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Distinguish between the terms hazard and risk.
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hazard is the property of a substance that could cause harm/intrinsic potential to cause harm; risk is the likelihood/chance that harm actually occurs under the specific conditions of use
Suggest two ways the risk of using a flammable, volatile organic solvent in a school laboratory can be reduced.
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work on a smaller scale to reduce the quantity of vapour produced; use a fume cupboard / remove naked flames / heat using a water bath or electric mantle (any two distinct precautions)
Discuss how a student could carry out a reaction safely using bromine, a toxic and corrosive liquid, and ethene gas. In your answer you should: - identify the main hazards; - suggest specific precautions linked to each hazard; - suggest one alternative reagent or method that lowers the overall risk.
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bromine is toxic by inhalation, so the reaction must be carried out in a fume cupboard to prevent breathing in vapours bromine is corrosive to skin and eyes, so gloves and safety goggles must be worn ethene is flammable, so naked flames must be removed and the apparatus checked for leaks working on a smaller scale reduces the quantity of bromine vapour and ethene used using bromine water (aqueous) instead of liquid bromine reduces the volatility and therefore inhalation risk alternative: testing for unsaturation could be done on a smaller scale using a few drops in a test tube safe disposal: bromine residues neutralised with sodium thiosulfate before disposal
Define the term homologous series.
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a family/series of compounds with the same general formula/same functional group; successive members differ by –CH₂– / show a gradation in physical properties
State the general formula of the alkene homologous series and give the molecular formula of the alkene with five carbon atoms.
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CₙH₂ₙ; C₅H₁₀
Explain why all members of a homologous series undergo similar chemical reactions.
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they contain the same functional group; the functional group determines the reactivity / type of reaction the molecule undergoes
Name the compound with the structural formula CH₃CH(CH₃)CH₂CH₂CH₃.
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2-methylpentane
Draw the displayed formula and the skeletal formula of 2-chloropropane.
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displayed formula shows H₃C–CHCl–CH₃ with all bonds drawn (1); skeletal shows a zigzag of two segments with Cl labelled on the central vertex (1)
Name the compound CH₃CH₂CH(OH)CH₂CH₃ using IUPAC rules.
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identify longest chain = 5 carbons (pentane stem); –OH given lowest locant gives position 3; name = pentan-3-ol
Explain why the compound CH₃CH(CH₃)CH(CH₃)CH₃ is named 2,3-dimethylbutane and not 2,3-dimethylbutane numbered from the other end.
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the longest chain has four carbons (butane); numbering from either end gives substituent locants 2 and 3, which is identical; the lowest set of locants is 2,3 so the name is 2,3-dimethylbutane
Classify the reaction CH₂=CHCH₃ + HBr → CH₃CHBrCH₃.
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addition (reaction)
State and explain the type of reaction occurring when ethanol is heated with acidified potassium dichromate(VI) to form ethanal.
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oxidation; ethanol loses two hydrogen atoms / oxygen is added to the carbon framework / oxidation number of carbon increases
Identify the reaction type when many propene molecules join to form poly(propene), and write a general equation.
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polymerisation (1); n CH₂=CHCH₃ → (–CH₂–CH(CH₃)–)ₙ (1)
Define homolytic fission and give the type of species produced.
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a covalent bond breaks so each atom takes one electron from the shared pair (1); two free radicals are produced (1)
Explain why the C–Cl bond in chloromethane is more likely to undergo heterolytic fission than homolytic fission in solution.
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the C–Cl bond is polar with δ⁺ on carbon and δ⁻ on chlorine; the more electronegative chlorine takes both bonding electrons forming Cl⁻ and a carbocation, favouring heterolytic fission
Describe the difference between a half-headed (fish-hook) curly arrow and a full curly arrow in mechanism diagrams.
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a half-headed arrow represents the movement of a single electron, used in homolytic/radical processes (1); a full curly arrow represents the movement of a pair of electrons, used in heterolytic/ionic mechanisms (1)
Define the term free radical.
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a species (atom, molecule or ion) with an unpaired electron
Define the term electrophile and give one example.
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an electron-pair acceptor / a species attracted to an electron-rich region of a molecule (1); example: H⁺ / NO₂⁺ / Br⁺ / δ⁺ carbon in a polar bond (1)
Explain why a chlorine radical is highly reactive.
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it has an unpaired electron in its outer shell (1); it readily reacts to pair this electron, achieving a stable noble-gas configuration (1)