HCI_H2_CHEM_P3_ANSWERS Prelim

2014 HCI H2 Chemistry Prelims Paper 3 Mark Scheme 1 (a) Description: Cl2 reacts explosively with H2 under direct sunlight. Br2 reacts slowly with H2 upon heating. I2 reacts partially/reversibly with H2 upon heating. Explanation: Down the group, the bond strength for HX decreases in order of HCl > HBr > HI. The enthalpy of reaction to become less exothermic, thus the reaction is less vigorous down the group. [2] (b) (i) Name of mechanism: Free radical substitution Initiation: Propagation: + Br + HBr + Br2 Br + Br Termination: + Br Br + [3] (ii) Large excess of alkane / limited amount of bromine. [1] (c) (i) The bromine is added on the least substituted carbon atom. [1] (ii) Bromine radical is electron deficient, while C=C bond is electron rich. OR The  bond of the C=C bond is weaker than the  bond of the C-H bond. Hence, the radical will attack the alkene preferentially. [1] (iii) Br The intermediate formed in the reaction ha s more electron donating alkyl group s attached to the carbon radical compared to that drawn above . Hence it is more stable, which will lead to the formation of the major product. [2] (iv) [3] (d) (i) Unit of k: s-1 (or h1 or min-1) [1]

(ii) At higher temperatures, the proportion of molecules with kinetic energy greater than or equal to activation energy increases . Thus, frequency of effective collision increases, rate constant increases. [3] (iii) Chlorine radical acts as homogeneous catalyst. It exists as same phase as reactants and is regenerated. [2] (iv) C-Cl bond in CFCs can be broken down by UV light to form chlorine radicals, which react with ozone. [1]

2 (a) Longifolene is a non–polar molecule. It is does not form favourable interactions with water. [1] (b) In order of increasing basicity: Hydrazine, ammonia, ethylamine Each nitrogen atom on hydrazine is bonded to an electron withdrawing nitrogen atom, which reduces the availability of the lone pair of electrons to accept a proton . Hence, it is a weaker base compared to ammonia. Ethylamine has an electron donating alkyl group bonded to the nitrogen atom, increasing the availability of the lone pair of electrons to accept a proton compared to ammonia and hydrazine. Hence it is the strongest base. [3] (c) (i) I: LiAlH4/ dry ether or H2/Ni or NaBH4/ methanol II: KMnO4 or K2Cr2O7/ H2SO4(aq) / heat IV: SOCl2 or PCl3 or PCl5 or conc HCl V: Ethanolic NaOH, heat [4] (ii) Add 2,4–dinitrophenylhydrazine to the sample. Orange precipitate observed with A. No orange precipitate with B. Accept other tests, i.e. PCl3, PCl5, oxidising agent tests, etc. [2] (d) (i) OR Trigonal pyramidal [2] (ii) HCl(aq) or HNO3(aq) or H2SO4(aq) or H2O(l) [1] (iii) Name: Nucleophilic addition R R O CH3 R CH3 R O  slow R CH3 R O