2019 TMJC H2 Chem Prelim P2 ANS

1 Tampines Meridian Junior College 2019 JC2 Prelim Exam H2 Chemistry 1 (a) Carbon forms compo unds with Group 16 elements such as oxygen, sulfur and selenium. The properties of some of these compounds are given below. compound structure net dipole moment boiling point / C CO2 O=C=O 0 sublimes CS2 S=C=S 0 46 COS S=C=O 0.71 –50 COSe Se=C=O 0.73 –22 (i) Explain, in terms of structure and bonding, the difference in the boiling points of CS2 and COS. CS2 has a higher boiling point. Both CS 2 and COS have simple molecular structures. CS2 has a larger electron cloud (or larger number of electrons ) than COS. More energy is required to overcome the stronger instantaneous dipole- induced dipole interactions (or IMF/Dispersion forces) between CS 2 molecules than the instantaneous dipole -induced dipole interactions between COS molecules. [2] (ii) Explain why  CO2 has no net dipole moment.  COSe has a greater net dipole moment than COS. CO2 has not net dipole moment because it is linear and the dipole moments cancel out. COSe has a greater net dipole moment than COS. There is a smaller difference between the dipole moment of C=O and C=S in COS than that between C=O and C=Se in COSe since S is more electronegative than Se. OR polarity of C=S is greater than that of C=Se. [2] TAMPINES MERIDIAN JUNIOR COLLEGE 2019 JC2 H2 Chemistry Prelim Exam Paper 2 (Suggested Answers)

2 Tampines Meridian Junior College 2019 JC2 Prelim Exam H2 Chemistry Carbon forms the backbone of organic compounds. Hydrocarbons are the simplest organic compounds that contain carbon and hydrogen. (b) Compound A is an isomer of the hydrocarbon octane, C8H18. A Controlled chlorination of compound A in the presence of UV light produces different mono-chlorinated products with a molecular formula of C8H17Cl. (i) Suggest the total number of constitutional isomers which can be formed from the possible mono-chlorination of compound A. Draw the structural formulae of any two of these products. Total number of possible mono-chlorinated products = 4 [3] (ii) Draw the skeletal formula of the isomer of octane which could produce only one possible mono-chlorinated product if it undergoes free radical substitution. [1]

3 Tampines Meridian Junior College 2019 JC2 Prelim Exam H2 Chemistry (c) Besides reaction of alkanes with halogens, the process of thermal cracking, in which large alkane molecules are broken down into smaller alkanes and alkenes, proceeds via free radical mechanism. The following are reactions involved when propane undergoes thermal cracking. CH3CH2CH3   ●CH3 + ●CH2CH3 -- (1) CH3CH2CH3   ●H + ●CH2CH2CH3 -- (2) ●CH3 + CH3CH2CH3   CH4 + ●CH2CH2CH3 -- (3) ●CH2CH3 