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Crude oil contains many hydrocarbons. Fractional distillation heats and vaporises the mixture, then separates fractions in a column cooler at the top. Longer molecules usually have higher boiling points, greater viscosity and lower volatility. Each fraction remains a mixture. Distillation does not turn a long hydrocarbon into a short one; it separates molecules already present.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain fractional distillation of crude oil
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does fractional distillation separate by?
Boiling range. Different components condense at different levels.
After watching 2: Which formula can be a monoalkene?
C3H6. Acyclic monoalkenes follow CnH2n.
What you'll be able to do
Crude oil contains many hydrocarbons. Fractional distillation heats and vaporises the mixture, then separates fractions in a column cooler at the top. Longer molecules usually have higher boiling points, greater viscosity and lower volatility. Each fraction remains a mixture. Distillation does not turn a long hydrocarbon into a short one; it separates molecules already present.
Cracking chemically breaks larger hydrocarbons into smaller alkanes and alkenes using heat and suitable conditions, often a catalyst. Alkanes are saturated and have only single carbon–carbon bonds; an acyclic alkane has formula CnH2n+2. A monoalkene has a C=C bond and formula CnH2n. The alkene’s double bond undergoes addition and decolourises bromine water in the standard test. Use balanced atom counts to deduce missing cracking products.
Complete hydrocarbon combustion gives carbon dioxide and water. In insufficient oxygen, carbon monoxide and soot may form. Sulfur impurities produce sulfur dioxide; high temperatures can form nitrogen oxides from air. These pollutants have different effects, including toxicity, respiratory irritation and acid rain. Carbon dioxide contributes to the enhanced greenhouse effect. A claim about a fuel should specify energy output and emissions across its production and use, not only whether it burns cleanly in one test.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Describe the standard bromine-water result for ethene.
Orange bromine water becomes colourless.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.