VJC Prelim P2

 VJC 2015 9647/02/PRELIM/15 [Turn over 2 1 Planning You are provided with 5 unlabelled bottles containing pure natural acids from the extracts of roasted coffee. Each bottle contains one of the following natural acids:  lactic acid,  maleic acid,  oxalacetic acid,  pyruvic acid,  quinic acid, All the acids above are soluble in water. You are also provided with any other common laboratory reagents and apparatus. (a) All natural acids contain one or more carboxylic acid functional groups. Other than the carboxylic acid functional grou p, what other functional groups are also present in these natural acids? lactic acid: ………………………………………………………..… maleic acid: ………………………………………………………..… oxalacetic acid: ………………………………………………………..… pyruvic acid: ………………………………………………………..… quinic acid: ………………………………………………………..… [2] (b) Two of these natural acids are colourless liquids and the rest are white crystalline solids. Using relevant chemical knowledge, identify the two liquids. Explain your choices. The two liquids are ………………………………….……………………………..… Explanation: ……………………………………………………..…………………….. ……………………………………………………………………….………………..… ……………………………………………………………………….………………..… [2] OH CO2H HO HO HO CH3COCO2H HO2CCOCH2CO2H CH3CH(OH)CO2H HO2CCH=CHCO2H

 VJC 2015 9647/02/PRELIM/15 [Turn over 3 (c) Suggest a reagent that could be used to carry out a test -tube ex periment to distinguish the two liquids. Reagent: ………………………………….……………………………..… Describe what would be observed for each compound in the experiment. Observation: ………………………………….……………………….……………..… …………………………………………………………………………………………… [2] (d) Outline a logical sequen ce of chemical tests that would enable you to identify the remaining 3 solids. You should aim for a minimum number of reactions. Your plan should include  a positive test to confirm the identity of each compound;  detailed procedure (including quantities of chemicals and conditions used);  expected observations for each compound in each test. [2]

 VJC 2015 9647/02/PRELIM/15 [Turn over 4 (e) How would you ensure the reliability of the test result for quinic acid? ……………………………………………………………………….………………..… ……………………………………………………………………….………………..… ……………………………………………………………………….………………..… [1] (f) Suggest a safety measure that you would consider in carrying out your plan. (g) ……………………………………………………………………….………………..… ……………………………………………………………………….………………..… ……………………………………………………………………….………………..… [1] Draw a set -up of the apparatus for the synthesis of pyruvic acid from lactic acid. State the required reagents and conditions. Reagents and conditions: ………………………………….…..…………………..… Diagram of the set-up: [2] [Total: 12]

 VJC 2015 9647/02/PRELIM/15 [Turn over 5 2 Use of the Data Booklet is required for this question. (a) A 2.85 g sample of haematite iron ore, Fe2O3, was dissolved in hydrochloric acid and the solution diluted to 250 cm 3 in a standard flask. A 25.0 cm 3 of this solutio n was completely reduced with excess tin (II) chloride to form a solution of iron(II) ions. After t After the remaining tin(II) ions were removed with a suitable reagent, the solution of iron(II) ions was titrated against an acidified solution of 0.020 mol dm -3 potassium dichromate(VI) and required 26.40 cm3 for complete oxidation back to iron(III) ions. (i) Give the balanced equatio n for the reaction between iron (II) and dichromate(VI) ions. ………………………………….……………………………………………………..… [1] (ii) Calculate the percentage of iron(III) oxide, Fe2O3, in the ore. [3] (b) (i) Suggest whether the acidified potassium dichromate (VI) can be replaced by potassium manganate (VII) for oxidising iron (II) back to iron (III). Explain your answer. ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… [2]

 VJC 2015 9647/02/PRELIM/15 [Turn over 6 (ii) Explain why the excess tin (II) ions have to be removed before titr ation with potassium dichromate(VI). ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… [1] (c) The compounds of manganese catalyse a wide variety of reactions, one instance being manganese dioxide, MnO 2, which catalyses the decomposition of hydrogen peroxide: (i) With the aid of a sketch of the Boltzmann distribution curve, explain how the presence of a catalyst increases the rate of reaction. [2] (ii) The relationship between the rate constant and temperature is generally governed by the Arrhenius equation: k = A where k = rate constant A = Arrhenius constant Ea = activation energy in J mol-1 R = molar gas constant = = 8.31 J K-1 mol-1 T = temperature in K

 VJC 2015 9647/02/PRELIM/15 [Turn over 7 To determine the activation energy of the above reaction, the rate constant was determined at various temperatures and the experimental results were then processed by plotting a graph of ln k vs 1/T (K-1): Use the given information to determine a value for the activation energy of the reaction. [2] (d) Manganese compounds are also involved in many redox reactions. For example, MnO4 − ions oxidize C 2O4 2− ions in an acidic medium according to the following equation: 2MnO4 – + 5C2O4 2− + 16H+  2Mn2+ + 10CO2 + 8H2O A student decided to investigate the rate of the above reaction. He placed the reaction flask in a thermostatically contr olled water bath to keep the temperature constant and measured how the conductivity of the solution changed with time.

 VJC 2015 9647/02/PRELIM/15 [Turn over 8 He observed that the conductivity changes slowly at first but in the course of time, the conductivity changes at a progressively faste r rate. Towards the end of the reaction, the conductivity is again detected to change slowly. (i) Deduce whether the conductivity readings increased or decreased as the reaction progressed. ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… [1] (ii) Suggest reasons why the conductivity changes at different rates as observed by the student. ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… ………………………………….……………………………………………………..… [2] (iii) Sketch on the axes below to show how the amount of CO 2 formed varies with time. [1] [Total: 15] 0 Amount of CO2 time

 VJC 2015 9647/02/PRELIM/15 [Turn over 9 3 Hydrogen halides (or hydrohalic acids ) are inorganic compounds with the formula HX where X is one of the halogens. The hydrogen halides are colourless ga ses at s.t.p., except for hydrogen fluoride, which boils at 19°C. (a) The hydrogen halides