2025 Christ Church Prelim P2 MS

Christ Church Secondary SchoolSecondary 4 Express2025 Preliminary ExaminationChemistry Page | 1 Marking Scheme1aisulfur dioxide1iicopper (II) chloride1iiizinc sulfate1ivsulfur dioxide1biArrangement: particles closely packed in an unorderly manner Movement: particles able to slide past each other 2iiNa2O + H2O  2NaOH1iiiBlue precipitate formed12aNo. of moles of S2O32 = x 0.1 = 0.005 mol No of moles of I2 = x 0.1 = 0.004 mol [1]Mole ratio of S2O32 : I2  2 : 10.005 mol of S2O32- requires 0.0025 mol of I2 to react [1]Thus, the limiting reactant is S2O32- [1]3bMole ratio of S2O32 : I-  1 : 1No. of moles of I- = 0.005 mol [1]% yield = x 100 % = 26.0% [1][Accept ECF from (a)]2cThe oxidation state of iodine decreases [1] from 0 in I2 to 1 in I- [1] Accept answer where student show thiosulfate being oxidised and link it back to the characteristic of a reducing agent. 2dNo. Different elements are being reduced and oxidised at the same time.13a2Mg(s) + O2(g)  2MgO(s)[Reject: state symbols not written or incorrect]1bNitrogen.1

Christ Church Secondary SchoolSecondary 4 Express2025 Preliminary ExaminationChemistry Page | 2 cvolume of O2 reacted = 90 – 72= 18 cm3number of moles of O2 reacted= (18/1000) / 24= 0.00075 mol [1]From the balanced chemical equation, 2 mol of Mg reacts with 1 mol of O2.Hence, 0.0015 mol of Mg will react with 0.00075 mol of O2. [1]Mass of Mg reacted= 0.0015 x 24= 0.036 g [1][Accept: ECF from (a)] 3 4aMost LeastR, S, P, Copper, Q1bAnode: R(s)  R+(aq) + eCathode: Cu2+(aq) + 2e  Cu(s)[Reject: state symbols not written or incorrect.]2cAny two:Electrical energy is not supplied but produced through chemical reactions in a simple cell.Electrons move spontaneously from the anode to the cathode.The anode is the negative terminal in a simple cell.25aN2 (g) + 3H2 (g) 2NH3 (g)[1]From the equation, 1 mole / volume of N2 reacts with 3 moles / volumes of H2. [1]2bTemperature: about 450 oCPressure: about 250 atmCatalyst: finely divided iron[1] for all 3 conditions3cMethod: upward delivery [1]Reason: ammonia is less dense than air [1]2diWhen pressure increases, particles come closer together. [1]Frequency of effective collisions increases; rate of reaction increases.[1]2iiSince ammonia and nitrogen monoxide are gases, Volume ratio of NH3 : NO  1 : 1Theoretical volume of NO = 440 dm3 [1]40% = x 100 % x = 176 dm3 [1]2

Christ Church Secondary SchoolSecondary 4 Express2025 Preliminary ExaminationChemistry Page | 3 6aSilicon(IV) dioxide is a covalent compound and with no mobile charge carriers.1bThey both have a macromolecular structure. [1]They both have atoms in a rigid tetrahedral arrangment. [1]2ciAny twoSilicate contains only covalent bonds while wine glass contains both covalent and ionic bonds.Silicate has a regular arrangement while wine glass has an irregular arrangement.In silicate, each silicon atom is bonded to three oxygen atoms but in wine glass, each silicon atom can be bonded to one or more oxide i