The shape of the lesson
By the end of the lesson, learners can
- Fossil fuels and crude oil: A fraction is not a single compound: it is a mixture of hydrocarbons with similar chain lengths and a narrow range of boiling points.
- Why the fractions separate: Compare the boiling points of short and long alkanes in the simulator.
- Fossil fuels: coal, natural gas (mainly methane), petroleum.
- Order: refinery gas, gasoline, naphtha, kerosene, diesel, fuel oil, lubricating oil, bitumen.
- Down the column: longer chains, higher b.p., less volatile, more viscous.
- Separation: by boiling point; a physical process, no bonds broken.
One picture every idea returns to
The fossil fuels are coal, natural gas and petroleum (crude oil). The main constituent of natural gas is methane, CH4. Petroleum is a mixture of hydrocarbons (compounds of hydrogen and carbon only) that is separated by fractional distillation: the oil is vaporised, the vapour rises up a column that is hot at the bottom and cooler at the top, and each fraction condenses at the level where the temperature falls below its boiling point. Going down the column the molecules have longer chains, higher boiling points, lower volatility and higher viscosity.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Splitting crude oil into fractions.. Coal, natural gas and petroleum formed from ancient living things and power much of the world. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Fossil fuels and crude oil | A fraction is not a single compound: it is a mixture of hydrocarbons with similar chain lengths and a narrow range of boiling points. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why the fractions separate | Compare the boiling points of short and long alkanes in the simulator. Project tab 1 of the lab (Hydrocarbons lab, alkanes) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why the fractions separate, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Fractional distillation is a physical separation: no bonds inside molecules are broken, so it is not cracking. | Do not say the fractions are separated by melting point or by density; they condense at different temperatures because they have different boiling points. A volatile fraction evaporates easily, so high volatility goes with a low boiling point. |
Fuels and fractional distillation: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Name the three fossil fuels and state the main constituent of natural gas.
Give a use for each fraction: (a) refinery gas; (b) diesel oil; (c) fuel oil; (d) bitumen.
Describe how petroleum is separated into fractions in a fractionating column.
Compare the kerosene and lubricating oil fractions in terms of chain length, volatility and viscosity.
A fraction has molecules with 5 to 10 carbon atoms and boils between about 40 °C and 180 °C. Identify the fraction and give its use.
A student says that fractional distillation produces pure compounds. Explain why this is incorrect.
Answer key and marking notes
- Coal, natural gas and petroleum (crude oil).
- Natural gas is mainly methane, CH4.
- (a) bottled gas for heating and cooking.
- (b) fuel for diesel engines.
- (c) fuel for ships and home heating systems.
- (d) making roads.
- Petroleum is heated and vaporised; the vapour enters the column.
- The column is hot at the bottom and cooler at the top; vapours rise and cool.
- Each fraction condenses at the level where the temperature is below its boiling point and is drawn off there.
- Lubricating oil has longer chains than kerosene.
- Lubricating oil is less volatile (higher boiling point).
- Lubricating oil is more viscous.
- Gasoline (petrol).
- Fuel for cars.
- Each fraction is a mixture of hydrocarbons.
- The molecules in a fraction have similar chain lengths and a range of boiling points, not a single boiling point.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the main constituent of natural gas?
- Amethane
- Bethane
- Cbutane
- Dhydrogen
Which process separates petroleum (crude oil) into useful fractions?
- Afractional distillation
- Bcracking
- Cfiltration
- Dcrystallisation
The diagram shows a fractionating column used to separate petroleum. Which fraction contains the molecules with the lowest boiling points?
- AP
- BQ
- CR
- DS
Which fraction from petroleum is matched with its correct use?
- Abitumen: making roads
- Bkerosene: fuel for cars
- Cnaphtha: heating and cooking in homes
- Dlubricating oil: jet fuel
Compared with gasoline, the diesel oil fraction contains molecules that
- Ahave longer chains and a higher boiling point
- Bhave shorter chains and a lower boiling point
- Chave longer chains and are less viscous
- Dhave shorter chains and are more viscous
Look again at the column. Which statement about fraction S compared with fraction P is correct?
- AS is more viscous and less volatile than P
- BS is less viscous and more volatile than P
- CS has shorter chain molecules than P
- DS has a lower boiling point range than P
Why do the larger hydrocarbon molecules in petroleum have higher boiling points?
- Athe attractive forces between larger molecules are stronger, so more energy is needed to separate them
- Blarger molecules have stronger covalent bonds inside them that must be broken when they boil
- Clarger molecules contain double bonds that hold them together
- Dlarger molecules are ionic, so they form a giant lattice
Coal, natural gas and petroleum are all fossil fuels. Which statement about them is correct?
- Athey are non-renewable, and burning them releases carbon dioxide
- Bthey are renewable because plants are still growing today
- Cburning them produces only water
- Dthey are pure compounds with fixed boiling points
Quiz key
Natural gas is mainly methane, CH4. It contains small amounts of ethane and other alkanes, but not as the main part. Butane is sold separately as bottled gas from refinery gas. Hydrogen is not a significant part of natural gas.
Petroleum is a mixture of hydrocarbons with different boiling points, so it is separated by fractional distillation. Cracking breaks large molecules into smaller ones; it is a chemical change used after distillation, not a separation. Filtration separates an insoluble solid from a liquid. Crystallisation obtains a solid solute from a solution.
The column is cooler near the top, so only the molecules with the lowest boiling points stay as gas long enough to reach the top and leave as fraction P (refinery gas). Q and R condense lower down because their boiling points are higher. S leaves near the bottom, where it is hot, so it contains long chain molecules with high boiling points.
Bitumen is the thick residue used for surfacing roads. Kerosene (paraffin) is jet fuel, while gasoline (petrol) fuels cars. Naphtha is a feedstock for making chemicals; refinery gas is used for heating and cooking. Lubricating oil is used for lubricants, waxes and polishes, not as jet fuel.
Fractions collected lower down the column have longer carbon chains, higher boiling points and higher viscosity. Diesel oil is below gasoline, so its molecules are longer with higher boiling points. Shorter chains and lower boiling points describe gasoline. Longer chains make a liquid more viscous, not less, so the last two options mix up the trend.
S condenses near the bottom, so its molecules are long chains with strong attractive forces between them: high boiling point, high viscosity and low volatility. P is the opposite. Saying S is less viscous and more volatile reverses the trend. S has longer, not shorter, chains, and a higher, not lower, boiling point range.
Boiling separates whole molecules from each other. Larger molecules have stronger intermolecular attractive forces, so more energy is needed and the boiling point is higher. Covalent bonds within molecules are not broken on boiling. The alkanes in petroleum are saturated, so they have no double bonds. Hydrocarbons are covalent molecules, not ionic compounds.
Fossil fuels formed over millions of years, so they are being used far faster than they form: they are non-renewable. They contain carbon, so burning them releases carbon dioxide, a greenhouse gas. Plants growing today will not become fossil fuels on a human timescale. Burning hydrocarbons gives carbon dioxide as well as water. Petroleum and coal are mixtures, not pure compounds, so they have no single boiling point.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.