NYJC 2020 Prelim 9729 P3 Answers

H2 Chemistry 9729/03 NYJC J2/20 PX [Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME 1 9 CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 16 September 2020 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /23 2 /19 3 /18 4 or 5 /20 Total /80 This document consists of 32 printed pages and 0 blank page.

2 H2 Chemistry 9729/03 NYJC J2/20 PX For Examiner's Use Section A Answer all questions in this section. 1 Lithium metal and its compounds have many uses, ranging from nuclear che mistry, batteries, air purification and pharmaceuticals. (a) A button cell is a small single cell battery shaped like a button, with sizes typically 5 to 25 mm. Lithium -iodine button cells are widely used to power small portable electronics devices such as watches and calculators. (i) Using appropriate data from the Data Booklet, calculate Eʅ cell of a lithium - iodine button cell, and write the equations that take place at the cathode and anode. [3] From Data Booklet, Eʅ /V (1) Li+ + e ∏ Li – 3.04 (2) I2 + 2e ∏ 2I– +0.54 Eʅ cell = 0.54 – (– 3.04) = + 3.58 V [1] with (+) sign Cathode: ½ I2 + e → I– OR I2 + 2e → 2I– [1] with single arrow Anode: Li → Li+ + e [1] with single arrow (ii) State the polarities of the cathode and anode. [1] Cathode : positive Anode : negative [1] (iii) The actual voltage of a lithium-iodine button cell is +2.80 V. Suggest why this is different from the value you have obtained from (i). [1] In the part (i ) calculations, we have assumed that the concentrations of the ions are at 1 mol dm -3, and conditions are at 25 OC and 1 bar but the working conditions of the button cell are not at standard conditions. [1] (iv) After a current has been drawn for some time, the voltage of the button cell decreases.  Describe how the masses of the lithium and iodine will have changed.  Suggest why the voltage decreases. [3] Eʅ /V (1) Li+ + e ∏ Li – 3.04 (2) ½ I2 + e ∏ I– +0.54 Overall eqn: Li + ½ I2 → Li+ + I– Ecell = E(I2/I–)  E(Li+/Li) When current is drawn,  Lithium oxidises to Li+ by losing electrons. Mass of Li decreases.  Iodine reduces to iodide ions by gaining electrons. Mass of iodine decreases.

3 H2 Chemistry 9729/03 NYJC J2/20 PX [Turn Over For Examiner's Use  Since [I–] increases, POE of (2) shifts to the LEFT to remove the added I –, causing E(I2/I–) to be less positive, and hence Ecell will become less positive  [Li+] increases, POE of (1) shifts to the RIGHT to remove the added Li+, causing E(Li+/Li) to be less negative/more positive, and hence Ecell will become less positive 4 pt 3 mk; 3 pt 2 mk; 1 or 2 pts 1 mk (b) Lithium is also used to make rechargeable batteries like the lithium -ion battery. A type of lithium-ion battery consists of a cathode made from cobalt oxide, CoO 2, and anode made from graphite with lithium atoms inserted between the layers; and an electrolyte of LiBF4 dissolved in an organic solvent. During discharge, Li atoms at the a node give up electrons to become Li + ions. The electrons travel round the external circuit, and are picked up by the cathode. A Li + ion from the electrolyte also moves to the cathode, and get inserted between the CoO2 lattice. Cobalt is reduced from a +4 t o +3 oxidation state in the process. This is illustrated in the following diagram in which C –C–C–C–C is a simplified representation of a layer of carbon atoms in graphite. (i) For safety reasons, an organic solvent, rather than an aqueous solvent is used in this battery. Explain your answer with a suitable equation. [1] Lithium reacts explosively/vigorously/exothermically with water / gives off flammable hydrogen gas. Thus, a non-aqueous electrolyte is typically used. 2Li(s) + 2H2O (l) → 2LiOH (aq) + H2 (g) [1] for explanation and eqn (State symbols not required) (ii) Suggest the type of bonding between  the lithium atoms and the layers of carbon atoms in graphite

4 H2 Chemistry 9729/03 NYJC J2/20 PX For Examiner's Use  the lithium ions and the cobalt oxide lattice. Explain your answers. [2]  Metallic bonds are present in Li, with Li + ions in a sea of delocalised electrons. In graphite, there are delocalised electrons between each layer of C atoms. Hence metallic bonding is formed between the Li atoms and C atoms in graphite. OR Metallic bonding exists between Li + ions and delocalised electrons between the layers of C atoms  [1] for stating the type of bonding and explanation Ionic bonding is formed between Li + and the O – ions present in the CoO 2 lattice. [1] (Refer to diag ram of cathode during discharde. O– ions are formed in the process.) (iii) During the charging process of the battery, lithium ions from the cathode are inserted into the anode. When charging is carried out at temperatures below 0 C, the rate of reaction slows down, and lithium plating may occur instead. Lithium plating is the formation of metallic lithium around the anode and this results in a poorer battery performance. Calculate the mass of lithium that will be deposited when a lithium-ion battery is charged for 1 hour at a current of 1.2 A, under conditions that results in lithium plating. [2] Li+ + e– → Li Q = It = neF Li e Itnn F m Li g 1.2 60 60 96500 1.2 60 60( ) 6.9 0.3089 0.30996500          1 mk for calculation of n(Li); 1 mk for conversion to mass (allow ecf) (c) Latest research has found that replacing the graphite anode with graphene in lithium-ion batteries can help extend battery life. (i) Graphene is a single, one atom thick layer of graphite. Describe the lattice structure of graphene with the aid of a labelled diagram. Your answer should include  the type of hybridisation of the carbon atoms  the type of orbital overlap to form the bonds [3]

5 H2 Chemistry 9729/03 NYJC J2/20 PX [Turn Over For Examiner's Use  Graphene has a giant covalent structure with each C atom covalently bonded to three other carbon atoms (trigonal planar geometry) to form hexagonal rings.  Each carbon atom is sp2 hybridised, and forms 3  bonds and 1  bond.  The  bond is form ed through head-on overlap between sp 2 hybrid orbital from an adjacent C atom. The  bond is formed through sideways overlap of the unhybridised p orbitals from an adjacent C atom.  Diagram (should show at least 1 C that forms 3 sigma bonds): 4 pts 3 mks; 3 pts 2 mks; 1 or 2 pts 1 mk. (deduct 1 mk overall if student drew graphite instead) (ii) State and explain how you would expect the C–C bonds in graphene to differ in bond strength compared to that in diamond. [2]