2022 TMJC H2 Chem Prelim P2 (Ans)

2022 H2 Chem Prelim P2 (Ans) 1 (a) The elements of Period 3 form different oxides when reacted with oxygen. (i) State and explain the variation in bonding within the oxides across Period 3. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………..... [2] Compounds A and B are oxides of Period 3 elements which exist in the solid state at room temperature. When excess water is added to a sample of A, the solid dissolves completely and an acidic solution is obtained. However, when excess water is added to B, the solid remains insoluble and a neutral solution is obtained. (ii) Identify compound A. ………………………………………………………………………………………………….. [1] (iii) Suggest two possible identities of compound B and describe how the identity of compound B can be confirmed using the acid-base behaviour of Period 3 oxides. Include all relevant observations and an equation in your answer. …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... The bonding within the oxide changes from ionic to covalent across the Period. [1] Large difference in electronegativity between metallic atoms (e.g. Na/Mg/Al) and oxygen results in the transfer of electrons and hence formation of ionic bonds. Small difference in electronegativity between non-metallic atoms (e.g. Si/P/S) and oxygen results in sharing of electrons and hence formation of covalent bonds. [1] P4O10 [1] Possible identities of B: aluminium oxide / Al2O3 and silicon dioxide / SiO2. [1] Add HCl(aq) (or H2SO4(aq)) to a sample of compound B. If it is soluble / dissolves in HCl(aq), B is Al2O3. If it is insoluble, B is SiO2. [1] for test, observation Al2O3 + 6HCl → 2AlCl3 + 3H2O OR Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O [1] for equation Markers’ Comments • Many candidates misinterpreted the question and gave the structures (or even the acid-base nature) of the Period 3 oxides instead of the bonding. Even so, the structures of the oxides were also incorrect e.g. giant metallic structure for the metallic oxides or simple molecular structure for Al2O3. This is a book work question and candidates are advised to revise and distinguish accurately between the different types of structures and chemical bonding. • Many candidates were also unable to explain why the type of bonding changes across the Period. Markers’ Comments • SO3 is a colourless liquid that fumes in air at r.t.p. and hence is not accepted as the identity of compound A. • Many candidates struggled to give the correct formula of phosphorus (V) oxide e.g. PO4, or suggested the identity of the element rather than the compound (oxide). OR Add NaOH(aq) to a sample of compound B. If it is soluble / dissolves in NaOH(aq), B is Al2O3. If it is insoluble, B is SiO2 [1] for test, observation; Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4 [1] for equation

2 2022 H2 Chem Prelim P2 (Ans) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry Na2O MgO Al2O3 SiO2 P4O10 SO3 ………………………………………………………………………………………………….. [3] (b) (i) Complete the variation in the melting points of Period 3 oxides. [1] melting point / °C [1] for last 3 points Markers’ Comments • Several candidates wrote SiO4 instead of SiO2 for the formula of silicon dioxide. • It was disappointing to see that candidates still did not write proper observation statements (e.g. “react” instead of “dissolve”) given that this has been repeatedly emphasised during both tutorial and practical sessions. • Some candidates did not read the question carefully and went on to explain why the compound was insoluble in water, which is irrelevant. In addition, the question only asked for “an” equation, hence just one was sufficient – either the addition of HCl (aq) or NaOH (aq) would be sufficient to confirm the identity. There was no need for both. • It should be noted that SiO2 is an acidic oxide, but it can only react with hot and concentrated NaOH. Markers’ Comments • Of the Period 3 oxides, MgO has the highest melting point, not SiO2, which was a common error for this question.

3 (ii) Explain the variation in melting points from Na2O to Al2O3. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ………………………………………………………………………………………………... [2] (c) Sodium azide, NaN3, is commonly used in car airbags to produce nitrogen gas upon collisions. (i) Draw a ‘dot -and-cross’ diagram for the azide ion , N3 – , showing all of the outer shell electrons. OR [1] [1] (ii) State and explain, with reference to the Valence Shell Electron Pair Repulsion theory, the shape and bond angle around the central atom of N3 – . …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... ……………………………………………………………………………………………….......... ………………………………………………………………………………………………….. [2] There are 2 bond pairs and 0 lone pair around the central N atom. [1] allow ECF To minimise repulsion and maximise stability, the shape around the central N atom is linear. As there are equal bond pair-bond pair repulsions, the bond angle is 180°. [1] allow ECF • Ionic charge: Mg2+ > Na+ and Ionic radius: Mg2+ < Na+ Lattice energy (L.E.)  | q+∙q- r++r- |, L.E. magnitude / ionic bond strength: MgO > Na2O [1] Energy required to break ionic bond: MgO > Na2O, hence melting point: MgO > Na2O Markers’ Comments • For candidates who revised their Chemical Bonding and Periodic Table concepts thoroughly, they generally did well for this part. • However, a sizeable number of candidates were still unable to explain the variation in melting points accurately, reflecting some gaps in conceptual understanding. One common error was stating that Mg2+ has a higher charge density than Na+ and hence MgO has a higher magnitude of lattice energy. They then used the same charge density reasoning to explain why Al2O3 has a partial covalent character and hence has weaker ionic bonds as compared to MgO, failing to see that they are contradicting themselves. • It was concerning to see a sizeable number of candidates going on to address the melting point trends from SiO2 to SO3 when the question did not even ask for it. From several parts of Question 1, it is clear that these candidates need to work and improve on their question reading. Markers’ Comments • Common errors were not ensuring that each N atom had just 8 electrons around it. Many candidates had 10 or 12 electrons around the central N atom. N cannot expand octet structure due to lack of energetically accessible and vacant 3d orbitals. • The additional electron from the negative charge should not go to the central N atom as the central atom is generally the least electronegative and hence will be least likely to accept the additional electron. Even though they are all N atoms in this case, but we will still follow the rule. Markers’ Comments • It has been repeatedly emphasised that regardless of the bond order (single, double, or triple bond), it is regarded as ONE bond pair. However, there are candidates still making the same mistake till this point. • Given that there are zero lone pairs, it is unnecessary to mention about lone pair-lone pair and lone pair- bond pair repulsions. • Charge density ( q+ r+ ) : Al3+ > Mg2+, Al3+ polarises O2– electron cloud to a larger extent. Al2O3 has partial co