CHEMISTRY QUESTIONSHEETS AS Level AS TOPIC 16 LABORATORY CHEMISTRY Questionsheet 1 Questionsheet 2 Questionsheet 3 Questionsheet 4 Questionsheet 5 Questionsheet 6 Questionsheet 7 Questionsheet 8 Questionsheet 9 Questionsheet 10 Questionsheet 11 TESTS FOR ANIONS IDENTIFICATION OF GASES BOILING UNDER REFLUX FILTRATION RECRYSTALLISATION MELTING POINTS AND BOILING POINTS TESTS FOR FUNCTIONAL GROUPS ORGANIC TEST QUESTION I ORGANIC TEST QUESTION II (Preperation of 1-bromobutane) ORGANIC TEST QUESTION III (Preperation of ethyl ethanoate) ORGANIC TEST QUESTION IV (Using IR Spectra) 15 marks 16 marks 13 marks 16 marks 17 marks 16 marks 15 marks 19 marks 14 marks 17 marks 17 marks Authors Trevor Birt Donald E Caddy Andrew Jones Adrian Bond Editors John Brockington Stuart Barker John Brockington Kevin Frobisher Andy Shepherd Stuart Barker Curriculum Press Licence Agreement: Paper copies of the A-Level Chemistry Questionsheets may be copied free of charge by teaching staff or students for use within their school, provided the Photocopy Masters have been purchased by their school. No part of these Questionsheets may be reproduced or transmitted, in any other form or by any other means, without the prior permission of the publisher. All rights are reserved. This license agreement is covered by the laws of England and Wales Curriculum Press March 2008. Curriculum P ress www.curriculum-press.co.uk Curriculum Press Bank House 105 King Street Wellington Shropshire TF1 1NU
TOPIC 16 Questionsheet 1 TEST FOR ANIONS a) It can be difficult to distinguish between sodium nitrate and sodium bromide because each of these compounds, in the solid state, can give off brown fumes when treated with concentrated sulfuric acid. Identify the brown fumes in each case. Brown fumes from NaNO 3... [1] Brown fumes from NaBr... [1] How could you demonstrate conclusively the presence of nitrate ions in an aqueous solution? State the reagent (or reagents) you would use, the reaction conditions, the expected observation, and briefly explain the underlying chemistry. Reagent(s)... [2] Conditions... [1] Observation... [1] Chemistry... [1] b) By stating reagents and observations show how, in aqueous solution, you could detect: sodium sulfate in the presence of sodium sulfite. Reagent(s)... [2] Observation... [1] sodium sulfite in the presence of sodium sulfate. Reagents(s)... [1] Observation... [1] c) How, by means of a chemical test, could you distinguish between an aqueous solution of sodium carbonate and one of sodium hydrogencarbonate? Name the reagent you would use, and state the observation that would be expected with each compound. Reagent... [1] Observation with Na 2 CO 3... [1] Observation with NaHCO 3... [1] TOTAL / 15
TOPIC 16 Questionsheet 2 IDENTIFICATION OF GASES a) Complete the table below so as to show simple tests for some common gases. (The first one has been done for you.) Gas Test Result Carbon dioxide Hydrogen Oxygen Ammonia Pass through limewater Limewater turns milky [6] b) Identify each of the gases described below. A colourless gas which changes the colour of moist litmus paper from blue to red, and gives a white smoke when mixed with gaseous ammonia. A green gas which turns moist litmus paper from blue to red and then bleaches it. (iii) A brown gas which can be condensed to a yellow liquid by passing it through a U-tube surrounded by ice. (iv) A pungent, colourless gas which changes the colour of acidified potassium dichromate(vi) solution from orange to green. c) Described below are some observations made during three experiments. Read them carefully, and suggest the identity of substances U to Z. When burning magnesium was plunged into a gar jar of gas U, it continued to burn with a brilliant white flame to give a white deposit V with black specks of a solid W. U =... [1] V =... [1] W =... [1] When burning sodium was plunged into a gas jar of gas X, it continued to burn with a golden yellow flame to give a white solid Y which dissolved in water to give a neutral solution. X =... [1] Y =... [1] (iii) When crystals of ammonium dichromate(vi), (NH 4 ) 2 Cr 2 O 7, were heated, a colourless, odourless gas Z was given off, which did not affect litmus paper, did not burn and did not support combustion. Z =... [1] TOTAL / 16
TOPIC 16 Questionsheet 3 BOILING UNDER REFLUX a) Give three reasons why it is inadvisable to boil an organic liquid in an open beaker. b) A reflux condenser is always used when boiling an organic liquid. State what is meant by a reflux condenser and explain how it works. c) Why is cold water always led into the bottom of the condenser and allowed to flow out from the top? d) Explain whether or not a stopper should be inserted in the top of a reflux condenser. e) Give two reasons why it is preferable to heat a distillation flask with an electric mantle rather than a Bunsen burner. f) Explain why it is common practice to add a few anti-bumping granules to a liquid which is being boiled under a reflux condenser. TOTAL / 13
TOPIC 16 Questionsheet 4 FILTRATION a) Why can it take a long time, using a filter funnel and filter paper, to separate a solid from an organic liquid? b) The problem can be lessened by careful folding of the filter paper. Study Figures 1 and 2 and answer the questions below. Figure 1 Filter paper folded normally Figure 2 Wide angle filter paper Which arrangement (Figure 1 or Figure 2) will give the greater rate of filtration? Explain your answer to b). c) Fast filtration is usually achieved by means of a Büchner apparatus. Draw a labelled diagram of this apparatus, showing clearly the position of the filter paper, and explain briefly the technique of using it. Sketch [5] Technique... d) A drying agent (such as anhydrous calcium chloride) is often separated from an organic liquid by decanting. What is meant by decanting? State two advantages of decanting over filtering. 1... 2... [2] (iii) In what circumstances is decanting unsatisfactory? TOTAL / 16
TOPIC 16 Questionsheet 7 RECRYSTALLISATION Ethanol added under reflux Filtration through warm Büchner funnel Naphthalene is a colourless, crystalline solid used in the manufacture of firelighters and traditional mothballs. It is very soluble in benzene at all temperatures, but can be successfully purified by recrystallisation from ethanol. A sample of impure naphthalene, contaminated with sand, water and an ethanol-soluble organic compound, was put into a distillation flask and covered with ethanol. A reflux condenser was fitted, the mixture was boiled and more ethanol was added cautiously until all the naphthalene had dissolved. The hot solution was then filtered through a preheated Büchner funnel, after which the filtrate was allowed to cool to room temperature and crystallise. The resulting crystals were filtered off through a cold Büchner funnel, washed with a small amount of ethanol and finally dried in a desiccator. a) Why is benzene unsuitable as a solvent for recrystallising naphthalene? b) State the essential features of ethanol that make it a suitable solvent for recrystallising naphthalene. c) What is the purpose of the reflux condenser?... d) Why must care be taken not to use too much ethanol when dissolving the impure naphthalene? e) What is the purpose of the hot filtration? TOTAL (Continued...) /
TOPIC 16 Questionsheet 7 Continued RECRYSTALLISATION f) In what fundamental way does a Büchner filtration apparatus differ from an ordinary filtration apparatus? g) Why is the Büchner funnel preheated for the hot filtration? h) Why is it important to wash the crystals with only a small amount of ethanol? i) Sketch a labelled diagram of a desiccator and explain briefly how it works. [5] j) How could a chemist be sure that the naphthalene finally obtained was pure? TOTAL / 17
TOPIC 16 Questionsheet 6 MELTING POINTS AND BOILING POINTS a) Determining the melting point of a solid compound (or the boiling point of a liquid compound) can serve two purposes in organic chemistry. State these purposes, and in each case explain how the results of a melting point determination are used to achieve that purpose. Purpose... [1] Explanation...... [2] Purpose... [1] Explanation...... [2] b) For a compound whose melting point in completely unknown, it is usual to conduct two melting point determinations; one rough and one accurate. What is the advantage of this procedure? In what ways do the experimental techniques for the two determinations differ from each other? (iii) What precaution must be taken between carrying out the rough and accurate determinations? c) The boiling point of a liquid is usually determined by performing a simple distillation. Sketch the apparatus, paying particular attention to the location of the thermometer bulb. [2] Explain why an inaccurate result may be obtained if the thermometer bulb is wrongly positioned. (iii) When distilling high boiling point liquids, why should an air condenser be used instead of a water cooled condenser? TOTAL / 16
TOPIC 16 Questionsheet 7 TESTS FOR FUNCTIONAL GROUPS a) Complete the following table, by inserting suitable reagents and conditions to test for the functional groups shown, and the observations you would expect to make if these functional groups were present. Functional group Reagents & conditions Observations Alkene Alcohol Carboxylic acid b) 0.495g of a compound X, a chloroalkane M r = 99, was boiled with NaOH(aq), then acidified with dilute nitric acid and treated with excess AgNO3(aq). The resulting precipitate had a mass of 1.435 g. [6] Explain why, in testing organic compounds for halogens, it is necessary to boil a compound with NaOH(aq) and then acidify with dilute nitric acid before adding AgNO 3 (aq). Reason for boiling with NaOH(aq)...... [2] Reason for acidification... [1] Calculate how many moles of chloride ions were released from one mole of compound X in the above experiment. (iii) Deduce the molecular formula and hence suggest TWO possible structural formulae for X. TOTAL / 15
TOPIC 16 Questionsheet 8 ORGANIC TEST QUESTION I Ethanal (b.p. 21 o C) can be prepared in the laboratory by the oxidation of ethanol (b.p. 78 o C) by oxidation with a solution of sodium dichromate(vi) in dilute sulfuric acid. The technique involves running the acidified dichromate solution slowly into ethanol maintained at a temperature of about 50 o C in a two-necked distillation flask. Ethanal vapour is led into a water cooled condenser, from which liquid ethanal is collected in a cooled receiver. a) Explain the difference in boiling points between ethanal and ethanol. b) Draw a labelled diagram of the required apparatus. c) Why should the temperature be maintained at approximately 50 o C? d) Why is the dichromate(vi) solution run into the ethanol slowly? e) Why is the experiment designed so that the dichromate solution is run into ethanol, rather than the other way round? f) Why is ethanal collected in a cooled receiver? g) How would the apparatus and experimental technique be modified so that, using the same reagents, ethanoic acid could be prepared rather than ethanal?... [4] [5] TOTAL / 19
TOPIC 16 Questionsheet 9 ORGANIC TEST QUESTION II THE PREPERATION OF 1-BROMOBUTANE 1-bromobutane can be prepared as follows: Potassium bromide and water are added to butan-1-ol in a flask that is immersed in cold water. Concentrated sulfuric acid is slowly added with stirring and the mixture heated under reflux for about 30 minutes. The oily product is distilled and then shaken with concentrated hydrochloric acid. The aqueous layer is removed and the remainder shaken with sodium carbonate solution then water, each successive aqueous layer being rejected. The residual liquid is left overnight with anhydrous calcium chloride and the liquid then redistilled. a) Give two purposes for the concentrated sulfuric acid. b) Why is it added slowly? c) Before refluxing, what else is included in the flask and why? d) Why is prolonged reflux necessary? e) Why is the initial distillate shaken with concentrated hydrochloric acid? f) What is removed by shaking with: sodium carbonate?... water?... g) Name the apparatus used in f). h) What is the purpose of the anhydrous calcium chloride? i) On redistillation, how would you know if the purity had improved? TOTAL / 14
TOPIC 16 Questionsheet 10 ORGANIC TEST QUESTION III THE PREPERATION OF ETHYL ETHANOATE Ethyl ethanoate can be prepared from ethanol and ethanoic acid by the following method. Mix 20 cm 3 of ethanol and 50 cm 3 of pure ethanoic acid in a 250 cm 3 round bottomed flask. Carefully add 2 cm 3 of concentrated sulfuric acid, then a small quantity of anti-bumping material. Fit a water cooled condenser and gently heat under reflux for about three to six hours. When cool pour the product into a beaker containing an equal volume of saturated aqueous sodium chloride and stir. The sodium chloride reduces the solubility of the ethyl ethanoate in the water. Using a separating funnel separate the ester layer and wash with sodium carbonate solution in the separating funnel (CARE!). Separate the ester layer and run it into a dry conical flask containing 2 spatula measures of anhydrous sodium sulfate. Stopper, shake gently and leave for at least 30 minutes. Filter or decant the ester into a distilling flask and distil slowly. Collect the fraction, which boils between 70-79 C, in a dry, pre-weighed container. boil to dryness. Weigh the product and calculate the % yield. Data M r Density/g cm -3 Boiling Point/ C Ethanol 46 0.79 78 Ethanoic acid 60 1.05 118 Ethyl ethanoate 88 0.925 77 a) Which of the reactants is in excess? Explain, with details of calculations, your answer. Suggest why this reactant has been chosen to be in excess. b) What does the term reflux mean and why is reflux used in this preparation? TOTAL (Continued...) /
TOPIC 16 Questionsheet 10 Continued ORGANIC TEST QUESTION III THE PREPERATION OF ETHYL ETHANOATE c) Which substances will dissolve in the aqueous sodium chloride layer? Why is an aqueous sodium chloride used, rather than water. (iii) Is the ester the upper or lower layer? Explain the reason for your choice. d) What is the purpose of the anhydrous sodium sulfate? Why is it filtered off? e) If 21.2 g of ethyl ethanoate were obtained in a preparation, calculate the % yield.... [5] TOTAL / 17
TOPIC 16 Questionsheet 11 ORGANIC TEST QUESTION IV USING IR SPECTRA Bond Functional group Wavenumber/cm -1 C O alcohols, esters. 1000-1300 C=O aldehydes, ketones carboxylic acids and esters. 1680-1750 O H carboxylic acids. 2500 3300 (broad) O H alcohols, phenols 3230 3550 (broad) a) Compound A, with the molecular formula C 3 H 8 O, had a broad absorption in the IR at 3300 cm -1. On heating under reflux with acidified aqueous potassium dichromate(vi), it was converted to Compound B, which had a broad absorption in the IR at 3000 cm -1 and an absorption at 1716 cm -1. Identify the two compounds. A =... [1] B =... [1] Explain your reasoning... b) Compound C, with the molecular formula C 3 H 8 O, had a broad absorption in the IR at 3300 cm -1. On heating under reflux with acidified aqueous potassium dichromate(vi), it was converted to Compound D, which had an absorption at 1715 cm -1. Identify the two compounds, given that Compound D had no strong absorption in the 2500 3300 cm -1 region. C =... [1] D =... [1] Explain your reasoning... c) Compound E, with the molecular formula C 2 H 6 O, had a broad absorption in the IR at 3300 cm -1. On heating under reflux with Compound F and concentrated sulfuric acid, it was converted to Compound G, which an absorption at 1743 cm -1. Identify the compounds, given that Compound F (molecular formula C 2 H 4 O 2 ) had a broad absorption in the IR at 3000 cm -1 and an absorption at 1716 cm -1. E =... [1] F =... [1] G =... [1] Explain your reasoning... Quality of language [1] TOTAL / 17