2019 MI Prelim H2 Chem P4 ANS

Class Adm No Candidate Name: This question paper consists of 19 printed pages. 2019 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 4th Sept 2019 2 hour 30 mins Candidates answer on the Question paper. READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number 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. 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 at the back of the Question Paper. 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. Question 1 2 3 4 Total Marks 18 22 9 6 55 Shift Laboratory

2 1 Determination of titration value at equivalence point The reaction between acid and carbonates is well known. In the presence of excess acid, the following reaction occurs: CO32–(aq) + H+(aq) ⇌ HCO3–(aq) pKa(HCO3–) = 10.32 equation 1 HCO3–(aq) + H+(aq) ⇌ H2CO3(aq) pKa(H2CO3) = 6.37 equation 2 H2CO3(aq) ⇌ CO2(aq) + H2O(l) equation 3 CO2(aq) is then released from the solution as CO2(g), which is observed as effervescence. The entire reaction is known to release heat. FA 1 is 1.8 mol dm-3 of aqueous potassium carbonate, K2CO3. FA 2 is nitric acid, HNO3, of concentration between 1.9–2.1 mol dm-3. Assuming that the first equivalent of H + fully reacts with CO32– before reacting with the HCO3– produced, the reaction between FA 1 and the first equivalent of FA 2 can be simplified as: CO32–(aq) + H+(aq) → HCO3–(aq) ∆Hrxn And the reaction between FA 1 and the second equivalent of FA 2 can be simplified as: HCO3–(aq) + H+(aq) → H2CO3(aq) As the precise concentration of FA 2 is unknown, determination of ∆Hrxn can be done using a thermometric titration to simultaneously determine both the concentration of FA 2 as well as ∆Hrxn. Thermometric titration is a technique whereby equivalence points of a reaction can be located by observing temperature changes, hence eliminating the need for an indicator. In 1(a), you will perform an acid -carbonate thermometric titration. The data from this titration will be used to determine:  the titration value at the first equivalence point, Veq1,  the precise concentration of FA 2, [HNO3],  the maximum temperature change, ∆Tmax,  the enthalpy change of reaction, ∆Hrxn. For Examiners’ Use (a) (i)