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📘 GCSE Chemistry · Notes & worksheet

Organic Chemistry.

GCSE Higher (Grades 6-9)board-neutral (AQA / Edexcel / OCR)Worksheet: 29 marks

Notes

Crude oil and fractional distillation

Crude oil is a mixture of hydrocarbons. It is separated by fractional distillation into fractions. Shorter chains: lower boiling points, more flammable, less viscous, collected near the top of the column. Longer chains: higher boiling points, more viscous, near the bottom.

Alkanes

General formula CₙH₂ₙ₊₂. Saturated (only single bonds). Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
Complete combustion: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Incomplete combustion gives carbon monoxide (toxic) and/or carbon (soot).

Cracking

Breaking long-chain hydrocarbons into shorter, more useful ones using heat and a catalyst (catalytic cracking) or steam. It produces alkanes and alkenes.
C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Alkenes

General formula CₙH₂ₙ. Unsaturated (contain a C=C double bond). Ethene C₂H₄, propene C₃H₆.

  • Test: bromine water goes from orange to colourless with an alkene.
  • Reactions: addition of hydrogen (makes alkanes), steam (makes alcohols) and halogens.

Addition polymers

Many monomers with C=C join to form a long polymer chain with no other product. Ethene gives poly(ethene); propene gives poly(propene). In a repeat unit the double bond becomes a single bond.

Alcohols

Contain –OH. Methanol, ethanol, propanol.

  • Ethanol by fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast, about 30 °C, no oxygen).
  • Ethanol by hydration of ethene: C₂H₄ + H₂O ⇌ C₂H₅OH (steam, phosphoric acid catalyst, high temperature and pressure).

Carboxylic acids

Contain –COOH. Methanoic, ethanoic and propanoic acid. They are weak acids, react with carbonates to give a salt, water and carbon dioxide, and react with alcohols to make esters.
Ethanoic acid + ethanol → ethyl ethanoate + water (acid catalyst).

Common mistakes

  • Writing "the bromine water turns clear". The correct term is colourless.
  • Drawing a polymer repeat unit with the double bond still in place.
  • Forgetting that cracking produces an alkene as well as an alkane.

Exam tips

  • Balance equations by counting carbon, then hydrogen, then oxygen.
  • Name using the stem for chain length, then the suffix: -ane, -ene, -anol, -anoic acid.

Worksheet

Prints the questions only, without answers. Choose “Save as PDF” in the print window to download.

Total: 29 marks.

  1. Write a balanced equation for the complete combustion of propane (C₃H₈). (2)
  2. Explain how fractional distillation separates crude oil, and explain why the shorter fractions are collected near the top. (4)
  3. Decane (C₁₀H₂₂) can be cracked to make octane (C₈H₁₈) and one other product.
    (a) Give the formula of the other product. (1)
    (b) Describe a test to show the other product is an alkene, and give the result. (2)
    (c) Explain why cracking is carried out. (2)
  4. Describe the structure of an alkene and explain why alkenes are described as unsaturated. (2)
  5. Propene (C₃H₆) can form an addition polymer.
    (a) Name the polymer. (1)
    (b) Draw (or describe) the repeat unit and explain how it differs from the monomer. (3)
  6. Write the equation for the manufacture of ethanol by fermentation and state two conditions needed. (4)
  7. Compare the two methods of making ethanol, fermentation and hydration of ethene, giving one advantage of each. (2)
  8. Write the word equation for the reaction between ethanoic acid and ethanol, and name the type of compound formed. (3)
  9. Explain why carbon monoxide is a danger when hydrocarbon fuels are burned in a poorly ventilated room. (3)
Show mark scheme and answers (tutor copy)

Try the worksheet first, then check your work here.

  1. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. (B1 correct formulae, B1 balanced)
  2. Crude oil is heated and vapour enters a column that is hot at the bottom and cooler at the top (B1). Hydrocarbons rise until they reach a temperature below their boiling point, where they condense (B1). Short-chain molecules have weaker intermolecular forces and lower boiling points (B1), so they travel higher before condensing and are collected near the top. (B1)
  3. (a) C₂H₄ (ethene). (B1)
    (b) Add bromine water (B1). It changes from orange to colourless. (B1)
    (c) Longer hydrocarbons are less in demand than shorter ones (B1). Cracking makes shorter, more useful alkanes for fuel and alkenes as feedstock for polymers and chemicals. (B1)
  4. Alkenes contain a carbon–carbon double bond (C=C) (B1). They are unsaturated because they do not contain the maximum number of hydrogen atoms (they can add more atoms across the double bond). (B1)
  5. (a) Poly(propene). (B1)
    (b) Repeat unit has a C–C single bond in the backbone, with a CH₃ side group on one carbon and hydrogens (B1, B1). In the monomer there is a C=C double bond, which opens up so that each carbon can bond to the next monomer. (B1)
  6. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (B1 reactants, B1 products and balance). Conditions: yeast (B1) and about 30 °C, in the absence of oxygen. (B1)
  7. Fermentation: uses renewable resources, low temperature, low cost equipment. Hydration: continuous process, fast, produces pure ethanol. (B1 each)
  8. Ethanoic acid + ethanol → ethyl ethanoate + water (B1, B1). The compound formed is an ester. (B1)
  9. In limited oxygen incomplete combustion occurs and carbon monoxide forms (B1). CO is colourless and odourless so it cannot be detected without an alarm (B1). It combines with haemoglobin and reduces the oxygen carried in the blood, which can cause fainting or death. (B1)
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