TKSS 2026 Chem Prelim P2 MS

1 Chemistry 6092 TKSS Sec 4 Preliminary Examination 2026 Mark Scheme Qn Ans marks 1 allow chemical formulae (a) zinc [1] (b) nickel [1] (c) chlorine [1] (d) hydrogen [1] (e) zinc [1] 2(a) In lightbulbs / in the manufacture of steel Accept: to provide an inert atmosphere for reactions to take place in Reject: to store … / to replace nitrogen / to fill balloons [1] (b) Argon has a fully-filled outermost electron shell / has 8 electrons in the outer shell / has a stable electronic structure/arrangement Chlorine gains one electron (to give a full (outer) shell) reject chlorine loses or gains electrons IGNORE argon does not gain/lose electrons IGNORE chlorine has 7 electrons in the outer shell alone / chlorine does not have a full outer shell / is missing an electron [1] [1] (c)(i) True False True False [1] for every 2 correct (ii) Any 1 from: sodium / potassium / rubidium / caesium / francium reject formulae [1] 3 a) i) Particle X= cation / positive ion / copper ion / metal ion Particle Y = electrons [1] for both ii) X does not move AND Y moves [1] b) i) The total percentage of elements does not add up to 100% (idea of calculation of all being less than 100%) [1] ii) Wrong ratio (show calculations) Or Size of copper atoms is bigger than zinc in the diagram but from the table copper should be smaller [1] iii) Different sizes ion/atoms disrupt the orderly arrangement of (copper) atoms. [1]

2 Qn Ans marks Layers of atom/ions cannot easily slide over [1] (c) zinc is more reactive than copper, zinc displaces copper from solution (forming orange coating) copper(II) ions decreasing / used up / reacted (causing paler blue solution) Zn + Cu2+ → Cu + Zn2+ [1] [1] [1] (d) Copper(II) ion is lower than hydrogen ion in the electrochemical series (thus preferentially discharged) Zinc ion higher than hydrogen ion in the electrochemical series Reject zinc ion higher than copper(II) ion Allow explanation in terms of reactivity of atom or tendency for ion to gain electron [1] 4 a) i) They have same number of protons. [1] ii) [1] correct protons and neutrons [1] electron arrangement b) Magnesium loses/transfers electrons to oxygen, forming magnesium ion and oxide ion. Ionic bond is formed due to strong electrostatic forces of attraction between the ions. Carbon shares electron with oxygen forming covalent bonds. [1] [1] c i) Graph intersects origin (0,0) AND Graph gradient is constant (y= 2x) OR When moles of oxygen doubled (1 to 2), the moles of element also doubles (2 to 4). [1] for proving constant gradient or proportional nucleus containing: ………8……… protons ………10 ……… neutrons

3 Qn Ans marks [1] quoting values from graph ii) 0.9 mol of oxygen needed Mass of oxygen = 0.9 x 32 = 28.8 g [1] iii) Beth. 1 mole of oxygen gas reacts with 2 moles of element. [1] 5 a) i) Propanoyl chloride , C2H5COCl [1] 2 correct [2] for all 3 correct ii) Each successive member differ by -CH2 group / All have the same general formula of CnH2n+1COCl / Same functional group, COCl [1] iii) C8H17COCl b i) Ethyl ethanoate [1] ii) HCl Reject name [1] c) i) C-O and O-H [1] ii) The same number AND type of bonds are broken and formed. Energy absorbed to break bonds is equal to the energy released to form bonds [1] [1] [1] 6 a) i) Ester linkage [1] ii) Water is formed / more than 1 product formed / monomer contains functional group at both ends. [1]

4 Qn Ans marks b) C F Br Mass in 100g sample/ g 10.8 17.1 72.1 No of moles 10.8/12 =0.9 17.1/19 = 0.9 72.1/80 =0.9013 Divide by smallest number 0.9/0.9 = 1 0.9/0.9 = 1 0.9013/0.9 =1.001 Empirical formula = CFBr (any order) [1] [1] [1] c i) [1] for equation [1] for correct polymer drawn b) i) Both needs only 1 type of monomer. Any 2 from: In addition polymerisation, there is only 1 product while in condensation polymerisation there is more than 1 (or 2 products) In addition polymerisation, monomer contains C=C while in condensation polymerisation monomer has functional group at both ends. In addition polymerisation, the polymer has C-C linkage while in condensation polymerisation the polymer has ester linkage. [1] [2] 7a) i) ClO-: +1 Cl-: -1 [1] [1] ii) Colourless potassium iodide turns yellow/brown [1] b)i) There are more than 8 (valence) electrons around phosphorus atom [1] ii)

5 Qn Ans marks structure bonding NaCl giant ionic lattice a lot of energy is needed to overcome strong electrostatic forces of attraction/ ionic bonds between ions PCl5 simple covalent little energy is needed to overcome weak intermolecular forces of attraction (between PCl5 molecules) PVC macro - molecular little energy is needed to overcome weak intermolecular forces of attraction (between PVC molecules) OR intermolecular forces of attraction are stronger due to larger Mr OR melts over a range due to different chain lengths [2]: one column /row correct [3]: two columns /rows correct [4]: all three columns /rows correct 8 a) As the particle radius increases, the time taken for dissolution increases. As particle radius increases, exposed surface area for collision decreases. Hence the frequency of effective collision is lowered, (so the rate of reaction is lowered/time taken for reaction to complete increases). [1] [1] b) Acidity decreases AND Carbonic acid (formed from carbon dioxide and water) is reacted with mineral olivine [1] c) SiO44- [1]

6 Qn Ans marks d) Energy level of product lower than reactant Downward single-headed arrow of enthalpy change approx. same length (both diagrams R and P level must be the same) Upward single-headed arrow of activation energy (smaller for reaction with enzyme) [1] [1] ECF [1] ECF e) Mg2SiO4(s) + 4CO2(g) + 2H2O(l)  2Mg2+(aq) + 4HCO3-(aq) + SiO2(s) Moles of CO2 = 48000/ 24 = 2000 mol CO2 : Mg2SiO4 4 : 1 2000 mol : 500 mol Mass of = 500 x [ 2(24) + 28 + 4(16) ] = 500 x 140 = 70 000g or 70 kg [1] [1] ECF [1] ECF f i) More particles have energy equal to or greater than the activation energy (so the frequency of effective collision increases) [1] ii) Enzymes denatured / deformed [1]

7 Qn Ans marks 9 a) There is a range of boiling point Reject: does not have fixed mp/bp, data shows a range, also should not give negative response [1] b) Crude oil is boiled/ vaporised Any 1 from: -Fractionating column is cooler at the top. -The fractions condense when temperature is lower than the boiling point. Any 1 from: -Heavier fractions/molecules with higher boiling point condense/collected at the bottom -Lighter fraction/molecules with lower boiling point condense/collected at the top. [1] [1] [1] c) Hydrogen is a gas while diesel is a liquid. OR Particles in hydrogen are far apart while particles in diesel liquid are closely packed. For the same mass 1g, hydrogen occupies volume of 0.09 cm3 while diesel occupies larger volume of 0.85cm3 [1] [1] d) i) 2H2O  2H2 + O2 [1] ii) State at least 1 advantage and 1 disadvantage: Any 3 from: Advantage of hydrogen over diesel: -obtained from water which is renewable resource while petrol comes from limited resource, crude oil. -only formed water which is non-pollutant while petrol form carbon dioxide which is greenhouse gas. Disadvantage of hydrogen over diesel: - occupies a large volume so large tank is requi