ASRJC_H2_Chem_2022_Prelim_P2_Soln

ASRJC JC2 PRELIM 2022 9729/02/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2022 JC 2 PRELIMINARY EXAMINATION SOLUTIONS WITH MARKERS COMMENTS NAME:______________________________ ( ) CLASS: 22 / _____ CHEMISTRY 9729/02 Paper 2 Structured Questions 14 September 2022 2 hours READ THESE INSTRUCTIONS FIRST Please look through the marks scheme and pay attention to the comments from markers. As you read, take note of:  What are your misconceptions?  What are common errors to avoid?  How should you improve next time? Do pay attention as your tutor go through in class and be proactive in asking questions. If you do not ask now, then when? Remember the power of tiny gains and be a little better every day!

1 (a) (i) • Ca has more filled principal quantum shells than Mg. • Nuclear charge and shielding effect of Ca are higher • Outermost electrons are of Ca further from the nucleus. [1] • Less energy is required to overcome the weaker electrostatic forces of attraction between the nucleus and the electron to be removed for Ca. • Second ionisation energies of Ca is lower than that of Mg. [1] Comments: This is a simple comparison of IE down the group. Although it is about the 2nd IE, the electron to be removed is from the same orbital as that of the 1 st IE for both Ca and Mg. Students commonly missed out:  electron to be removed from Ca is further away from the nucleus  electrostatic forces of attraction is between the nucleus and the electron removed Other error:  spelled “principal” wrongly as “principle”  thought that Ca has more valence shells than Mg when there should be only one valence shell for both Ca and Mg.  wrote that the 2nd IE of Ca is higher when the values of ionisation energies can be found in the Data Booklet. (ii) Thermal stability: CaCO3 > ZnCO3 > MgCO3 Zn, Ca and Mg have the same cationic charge of +2 Ionic radius of Ca2+: 0.099 nm Ionic radius of Mg2+: 0.065 nm Ionic radius of Zn2+: 0.074 nm Ionic radius: Ca2+ > Zn2+ > Mg2+ [1] with quoted values Charge density (q/r): Ca2+ < Zn2+ < Mg2+ Hence, polarising power of cation, ease of distortion of anion and weakening of C-O bond increases in the same order. [1] Comments: Many students were not able to use the ionic radius of the 3 cations to relate to the charge density, polarising power etc. Instead, some incorrectly quoted E o values or atomic radius which have NEVER been used to explain thermal stability of carbonates! Several students also missed out key words like the electron cloud of CO 32- was distorted to a greater extent or the C-O bond is weakened to a greater extent etc. Some students spelled “extent” wrongly as “extend” in their answer. (b) (i) [Be(H2O)4]2+ +H2O ⇌ [Be(H2O)3(OH)]+ + H3O+ OR [Be(H2O)4]2+ ⇌ [Be(H2O)3(OH)]+ + H+ [1] Comments: Many students missed out the information from the qns that Be forms a