2024 YIJC JC2 H1 CHEM PE P2 (suggested answers)

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUGGESTED ANSWERS CG H1 GROUP 1 / 2 / 3 CHEMISTRY Paper 2 Structured and Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 8873/02 10 September 2024 2 hours READ THESE INSTRUCTIONS FIRST This document consists of 16 printed pages and 1 blank page. For Examiner’s Use Section A 1 / 9 2 / 8 3 / 10 4 / 10 5 / 12 6 / 11 Section B 7 or 8 / 20 Penalty units significant figures Total / 80 Write your name and class in the spaces at the top of this page. 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. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question.

©YIJC [Turn over 2 Section A Answer all the questions in this section in the spaces provided. 1 (a) A, B and C are three consecutive period 3 elements in the periodic table. D is an isotope of one of the elements. Table 1.1 shows information on the number of protons, neutrons and electrons in species of A, B, C and D. Table 1.1 species protons neutrons electrons A 15 16 15 B2− 16 17 q C 17 18 17 D− 17 17 18 (i) Deduce the value of q. [1] 18 (ii) Identify the element, A, B or C, that is isotopic to D. Explain your answer. [2] C. They have the same number of protons but different number of neutrons.

©YIJC [Turn over 3 (b) Element E is a Period 3 element. The first nine successive ionisation energies, in kJmol−1, of element E are given in Table 1.2. Table 1.2 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 1251 2298 3822 5159 6542 9362 11018 33604 38600 (i) With reference to E, explain what is meant by first ionisation energy of E. [1] First ionisation energy of E is the energy required to remove one mole of the outermost electrons from one mole of gaseous E atom to form one mole of gaseous E+ cations. (ii) Use the information given in Table 1.2 to deduce the identity of element E. Explain your answer. [2] Element E is chlorine. There is a large increase between the 7th and 8th ionisation energy suggesting that there are 7 valence electrons in element E. (c) Sulfur is also an element found in the Period 3 of the Periodic Table. (i) Complete the diagram to show the arrangement of electrons in the orbitals of a sulfur atom. ↿⇂ 1s 2s 2p 3s 3p [1] ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿ ↿ 1s 2s 2p 3s 3p

©YIJC [Turn over 4 (ii) State and explain the general trend in first ionisation energy across Period 3 elements. Explain why sulfur does not follow this general trend. [2] The first ionisation energy generally increases across Period 3. Across Period 3, nuclear charge increases, shielding effect is relatively constant. Hence, effective nuclear charge increases (or the attraction between the nucleus and the valence electrons becomes stronger ), and more energy is required to remove the valence electrons from the atom. Sulfur does not follow this general trend as the outermost electron to be removed from sulfur in the 3p orbital is paired and there is inter-electronic repulsion. Hence, less energy is required to remove the valence electron from S. [Total: 9]

©YIJC [Turn over 5 2 (a) Magnesium exists as three principal isotopes, 24Mg, 25Mg and 26Mg. isotope relative isotopic mass natural abundance (%) 24Mg 23.99 78.99 25Mg 24.99 10.00 26Mg 25.98 11.01 Use the data to calculate the relative atomic mass of magnesium to two decimal places. [1] relative atomic mass = (78.99 × 23.99) + (10.00 × 24.99) + (11.01 × 25.98) 100 = 24.31 (b) Gastric juice is a thin, acidic digestive fluid secreted by glands in the mucous membrane that lines the stomach. An average adult produces about 1 mole of gastric juice when pain is experienced. 1 mole of gastric juice contains 0.002 mole of hydrochloric acid. An antacid is a substance which counteracts stomach acidity by raising the pH. One example is Brand F antacid which contains 20% magnesium carbonate, MgCO3. One tablet of antacid in Brand F has a mass of 325 mg. (i) Write a balanced equation for the reaction of Brand F antacid with hydrochloric acid. [1] MgCO3 + 2HCl → MgCl2 + CO2 + H2O (ii) Calculate the amount of magnesium carbonate in one tablet of Brand F antacid. [1] Amount of MgCO3 = (0.2 × 0.325)  84.3 = 7.71 × 10−4 mol (iii) Calculate the number of tablets a person has to consume to remove the gastric juice. [2] Amount of HCl present in 1 mole of gastric juice = 1 × 0.002 = 0.002 mol Amount of MgCO3 required = 0.002  2 = 0.001 mol Number of tablets needed = 0.001  (7.71 × 10−4) = 1.29 = 2

©YIJC [Turn over 6 (c) Compound G contains carbon, hydrogen and oxygen only. G contains 66.7% by mass of carbon, 11.1% by mass of hydrogen and 22.2% by mass of oxygen. The relative molecular mass, Mr, of G is 72. (i) Calculate the empirical formula and deduce the molecular formula of G. [2] C H O moles 66.7  12.0 = 5.558 11.1  1.0 = 11.1 22.2  16 = 1.387 ratio 4 8 1 empirical formula of G = C4H8O Let (C4H8O)n be the molecular formula of G. n × [(4 × 12.0) + (8 × 1.0) + (1 × 16.0) n = 1 molecular formula of G = C4H8O (ii) Compound G is a ketone. Draw the displayed formula of G. [1] CH H H C O C H H C H H H [Total: 8]

©YIJC [Turn over 7 3 (a) Phosphorus reacts with fluorine to form , phosphorus trifluoride, PF3 and phosphorus pentafluoride, PF5. (i) Draw the dot -and-cross diagram for one molecule of PF 3. Show only the outer shell electrons. [1] (ii) State the bond angle in a molecule of PF3 and name the shape of the molecule. Explain your answers using Valence Shell Electron Pair Repulsion Theory. [3] Shape: trigonal pyramidal Bond angle: 107o Explanation: The 4 electrons pairs around the P atom orientate themselves as far apart to minimise repulsion thus it has a basic shape of a tetrahedral. Of the 4 electron pairs, 3 are bond pairs and 1 is a lone pair. As the lone pair -bond pair repulsion is greater than the bond pair -bond pair repulsion , it adopts a trigonal pyramidal shape with a bond angle of 107o. (iii) Deduce whether PF3 is a pol