ASRJC 2021 J2Prelims H2Chem P3 soln

ASRJC JC2 PRELIMS 2021 9729/03/H2 [Turn over Anderson Serangoon Junior College 2021 JC2 Preliminary Examination H2 Chemistry (9729) Paper 3 Suggested Solutions 1 (a) SiH4, PH3 and H2S are three compounds. (i) State and explain the bond angles in the molecules of these three compounds. [4] To minimise repulsion, the valence electron pairs around a central atom are arranged as far apart as possible. Lone pairs exert greater repulsion than bond pairs. [1] There are 4 bond pairs and (0 lone pair) around Si. Hence, SiH4, is tetrahedral with a bond angle of 109.5°. [1] There are 3 bond pairs and 1 lone pair around P. Since lone pair–bond pair repulsion is greater than bond pair–bond pair repulsion, PH3 is trigonal pyramidal with a bond angle of 107°. [1] There are 2 bond pairs and 2 lone pairs around S. Since lone pair–lone pair repulsion is greater than lone pair–bond pair repulsion which is greater than bond pair–bond pair repulsion, H2S is bent with a bond angle of 104.5°. [1] (ii) Phosphorus and hydrogen have same electronegativity. Draw the three molecules SiH4, PH3 and H2S and indicate for each one the polarity of each of the bonds it contains and the overall polarity of the molecule. [4] overall: non polar overall: polar overall: polar *Note: must indicate the polarity of each bond as question mentioned it. [1]: all 3 correct drawings [1]: polarity of each of the bonds [1]: overall polarity of each molecule [1]: correct direction of polarity for PH3 and H2S Thinking process Electronegativity increases across a Period. As P and H have same electronegativity, Si will be less electronegative than H while S is more electronegative than H. (iii) State the properties of a gas necessary for it to approach ideal behavior. [2] Intermolecular forces of attraction between gas molecules are negligible. [1] The volume of the gas particles/molecules is negligible compared to the volume of the container. [1] Si d d d d d H H H H P H H H S H H d d d

2 ASRJC JC2 PRELIMS 2021 9729/03/H2 [Turn over (iv) Of the three gases at 400 K, the behaviour of SiH4 is closest to ideal gas behavior. Suggest why H2S deviate more from ideal behaviour than SiH4. [1] H2S is polar and has stronger permanent dipole-permanent dipole attractions between its molecules. Hence, it has more significant intermolecular forces and deviate more from ideal behaviour. [1] (b) (i) Define the term lattice energy. [1] The amount of heat evolved when one mole of the solid ionic compound is formed from its constituent free gaseous ions under s