YJC_H1_CHEM_P1_P2_ANS(1)

Yishun Junior College H1 Chemistry Preliminary Examinations 2015 Paper 1 Answers 1 A 7 A 13 A 19 D 25 B 2 C 8 A 14 B 20 C 26 B 3 B 9 D 15 C 21 C 27 A 4 C 10 D 16 D 22 B 28 C 5 C 11 B 17 D 23 C 29 B 6 D 12 B 18 C 24 A 30 D (A) : 7 (B) : 8 (C) : 9 (D) : 6 Paper 2 Answers 1 (a) Nitrogen has lesser nuclear charge / lesser number of protons and lesser shielding effect/lesser number of electron shells than phosphorus. [1] As the increase in shielding effect outweighs the increase in nuclear charge, the attraction between the nucleus and valence electrons is stronger for nitrogen. [1] Hence, more energy is needed to remove the first electron from nitrogen. [2] (b) Both ammonia and phosgene have simple molecular structure. Lesser energy is needed to overcome the weaker intermolecular permanent dipole-permanent dipole attractions [1] in phosgene compared to the strong intermolecular hydrogen bonds [1] in ammonia. [2] (c) (i) A dative bond is formed when a shared pair of electrons between two atoms is contributed by only one of the atoms. [1] (ii) Dative bond (e− pair from N atom donated to B atom) – [1] Correct dot-and-cross diagram – [1] [2]

(iii) N in ammonia has 3 bond pairs and 1 lone pair of electrons while N in H3NBF3 has 4 bond pairs and no lone pairs of electrons. [1] Lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion. OR Lone pair of electrons repel more than bond pairs. [1] Hence, the H-N-H bond angle is larger in H3NBF3. [2] [Total:9] 2 (a) alkene [1] and ester [1] [2] (b) [1] (c) [2] (d) (i) The standard enthalpy change of combustion of a substance is the heat evolved when one mole of the substance is completely burnt in oxygen gas at 25oC and 1 atm. [1] (ii) heat released = 100 × 4.18 × (56.5 − 25) = 13200 J [1] (iii) ηA = 2.50 268 = 9.33 × 10−3 mol [1] (iv) ∆HC = − 13.2 9.33 × 10−3 = −1420 𝑘𝐽𝑚𝑜𝑙−1 working – [1] sign + units – [1] [2] [Total:10] O O-Na+ CH3OH [1] [1] O O

3 (a) Test 1: Compound C is a carbonyl compound OR a ketone or aldehyde [1] Test 2: Compound C is an aldehyde or benzaldehyde [1] Test 3: Compound C is not an aliphatic aldehyde OR Compound C is a benzaldehyde [1] [3] (b) [1] (c) OR [1] (d) Each carbon atom has 3 sp2 hybrid orbitals and 1 unhybridised p- orbital. [1] 1 sp2 orbital overlap head-on with the s orbital of H atom to form a C−H single (sigma bonding). The other 2 sp2 orbitals are used to form sigma bonds with the neighbouring carbon atoms (C−C single bonds). [1] The unhybridised p-orbitals for the 6 carbon atoms overlap side