2020_ASRJC-NYJC_H3_Prelim_Key

A COOLABORATION BETWEEN ANDERSON SERANGOON JUNIOR COLLEGE & NANYANG JUNIOR COLLEGE H3 Chemistry Preliminary Examination 2020 Suggested Solutions 1 (a) The active site bind specifically to one enantiomer better than the other enantiomer. The enantiomers of a drug can have different effect on the body. (b) The Lewis acid catalyst interact with the reactant and its ster ically bulky groups effectively shields on one of the stereotopic faces of the reactant, leavin g the other face available for reaction to occur. (c) (i) H 2 gas is evolved at the Pt electrode (cathode). 2H+ + 2e‒ → H2 (ii) N N Ph O Ph + D + 2H+ + 2e Ph O N N Ph O Ph O Ph (iii) N N Ph O Ph + D + H2Ph O N N Ph O Ph O Ph ( i v ) Hr = 2×BE(C‒H) ‒ BE(C‒C) ‒ BE(H‒H) = 2(410) ‒ 350 ‒ 436 = +34 kJ mol ‒1  ( v ) The increase in the total number of gaseous molecules in the sy stem leads to and increase in the entropy of the system. S is positive. Since H is positive and −T S is negative, G is negative only at high temperature. Hence, the reaction is spontaneous at high temperature. ( v i ) Amount of D (77 % yield) = 1.0 406.0 = 2.463 × 10‒3 mol Theoretical Amount of D = 2.463 × 10‒3 × 100 77 = 3.199 × 10‒3 mol Amount of limiting reactant = 3.199 × 10‒3 mol Amount of catalyst = 3.199 × 10‒3 × 5 % = 1.599 × 10‒4 mol Mass of catalyst = 1.599 × 10‒4 × 857.3 = 0.137 g (vii) Theoretical amount of electrons transferred = 2 × 3.199 × 10‒3 = 6.398 × 10‒3 mol I × 11 × 60 × 60 = 6.398 × 10‒3 × 96500 I = 0.0156 A

2 (d) From the Data Booklet, Cu2+ + 2e‒ ⇌ Cu (+0.34 V) Zn2+ + 2e‒ ⇌ Zn (‒0.76 V) Zn → Zn2+ + 2e‒ Since the Eʅ value of Zn is the most negative, Zn will be preferentially oxidized. (e) From Abstract 1, -Rh1 produces R configuration product while -Rh2 produces S configuration product. Hence, -Rh2 is used in reaction 1 as D has a S configuration. N N Ph O O E: must show S configuration (f) 1st order reaction. Rate of formation of bubbles is proportional to the concentration of A. The concentration of B and (CH3)3SiCl are in large excess relative to A, and -Rh1/-Rh2 is a catalyst. Hence, concentration of B, (CH 3)3SiCl and -Rh1/-Rh2 remain constant throughout the reaction. It is a pseudo first order reaction. (g) O Ph [Rh] N O Ph [Rh] N Ph OSi(CH3)3 OSi(CH3)3 Ph 2 (a) Ph CH2Cl HHHH Ph CH2Cl H H H H Ph CH2Cl H HHH I: eclipsed (syn) (most unstable) II: gauche III: eclipsed Ph CH2Cl H H H H Ph CH2Cl H HHH Ph CH2Cl H H H H IV: staggered (anti) (most stable) V: eclipsed VI: gauche

3 (b) (i) In an external applied magnetic field, all protons have their m agnetic moments either parallel with the field or antiparallel to it. As a result, the degenerate spin states split into two states of unequal energy. The energy difference between the two levels is small and falls within the ra