RVHS Prelim_H2_Chemistry_P4_QP

River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination [Turn over RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS 2 4 J CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9729/04 Paper 4 Practical 26 August 2025 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, class and name on all the work that you hand in. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. 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. DO NOT WRITE IN ANY BARCODES. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 19 and 20. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use 55 This document consists of 20 printed pages.

2 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination 1 Determination of the equilibrium constant Aqueous iron(II) ions, Fe2+(aq), are usually kept in acidic conditions to prevent them from readily oxidising to aqueous iron(III) ions, Fe3+(aq). Fe2+(aq) ions react with Ag +(aq) ions in a redox reaction. The following equilibrium is established. Fe2+(aq) + Ag+(aq) = Fe3+(aq) + Ag(s) The concentration of Ag+(aq) at equilibrium can be found by a titration with a standard solution of aqueous potassium thiocyanate, KSCN(aq). The equilibrium constant for the reaction can be found using the following equation. Kc = 3+ eqm 2+ + eqm eqm [Fe (aq)] [Fe (aq)] ×[Ag (aq)] FA 1 is the equilibrium mixture. FA 2 is 0.0100 mol dm–3 potassium thiocyanate, KSCN. The equilibrium mixture has been prepared for you using the following instructions. You do not need to perform the preparation. Step 1 Add 100.0 cm3 of 0. 100 mol dm–3 Ag+(aq) to 100.0 cm3 of acidified 0.100 mol dm–3 Fe2+(aq) in a 500 cm3 conical flask and stopper the flask. Step 2 Ag precipitate forms. Leave the resultant solution for 4 hours. This is the equilibrium mixture, FA 1. When the equilibrium mixture is titrated against a standard solution of potassium thiocyanate, white ppt of silver thiocyanate is formed as shown in equation 1. equation 1 Ag+(aq) + SCN–(aq) = AgSCN(s) The end point is indicated by formation of complex ion between thiocyanate and iron(III) ion when one excess drop of aqueous potassium thiocyanate is added. In this experiment, you will determine the equ