2019 RVHS Prelim P4 ANS

River Valley High School Pg 1 of 18 JC 2 H2 Chemistry 9729 Practical Examination 2019 Suggested Solutions for 9729 2019 Prelim Practical Examinations 1 An experiment to investigate the behaviour of acids and bases in aqueous solution For Examiner’s Use For Examiner’s Use FA 1 is 1.00 mol dm–3 sodium hydrogencarbonate, NaHCO3 FA 2 is sodium hydroxide, NaOH, between 1.5 to 2.5 mol dm–3 FA 3 is 2.00 mol dm–3 sulfuric acid, H2SO4 According to the Arrhenius theory of acids and bases, an acid produces H+(aq) ions and a base produces OH–(aq) ions, in aqueous solution. Using the Arrhenius theory, an acid -base neutralisation reaction involves reacting together these two ions to produce water molecules. The equation for this neutralisation reaction is given below. H+(aq) + OH–(aq) H2O(l) ΔHneutralisation = –57.1 kJ mol–1 The reaction between H +(aq) and OH –(aq) involves bond formation and so is exothermic. It is possible to make use of this fact to perform a thermometric titration. This task involves two different acid-base reactions. The first reaction is between sodium hydrogencarbonate, FA 1, and sodium hydroxide, FA 2. Reaction 1: NaHCO3(aq) + NaOH(aq) Na2CO3(aq) + H2O(l) ΔHreaction 1 The molar enthalpy change for Reaction 1, ΔHreaction 1, is the enthalpy change when 1.00 mol of NaHCO3 reacts completely with NaOH. Instead of using an indicator to determine the endpoint, you will perform a thermometric titration to determine the equivalence point of the reaction. The equivalence point is that point where H +(aq) from the acid and OH –(aq) from the base are present in equal molar amounts. The second reaction is between sodium hydrogencarbonate, FA 1, and sulfuric acid, FA 3. Reaction 2: NaHCO3(aq) + ½H2SO4(aq) ½Na2SO4(aq) + H2O(l) + CO2(g) ΔHreaction 2 The molar enthalpy change for Reaction 2, ΔHreaction 2, is the enthalpy change when 1.00 mol of NaHCO3 reacts completely with H2SO4. In your first experiment, you will perform a thermometric titration by adding portions of FA 2 progressively to a known volume of FA 1. You will continue adding FA 2 until the equivalence point is reached and passed. Throughout the experiment you will note and record the temperature of the mixture after each addition.

River Valley High School Pg 2 of 18 JC 2 H2 Chemistry 9729 Practical Examination 2019 You will then analyse your results graphically in order to determine the • titration volume at the equivalence point, Vequivalence, • maximum temperature change, ΔTmaximum 1, • molar enthalpy change, ΔHreaction 1, for Reaction 1. In your second experiment, you will mix together a given volume of FA 1 with a suitable volume of FA 3. You will then determine the • maximum temperature change, ΔTmaximum 2, • molar enthalpy change, ΔHreaction 2, for Reaction 2. For Examiner’s Use Reaction between FA 1 and FA 2 (a) (i) In this task you will