AJC N2014_H2_P2 solns

©2015AndersonJC/CHEM 1 H2 Chemistry 9647 2014 ‘A’ Level P2 Suggested Solutions 1 (a) H+(aq) + OH–(aq) H2O(l) [1] Comments: Students should not give the molecular equation. Many students forgot to include state symbols in their equations. (b) Justification for the volume of HNO3 required Assuming 20.00 cm3 of Ba(OH)2 was used for the reaction, No of moles of Ba(OH)2 used = 00.11000 00.20  = 0.0200 mol No of moles of HNO3 required = 0.0200 x 2 = 0.0400 (2 HNO3 ≡ 1 Ba(OH)2) Minimum volume of HNO3 required for complete neutralisation = 100050.1 0400.0  = 26.7 cm3 The maximum temperature change should occur when equ al number of moles of hydroxide and acid reacts. Hence, there should be at least 3 convenient volumes that are less than 26.7 cm3, and at least 3 convenient volumes that are more than 26.7 cm3, so as to obtain 3 points each to plot for the 2 straight line graphs. (accept other suitable volumes of Ba(OH)2 used, e.g. 10.00 cm3) [1] volume of Ba(OH)2 used = 20.00 cm3 initial burette reading / cm3 final burette reading / cm3 volume of HNO3 added / cm3 initial temperature / oC highest temperature of mixture / oC 0.00 8.00 a b 16.00 – 24.00 – 32.00 – 40.00 – 48.00 – [1]: appropriate volumes of HNO3 added. (Accept 3 convenient volumes before/after 26 cm3.) [1] Procedure 1. Using a 50.00 cm3 burette, transfer 20.00cm3 of Ba(OH)2 into a Styrofoam cup. 2. Place the Styrofoam cup in a 250 cm3 beaker to prevent it from toppling. 3. Fill up another 50.00 cm3 burette with HNO3. 4. Take the initial temperature of the solution in the Styrofoam cup. Record the value. 5. Take the initial burette reading and record the value into the table. 6. Add 8.00 cm3 of HNO3 from the burette into the Styrofoam cup. 7. Stir the mixture gently with the thermometer, measure and record the highest temperature reached. 8. Immediately, add another 8.00 cm3 of HNO3 from the burette into the Styrofoam cup. For every 8.00 cm3 of HNO3 added, stir the mixture gently, and measure and record its highest temperature reached. [3]

©2015AndersonJC/CHEM 2 [1]: appropriate sequence of procedure [S] [1]: appropriate apparatus and their respective capacities [T] [1]: ensuring reliability of results (i.e: stir gently, immediately) [R] How to recognise if the equivalence–point has passed The equivalence point has passed if the temperature of solution shows a decrease. [1] Graph expected to be obtained [1]: rough sketch of points, with Tmax at around 26.70 cm3. Explanation of the shape of the graph • before equivalence point, HNO3 is the limiting reagent. • with each addition of HNO3, n H2O increases and hence temperature of solution increases. • after equivalence point , Ba(OH) 2 has been completely neutrali sed and HNO 3 is in excess. • Temperature of solution decreases due to (i) coolin