PJC H2 Chem 2012 Prelim P2 Soln

© PJC 2012 9647/02/JC2 Prelim/2012 2 Answer all the questions in the spaces provided. 1 Planning (P) You are provided with solutions FA 1, FA 2 and FA 3. FA 1 and FA 2 are either 1.0 mol dm –3 nitric acid or 1.0 mol dm–3 ethanoic acid whereas FA 3 is 2.0 mol dm–3 sodium hydroxide. You are to plan an experiment that will enable you to identify FA 1 and FA 2 , and hence, calculate the enthalpy change of neutrali sation for the reaction between ethanoic acid and sodium hydroxide. You are provided with the following apparatus: a thermometer, a polystyrene cup, other common apparatus in the laboratory (a) State the independent and dependent variable s when determining the identity of each solution, FA 1 and FA 2. [1] Independent variable – Strength of acid Dependent variable – Rise in temperature (b) Plan an experiment to identify which of the two solutions, FA 1 and FA 2 , is 1.0 mol dm–3 nitric acid and which is 1.0 mol dm–3 ethanoic acid. Your plan must identify the acids solely based on the change in temperature. Mathematical processing of the temperature change is thus not required. Your plan should give a step by step description of the method including: · the apparatus used for measurement · appropriate volumes of reagents · how you would measure the various variables [3] Step 1: Using a 100 cm 3 measuring cylinder, measure out 25 cm3 of FA 3 into the polystyrene cup and record its initial temperature using a thermometer. Rinse the thermometer. Step 2: Using ANOTHER 100 cm3 measuring cylinder, measure 50 cm3 of FA 1. Record its initial temperature. Step 3: The initial temperature is given by the average of the initial temperature of FA 1 and FA 3.

© PJC 2012 9647/02/JC2 Prelim/2012 3 Step 4: Carefully transfer the 50 cm3 of FA 1 into the polystyrene cup containing FA 3 , stir with the thermometer and record the highest temperature reached. Rinse the cup. Step 5: Repeat steps 1 to 4, this time replacing FA1 with FA2. (c) Either a burette with 0.1 cm3 interval or a measuring cylinder with 1 cm3 interval can be used to measure the volume of FA 3 required. It is known that the error (or uncertainty) that is associated with each reading when using a measuring cylinder with 1 cm3 interval is ±0.5 cm3, while that using a burette with 0.1 cm3 interval is ±0.05 cm3. Assuming that the volume of FA 3 measured is V cm 3, calculate the maximum total percentage error (or uncertainty) in the measurement of the volume of FA 3, in terms of V, when using: (i) a measuring cylinder with 1 cm3 interval, Percentage error in measuring V cm3 = ± (0.5/ V) x 100% = ± 50/V % (ii) a burette