WJEC GCSE Chemistry
WJEC GCSE Chemistry 2.5 Crude Oil, Fuels and Organic Chemistry Exam Questions
Free WJEC GCSE Chemistry revision for 2.5 Crude Oil, Fuels and Organic Chemistry: what you need to know, common mistakes and exam-style questions with answers. Covers fractional distillation, alkanes, alkenes, cracking, combustion, hydrogen fuel, addition polymers, alcohols, fermentation, ethanoic acid and infrared spectroscopy.
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About WJEC GCSE Chemistry 2.5 Crude Oil, Fuels and Organic Chemistry
This is the largest topic in Unit 2 and covers WJEC GCSE organic chemistry. You need to explain fractional distillation of crude oil and the trends in properties of the fractions, combustion and the energy released per gram of fuel, hydrogen as a fuel and the fire triangle. The organic section covers alkanes and alkenes, cracking, isomers and naming, addition reactions, addition polymers and plastic waste. It continues with alcohols: fermentation, the test with potassium dichromate, uses of ethanol, oxidation to carboxylic acids and identifying bonds with infrared spectroscopy. The specified practical measures the energy released by burning alcohols.
What you need to know
- Crude oil A mixture of hydrocarbons separated by fractional distillation: vaporised oil rises through a column that is cooler at the top, and fractions with lower boiling points condense higher up. Longer chains have higher boiling points, are more viscous, darker and harder to ignite.
- Combustion and fuels Complete combustion of hydrocarbons gives carbon dioxide and water. Hydrogen burns to give only water but is made by energy-intensive electrolysis and is hard to store. The fire triangle needs fuel, heat and oxygen; removing one puts a fire out.
- Alkanes and alkenes Alkanes, CnH2n+2, are saturated (single bonds only). Alkenes, CnH2n, contain a C=C double bond. Cracking breaks long alkanes into shorter alkanes and alkenes using heat and a catalyst. Know isomers of C4H10 and C5H12 and names such as but-1-ene and but-2-ene.
- Addition reactions and polymers Alkenes decolourise bromine water (orange to colourless). Monomers with a C=C bond join to form addition polymers such as poly(ethene), poly(propene), PVC and PTFE. Plastics do not biodegrade; recycling saves crude oil and energy.
- Alcohols General formula CnH2n+1OH. Ethanol is made by fermenting sugar with yeast: C6H12O6 → 2C2H5OH + 2CO2. Acidified potassium dichromate turns from orange to green with alcohols. Ethanol is oxidised to ethanoic acid, which is why wine goes sour.
- Energy from fuels Specified practical: burn alcohols in spirit burners under a measured volume of water, recording mass lost and temperature rise. Energy (J) = mass of water x 4.2 x temperature rise; divide by mass of fuel burned for energy per gram. Infrared spectra identify bonds such as C=C, O-H and C=O.
Common mistakes in 2.5 Crude Oil, Fuels and Organic Chemistry
- Saying alkenes are saturated. Alkenes have a C=C double bond, so they are unsaturated; alkanes are saturated.
- Writing that the bromine water test turns alkenes colourless. The bromine water changes from orange to colourless.
- Stating that fractions with high boiling points collect at the top of the column. The top is coolest, so low boiling point fractions such as petroleum gases collect there.
- Drawing the polymer with the double bond still present. In the repeat unit the C=C becomes a single bond, with bonds extending out of the brackets and n after them.
Try three 2.5 Crude Oil, Fuels and Organic Chemistry exam-style questions
1. Burning 0.50 g of ethanol raised the temperature of 100 g of water by 20 °C. Using energy = mass of water x 4.2 x temperature rise, calculate the energy released per gram of ethanol in kJ/g. (3 marks)
Show the mark scheme answer
100 x 4.2 x 20 = 8400 J; 8400 / 0.50 = 16 800 J/g; = 16.8 kJ/g.
2. Describe the test for an alkene and give the result. (2 marks)
Show the mark scheme answer
Add bromine water; turns from orange to colourless.
3. Give one advantage and one disadvantage of using hydrogen as a fuel for cars. (2 marks)
Show the mark scheme answer
Advantage: only product is water / no carbon dioxide / renewable; disadvantage: made by electrolysis which needs much electricity / difficult to store / explosive with air.
2.5 Crude Oil, Fuels and Organic Chemistry FAQs
What does WJEC GCSE Chemistry 2.5 Crude Oil, Fuels and Organic Chemistry cover?
It covers crude oil and fractional distillation, fuels and combustion, hydrogen fuel, the fire triangle, alkanes, alkenes, cracking, isomers, addition polymers and plastics, alcohols, fermentation, oxidation of ethanol and infrared spectroscopy.
Which unit is Crude Oil, Fuels and Organic Chemistry examined in?
It is examined in Unit 2: Chemical Bonding, Application of Chemical Reactions and Organic Chemistry, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
Which organic names and formulae should I know?
Learn methane to pentane, ethene and propene, methanol to pentanol, the general formulae for alkanes, alkenes and alcohols, the four polymers and their uses, and how to name simple branched isomers.
