ASRJC_H2_Chem_2022_Prelim_P3_Soln

ASRJC JC2 PRELIM 2022 9729/03/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2022 JC 2 PRELIMINARY EXAMINATION SOLUTIONS WITH MARKERS COMMENTS NAME:______________________________ ( ) CLASS: 22 / _____ CHEMISTRY 9729/03 Paper 3 Free Response Questions 16 September 2022 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet 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) N OO [1] (ii) bent, sp2 [1] Comments: (i) Many students did not know that in some molecules (eg NO2), the total number of valence electrons is an odd number. You can easily check by summing the total number of valence electrons before drawing the dot -and-cross diagram. In this case NO2 has 5+6+6=17 valence electrons. This means that there will b e at least one unpaired electron in the molecule! You can see this in your Chemical Bonding lecture notes, Pg 20 for more example and details. NO. .. x O.. ... x. x .x.x. is incorrect as there are 9 electrons around N. N, in period 2, CANNOT expand octet. It will only have a maximum of 8 electrons. (ii) In counting the number of bond pairs, each double bond or dative bond is counted as one bond pair. One lone electron is also counted as one lone pair as it also exerts repulsion, though less than a full 2-electron lone pair. This concept has been covered in our JC1 Chemical Bonding tutorial question 8! When writing the hybridisation state such as sp2, ‘sp’ is written in small letters and ‘2’ is written in superscript (raised). (iii) Bond length is the distance between the nuclei of the two atoms in the bond. [1] (iv) The 2p orbital of N overlaps with the 2p orbital of the singly bonded O atom allows the lone pair of electrons on the O atom to be delocalised into the N=O bond. [1] As a result, both the N –O bonds have partial double bond character . And the observed bond length is intermediate between a single and double bond. [1]  Mention delocalization of electron pair of O into the N=O bond  Leading to partial double bond character Comments: (iii) The words ‘nuclei’ and ‘bond’ are often missing. (iv) Many students attempted to explain the difference in bond length/bond strength of N=O and N -O instead of answering to the question on why the N -O in NO 2 is intermediate between a single and double bond. (FYI) Delocalisation of lone pair on O in N=O bond:

ASRJC JC2 PRELIM 2022 9729/03/H2 [Turn over (b) (i) Energy is released as a N-N covalent bond is