]; vapours enter column [1]; column has temperature gradient [1]; fractions condense at different boiling points [1].
Q2 [2] propane [1]; alkane [1].
Q3 [2] same molecular formula [1]; different structural formula/arrangement [1].
Q4 [3] ΔT = 58 - 18 = 40°C [1]; Q = mcΔT [1]; Q = 250 × 4.2 × 40 = 42,000 J = 42 kJ [1].
Q5 [3] knocking is premature ignition [1]; causes inefficient/rough engine running [1]; higher octane rating means greater resistance to knocking [1].
HL Q6 [6] write formula/cycle [1]; correct substitution [2]; correct sign handling [1]; final calculation [1]; units kJ mol⁻¹ [1].
Q2 [2] propane [1]; alkane [1].
Q3 [2] same molecular formula [1]; different structural formula/arrangement [1].
Q4 [3] ΔT = 58 - 18 = 40°C [1]; Q = mcΔT [1]; Q = 250 × 4.2 × 40 = 42,000 J = 42 kJ [1].
Q5 [3] knocking is premature ignition [1]; causes inefficient/rough engine running [1]; higher octane rating means greater resistance to knocking [1].
HL Q6 [6] write formula/cycle [1]; correct substitution [2]; correct sign handling [1]; final calculation [1]; units kJ mol⁻¹ [1].
HL Syllabus Patch: Sources of Hydrocarbons and Methane
Higher Level students must connect hydrocarbon fuels to their natural and industrial sources. Hydrocarbons are obtained from petroleum, natural gas, and coal. Methane can also form naturally by the decomposition of vegetation and animal waste in oxygen-poor conditions.
organic matter → methane + carbon dioxide + other gases
| Source | Main points | Exam link |
|---|---|---|
| Petroleum | Mixture of hydrocarbons separated by fractional distillation. | Main source of petrol, diesel, kerosene, lubricants. |
| Natural gas | Mainly methane with smaller amounts of other gases. | Clean-burning fuel, domestic and industrial use. |
| Coal | Solid fossil fuel; important historically and still used industrially. | Energy source and chemical feedstock. |
| Biogas / decomposition | Methane produced in slurry pits, refuse dumps, and marshy conditions. | Explosion hazard and greenhouse-gas source. |
Examiner Trap: Methane is useful as a fuel, but it is also dangerous in enclosed spaces such as slurry pits, coal mines, and refuse dumps because it is flammable and can form explosive mixtures with air.
HL Link: Methane is a greenhouse gas, so its release contributes to the greenhouse effect.
HL Syllabus Patch: Aromatic Hydrocarbons
Aromatic hydrocarbons contain the benzene ring. At Higher Level you should know benzene, methylbenzene (toluene), and ethylbenzene.
| Compound | Formula | Key point |
|---|---|---|
| Benzene | C6H6 | Planar ring with delocalised electrons. |
| Methylbenzene | C6H5CH3 | Also called toluene. |
| Ethylbenzene | C6H5C2H5 | A benzene ring with an ethyl side-chain. |
Aromatic compounds are generally non-polar and are much more soluble in non-polar solvents than in water.
Solubility idea: Benzene and methylbenzene do not mix well with water because water is polar and aromatic hydrocarbons are largely non-polar.
HL Syllabus Patch: Other Chemical Fuels
Ethyne
Ethyne (acetylene), C2H2, is an alkyne. It burns with a very hot flame and is used in oxyacetylene welding.
2C2H2 + 5O2 → 4CO2 + 2H2O
Hydrogen
Hydrogen is a clean fuel at the point of use because its combustion forms water.
2H2 + O2 → 2H2O
| Method of production | Outline |
|---|---|
| Electrolysis of water | Electric current decomposes water into hydrogen and oxygen. |
| Steam reforming | Methane reacts with steam to produce hydrogen and carbon monoxide. |
CH4 + H2O → CO + 3H2
HL Note: Hydrogen is attractive as a fuel, but storage, transport, and production cost are important practical issues.
HL Syllabus Patch: Bomb Calorimeter
A bomb calorimeter is used to measure the heat released when a fuel burns completely in excess oxygen. It is more accurate than a simple school calorimeter because heat loss is reduced and combustion is more complete.
| Part | Function |
|---|---|
| Steel bomb | Contains the fuel sample and oxygen at high pressure. |
| Water jacket | Absorbs the heat released. |
| Thermometer / sensor | Measures the temperature rise. |
| Ignition wire | Starts the combustion reaction. |
Q = mcΔT
Exam link: Once the heat absorbed by the water is known, the heat released per mole of fuel can be calculated.
HL Practical Link: Bond Energy and Methane
The combustion of methane is a standard Higher Level context for comparing bonds broken and bonds formed. Energy must be supplied to break bonds, and energy is released when new bonds form.
CH4 + 2O2 → CO2 + 2H2O
ΔH = Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed)
HL Reminder: For exothermic reactions like methane combustion, the total energy released when product bonds form is greater than the energy needed to break the reactant bonds, so ΔH is negative.