2023_ASRJC_H2_Chem_Prelim_P3_MS

ASRJC JC2 PRELIM 2023 9729/03/H2 [Turn over Anderson Serangoon Junior College 2023 JC2 H2 Chemistry Preliminary Examination Paper 3 Suggested Solutions 1 (a) (i) Describe and explain the relative basicities of methylamine, dimethylamine and trimethylamine in the gas phase. [3] Order of increasing basicity in the gas phase: methylamine < dimethylamine < trimethylamine The increasing number of electron-donating methyl / alkyl group from methylamine to dimethylamine to trimethylamine increases the availability of the lone pair of electrons on N atom to accept a proton. [1]: trend [1]: more electron donating alkyl group [1]: greater availability of lone pair (ii) Explain why amides are neutral. [1] Amides are neutral because the lone pair on nitrogen atom is delocalised into the  bond of the adjacent C=O by resonance and hence not available for donation to a proton. [1] (b) Deuterium (symbol D or 2 1H ) was discovered in 1931. Deuterium accounts for 0.0156% of all the naturally occurring hydrogen in the oceans, while the most common isotope 1 1H accounts for 99.98%. Tritium (symbol T or 3 1H ), a rare and radioactive isotope of hydrogen account for only 0.0044%. Chemically, deuterium behaves similarly to ordinary hydrogen. (i) Calculate the average Ar of hydrogen. Give your answer to four decimal places. [1] Ar = 0.0156 99.98 0.0044( 2) ( 1) ( 3)100 100 100 +  +  = 0.000312 + 0.9998 + 0.000132 = 1.000244 = 1.0002 (4 decimal places) [1] (ii) On the same diagram, sketch how a beam of singly positively charged deuterium ions and a beam of hydrogen ions will behave in an electric field. In your diagram, indicate clearly the relative angle of deflection for each beam. (You may let the angle of deflection of hydrogen ions be x°) [2] Diagram should show charged deuterium ion deflecting less than hydrogen ion. Deuterium ion and hydrogen ion must deflect in the same direction and towards negative plate. Source xo 1 1H+ + ‒ x/2o 2 1H+

2 ASRJC JC2 PRELIM 2023 9729/03/H2 Above two points [1] Since angle of deflection  e/m ratio:  of Deuterium ion is half of  of hydrogen ion [1] (iii) Explain the difference in the thermal stability of DCl, DBr and DI. [2] Down Group 17 from chlorine to bromine to iodine, the size of valence orbitals increases and become more diffused. This causes a decrease in effectiveness of orbital overlap between the valence orbital of halogen and s–orbital of hydrogen. / Electronegativity difference between the halogen and deuterium decreases, resulting in a decrease in bond polarity. [1] D–X bond becomes weaker and hence the D–X bond energy decreases from DCl to DBr t