TJC 08 TJC H1 Prelim Paper 2 Questions

T CANDIDATE NAME CIVICS GROUP / CHEMISTRY 8872/02 Paper 2 17th September 2008 2 hours Candidates answer Section A on the Question Paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group number and candidate name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer two questions on separate answer 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. This document consists of 15 printed pages. For Examiner’s Use Paper 1 /30 Paper 2 A1 /10 A2 /8 A3 /11 A4 /11 B /20 B /20 Total /110 Percentage % TEMASEK JUNIOR COLLEGE Preliminary Examinations Higher 1

For Examiner’s Use 2 Section A Answer all questions in this section in the spaces provided. 1 (a) For decades, scientists were puzzled by why dinosaurs suddenly became extinct 65 million years ago. In studying core samples of rock dating back to that period, scientists found an unusual high level of iridium. This possibly came from an iridium-rich asteroid that struck the Earth’s surface and the mystery to dinosaurs’ extinction was solved. Iridium exists as two naturally occurring isotopes, iridium-191 and iridium-193. (i) Define the term relative atomic mass. ........................................................................................................................................... ........................................................................................................................................... (ii) The relative atomic mass of iridium is 192.2. Calculate the natural abundance, in percentage, of each isotope. (iii) Iridium is one of the rarest metals in the Earth’s crust. It is thought to exist in 2 parts per billion (ppb). Calculate the mass of iridium in 10 tonnes of Earth’s crust. [1 billion = 10 9, 1 tonne = 1000 kg] [4] TJC 2008 8872/02/PRELIM 2008 [Turn over

For Examiner’s Use 3 (b) Aqua regia (a mixture of 75% nitric acid and 25% hydrochloric acid by volume) is highly corrosive. Only noble metals like iridium are inert to this solution. A 5 g sample of platinum- iridium alloy required 24.6 cm 3 of aqua regia for complete reaction. Platinum was completely oxidised to platinum(IV) ions by nitric acid and 0.5 g of metal was recovered. (i) Find the percentage of each metal in the alloy. (ii) The concentration of nitric acid used to make aqua regia is 5.0 mol dm -3. Assuming that the reaction between the sample and aqua regia is complete, construct the balanced equation for the reaction. [4] TJC 2008 8872/02/PRELIM 2008 [Turn over

For Examiner’s Use 4 (c) Man-made iridium-192 is an important radioactive isotope of iridium. It has a half-life of 74 days. The decay of a sample of iridium-192 was monitored. (i) Calculate the original mass of the sample if the sample weighed 0.05 g after 222 days. (ii) Suggest an application for iridium-192. ........................................................................................................................................... [2] [Total: 10] TJC 2008 8872/02/PRELIM 2008 [Turn over

For Examiner’s Use 5 2 (a) Liquid H2O2 and liquid Br2 are two common oxidising agents used in the laboratory. The diagram below shows an energy cycle involving liquid H2O2. H 2O2(l) H2O2(g) 2H(g) + 2O(g) H 2(g) + O2(g) (i) Define the term standard enthalpy change of reaction. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... (ii) In the energy cycle above, name the enthalpy change represented by ∆H4 o. ........................................................................................................................................... (iii) Using the following information and data from the Data Booklet, calculate ∆H3 o and ∆H4 o. ∆H1 o = +51.6 kJ mol-1 ∆H2 o = +1070 kJ mol-1 [5] ∆H3 o ∆H1 o ∆H2 o ∆H4 o TJC 2008 8872/02/PRELIM 2008 [Turn over

For Examiner’s Use 6 (b) The boiling points of liquid H2O2 and liquid Br2 are 150 oC and 59 oC respectively. Explain this difference in terms of bonding and structure. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .........................................................................................