2023 NJC H2 Chem Prelim P1 (Ans)

1 NJC SH2 Preliminary Examination 9729/01/23 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 1 Multiple Choice Additional Materials: Optical Answer Sheet Data Booklet 9729/01 14 September 2023 1 hour READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. Write your name, subject class and registration number on the Answer Sheet in the spaces provided unless this has been done for you. There are thirty questions on 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 separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. 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. A Data Booklet is provided. Instructions on how to fill in the Optical Mark Sheet Shade the index number in a 5 digit format on the optical mark sheet: 2nd digit and the last 4 digits of the Registration Number. Example: Student Examples of Registration No. Shade: 2205648 25648 This document consists of 8 printed pages.

2 NJC SH2 Preliminary Examination 9729/01/23 1 Use of the Data Booklet is relevant to this question. Which particle has more protons than electrons and more protons than neutrons? (D = H1 2 ) A D3O+ B H3O+ C NH2− D OD− Ans: (B) Particle No of Protons No of Neutrons No of Electrons D3O+ 11 11 10 H3O+ 11 8 10 NH2− 9 7 10 OD− 9 9 10 *Only cations have more protons than electrons *D atom has 1 proton and 1 neutron while H atom has 1 proton and 0 neutron. 2 Cl2 reacts with hot dilute NaOH to give Cl− and ClOx− in a 5:1 ratio. What is the value of x in ClOx−? A 1 B 2 C 3 D 5 Ans: (C) [R] ½ Cl2 + e− ⎯→ Cl− --------- x5 [O] ½ Cl2 ⎯→ ClOx− + ne− When 5 moles of Cl− is produced, 5 moles of electrons are gained. Hence, when 1 mole of ClOx− is produced, 5 moles of electrons are lost. Oxidation state of Cl increased by 5 units, from 0 (in Cl2) to +5 (in ClOx−) Hence, x = 3

3 NJC SH2 Preliminary Examination 9729/01/23 [Turn over 3 The boiling point of water (100 C) is greater than that of HF (20 C). Which statement is a correct explanation of this? A Each hydrogen bond formed between water molecules is stronger than that formed between HF molecules. B There are more atoms in a water molecule than there are in an HF molecule, resulting in stronger intermolecular forces in water. C There are on average, more hydrogen bonds between water molecules than there are between HF molecules. D The water molecule has stronger permanent dipole-permanent dipole interactions than the HF molecule. Ans: (C) Both HF and H 2O are simple covalent molecules and their boiling points are affected by the intermolecular forces between its molecules. Both HF and H 2O are able to form hydrogen bonds between its molecules. H -F bond is more polar than H-O bond, hence each hydrogen bond between HF is stronger than that between H2O (option A is incorrect). HF and H2O can form different number of H-bonds per molecule. The greater average number of H-bonds formed per molecule • the more extensive the H-bonds present in the compound • the higher the energy required to break the larger number of H-bonds • the higher the boiling point HF H2O No. of protonic H 1 2 No. of lone pair electrons 3 2 Average no. of H-bonds per molecule 1 2 4 When the carbon-oxygen bonds in methanol, CH3OH, methanal, HCHO, and methanoate, HCO2– are arranged in order of increasing length, what is the correct order? A CH3OH, HCO2–, HCHO B HCO2–,CH3OH, HCHO C HCHO, HCO2–,CH3OH D HCO2–, HCHO, CH3OH Ans: (C) HCHO, = double bond HCO2– = partial double bond CH3OH = single bond Single bond longer than double bond Double bond longer than partial double bond

4 NJC SH2 Preliminary Examination 9729/01/23 5 Histamine is produced in the body to help fight infection. Its shape allows it to fit into receptors which expand blood vessels. What are the bond angles x, y and z in histamine, from the smallest to the largest? smallest bond angle largest bond angle A x y z B y x z C y z x D z y x Ans: (A) x: 3 bond pair regions, 1 lone pair region (trigonal pyramidal), 107° y: 4 bond pair regions (tetrahedral), 109.5° z: 3 bond pair regions (trigonal planar), 120° 6 When a sample of a gas is compressed at constant temperature from 1500 kPa to 6000 kPa, its volume changes from 76.0 cm3 to 20.5 cm3. Which statements are possible explanations for this behaviour? 1 The gas behaves non-ideally. 2 The gas partially liquefies. 3 The gas molecules have undergone dimerisation. A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 1 only Ans: (D) Under a fourfold increase in pressure, the volume of an ideal gas will decrease from 76.0 cm3 to 19.0 cm3 Option 1 is correct. The change in volume is not inversely proportional to the increase in pressure (pV = nRT), hence the gas behaves non-ideally. Option 2 is incorrect. If th e gas part ially liquefies, the amount of gas molecules would decrease and thus the volume would be much smaller than 19.0 cm3. Option 3 is incorrect. If the gas molecules have undergone dimerization, the amount of gas molecules would decrease and thus the volume would be much smaller than 19.0 cm3.

5 NJC SH2 Preliminary Examination 9729/01/23 [Turn over 7 The table below describes some of the chemistry and thermodynamic properties of the halogens. process name and symbol of quantity 2HX(g) ⎯→ H2(g) + X2(g) H2(g) + X2(g) 2HX(g) X(g) ⎯→ X+(g) + e- enthalpy change of reaction, ∆Hꝋ equilibrium constant, Kp ionisation energy, I.E. Which statements are correct? 1 |∆Hꝋ| for HCl > |∆Hꝋ| for HBr 2 Kp for HBr > Kp for HI 3 I.E. for I > I.E. for Cl A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 1 only Ans: (B) Option 1 is correct: H-Cl bond energy is 431 versus H-Br bond energy is 366. Option 2 is correct: H-I bond energy is 299 versus H-Br bond energy is 366, therefore more HBr will be formed as more energy is released. Since Kp = (P HX)/(PH2)(PX2), a greater proportion of HX will result in a larger Kp value. Option 3 is incorrect. First IE of I is 1010 kJ mol-1 and first IE of Cl is 1260 kJ mol-1 8 In a calorimetric experiment 1.60 g of a fuel are burnt. 45.0 % of the energy released is absorbed by 200 g of water. The temperature of the water rises from 18 C to 66 C. What is the total energy released per gram of fuel burnt (to 3 significant figures)? A 25 100 J B 55 700 J C 89 200 J D 373 000 J Ans: (B) Energy required = mc∆T  100 45 = 200  4.18  (66 – 18)  100 45 = 89 173J Total energy released per gram of fuel = 89 173  1.6 = 55 733 J = 55 700 J (3 sf)

6 NJC SH2 Preliminary Examination 9729/01/23 9 Consider this gas phase reaction. Cl2(g) + CHCl3 (g) ⎯→ HCl (g) + CCl4 (g) The rate equation for the above reaction is given below. rate = k [CHCl3] [Cl2]1/2 Based on this information, what conclusions can be drawn about this proposed mechanism? Step 1 Cl2 (g) 2Cl (g) Step 2 Cl (g) + CHCl3 (g) ⎯→ H