State the general formula of an alkane and a cycloalkane.
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alkane CₙH₂ₙ₊₂; cycloalkane CₙH₂ₙ
Alkanes
Explain why alkanes are described as saturated hydrocarbons.
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contain only carbon and hydrogen atoms (hydrocarbon); contain only single bonds / no C=C double bonds (saturated)
Define the term structural isomerism.
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compounds with the same molecular formula; but different structural formulae / different arrangement of atoms
State the type of structural isomerism shown by alkanes.
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chain isomerism
Draw and name the three structural isomers of C₅H₁₂.
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pentane (straight chain); 2-methylbutane (four-carbon chain with methyl branch on C2); 2,2-dimethylpropane (three-carbon chain with two methyl branches on C2)
Name the cycloalkane with molecular formula C₅H₁₀.
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cyclopentane
Explain why pentane and 2-methylbutane are structural isomers.
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same molecular formula C₅H₁₂; different structural formulae / different arrangement of carbon skeleton
Explain how fractional distillation separates the hydrocarbons in crude oil.
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crude oil is heated/vaporised and rises up the column; column has a temperature gradient (hotter at bottom, cooler at top); each fraction condenses at the tray where the temperature equals its boiling point / shorter chains condense higher up
Write a balanced equation for the cracking of decane (C₁₀H₂₂) into octane and one other product.
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C₁₀H₂₂ → C₈H₁₈ + C₂H₄; correct balancing and identification of ethene
State the difference between cracking and reforming.
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cracking breaks long-chain alkanes into shorter alkanes and alkenes; reforming converts straight-chain alkanes into branched/cyclic/aromatic compounds (with a Pt catalyst)
State two pollutants formed by incomplete combustion of an alkane fuel.
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carbon monoxide; carbon particulates / soot / unburned hydrocarbons
Explain why carbon monoxide is toxic.
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binds to haemoglobin (more strongly than oxygen); prevents oxygen being transported around the body
Explain how oxides of nitrogen are produced in a car engine and why they cause environmental damage.
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high temperature in engine causes N₂ and O₂ to react; forming NO/NO₂; oxides dissolve in atmospheric water producing acid rain (which damages forests, lakes, buildings)
Explain why CO₂ emissions from alkane combustion are linked to climate change.
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CO₂ is a greenhouse gas; absorbs infrared radiation emitted from Earth’s surface; re-emits some back, raising atmospheric temperature
Give two reasons for developing alternative fuels.
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crude oil is a finite/non-renewable resource; reduces CO₂ emissions / reduces other pollutants / lowers contribution to climate change
Explain why bioethanol is sometimes considered carbon-neutral but is not truly neutral in practice.
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CO₂ released on combustion of bioethanol equals the CO₂ absorbed by the crop during photosynthesis; giving zero net CO₂ in theory; however energy is needed for farming/transport/fermentation/distillation; this energy often comes from fossil fuels which release additional CO₂
State why hydrogen is considered a clean fuel.
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combustion produces only water; no CO₂/CO/particulates emitted at point of use
Write balanced equations for the complete and incomplete combustion of propane forming CO.
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complete: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O; incomplete: 2C₃H₈ + 7O₂ → 6CO + 8H₂O
State the condition required for an alkane to react with chlorine.
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ultraviolet (UV) light
Define the term free radical.
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a species with an unpaired electron