2025 H1 Chem Prelim Paper answers_HCI

2025 Prelim Exam H1 Chemistry For internal circulation only 1 2025 Preliminary Examination H1 Chemistry (8873) Suggested Solutions Paper 1 1 D 11 B 21 C 2 A 12 D 22 A 3 C 13 B 23 D 4 B 14 D 24 B 5 B 15 A 25 A 6 C 16 B 26 D 7 C 17 C 27 B 8 A 18 B 28 C 9 C 19 B 29 D 10 B 20 D 30 C

2025 Prelim Exam H1 Chemistry For internal circulation only 2 Paper 2 Section A 1(a)(i) E and F have the same number of protons but different number of neutrons. [1] no partial credit (ii) G has 7 electrons pairs [1] (iii) A and C [1] (iv) C and D have the same number of protons, but C has one more quantum shell than D. Hence, C has a larger shielding effect and smaller effective nuclear charge than D, resulting in weaker attraction between the nucleus and outermost electrons. [1] same no. of proton + larger shielding effect. Award 0.5 mark if only say ‘smaller effective charge’ [1] C has one more quantum shell (v) B and G are deflected to the negative plate as they are cations. [0.5] E is deflected to the positive plate as it is an anion. [0.5] 0.5 mark for ‘opposite directions’ only angle of deflection α charge mass angle of E equals angle of G as they have the same charges and masses [0.5] angle of B is smaller than angle of E (or G) as B has a greater mass [0.5] • Directions + reasons in terms of identity or charges • Relative angle + reasons in terms of charges and masses (b)(i) A large difference in electronegativity between the element and chlorine results in (transfer of electrons and) ionic bonding, while a small difference results in (sharing of electrons and) covalent bonding. [0.5] explanation for ionic bonding [0.5] explanation for covalent bonding (ii) NaCl(s) → Na+(aq) + Cl−(aq) [1] PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq) [1] state symbols not required pH of mixture = 1 [1] award 0.5 mark for correct pH of separate solutions

2025 Prelim Exam H1 Chemistry For internal circulation only 3 2(a) It states that if the conditions of a system in equilibrium are changed, the position of equilibrium will shift in a way to reduce that change. [1] no partial credit for definition (b)(i) From Fig 2.1, w hen temperature increases, yield increases which implies position of equilibrium shifted right. Hence, forward reaction is endothermic as it absorbs the additional heat to offset the increase in temperature. [1] correct shift in p.o.e. [0.5] endothermic [0.5] reasons in terms of heat (ii) From Fig 2.1, w hen pressure increases, yield increases which implies position of equilibrium shifted right. Hence, forward reaction results in a decrease in total number of gas molecules as this will offset the increase in pressure. [1] correct shift in p.o.e. [1] decrease in total no. of gas molecules (c)(i) Kc = [H2][CO] [H2O] [1] (ii) H2O(g) + C(s) H2(g) + CO(g) initial / mol 2.0 0 0 change / mol -0.3 +0.3 +0.3 eqm