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IB DemystifiedMYP Sciences

Hydrocarbons and fuels

Crude oil is a thick black mixture of thousands of compounds of just two elements, carbon and hydrogen. Separating and reshaping those hydrocarbons gives us the fuels that move the world and the raw materials for plastics, and burning them is at the heart of today's energy and pollution challenges.

Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

Refinery gasesbelow 25 °Cbottled gas (LPG)Petrol25–75 °CcarsNaphtha75–150 °Cmaking chemicalsKerosene150–240 °Caircraft fuelDiesel240–350 °Clorries, busesFuel oilover 350 °Cships, power stationsBitumenresidueroads, roofingheated crudeoil vapourcoolerhotter
Figure 1. Fractional distillation separates crude oil by boiling point.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Combustion and pollution
  6. Fuels in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • explain how fractional distillation separates crude oil
  • describe alkanes and how their properties change with chain length
  • explain complete and incomplete combustion and the pollutants produced
  • explain cracking and test for alkenes with bromine water
  • compare fuels using energy data and evaluate calorimetry experiments
  • discuss choices between fuels, including biofuels and gas

Before you start

You will use these skills. If any feel shaky, review them first.

  • covalent bonding and intermolecular forces (see Covalent bonding and structures)
  • balancing equations and the mole (see The mole and reacting masses)
  • energy changes in reactions (see Energy changes)

Key vocabulary

Hydrocarbon
A compound of hydrogen and carbon only.
Alkane
A saturated hydrocarbon with only single bonds, general formula CₙH₂ₙ₊₂.
Alkene
An unsaturated hydrocarbon with a C=C double bond, such as ethene, C₂H₄.
Fraction
A group of hydrocarbons with similar boiling points.
Cracking
Breaking large hydrocarbons into smaller alkanes and alkenes using heat and a catalyst.
Incomplete combustion
Burning with too little oxygen, producing carbon monoxide or soot.

Understanding the ideas

  1. What is it?

    Crude oil is a mixture of hydrocarbons formed over millions of years from the remains of tiny sea organisms. Most are alkanes: chains of carbon atoms with hydrogen atoms attached, all joined by single covalent bonds.

  2. Why does it happen?

    Larger hydrocarbon molecules have stronger forces between them, so they have higher boiling points, are thicker and are harder to ignite. This difference lets a fractionating column separate crude oil: each fraction condenses at the height where the temperature falls below its boiling point.

  3. How do we know?

    Refineries measure the boiling ranges and compositions of fractions precisely. In the laboratory, you can compare fuels by burning them under a can of water, and test for alkenes with bromine water.

  4. Why does it matter?

    Fuels from crude oil power most transport and much industry, and cracking provides the alkenes used to make plastics. But burning fuels releases carbon dioxide, a greenhouse gas, and pollutants such as carbon monoxide, soot, sulfur dioxide and nitrogen oxides.

  5. What does it connect to?

    This topic links to bonding and intermolecular forces, energy changes, the mole and polymers in chemistry; to climate change and air pollution in Earth science; and to health effects of pollutants in biology.

Combustion and pollution

  • Complete combustion (plenty of oxygen): hydrocarbon + oxygen → carbon dioxide + water.
  • Incomplete combustion (limited oxygen): also produces carbon monoxide (toxic) and carbon (soot, which harms lungs).
  • Sulfur dioxide from sulfur impurities causes acid rain.
  • Nitrogen oxides form when nitrogen and oxygen from the air react in hot engines.
  • Cracking, for example C₁₀H₂₂ → C₈H₁₈ + C₂H₄, turns large molecules into petrol and alkenes; alkenes decolourise bromine water.

Fuels in the real world

Pakistan's refineries process crude oil into petrol, diesel, kerosene and fuel oil. Many cities struggle with smog made worse by vehicle exhaust and burning. Catalytic converters, low-sulfur fuels and the global phase-out of leaded petrol have all reduced harmful emissions.

Worked examples

Example 1: an alkane formula

What is the formula of the alkane with 6 carbon atoms?

  1. Use CₙH₂ₙ₊₂ with n = 6.
  2. H = 2 × 6 + 2 = 14.
  3. Hexane, C₆H₁₄.

Example 2: energy per gram

Burning 0.80 g of a fuel raises 100 g of water by 30 °C. Find the energy per gram.

  1. Energy = 100 × 4.2 × 30 = 12 600 J.
  2. Per gram = 12 600 ÷ 0.80 = 15 750 J/g ≈ 15.8 kJ/g.

In the eAssessment

Fuel questions combine structure, equations, data and decisions. Expect:

  • Explain fractional distillation, cracking and combustion using boiling points and bonding.
  • Write and balance combustion and cracking equations.
  • Process calorimetry data and evaluate the method.
  • Discuss fuel choices, weighing energy, pollution, cost and supply.

Common ways to lose marks: saying larger molecules have weaker forces; confusing alkanes and alkenes; saying nitrogen oxides come from the fuel; and forgetting heat losses when evaluating calorimetry.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: which fuel produces the most soot?

Partially guided investigation · about 45 minutes · pairs, with teacher supervision

Research question
How does the type of fuel (candle wax, ethanol in a spirit burner, and paraffin oil in a small lamp) affect the mass of soot collected on a cold surface held above the flame for 60 seconds?
Scientific background
When hydrocarbons burn with limited oxygen, some carbon is released as soot. Larger molecules are more likely to burn incompletely. The soot can be collected on a cold, weighed surface.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Candle, spirit burner with ethanol, small paraffin lamp, heat-resistant mat, clamp stand, clean glass watch glasses or boiling tubes filled with cold water, balance (0.001 g if available), stopwatch.
Method
  1. Weigh a clean, dry watch glass.
  2. Clamp it the same distance (for example 5 cm) above the flame for exactly 60 seconds.
  3. Let it cool, then reweigh it to find the mass of soot.
  4. Repeat three times for each fuel with a clean watch glass, and compare the means.

Safety. Wear eye protection, tie back hair, and keep flammable liquids capped and away from flames. Your teacher should fill and light burners. Glassware gets very hot; let it cool before handling. Work in a well-ventilated room.

Then evaluate: was the balance precise enough, and how could you collect more soot to reduce the uncertainty?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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