BLSS Prelim P2 ans

2 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 Section A [70 marks] Answer all questions. 1 The formulae of some elements are given in the following list. Al Cu Fe H 2 Na O 2 Zn (a) Choose elements from the list to answer the questions. (i) Which element is used as a catalyst in the Haber process? Fe [1] (ii) Choose one element which shows both metallic and non-metallic character. H2 [1] (iii) Which element can be extracted from its ore by heating with carbon and is purified by electrolysis? Cu [1] (iv) Which element is produced at the graphite anode during electrolysis of aqueous sodium sulfate? O2 [1] (v) Which element is used for galvanizing? Zn [1] (b) (i) Which elements in the list are transition elements? Cu, Fe [1] (ii) Give three characteristic properties of transition elements. Form colored compounds. Have variable oxidation states in compounds. Are used as catalysts. [2] [Total: 8] 4E Chem Prelim 2024 P2 Answers

3 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 2 Read the information about the electrolysis of potassium chloride. Potassium chloride has a melting point of over 700 oC. Molten potassium chloride can be electrolysed. Chlorine gas forms at the anode. We might expect potassium metal to form at the cathode, but in practice, potassium is very soluble in molten potassium chloride. A solution of potassium in molten potassium chloride forms. This problem means that electrolysis cannot be used to extract potassium from potassium chloride. (a) The information contains an example of a compound and an example of a mixture. (i) Identify the compound and mixture in the information. compound potassium chloride mixture solution of potassium in molten potassium chloride [1] (ii) Explain the difference between a compound and a mixture. A compound is a PURE substance containing two or more ELEMENTS that are chemically combined in a FIXED RATIO while a mixture is made up of two or more SUBSTANCES that are NOT chemically combined and not in any fixed ratio. The ELEMENTS in a compound are combined in a FIXED RATIO while The COMPONENTS of a mixture can be mixed in any proportion. The physical and chemical properties of a compound are different from the elements it is made of while the chemical properties of a mixture are the same as those of its components. [any one] [1] (b) The electronic configuration of a potassium atom is 2,8,8,1. Complete Table 2.1 to show the electronic configuration of a potassium ion and a chloride ion. Table 2.1 Ion electronic configuration K+ 2,8,8 Cl – 2,8,8 [1] (c) (i) Write the ionic half-equation for the reaction that happens at the anode. 2Cl – (l)  Cl2 (g) + 2e [1] (ii) Describe a simple test that would confirm the identity of the element at the anode. Include the result of the test in your answer. Test: Insert a moist blue litmus paper into the gas. Result: The moist blue litmus paper turns red, then bleached. [1]

4 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 (d) Dilute aqueous potassium chloride can also be electrolysed. Describe two differences between the products of the electrolysis of dilute aqueous potassium chloride and the products of the electrolysis of molten potassium chloride. 1 Electrolysis of dilute aqueous potassium chloride produces hydrogen at the cathode while electrolysis of molten potassium chloride produces potassium at the cathode. 2 Electrolysis of dilute aqueous potassium chloride produces oxygen gas at the anode while electrolysis of molten potassium chloride produces chlorine gas at the anode. [2] [Total: 7]

5 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 3 37Cl is an isotope of chlorine. (a) Explain what is meant by the term isotope. Isotopes are atoms of the same element with the same number of protons but different number of neutrons. [2] (b) An ion of 37Cl contains the following particles in Table 3.1. Table 3.1 particle number X 20 Y 18 Z 17 Identify particles X, Y and Z. particle X neutron particle Y electron particle Z proton [2] (c) 35Cl is another isotope of chlorine. Complete Table 3.2 to show the numbers of each type of particle in an atom of 35Cl. Table 3.2 particle number X 18 Y 17 Z 17 [2] [Total: 6]

6 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 4 Nitrogen oxides in the upper atmosphere cause damage to the ozone layer. Aircraft engines are one source of nitrogen oxides. (a) (i) Explain how nitrogen oxides are formed in the engine of an aircraft. At high temperatures, nitrogen combines with oxygen to form nitrogen monoxide (NO). [1] NO reacts with oxygen to produce nitrogen dioxide (NO2). [1] [2] (ii) Give one natural source of nitrogen oxides in the atmosphere. From lightning activity. [1] (b) Nitrogen monoxide, NO, damages the ozone layer by reacting with ozone in a two step reaction. NO + O3  NO2 + O2 step 1 NO2 + O3  NO + 2O2 step 2 (i) Use oxidation states to identify which element is oxidised in step 1. element Nitrogen change in oxidation state 2+ to 4+ [2] (ii) One nitrogen monoxide molecule can destroy thousands of ozone molecules. Use the equations for steps 1 and 2 to explain why. In step 1, one molecule of NO reacts with 1 molecule of O3 to form 1 molecule of NO2. The NO2 formed in step 1 then reacts with another molecule of O3 to form 1 molecule of NO. [1] Since NO can be formed in step 2, the cycle is continuous with the large consumption of O3 molecules. [1] [2] (c) Nitrogen oxides are removed from car exhaust emissions by catalytic converters. In a converter, nitrogen monoxide reacts with carbon monoxide. (i) Write an equation for this reaction. 2NO + 2CO  N2 + 2CO2 [1] (ii) Cars fitted with catalytic converters still give out environmentally harmful gases. Name one environmentally harmful gas that is emitted in large amounts, and describe the problem it causes. Carbon dioxide Carbon dioxide is a greenhouse gas which absorbs heat, thus causing global warming. [1] [Total: 9]

7 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 5 Chlorofluorocarbons (CFCs) were widely used as aerosol propellants until the mid- 1980s. They are now banned in most countries because they lead to the depletion of the ozone layer. Fig. 5.1 shows two CFCs have the trade names Freon-11 and Freon-12. Freon-11 Freon-12 Fig. 5.1 (a) Draw a ‘dot-and-cross’ diagram to show the bonding in Freon-11. Show outer shell electrons only. [2] (b) Suggest the full structural formula for Freon-13. [1] (c) Another Freon has the formula CF4. Suggest the trade name of this Freon. Freon-14 [1] (d) Can all of these Freons be classified as CFCs? Give a reason for your answer. Not all Freons can be classified as CFCs as Freon-14 does not contain chlorine. CFC stand for chlorofluorocarbon. Hence, a CFC should contain carbon, fluorine and chlorine atoms. [1]

8 BLS/PRELIM/2024/4E/CHEMISTRY/6092/02 (e) Freon-11 h