2022 TMJC H2 Chem Prelim P2 (QP)

2022 H2 Chem Prelim P2 (QP) Answer all questions in the space provided. 1 (a) The elements of Period 3 form different oxides when reacted with oxygen. (i) State and explain the variation in bonding within the oxides across Period 3. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………..... [2] Compounds A and B are oxides of Period 3 elements which exist in the solid state at room temperature. When excess water is added to a sample of A, the solid dissolve s completely and an acidic solution is obtained. However, when excess water is added to B, the solid remains insoluble and a neutral solution is obtained. (ii) Identify compound A. ………………………………………………………………………………………………….. [1] (iii) Suggest two possible identities of compound B and d escribe how the identity of compound B can be confirmed using the acid-base behaviour of Period 3 oxides. Include all relevant observations and an equation in your answer. …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... …………………………………………………………………………………………………...... ………………………………………………………………………………………………….. [3]

2 2022 H2 Chem Prelim P2 (QP) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry × Na2O MgO Al2O3 SiO2 P4O10 SO3 (b) (i) Complete the variation in the melting points of Period 3 oxides. [1] (ii) Explain the variation in melting points from Na2O to Al2O3. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………..... [2] melting point / °C × ×

3 [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (c) Sodium azide, NaN3, is commonly used in car airbags to produce nitrogen gas upon collisions. (i) Draw a ‘dot -and-cross’ diagram for the azide ion , N3 – , showing all of the outer shell electrons. [1] (ii) State and explain, with reference to the Valence Shell Electron Pair Repulsion theory, the shape and bond angle around the central atom of N3 – . …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... ……………………………………………………………………………………………….......... ………………………………………………………………………………………………….. [2]

4 2022 H2 Chem Prelim P2 (QP) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry When a car is involved in a collision, sensors send an electric signal to the container inside the airbag where the sodium azide is stored. This triggers the decomposition of sodium azide and inflates the airbag with nitrogen gas to nearly its full volume in a very short period to protect the driver, after which the airbag slowly deflates. The typical volume of a fully inflated airbag is 60 dm3. The decomposition of sodium azide is shown in reaction 1. reaction 1 2NaN3(s) → 2Na(s) + 3N2(g) As the sodium metal by-product of reaction 1 is highly reactive, airbag manufacturers mix sodium azide with potassium nitrate to remove the sodium by-product as shown in reaction 2. reaction 2 2KNO3(s) + 10Na(s) → 5Na2O(s) + K2O(s) + N2(g) This also produces additional nitrogen gas which contributes to the airbag. (iii) Given that an airbag contains 100 g of sodium azide , calculate the volume of nitrogen gas that can be produced at room temperature and pressure. [3] [Total: 15]

5 [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry 2 Wastewater comprises of various organic matter and is treated to remove all harmful contaminants before being discharged into water bodies. (a) Tonalide is an organic compound commonly found in sewage sludge produced by wastewater treatment plants. In an experiment, a sample of tonalide was combusted underneath a container containing 170 g of water. It was found that the temperature of the water increased b y 55 °C after 0.050 mol of tonalide had been combusted. (i) Calculate the enthalpy change of combustion of tonalide , Hc, in kJ mol –1. Assume the container absorbed negligible heat from the combustion. [2] (ii) The literature value for enthalpy change of combustion of tonalide is −865 kJ mol −1. Compare this value to the one you have calculated in (a)(i) and suggest a reason for the discrepancy. ............................................................................................................................................ ....................................................................................................................................... [1]

6 2022 H2 Chem Prelim P2 (QP) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (b) Tonalide has molecular formula CxHyO. 0.3 dm3 of gaseous tonalide was combusted in excess oxygen in a n enclosed vessel at 400 C. The gas mixture obtained at the end of combustion was then treated by: • passing it through a dehydrating agent , resulting in a volume contraction of 3.9 dm3; followed by • bubbling the re maining gas mixture through aqueous sodium hydroxide , with a further contraction of 5.4 dm3. Determine the values of x and y in the molecular formula of tonalide. Show your working clearly. [2]

7 [Turn over Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (c) Ammonia is also another contaminant commonly found in agricultural wastewater. It is harmful to the aquatic ecosystems if present in significant quantity. When ammonia is completely combusted, it produces nitrogen gas and water only. (i) Define, with aid of an equation, the standard enthalpy change of combustion of ammonia, NH3(g). …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... ………………………………………………………………………………………………….. [2] (ii) Using appropriate data from the Data Booklet , calculate the enthalpy change of combustion of ammonia. [2] (iii) With the advancement in combustion technology, ammonia has been studied as a potential source for clean energy. Presently however, oxides of nitrogen are often produced as by-products of the reaction. Suggest one advantage and one disadvantage of using ammonia as a fuel. Advantage ………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... Disadvantage ……………………………………………………………………………………. …………………………………………………………………………………………………….. ……………………………………………………………………………………………………... [2]

8 2022 H2 Chem Prelim P2 (QP) Tampines Meridian Junior College 2022 JC2 Preliminary Examination H2 Chemistry (iv) The process of nitrification is often used to convert ammonia into a less toxic nitrate. NH3(aq) + 2O2(g) → NO3−(aq) + H+(aq) + H2O(l) ∆Hr Using the information below, construct a suitable energy cycle to determine the enthalpy change of nitrification reaction, ∆Hr. NH3(aq) + O2(g) + H2(g) → NH2OH(aq) + H2O(l) ∆H1 = –284 kJ mol–1 NH2OH(aq) + H2O(l) + 1 2O2(g) → NO3−(aq) + 2H2(g) + H+(aq) ∆H2 = +1653 kJ mol–1 2H2O(l) → 2H2(g) + O2(g) ∆H3 = +572 kJ mol–1