Organic Chemistry
Organic chemistry at Combined Science GCSE level covers the hydrocarbons obtained from crude oil: what crude oil is, how fractional distillation separates it into fractions of similar chain length, and cracking, the process that breaks large hydrocarbon molecules into smaller, more useful alkanes and alkenes.
Before you start
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Method
- Learn that crude oil is a finite fossil fuel formed from the buried remains of plankton over millions of years, and that it is a mixture of hydrocarbons (molecules of hydrogen and carbon only), mostly alkanes with the general formula CnH2n+2.
- For fractional distillation questions, link the height a fraction is collected at in the column to its chain length and boiling point: short-chain fractions are collected near the top (cooler), long-chain fractions near the bottom (hotter).
- For a 'trend' question, name the specific property (boiling point, viscosity or flammability) and state clearly whether it increases or decreases as chain length increases, rather than giving a vague answer.
- For combustion questions, first decide whether the oxygen supply is described as plentiful (complete combustion, producing carbon dioxide and water) or limited (incomplete combustion, producing carbon monoxide and/or carbon soot, plus water), then write or balance the products.
- For a cracking equation, count the carbon and hydrogen atoms in the starting hydrocarbon, subtract the atoms already shown in the named product, and use what is left to find the missing product's molecular formula.
- For 'why is cracking used' questions, explain in terms of supply and demand: crude oil naturally yields more long-chain hydrocarbons than the market needs, and not enough short-chain fuels and alkenes, so cracking converts the surplus into what is actually in demand.
Worked example
A hydrocarbon, decane (C10H22), is cracked to produce octane (C8H18) and one other product. Work out the molecular formula of the other product, and state what type of hydrocarbon it is.
- Count the atoms in the starting molecule: decane, C10H22, has 10 carbon atoms and 22 hydrogen atoms.
- Count the atoms already accounted for in the named product: octane, C8H18, has 8 carbon atoms and 18 hydrogen atoms.
- Find the atoms remaining for the other product: carbon = 10 - 8 = 2; hydrogen = 22 - 18 = 4.
- Write the formula of the other product using these totals: C2H4.
- Final answer: the other product is C2H4 (ethene), an alkene, since a 2-carbon alkane would need the formula C2H6 (CnH2n+2), so the missing hydrogen atoms show this product is unsaturated.
Practice questions
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Q1Define the term 'hydrocarbon'.Show answer
Answer: A compound made of hydrogen and carbon atoms only.
Q2State the general formula of the alkanes.Show answer
Answer: CnH2n+2
Q3Name the process used to separate crude oil into fractions.Show answer
Answer: Fractional distillation.
Q4State what happens to the boiling point of hydrocarbons as their chain length increases.Show answer
Answer: It increases.
Q5Balance the equation for the complete combustion of methane: CH4 + O2 -> CO2 + H2OShow answer
Answer: CH4 + 2O2 -> CO2 + 2H2O
Q6State two products, other than water, that can be formed during the incomplete combustion of a hydrocarbon.Show answer
Answer: Carbon monoxide and carbon (soot).
Q7Name the two industrial methods of cracking.Show answer
Answer: Catalytic cracking and steam cracking.
Q8Hexadecane (C16H34) is cracked to produce dodecane (C12H26) and one other product. Calculate the molecular formula of the other product.Show answer
Answer: C4H8 (carbon: 16 - 12 = 4; hydrogen: 34 - 26 = 8), an alkene called butene.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain why crude oil can be separated into fractions using fractional distillation.
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Propane (C3H8) is used as a fuel in camping stoves. (a) Write a balanced symbol equation for the complete combustion of propane. (b) A different sample of propane burns with an insufficient supply of oxygen. State one hazard of this incomplete combustion, other than less heat being released, and explain why it occurs.
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Crude oil naturally contains a much larger proportion of long-chain hydrocarbons than short-chain hydrocarbons, but consumer demand is mostly for short-chain fuels such as petrol, and for alkenes to manufacture plastics. Explain how the oil industry addresses this mismatch between supply and demand, and describe how the process works.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
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This topic is chapter 8 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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