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Crude oil is a mixture of hydrocarbons. Fractional distillation separates groups with different boiling ranges: vapour rises through a column that is cooler at the top and condenses when the temperature falls below the relevant boiling range. Longer alkanes generally have stronger London attractions, higher boiling points and lower volatility. Distillation is a physical separation; cracking is a chemical reaction that breaks larger hydrocarbons into smaller, more useful molecules, often including alkenes.
Video by Eliot Rintoul. 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 and cracking
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which process chemically breaks large hydrocarbon molecules?
Cracking. Cracking changes covalent connectivity.
After watching 2: What happens in a termination step?
Two radicals combine. No new radical is generated.
What you'll be able to do
Crude oil is a mixture of hydrocarbons. Fractional distillation separates groups with different boiling ranges: vapour rises through a column that is cooler at the top and condenses when the temperature falls below the relevant boiling range. Longer alkanes generally have stronger London attractions, higher boiling points and lower volatility. Distillation is a physical separation; cracking is a chemical reaction that breaks larger hydrocarbons into smaller, more useful molecules, often including alkenes.
In chlorination, UV light causes homolytic cleavage of Cl2 in initiation. A chlorine radical removes hydrogen from an alkane to form HCl and an alkyl radical. That radical reacts with Cl2, forming a haloalkane and regenerating a chlorine radical in propagation. Termination combines two radicals without creating another. Further substitutions and different hydrogen positions produce mixtures, limiting the method’s synthetic selectivity. UV provides energy for initiation rather than acting as a reactant consumed in each propagation step.
Complete combustion forms carbon dioxide and water. In limited oxygen, carbon monoxide and soot may form; carbon monoxide interferes with oxygen transport. Sulfur impurities can yield sulfur dioxide, while high combustion temperatures allow atmospheric nitrogen and oxygen to form nitrogen oxides. Catalytic converters promote reactions that reduce selected pollutants, but they do not eliminate carbon dioxide production. A useful evaluation compares energy supply, pollutant formation, resource use and practical alternatives, using the conditions and data supplied.
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: Balance complete combustion of propane.
C3H8 + 5O2 → 3CO2 + 4H2O.
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.