CJC 2022 JC2 H2 CHEM PRELIM P3 MS Examiners Comments

1 9729/03/CJC JC2 Preliminary Examination 2022 [Turn over CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response 13 September 2022 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this bookl et. The question number must be clearly shown. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculat or is expected, where appropriate. A Data Booklet is provided. At the end of examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 28 printed pages For Examiner’s Use Section A Q1 /15 Q2 /21 Q3 /24 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examinations Higher 2 MARK SCHEME & EXAMINER’S COMMENTS

2 9729/03/CJC JC2 Preliminary Examination 2022 Section A Answer all the questions in this section. 1 Chromium is a hard, steel-grey metal with a lustrous appearance. It is valued for its high corrosion resistance and hardness and is commonly used to manufacture alloys such as steel. Chromium plating is sometimes used to give a polished mirror finish to steel. Chromium compounds are also often used as pigments, known as chrome yellow. (a) The following sequence of reactions involving chromium illustrates many of the characteristics properties of transition metals. (i) Solutions of transition metals are frequently coloured. With reference to [Cr(H2O)6]2+,explain fully why it forms a blue solution. [2] .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… orange solution H2O2(aq) yellow solution chromium metal dilute H2SO4 [Cr(H2O)6]2+ blue solution [Cr(H2O)6]3+ green solution grey-green precipitate [Cr(OH)6]3─ dark green solution air NaOH(aq) In the presence of H2O ligands, the partially filled degenerate d-orbitals of Cr2+ ion are split into 2 groups of non-degenerate d orbitals with a small energy gap. In the presence of visible light, d electron in a d orbital of lower energy absorbs orange light and is promoted to the higher energy d* orbital (d-d* electronic transition). The complementary colour (blue) which is not absorbed appears as the colour of [Cr(H2O)6]2+ observed. NaOH(aq) dilute H2SO4 EXAMINER’S COMMENTS Most students were only able to get the second mark for stating the correct colour absorbed. Common mistakes made are as follows: • Missing keywords, eg ‘partially filled’ in the answer – note that if the d orbitals are empty (eg Sc3+ ion) or fully filled (eg Cu+ ion), there is no d- d* electronic transition (no colour observed ) even if splitting of the d orbitals takes place. • Stating that the a d orbital of lower energy state is promoted instead of a d electron in the orbital of lower energy.

3 9729/03/CJC JC2 Preliminary Examination 2022 [Turn over (ii) Suggest the identity of the grey-green precipitate formed in the reaction between [Cr(H2O)6]3+ and aqueous sodium hydroxide. With the aid of an equation, explain fully how it is formed. [2] .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… 1 (a) (iii) Chromium(III) ions can also react with iminodiacetate ions (tridentate ligand) to form a chelating complex ion. Draw the structure of the complex ion , showing the shape clearly. [1] Cr III O NH NH O O O - O O O O The grey-green precipitate is Cr(OH)3. [Cr(H2O)6]3+ + 3OH─ → Cr(OH)3 + 6 H2O When OH─ ions are added to [Cr(H2O)6]3+ solution, an precipitation/acid-base reaction occurs to form Cr(OH)3. OR The grey-green precipitate is Cr(OH)3(H2O)3. [Cr(H2O)6]3+ + 3OH─ → Cr(OH)3(H2O)3 + 3 H2O When OH ─ ions are added to [Cr(H2O)6]3+ solution, a ligand displacement reaction occurs to form Cr(OH)3(H2O)3. EXAMINER’S COMMENTS Most students were able to state the correct identity of the precipitate. Common mistakes made in the second mark are either giving an unbalanced equation or stating Na as the final product (which is not possible), for example: [Cr(H2O)6]3+ + 3NaOH → Cr(OH)3 + 3 Na + 6 H2O

4 9729/03/CJC JC2 Preliminary Examination 2022 Legend can also be accepted to represent ligand used. (iv) Identify the species present in the yellow and orange solutions. Hence write an equation to show the formation of the species in the orange solution from that in the yellow solution. [2] .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… Species present in yellow solution:CrO42─ Species present in orange solution: Cr2O72─ 2 CrO42─ + 2 H+ → Cr2O72─ + H2O EXAMINER’S COMMENTS This question was very badly done. Most students left this part blank. Students are reminded that if a negatively-charged ligand is used, the individual charge is not shown in the structure of the complex, but is instead considered in the overall charge of the complex, for instance, the following drawing is wrong: EXAMINER’S COMMENTS This question was very badly done. Many students just guessed and gave random identities of the two solutions, which shows they do not understand the chromium-containing species responsible for the two colours.

5 9729/03/CJC JC2 Preliminary Examination 2022 [Turn over 1 (b) Draw a fully labelled diagram of the experimental set -up used to measure the standard electrode potential of the Cr3+(aq)/Cr(s) half-cell, indicating the direction of electron flow. [3] (c) Chromium is electrolytically deposited on the cathode from a solution containing Cr3+(aq) using inert electrodes. Calculate the volume of oxygen, at room temperature and pressure, produced at the anode when 1.00 kg of chromium is deposited on the cathode. [2] [O]: H2O(l) → 2H+(aq) + 1 2 O2(g) + 2e─. (x3) [R]: Cr3+(aq) + 3e─ → Cr(s) (x2) Overall equation: 3H2O + 2Cr3+ → 6H+ + 3 2 O2 +2Cr Amount of chromium in 1.00 kg = 1000 52.0 = 19.2 mol Amount of O2 produced = 19.2 2 × 3 2 = 14.4 mol Volume of O2 produced = 14.4 x 24 = 346 dm3 (to 3 s.f.) High resistance voltmeter Cr3+(aq) EXAMINER’S COMMENTS This question was badly done. Students were unable to identify cathode and the anode, and thus drew the wrong direction of electron flow. In addition, there were many missing details in the answers i.e. stating of standard conditions, labelling the electrodes wrongly. Some students eve n drew an electrolytic cell diagram and showed poor understanding of how standard electrode potentials are obtained. EXAMI