TJC Prelim P3 Ans

1 2017 TJC H2 Chemistry Preliminary Exam [Turn Over CHEMISTRY 9729/03 Paper 3 Free Response 13th September 2017 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Graph Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a 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. 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. This document consists of 14 printed pages and 2 blank pages.

2 2017 TJC H2 Chemistry Preliminary Exam [Turn Over Section A Answer all the questions in this section. 1 Calcium carbonate is found in rocks and is the main component of pearls and the shells of eggs, snails and many marine organisms. It is the active ingredient in agricultural lime and is also medicinally used as a calcium supplement or as an antacid. (a) (i) Describe and explain the trend in thermal stability of the Group 2 carbonates. [3]  Down the group, charge of the cation remains constant but ionic radius increases. Hence charge density of the cation decreases.  (CO32- ion is a large anion so is easily polarised but) Polarising power of cation decreases resulting in less distortion of the electron cloud of CO32- ion, weakening the C-O covalent bond less.  More energy needed to decompose the carbonate, hence down the Group, carbonates become more stable to heat, therefore decomposition temperature increases and thermal stability increases down the group. (ii) Determine the minimum temperature for the spontaneous decompo sition of calcium carbonate, given that ΔH = +178.5 kJ mol-1 and ΔS = +163.2 J K-1 mol-1. [2] ΔG = ΔH – TΔS  ΔH – TΔS < 0 T > ΔH / ΔS T > 178.5 x 1000 / 163.2  T > 1090 K (3 sig fig) (b) Limestone is predominantly calcium carbonate, a slightly soluble salt with a K sp value of 3.3 x 10 -9. This rocky material began accumulating in the Earth over 400 million years ago. The Howe Caverns in the USA is a relatively young limestone cave which began forming 800 000 years ago. (i) Determine the solubility of calcium carbonate. [1] Let solubility of CaCO3 be x. CaCO3 Ca2+ + CO32- Ksp = x2  x = 5.74 x 10-5 mol dm-3 The principal cave-forming process is explained below. Gaseous CO2 is in equilibrium with aqueous CO2 in surface water. CO2(g) CO2(aq) ----- (1) As surface water trickles through cracks in the ground, it meets soil -trapped air which contains higher levels of CO2 (𝑃𝐶𝑂2 = 100 Pa) and hence [CO2(aq)] increases. When this CO2-rich water contacts limestone, more CaCO 3 dissolves. As a result, more rock is carved out, more water flows in and over centuries a cave is formed. CaCO3(s) + CO2(aq) + H2O(l) Ca2+(aq) + 2HCO3-(aq) ----- (2)

3 2017 TJC H2 Chemistry Preliminary Exam [Turn Over (ii) Show that the atmospheric partial pressure of CO 2, 𝑃𝐶𝑂2, is 40.5 Pa. You may assume air contains 0.04% by volume CO2. [1] volume of gas α no of mole of gas (Avogadro’s Law)  𝑷𝑪𝑶𝟐 = 0.04/100 x 101325 = 40.5 Pa (iii) State and explain whether CO 2 in the atmosphere or soil-trapped air is behaving less ideally. [2]  CO2 in soil-trapped air will behave less ideally.  As soil -trapped air will be under higher pressure the total volume occupied by the gas is reduced, the volume of the gas molecules becomes more significant compared with the total volume of the gas. Stalactites and stalagmites are rock formations that hang from ceiling of caves and rise from the floor of caves, respectively. They are formed when surface water seeps through rock and drips from the ceiling of a cave. (iv) With reference to equilibria (1) and (2), explain the formation of stalactites and stalagmites in limestone caves. [3]  When the surface water drips from the ceiling, CO2(g) escape as it comes into contact with the atmosphere, which has a lower 𝑷𝑪𝑶𝟐.  By Le Chatelier’s principle, equilibrium (1) shift to the left causing [CO2(aq)] to decrease. Equilibrium (2) will be be shifted to the left to counter the decrease in [CO2(aq)]  CaCO3 will then be precipitated out on the ceiling or on the floor resulting in the formation of stalactites and stalagmites. (v) Suggest a reason why some of these rock formations can appear reddish brown or bluish green. [1]  It is likely due to the presence of transition metal ions like Fe3+ and Cu2+. Lakes bounded by limestone -rich soil are less likely to be impacted by acid rain. Limestone dissolves sufficiently in lake water to form a buffer system capable of mitigating the effect of acid rain. (vi) Suggest, with the aid of an equation, how the buffer system will maintain the pH of the lake water. [1]  CO32-(aq) + H+(aq)  HCO3-(aq) As the carbonate ions neutralise the H + from acid rain, there is negligible change in pH. Acidification due to acid rain of lakes and rivers poses a serious environmental problem. The concentration of carbonate ions in these water is small due to the high acidity, hence affecting the survivability of some marine organisms.

4 2017 TJC H2 Chemistry Preliminary Exam [Turn Over (vii) Suggest a reason why the survivability of some marine organisms is affected. [1]  Carbonate ions, together with calcium ions, are the building blocks of their shells and skeletons. (c) Emission from power stations using coal, which contains sulfur , as fuel has significant environmental consequences. One of the components is sulfur dioxide, which can be oxidised by atmospheric hydroxyl radical to sulfur trioxide. (i) Write a balanced equation to explain how sulfur trioxide contribute to acid rain. [1]  SO3(g) + H2O(l)  H2SO4(aq) Current power plants are fitted with flue-gas desulfurization devices. These devices heat powdered limestone in air to remove sulfur dioxide from the gases produced during coal combustion. One of the products is calcium sulfate. (ii) Write a balanced equation for the reaction that had taken place. [1]  CaCO3(s) + ½O2(g) + SO2(g)  CaSO4(s) + CO2(g) (d) Sulfur and aluminium can both react with chlorine. Aluminium chloride is a white solid that sublimes while sulfur tetrachloride, SCl 4, is an unstable pale yellow solid. Both react with