DHS_H1_CHEM_P2_Suggested_Solutions

This document consists of 18 printed pages and 0 blank page. © DHS 2018 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination 2018 Year 6 H1 CHEMISTRY 8873/02 Paper 2 Structured Questions 13 September 2018 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your index number and name on all the work you hand in. Write in dark blue or black pen You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use P1 P2 Section A P2 Section B Total % 30 Q1 Q2 Q3 Q4 Q5 Q6 110 13 9 20 18 20 20

2 © DHS 2018 8873/02 Section A Answer all the questions from this section, in the spaces provided. 1 (a) Group 1 elements are strong reducing agents. State and explain the trend of reducing power of Group 1 elements down the Group. [3] Reducing power of Group 1 elements increases down the group. Number of filled principle quantum shell increases and valence electrons are increasingly further away from the nucleus Valence electrons are less strongly attracted to the nucleus and smaller amount of energy is needed to remove the valence electron so it is more easily oxidised. (b) Group 1 elements and its compounds have m any uses. Sodium hydrogencarbonate (NaHCO3) is often used to treat ant stings which contains methanoic acid (HCOOH), a weak acid. Write an equation for the dissociation of methanoic acid in water. Indicate which species are the acid, the base and their conjugate pairs in the reaction. [2] HCOOH + H2O ⇌ HCOO– + H3O+ Acid: HCOOH Conjugate base: HCOO– Base: H2O Conjugate acid: H3O+ (c) When the ant bites, it injects a solution containing 50 % by volume of unionised methanoic acid. A typical ant may inject around 6.0 × 10–3 cm3 of this solution. (i) Given that the density of methanoic acid is 1.2 g cm –3, what is the amount (in moles) of methanoic acid that a typical ant injects? [1] volume of HCOOH = 6.0 x 10–3 x 0.5 = 3.0 x 10–3 cm3 mass of HCOOH = 3.0 x 10–3 x 1.2 = 0.00360 g no of moles of HCOOH = 0.00360 46.0 = 7.83 x 10–5 mol (3 s.f.) As soon as the methanoic acid is injected, it dissolves in water in the body to produce a solution of methanoic acid with pH 2.43. (ii) Assuming that it dissolves fu