2019 SAJC H2 Chem Prelim P2 ANS

Prelims Paper 2 Answers 1. (a) P, Q, R and S are consecutive elements in Period 4. The table below shows the first four ionisation energies (in kJ mol–1) of the elements. Element 1st I.E. 2nd I.E. 3rd I.E. 4th I.E. 5th I.E. P 908 1730 3828 5980 7970 Q 577 1980 2960 6190 8284 R 762 1540 3300 4390 9020 S 947 1798 2735 4837 6043 (i) Explain why the second ionisation energy of Q is higher than that of R. Q is in group 13, there is a great jump in IE from 3rd IE to 4th IE Q: 4s24p1 Q+: 4s2 R: 4s24p2 R+ : 4s24p1 Q: the second electron is removed from the 4s orbital, whereas for R, the second electron is removed from the 4p orbital. The 4p orbital is further away from the nucleus than the 4s and experiences additional shielding effect by the two 4s electrons. These factors outweigh the effect of increase in nuclear charge from Q to R, resulting in a weaker attraction by nucleus . Less energy is required to remove an electron from 4p than the 4s orbital. [2] (ii) Explain why P is not considered to be a transition metal like most of the d -block metals. They do not form stable ions with a partially filled d subshell. [1] [1] (b) The idea of covalent bonding was first described in 1916 by an American physical chemist Gilbert Newton Lewis. HCOCl is an example of a polar covalent molecule. (i) Explain what is covalent bonding. Covalent bonding is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. [1] (ii) State what is meant by the term polar when applied to a covalent bond. [1]

A polar covalent bond is one in which the electron density is unequally shared due to the difference in electronegativity of atoms bonded, resulting in ∂+ and ∂ - across bond. (iii) State and explain with reference to the Valence Shell Electron Pair Repulsion theory, the shape of HCOCl molecule. 3 bond pair no lone pair so trigonal planar . The 3 electrons bond pairs arrange themselves to maximise stability and minimise electronic repulsion. [2] (iv) The molecule of HCOCl contains both σ (sigma) and π (pi) bonds. Draw labelled diagrams to show how orbitals overlap to form  a σ (sigma) bond  a π (pi) bond σ (sigma) bond is formed from head-on overlap of s orbital of H and sp2 of C or sp2 of O and sp2 of C or π (pi) bond is formed from side-on overlap of unhybridised p orbitals. [2] [Total:9] Unhybridised p orbitals of C Unhybridised p orbitals of O

2 Below is a labelled diagram of a hydrogen-oxygen fuel cell in an alkaline electrolyte. (i) Write the half equations for both electrodes and hence the overall equation. Cathode: O2(g) + 2H2O(l) + 4e- → 4OH-(aq) Anode: H2(g) + 2OH-(aq) → 2H2O(l) + 2e- Overall equation : 2H2(g) + O2(g) → 2 H2O(l) [2] (ii) Calculate ∆Go for the above reaction and state whether it is spontaneous. Eocell = Eo cathode- Eo anode = 0.40-(-0.83) = +1.23 V ∆Go = ─nFEocell = ─ 4x96500x1.23 = ─ 474780 Jmol-1 = ─475 kJmol-1 It is spontaneous. [3] (iii) The hydrogen-oxygen fuel cell produces 1.5 A of current. Hydrogen gas is contained in a 1 dm 3 tank at a pressure and temperature of 200 atm and 20 oC respectively. Determine the number of days the fuel cell can operate before the hydrogen gas runs out. Give your answer to the nearest whole number. You may assum e there is an unlimited supply of O2. PV = nRT 200 x 1.01325 x 105 x 1 x 10-3 = n x 8.31 x 293 n (amt of H2 gas) = 8.322 mol no. moles of electrons given out = 2n = 16.64 Q = 16.64 x 96500 = 1605760C Q = It [3]

t = 1605760/1.5 = 1070506 s =1070506/86400 = 12.39 days 12 days [Total:8] 3. Spider silk is a protein fiber. Major amino acids in the silk proteins are alanine and glycine. Serine and glutamine are also present in significant quantities in some types of silk. The table below shows the typical amino acids present in spider silk. Amino Acid Formula of side chain (R in RCH(NH2)CO2H) Glycine –H Alanine –CH3 Serine –CH2OH Glutamine –CH2CH2CONH2 (a) (i) Glycine exists as crystalline solids. Draw the zwitterionic structure of glycine and account for the high melting point of glycine. [2] Crystalline solid of glycine has giant ionic lattice structure with strong electrostatic forces of attraction exists between the zwitterions. A lot of energy is required to overcome these strong ionic bonds between zwitterions. (ii) Write an equation to show the reaction between alanine and aqueous sodium carbonate. [1] (iii) Suggest a chemical test to distinguish between serine and glutamine. [2]

Test: K2Cr2O7(aq),H2SO4(aq), heat Observation: For serine, orange dichromate turns green For glutamine, orange solution remains OR Test: NaOH(aq), heat Observation: For serine, no pungent gas observed and moist litmus paper remained red. For glutamine, pungent gas evolved turns moist red litmus blue (b) Natural spider silk has excellent mechanical properties. In a recent study, researchers discovered that graphene-based materials can be used to boost the properties of spider's silk up to three times the strength and ten times the toughness of the unmodified silks. Graphene is the thinnest material known to exist, yet it is stronger than steel. It is made from only carbon and is a single layer of graphite just one atom thick as shown in Figure 3.1 below. Figure 3.1 (i) With reference to structure and bonding, explain why graphene is strong. [1] Graphene is an allotrope of C and just like graphite it has giant covalent structure. Each layer of graphene is made of hexagonal “ring” of carbon atoms strongly covalently bonded. A lot of energy is required to break these strong covalent C-C bonds.

(ii) Since its discovery in 2004, graphene has been widely studied and manufactured as replacement materials for many touchscreen products. Suggest why graphene is a suitable touchscreen material in mobile phones. [1] It is two -dimensional which makes it thin/light/transparent, suitable as touchscreen material. Or it has high electrical conductivity due to presence of delocalized electrons that serve as mobile charge carriers. (c) Spiders are excellent treats for colonies of marauding ants. To protect their homes, spiders coat their webs using a chemical called 2 -pyrrolidone, which acts as a deterrent to many insects. 2-Pyrrolidone is a 5-membered cyclic amide (commonly known as lactam). It is a colourless liquid that is miscible with water and most common organic solvents. 2-Pyrrolidone 2-pyrrolidone can be synthesised using 3-chloropropan-1-ol as the starting organic compound as shown in the reaction scheme below:

(i) State the type of reaction in Step V. [1] (Intramolecular) condensation (intramolecular) nucleophilic (acyl) substitution) (ii) Suggest reagents and conditions for Step I, II and III. Step I : ………………………………………………………………… Step II : ………………………………………………………………… Step III : ………………………………………………………………… [3] Step I: KMnO4(aq), H2SO4(aq), heat OR K2Cr2O7, H2SO4(aq), heat Step II: ethanolic KCN, heat Step III: H2, Ni, heat (iii) Draw the structures of intermediate compounds A and B. [2] A B (iv) With the aid of an equation, explain why anhydrous condition is necessary for the reaction in Step IV. [1] PCl5 will hydrolyse readily in water as follows: PCl5 + 4H2O  H3PO4 + 5HCl (v) 2-pyrrolidirol can be sy