NJC H1 CHEM P1 answers

1 NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 1 Multiple Choice Additional Materials: Multiple Choice Answer Sheet Data Booklet 8872/01 Thursday 17 Sept 2015 50 minutes READ THESE INSTRUCTIONS FIRST Write in soft pencil. Write your name, subject class and registration number on the Answer Sheet in the spaces provided and shade the appropriate boxes for your registration number. There are thirty questions in this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the Answer Sheet. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. The use of an approved scientific calculator is expected, where appropriate. This paper consists of 10 printed pages including this cover page.

2 Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1. Use of the Data Booklet is relevant to this question. Element X forms ion X2+ with 4 unpaired electrons. What could be element X? You’ll need to do trial and error for this question by writing out the valence electronic configuration of X2+ for each option below. A Mg2+: 2s2 2p6  no unpaired electrons B Cr2+: 3d4  4 unpaired d electrons C Ti2+: 4s2  no unpaired electrons D Ni2+:3d7  3 unpaired d electrons Answer: B 2. A solution of Sn 2+ ions can reduce an acidified solution of MnO 4 – ions. The Sn 2+ ions are oxidised to Sn4+ ions in this reaction. How many moles of MnO4 – ions are required to react completely with a solution containing 9.5 g of SnCl2 (Mr = 190)? Sn2+  Sn4+ + 2e x 5 MnO4 – + 8H+ + 5e  Mn2+ + 4H2O x 2 No. of moles of SnCl2 = 190 5.9 = 0.05 mol No. of moles of KMnO4 = 0.05 x 5 2 = 0.02 mol Answer: B 3. Use of the Data Booklet is relevant to this question. Magnesium nitrate, Mg(NO3)2, will decompose when heated to give a white solid, MgO, and a mixture of gases. One of the gas is an oxide of nitrogen. 7.4 g of anhydrous magnesium nitrate is heated until no further reaction takes place. What is the mass of the white solid formed? No. of moles of Mg(NO3)2 = 3.148 4.7 = 0.0499 mol Since 1 mole of Mg(NO3)2 will decompose to give 1 mole of MgO, No. of moles of MgO = 0.0499 mol Mass of MgO = 0.0499 x 40.3 = 2.01 g Answer: B

3 4. AlCl3 vapour forms molecules with formula Al2Cl6 as it is cooled. What happens to the bond angles during the change from AlCl3 to Al2Cl6? In AlCl3, there are 3 bond pairs and 0 lone pairs around Al, the shape is trigonal planar about Al and bond angle is 120⁰. In Al2Cl6, there are 4 bond pairs and 0 lone pairs around Al, the shape is tetrahedral about Al and bond angle is 109.5⁰. All the bond angles about Al decreases from 120⁰ to 109.5⁰. Answer: C 5. The table below shows the physical properties of four substances. Which substance has a giant covalent structure? Melting point/ ⁰C Boiling point/ ⁰C Electrical conductivity of solid Electrical conductivity of liquid Electrical conductivity of aqueous solution A 119 239 Good Good Insoluble B 800 1210 Poor Good Good C 993 1695 Poor Poor Good D 1610 2230 Poor Poor Insoluble We need to consider all the properties of substance with giant covalent structure. Melting and boiling point: High (Eliminate option A) Electrical conductivity in solid: Can be good (if it’s graphite) or Poor (if it’s diamond, Si, SiO2) Electrical Conductivity in liquid: Can be good (if it’s graphite) or Poor (if it’s diamond, Si, SiO2). Electrical conductivity in aqueous solution: Both diamond and graphite are not soluble in aqueous solution. (Eliminate option B and C) Answer: D

4 6. What is the order of increasing melting point for the four chlorides shown? Lowest melting point Highest melting point A SiCl4 PCl5 MgCl2 NaCl B PCl5 SiCl4 NaCl MgCl2 C SiCl4 PCl5 NaCl MgCl2 D NaCl MgCl2 SiCl4 PCl5 Both SiCl4 and PCl5 are simple covalent molecules and since both molecules are non- polar, the intermolecular forces between the molecules are temporary dipole-induced dipole interactions (TDID). The larger the Mr, the larger the electron cloud size which is more easily distorted and the TDID between PCl5 is stronger than that in SiCl4 and requires more energy to overcome. Both NaCl and MgCl2 are ionic compounds. The strength of ionic bond is determined by the magnitude of lattice energy. LE    rr qq . Since Mg2+ has larger charge than Na+, and ionic radius of Mg2+ is smaller than Na2+ and both compounds have the same anion, the LE of MgCl2 is larger than NaCl and the ionic bond in MgCl2 is stronger than NaCl which requires more energy to overcome. The melting point increases from SiCl4 to PCl5 to NaCl to MgCl2. Answer: C 7. Which bond angle is not found in the following molecule? H O C C C O O H O H I I I I H H N H H 105o 105o 105o 107o 109.5o 120o 120o 120o 120o When considering the bond angles, you should not be fooled by the diagram drawn. You should consider all the bond pairs and lone pairs around the central atom to determine the shape. There is no linear shape about any central atom therefore 180⁰ is not possible. Answer: C

5 8. Which process is endothermic? A Condensation of steam Energy is given out when steam changes to water and the process is exothermic. B Dissolving glucose in water Dissolution can be endothermic or exothermic. In this case, the dissolution is endothermic and you may obtain this answer by eliminating other options. C Neutralization between strong acid and weak base All neutralization reactions are exothermic. D Burning of charcoal All combustions are exothermic. Answer: B 9. Enthalpy changes that are difficult to measure directly can often be determined using Hess’ Law to construct an enthalpy cycle. Which enthalpy change is indicated by X in the enthalpy cycle shown? C(s) + 2H2(g) + 2O2(g) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) Hc Hf Hf X X represents either 2 x enthalpy change of combustion of hydrogen (eliminate options A and B). or X represents 2 x enthalpy change of formation of water. The + or – sign is indicating the direction of arrow. Since the direction of arrow is in the direction of the definition, the sign of the arrow is positive. (i.e. Enthalpy change of formation of water is the energy change when one mole of water is formed FROM its elements in their standard states.) Answer: D 10. Which statement concerning the equilibrium reaction given below is correct? 2CrO4 2–(aq) + 2H+(aq) Cr2O7 2–(aq) + H2O(l) A A redox reaction is taking place. Incorrect because the oxidation state of Cr remains as +6 on both sides. You do not need to calculate the oxidation states of O and H because the oxidation of O is always -2 in all covalent and ionic compound except for ele ments O 2 and hydrogen peroxide H2O2. The oxidation state of H in all covalent compounds is +1.

6 B The equilibrium constant, Kc, has no units. Incorrect because for this equilibrium, 222 4 2 72 ]H[]CrO[ ]OCr[Kc    and the unit is mol–3 dm9 C A decrease in pH will result in an increase in the concentration of Cr2O7 2–. Correct because when pH decreases, [H+] increases and position of equilibrium shifts right to partially decrease [H+]. Therefore, more Cr2O7 2– will be formed. D The addition of a catalyst will result in an increase in the concentration of Cr2O7 2–. Incorrect because catalyst increases the rate of forward and backward reaction to the same extent. Therefore, the overall concentration of reactants a