WJEC Chemistry for AS Level Student Book: 2nd Edition

Choose two chemical tests that will enable you to determine which one it is. Complete the table below. Give the reagent(s) for each test and the observation expected for a positive result. For each test put a tick ( ✓ ) in the box to show the compound that gives a positive result and a cross ( ✗ ) for those that do not. [5] Reagent(s) Observation expected for positive result butan- 2-ol 2-methyl- propanoic acid 3-hydroxy- butanoic acid This is AO2 because you are asked to show how you would use chemical tests in a practical context to identify a compound. AO2 When handling qualitative data Two reactions of organic compound A are shown below. conc H 2 SO 4 Cr 2 O 7 2– /H + B <-------------------------------------- A -------------------------------------> C Compound B is a straight-chain hydrocarbon with the formula C 4 H 8 . Draw the displayed formulae of two possible isomers of A . [2] Analytical data has been provided which gives information about the functional group of an organic compound. It is non numerical and so is qualitative. AO2 When handling quantitative data Calculate the volume, in cm 3 , that 2.54 × 10 –3 mol of nitrogen occupy at a temperature of 120 °C and a pressure of 101 kPa. [4] The information given in this question is quantitative and you are asked to apply mathematical procedures to calculate a volume. Assessment Objective 3 (AO3) Analyse, interpret and evaluate scienti c information, ideas and evidence to: ▪ make judgements and reach conclusions ▪ develop and re ne practical design and procedures. The AO3 marks on a paper are awarded for developing and re ning practical design and procedures, for making judgements and for drawing conclusions. You may be asked to criticise a method or analysis or be asked how to improve aspects of it. You may be asked to design a method to test a particular hypothesis or why you might apply a particular test. You may be asked to interpret a test or draw a conclusion from evidence presented to you. Examination questions will give information in novel situations. As with AO2 questions, you may be presented with unfamiliar scenarios, but you will be tested on how well you use your knowledge to understand and interpret them. For all the experiments you have done, make sure you know how to improve the accuracy of your method and the repeatability of readings. AO3 questions tend to use words such as ‘evaluate’, ‘suggest’, ‘justify’, ‘improve’ or ‘design’ and often ask you to manipulate experimental data. Here are two examples: AO3 Make judgements and reach conclusions Chlorine forms molecules and ions with other halogens. Two examples are [ClF 2 ] + and [ClF 2 ] - A student said that since these ions have a different charge their shapes must be different. Is he correct? Justify your answer by using VSEPR theory. [3] This question is AO3 because it asks you to draw a conclusion about the shapes of these unfamiliar ions and to make a judgement on the student’s statement. AO3 develop and re ne practical design and procedures Kathryn is asked to determine the enthalpy change of solution of ammonium nitrate, NH 4 NO 3 , using the following method: ▪ Weigh 7.40 g of ammonium nitrate ▪ Weigh 50.0 g of water in a polystyrene cup ▪ Record the temperature of the water ▪ Add the ammonium nitrate and stir until it has all dissolved ▪ Record the minimum temperature of the solution She noted that the temperature fell from 20.0 °C to 10.8 °C. Kathryn’s results gave a lower value than the literature value. Suggest two improvements to her method and explain how they would lead to a more accurate value. [4] This question is AO3 because asking for a method to improve the accuracy of the results is asking how to re ne the experimental procedure. 7 About this book