NEW CARDS - Paper 2 Exam Qs

• explain why the carbon–halogen bond enthalpy influences the rate of reaction.

  • Allow carbon-halogen bond strength decreases down the group / from Chlorine to Iodine
  • So carbon iodide > Carbon bromide > Carbon chloride

  
• Stage 1 Bonding

  • 1a) Each C has three covalent bonds
  • 1b) Spare electrons in a p orbital overlap to form a pi cloud
  • 1c) delocalisation
• Stage 2 Shape of a Benzene molecule

  • 2a) Planar
  • 2b) Hexagon/6 carbon ring/120o bond angle
  • 2c) C–C bonds equal in length / C–C bond lengths between single and double bond
• Stage 3 Stability

  • 3a) Expected delta H of hydrogenation of cyclohexatriene = –360 kJ mol–1
  • 3b) Delta H o hydrogenation of benzene is less exothermic by 152 kJ mol–1
  • 3c) Benzene is lower in energy than cyclohexatriene / Benzene is more stable
• • Double bonds separated by one single bond / alternating (or shown in structure)
  • Allows some delocalisation/overlap of p orbitals WHICH MAKES STRUCTURES MORE STABLE
• Suggest one reason why the use of CFCs was not restricted until several years after Rowland and Molina published their research. [1 mark]

  • One of these reasons:
  • • lack of evidence that ozone was being depleted
  • • lack of alternatives to CFCs
  • • commercial interest to continue to use CFCs
  • • hard to obtain international agreement
• State the benefit to life on Earth of ozone in the upper atmosphere. [1 mark]

  • Absorbs (harmful) ultraviolet / uv (light / radiation)
• CFC-11 is a greenhouse gas that can contribute to global warming. State and explain how CFC-11 is able to contribute to global warming. [2 marks]

  • M1 absorbs infrared radiation
  • M2 molecule has polar bonds
• Step 1 Crude oil to napatha

  • Fractional distillation
  • Separated into mixtures of compounds with similar boiling points / similar sized molecules
  • Separate compounds by chain length / size / boiling point

  
• Step 2 Naphtha to propene

  • Thermal cracking
  • To make alkenes from alkanes / shorter molecules
  • NOT Catalytic cracking - this produces motor fuels and aromatic compounds

  
• Step 3 - propene to polypropene

  • Addition polymerisation
  • Molecules joined together // to produce long chain molecule

  
• 0 5 . 2 Poly(propene) is not biodegradable because it is unreactive. Explain why poly(propene) is unreactive. [1 mark]

  • no polar bonds (in chain) / non-polar
  • C-C and C-H bonds are BOTH non polar
• Scientists are developing new polymers, including some that are biodegradable. Suggest why it is beneficial for some polymers to be biodegradable. [1 mark]

  • to prevent build-up of waste (in landfill) OR
  • they can be broken down by natural processes
• Explain why the value given for the O=O bond enthalpy in Question 08.2 is not a mean value. [1 mark]

  • Oxygen / O2 is the ONLY substance that has O=O bond
• Explain why there is an attraction between a C=C double bond and Br2 [3 marks]

  • M1 C=C is electron rich / area of high electron density
  • M2 Br-Br becomes POLARISED
  • M3 slightly positive Br is attracted to C=C
• 1-chloropropane can also be produced by the reaction between propane and chlorine in the presence of ultraviolet light.

  State why ultraviolet light is needed for this reaction to occur.

  • provides ENERGY to break (covalent) bond in chlorine / Cl2 or
  • to form chlorine free radicals
• Importance of percentage yield

  • idea of getting as much product as possible in the reaction
  • idea of efficient conversion of reactants to products
• Importance of percentage atom economy

  • M6 idea of maximising the mass of reactants / atoms that ends up in desired product
  • idea of minimising the amount of by-products
• Pent-1-ene is formed by the elimination of water from pentan-2-ol.

  State the reagent and condition for this reaction.

  • REAGENT = CONCENTRATED SULFURIC ACID
  • OR CONCENTRATED PHOSPHORIC ACID
  • CONDITIONS = Hot (allow high temperature)
  • If stated temperature in range 150 - 200 C
• • Using the rule that carbon only forms 4 bonds - draw a displayed formula including all C, H, O (to get answer C24H38O4)
  • Makes it MORE FLEXIBLE (allow less brittle)

  
• Ethanedioic acid is formed by the oxidation of ethane-1,2-diol (HOCH2CH2OH). State suitable reagent(s) and a condition for this reaction. [2 marks]

  • acidified potassium dichromate(VI) or sulfuric acid & potassium dichromate(VI)
  • REFLUX!!!
• What statement is true about thermal cracking?

  • C-C bonds are broken
• A data book value for the enthalpy of combustion of cyclohexane is –3920 kJ mol−1.

  The student concluded that the temperature rise recorded in the experiment was smaller (less exothermic) than it should have been.

  Suggest a practical reason for this. [1 mark] (MAIN REASONS)

  • Heat loss
  • Incomplete combustion
• A data book value for the enthalpy of combustion of cyclohexane is –3920 kJ mol−1 The student concluded that the temperature rise recorded in the experiment was smaller than it should have been. Suggest a practical reason for this. [1 mark]

  • Evaporation
  • or idea of heat not being transferred to calorimeter
  • Allow idea that it is not under standard conditions
  • Allow no lid / poor insulation
• The bond angle around the oxygen atom in methanol is slightly smaller than the regular tetrahedral angle of 109.5° Explain why this bond angle is smaller than 109.5° [1 mark]

  • Idea that lone pairs have greater repulsion than bonding pairs
  • There must be a comparison between the repulsion of a lone pair and bonding pair
• Stage 1: Formation of product

  • Nucleophilic attack

  • Planar carbonyl group

  • H– attacks from either side (stated or drawn)

  
• Stage 2: Nature of product

  • Product of step 1 shown

  • This exists in two chiral forms (stated or drawn)

  • Equal amounts of each enantiomer/racemic mixture formed
• Stage 3: Optical activity

  • Optical isomers/enantiomers rotate the plane of polarised light equally in opposite directions

  • With a racemic/equal mixture the effects cancel
• One monomer used in synthesis of Kevalar is Benzene-1,4diamine

  To make this monomer Stage 1 ammonia reacts with benzene

  Suggest why this is considered unexpected

  • AMMONIA IS A NUCLEOPHILE / has a lone pair
  • BENZENE REPELS NUCLEOPHILES / benzene attracts electrophiles
• Justify the statement that polymer formed by U is not biodegradable

  • Strong / non-polar C-C bonds (in the chain)
  • cannot be attacked by nucleophiles/acids/cannot be hydrolysed.
• Suggest why aqueous ethanol is a suitable solvent when heating coconut oil with KOH

  • Allow to dissolve both oil and KOH
  • ALLOW to act as a mutual solvent
• Give a safety procaution when heating Coconut oil, ethanol and KOH

  • Precaution must be linked to heating e.g. Use a water bath for heating mixture
  • ALLOW electric heater / heat mantle
  • Ethanol is flammable / prevent the risk of fire
• Explain why NaBH4 reduces methylbutanal but has no reaction with 2-methylbut-1-ene

  • M3 H- ion / nucleophile is attracted to slightly positive C
  • M4 electron rich C=C (3.14.1 double bond has C=C)
  • M5 H- ion / nucleophile is repelled by C=C
  • C=C only attacked by/reacts with electrophiles
• State the meaning of the term complementary when referring to DNA strands [1 mark]

  (Complementary means the two strands must have base sequences) that match (all) A to T and C to G
• Draw structure of a different diol with molecular formula C6H14O2 that has 1H NMR spectrum that consists of two singlet peaks

  • To form as little peak as possible, use methyl groups to form a symmetrical molecule
  • TWO singlet peaks - One peak from OH, one from methyl (2 total)
• State why dicarboxylic acid can be distinguished by high resolution mass spectrometry using electrospray ionisation

  The PRECISE (relative molecular) masses are different
• Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at other times during the experiments. [1 mark]

  • (At time zero/start) the concentrations are known 1
• State how the initial rate is obtained from a graph of the concentration of the product against time. [2 marks]

  • M1 (Calculate) gradient of tangent/curve/graph)
  • which is change in concentration over time
  • M2 at t=0 or at start of graph/curve
• Suggest why the order with respect to iodine is zero. [1 mark]

  • Iodine is not involved in (or before) the rate determining / slow(est) / limiting step (in the mechanism)

  
• State the significance of this zero order for the mechanism of the reaction. for B

  rate = k [A]^2

  • B does not appear in the slow step or the rds
  • Slow step or rate determining step involves only A