2019 NJC Prelim P3 QP

[Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9729/03 Wednesday 18 September 2019 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 11 printed pages and 1 blank page.

NJC/H2 Chem Prelim/03/2019 2 Section A Answer all the questions from this section. 1 Potassium is an element found in Period 4 of the Periodic Table. (a) (i) Write an equation for the second ionisation energy of potassium. [1] (ii) Fig 1.1 shows the second ionisation energy for the consecutive elements from Period 3 and Period 4. Fig 1.1 Suggest which of the above elements, A to H, is potassium. [2] (b) When 0.400 g of potassium dichromate(VI), K2Cr2O7, was heated using a strong flame, a yellowish green solid mixture was obtained and a gas was evolved. The gas collected ignited a glowing splint. Upon adding water to the yellowish green solid, part of the solid dissolved to give a yellow solution of K2CrO4, leaving a green solid. (i) State the identity of the gas. [1] (ii) The green solid that remained was collected and dried. It weighed 0.052 g and was found to be 68.4% chromium by mass and contains only chromium and oxygen. Determine the chemical formula of the green solid. [2] (iii) Using your answers to b(i) and b(ii), write the equation for the decomposition of potassium dichromate(VI). [1] (c) State two physical properties of chromium that differs from calcium. Explain the reasons for those differences. [2]

NJC/H2 Chem Prelim/03/2019 3 [Turn over (d) Vanadate(V) ion, VO3−, changes into pale yellow oxovanadium(V) ions, VO2+, when acid is added. Upon addition of excess zinc, the pale yellow solution changes to blue, to green and finally to violet. (i) With the aid of an equation, explain the type of reaction that occurs when vanadate(V) ion changes into oxovanadium(V) ions. [2] (ii) Explain why vanadium is able to exhibit variable oxidation states. [1] (iii) With the aid of the Data Booklet, deduce the final oxidation state of vanadium in the violet solution. [3