HCI H2 CHEM P2 Prelim

This document consists of 17 printed pages and 1 blank page. HWA CHONG INSTITUTION JC2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 10S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Additional Materials: Data Booklet 9647/02 21 September 2011 2 hours INSTRUCTIONS TO CANDIDATES 1) Write your name , CT group, centre number and index number clearly in the spaces at the top of this page. 2) Answer all questions in the spaces provided in this Question Booklet. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided . Do not use staples, paper clips, highlighters, glue or correction fluid. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use 1 / 12 2 / 23 3 / 16 4 / 9 5 / 12 Total / 72

2 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 1 Planning (P) Ammonium salts are commonly used in instant cold p acks. The cold pack contains water, and in the water is another pouch containing the ammoni um salt. When the pack is squeezed, this inner pouch is broken, releasing the salt, which qu ickly dissolves and lowers the pack’s temperature. To determine whether ammonium chloride or ammonium nitrate is more effective as the ingredient in the cold pack, a student decided to c onduct an experiment to find out the enthalpy change of solution of the two salts, starting with ammonium chloride. He added ammonium chloride to water and found the t emperature change by plotting a suitable graph to correct for surrounding heat transfer. Solubility of ammonium chloride at 25 oC = 6.95 mol dm –3 4.3 J are required to raise the temperature of 1.0 cm 3 of any solution by 1° C. In a preliminary investigation, the enthalpy change of solution of ammonium chloride was found to be approximately +15 kJ mol –1 . (a) Draw a labelled diagram to show the apparatus you could use in a school laboratory to carry out the experiment. [1] (b) (i) By considering the apparatus chosen in your diagra m, state a volume of water you could use in the experiment. Volume of water = ……………………………………………….. cm 3

3 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 (ii) Calculate the maximum mass of ammonium chloride th at can be added to the volume of water stated in (b)(i) . [2] (c) Suggest a minimum mass of ammonium chloride that c ould be used. Justify your choice with relevant calculations, stating any assumptions you made. [2] (d) Describe the procedure to find the enthalpy change of solution of ammonium chloride. In your plan, you should: • allow for the plotting of a suitable graph • include appropriate apparatus, masses and volumes of reagents, using your answers in parts (a) , (b) and (c) , and • draw table(s) with headings to show the measuremen ts you would make. ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........

4 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ …………………………………………………………………………… ………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………… ……………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ ……………………………………………………………………………………………........ Table(s) with headings [4]

5 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 (e) Draw a sketch of the graph that you expect to obta in in the experiment. Indicate clearly on the graph the initial and final temperatures that you would read. [1] (f) In the student’s experiment, he dissolved m g of ammonium chloride in v cm 3 of water, and obtained a temperature change of T oC. Outline how you would use his results to calculate a value for the enthalpy change of solution of ammonium chloride. [1] (g) The student repeats the same procedure for ammoniu m nitrate. How might he conclude which of the two salts is more effective as the ingredient in the cold pack? …………………………………………………………………………………………………….. …………………………………………………………………………………………………….. …………………………………… …………… ………………………………………………….. [1] [Total: 12]

6 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 2 (a) Elements and compounds react with each other in num erous ways. Nearly every inorganic chemical reaction falls into one or more of four broad categories. Category of reaction Description Synthesis (or direct combination) Two or more reactants combine to form one product: A + B → AB Decomposition A compound breaks down into two or more substances: XY → X + Y Single displacement One element being displaced from a compound by another element: A + BX → AX + B Double displacement Elements from two compounds displace each other to form new compounds: AX + BY → AY + BX (i) Write a balanced chemical equation, including stat e symbols, of a reaction to illustrate each of the four categories of reactions above. Each equation must include at least one element or compound from Group II or Group VII. The first has been done for you. Category of reaction Equation from Group II or Grou p VII Synthesis (or direct combination) Mg(s) + ½O2(g) → MgO(s) Decomposition Single displacement Double displacement (ii) The reaction of aqueous potassium iodide with aque ous copper(II) sulfate does not fall neatly into any of the four categories above. Stat e the type of reaction and write a balanced equation, including state symbols, for this reaction Type of reaction: ……………………………………………………………………...... ......... Equation: ……………………………………………………...……………………… ………... (iii) Write equations for the two reactions that occur when solid sodium bromide is warmed with concentrated sulfuric acid. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. [7]

7 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 (b) In an experiment, 50.0 cm 3 of aqueous magnesium chloride were titrated with 1.00 mol dm – 3 sodium hydroxide. The pH of the solution changed as given in the diagram. (i) In the experiment, the hydrated magnesium ion, Mg( H 2O) 6 2+ , acts as an acid. Write the mathematical expression for the K a of this ion. ……………………………………………………………………………………………

8 © Hwa Chong Institution 2011 9647 / 02 / JC2 Prelim 2011 (ii) Explain the pH observed at the following points of the titration: I. when the volume of alkali added is between 5 cm 3 and 10 cm 3 …………………………………………………………………………………………… ………. …………………………………………………………………………………………………… ……………………