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LABORATORY CHEMISTRYAS TOPIC 16

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)

Questionsheet 1

Questionsheet 2

Questionsheet 3

Questionsheet 4

Questionsheet 5

Questionsheet 6

Questionsheet 7

Questionsheet 8

Questionsheet 9

Questionsheet 10

Questionsheet 11

QUESTIONSHEETS AS LevelCHEMISTRY

15 marks

16 marks

13 marks

16 marks

17 marks

16 marks

15 marks

19 marks

14 marks

17 marks

17 marks

Curriculum Press – Licence Agreement:Paper copies of the A-Level Chemistry Questionsheetsmay be copied free of charge by teaching staff or studentsfor use within their school, provided the PhotocopyMasters have been purchased by their school. No part ofthese Questionsheets may be reproduced or transmitted,in any other form or by any other means, without theprior permission of the publisher. All rights are reserved.This license agreement is covered by the laws of Englandand Wales © Curriculum Press March 2008.

Curriculum Press • Bank House • 105 King Street • Wellington • Shropshire • TF1 1NU

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AuthorsTrevor Birt John BrockingtonDonald E Caddy Kevin FrobisherAndrew Jones Andy ShepherdAdrian Bond Stuart Barker

EditorsJohn BrockingtonStuart Barker

Do notwrite inmargin

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TEST FOR ANIONS

a) (i) It can be difficult to distinguish between sodium nitrate and sodium bromide because each of thesecompounds, 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 NaNO3

.................................................................................................................... [1]

Brown fumes from NaBr ........................................................................................................................ [1]

(ii) How could you demonstrate conclusively the presence of nitrate ions in an aqueous solution? State thereagent (or reagents) you would use, the reaction conditions, the expected observation, and brieflyexplain 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:

(i) sodium sulfate in the presence of sodium sulfite.

Reagent(s) ............................................................................................................................................. [2]

Observation ........................................................................................................................................... [1]

(ii) 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 carbonateand one of sodium hydrogencarbonate? Name the reagent you would use, and state the observation thatwould be expected with each compound.

Reagent .......................................................................................................................................................... [1]

Observation with Na2CO

3........................................................................................................................... [1]

Observation with NaHCO3

........................................................................................................................... [1]

15

TOPIC 16 Questionsheet 1


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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

[6]

b) Identify each of the gases described below.

(i) A colourless gas which changes the colour of moist litmus paper from blue to red, and gives a whitesmoke when mixed with gaseous ammonia.

....................................................................................................................................................................... [1]

(ii) A green gas which turns moist litmus paper from blue to red and then bleaches it.

....................................................................................................................................................................... [1]

(iii) A brown gas which can be condensed to a yellow liquid by passing it through a U-tube surrounded by ice.

....................................................................................................................................................................... [1]

(iv) A pungent, colourless gas which changes the colour of acidified potassium dichromate(VI) solutionfrom orange to green.

....................................................................................................................................................................... [1]

c) Described below are some observations made during three experiments. Read them carefully, and suggestthe identity of substances U to Z.

(i) When burning magnesium was plunged into a gar jar of gas U, it continued to burn with a brilliant whiteflame to give a white deposit V with black specks of a solid W.

U = ........................................................................................................................................................ [1]

V = ........................................................................................................................................................ [1]

W = ....................................................................................................................................................... [1]

(ii) When burning sodium was plunged into a gas jar of gas X, it continued to burn with a golden yellowflame 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), (NH4)

2Cr

2O

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]

16

Pass through limewater Limewater turns milky



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BOILING UNDER REFLUX

13

a) Give three reasons why it is inadvisable to boil an organic liquid in an open beaker.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

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.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

c) Why is cold water always led into the bottom of the condenser and allowed to flow out from the top?

....................................................................................................................................................................... [1]

d) Explain whether or not a stopper should be inserted in the top of a reflux condenser.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

e) Give two reasons why it is preferable to heat a distillation flask with an electric mantle rather than a Bunsenburner.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

f) Explain why it is common practice to add a few anti-bumping granules to a liquid which is being boiled undera reflux condenser.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]



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16

FILTRATION

a) Why can it take a long time, using a filter funnel and filter paper, to separate a solid from an organic liquid?

....................................................................................................................................................................... [2]

b) The problem can be lessened by careful folding of the filter paper. Study Figures 1 and 2 and answer thequestions below.

(i) Which arrangement (Figure 1 or Figure 2) will give the greater rate of filtration?

....................................................................................................................................................................... [1]

(ii) Explain your answer to b) (i).

....................................................................................................................................................................... [1]

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 ...........................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

d) A drying agent (such as anhydrous calcium chloride) is often separated from an organic liquid by decanting.

(i) What is meant by ‘decanting’?

....................................................................................................................................................................... [1]

(ii) State two advantages of decanting over filtering.

1 .................................................................................................................................................................

2 ............................................................................................................................................................. [2]

(iii) In what circumstances is decanting unsatisfactory?

....................................................................................................................................................................... [1]

Figure 1 Filter paper folded normally Figure 2 Wide angle filter paper



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RECRYSTALLISATION

Naphthalene is a colourless, crystalline solid used in the manufacture of firelighters and traditional mothballs. It isvery 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, wasput into a distillation flask and covered with ethanol. A reflux condenser was fitted, the mixture was boiled andmore ethanol was added cautiously until all the naphthalene had dissolved. The hot solution was then filteredthrough a preheated Büchner funnel, after which the filtrate was allowed to cool to room temperature andcrystallise. The resulting crystals were filtered off through a cold Büchner funnel, washed with a small amountof ethanol and finally dried in a desiccator.

a) Why is benzene unsuitable as a solvent for recrystallising naphthalene?

....................................................................................................................................................................... [1]

b) State the essential features of ethanol that make it a suitable solvent for recrystallising naphthalene.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

c) What is the purpose of the reflux condenser? ...................................................................................................

....................................................................................................................................................................... [2]

d) Why must care be taken not to use too much ethanol when dissolving the impure naphthalene?

....................................................................................................................................................................... [1]

e) What is the purpose of the hot filtration?

....................................................................................................................................................................... [1]


(Continued...)

Ethanoladdedunderreflux

Filtrationthroughwarm

Büchnerfunnel


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f) In what fundamental way does a Büchner filtration apparatus differ from an ordinary filtration apparatus?

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

g) Why is the Büchner funnel preheated for the hot filtration?

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

h) Why is it important to wash the crystals with only a small amount of ethanol?

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

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?

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

RECRYSTALLISATION

TOPIC 16 Questionsheet 7 Continued

17


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MELTING POINTS AND BOILING POINTS

16

a) Determining the melting point of a solid compound (or the boiling point of a liquid compound) can serve twopurposes in organic chemistry. State these purposes, and in each case explain how the results of a meltingpoint determination are used to achieve that purpose.

(i) Purpose ................................................................................................................................................. [1]

Explanation ...............................................................................................................................................

............................................................................................................................................................... [2]

(ii) Purpose ................................................................................................................................................. [1]

Explanation ...............................................................................................................................................

............................................................................................................................................................... [2]

b) For a compound whose melting point in completely unknown, it is usual to conduct two melting pointdeterminations; one rough and one accurate.

(i) What is the advantage of this procedure?

....................................................................................................................................................................... [1]

(ii) In what ways do the experimental techniques for the two determinations differ from each other?

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(iii) What precaution must be taken between carrying out the rough and accurate determinations?

....................................................................................................................................................................... [1]

c) The boiling point of a liquid is usually determined by performing a simple distillation.

(i) Sketch the apparatus, paying particular attention to the location of the thermometer bulb.

[2]

(ii) Explain why an inaccurate result may be obtained if the thermometer bulb is wrongly positioned.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(iii) When distilling high boiling point liquids, why should an air condenser be used instead of a watercooled condenser?

....................................................................................................................................................................... [1]



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TESTS FOR FUNCTIONAL GROUPS

[6]

b) 0.495g of a compound X, a chloroalkane Mr = 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.

(i) 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]

(ii) Calculate how many moles of chloride ions were released from one mole of compound X in the aboveexperiment.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(iii) Deduce the molecular formula and hence suggest TWO possible structural formulae for X.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

15

a) Complete the following table, by inserting suitable reagents and conditions to test for the functional groupsshown, and the observations you would expect to make if these functional groups were present.

Functional group ObservationsReagents & conditions


Alkene

Alcohol

Carboxylic acid


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ORGANIC TEST QUESTION I

19

Ethanal (b.p. 21 oC) can be prepared in the laboratory by the oxidation of ethanol (b.p. 78 oC) by oxidation witha solution of sodium dichromate(VI) in dilute sulfuric acid. The technique involves running the acidified dichromatesolution slowly into ethanol maintained at a temperature of about 50 oC in a two-necked distillation flask. Ethanalvapour 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.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

b) Draw a labelled diagram of the required apparatus.

[5]

c) Why should the temperature be maintained at approximately 50 oC?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

d) Why is the dichromate(VI) solution run into the ethanol slowly?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

e) Why is the experiment designed so that the dichromate solution is run into ethanol, rather than the other wayround?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

f) Why is ethanal collected in a cooled receiver?

....................................................................................................................................................................... [1]

g) How would the apparatus and experimental technique be modified so that, using the same reagents, ethanoicacid could be prepared rather than ethanal?

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [4]



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ORGANIC TEST QUESTION IITHE 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 incold water. Concentrated sulfuric acid is slowly added with stirring and the mixtureheated under reflux for about 30 minutes.

The oily product is distilled and then shaken with concentrated hydrochloric acid. Theaqueous layer is removed and the remainder shaken with sodium carbonate solutionthen water, each successive aqueous layer being rejected. The residual liquid is leftovernight with anhydrous calcium chloride and the liquid then redistilled.

a) Give two purposes for the concentrated sulfuric acid.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]b) Why is it added slowly?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

c) Before refluxing, what else is included in the flask and why?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]d) Why is prolonged reflux necessary?

....................................................................................................................................................................... [1]e) Why is the initial distillate shaken with concentrated hydrochloric acid?

....................................................................................................................................................................... [1]f) What is removed by shaking with:

sodium carbonate? ...................................................... water? .......................................................................

g) Name the apparatus used in f).

....................................................................................................................................................................... [1]

h) What is the purpose of the anhydrous calcium chloride?

....................................................................................................................................................................... [1]i) On redistillation, how would you know if the purity had improved?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

14


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ORGANIC TEST QUESTION IIITHE PREPERATION OF ETHYL ETHANOATE


Ethyl ethanoate can be prepared from ethanol and ethanoic acid by the following method.

Mix 20 cm3 of ethanol and 50 cm3 of pure ethanoic acid in a 250 cm3 round bottomed flask.Carefully add 2 cm3 of concentrated sulfuric acid, then a small quantity of anti-bumpingmaterial. Fit a water cooled condenser and gently heat under reflux for about three to sixhours.

When cool pour the product into a beaker containing an equal volume of saturated aqueoussodium chloride and stir. The sodium chloride reduces the solubility of the ethyl ethanoate inthe water. Using a separating funnel separate the ester layer and wash with sodium carbonatesolution in the separating funnel (CARE!). Separate the ester layer and run it into a dryconical flask containing 2 spatula measures of anhydrous sodium sulfate. Stopper, shakegently and leave for at least 30 minutes.

Filter or decant the ester into a distilling flask and distil slowly. Collect the fraction, whichboils between 70 - 79°C, in a dry, pre-weighed container. Do not boil to dryness.Weigh the product and calculate the % yield.

Data

Mr

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) (i) Which of the reactants is in excess? Explain, with details of calculations, your answer.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) Suggest why this reactant has been chosen to be in excess.

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

b) What does the term reflux mean and why is reflux used in this preparation?

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(Continued...)


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ORGANIC TEST QUESTION IIITHE PREPERATION OF ETHYL ETHANOATE

TOPIC 16 Questionsheet 10 Continued

c) (i) Which substances will dissolve in the aqueous sodium chloride layer?

....................................................................................................................................................................... [3]

(ii) Why is an aqueous sodium chloride used, rather than water.

....................................................................................................................................................................... [1](iii) Is the ester the upper or lower layer? Explain the reason for your choice.

....................................................................................................................................................................... [1]

d) (i) What is the purpose of the anhydrous sodium sulfate?

....................................................................................................................................................................... [1]

(ii) Why is it filtered off?

....................................................................................................................................................................... [1]

e) If 21.2 g of ethyl ethanoate were obtained in a preparation, calculate the % yield.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [5]

17


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ORGANIC TEST QUESTION IVUSING 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 C3H

8O, 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 abroad 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 .......................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]b) Compound C, with the molecular formula C

3H


under reflux with acidified aqueous potassium dichromate(VI), it was converted to Compound D, which hadan absorption at 1715 cm-1. Identify the two compounds, given that Compound D had no strong absorption inthe 2500 – 3300 cm-1 region.

C = ................................................................................................................................................................. [1]

D = ................................................................................................................................................................. [1]


...........................................................................................................................................................................

....................................................................................................................................................................... [3]

c) Compound E, with the molecular formula C2H


under reflux with Compound F and concentrated sulfuric acid, it was converted to Compound G, which anabsorption at 1743 cm-1. Identify the compounds, given that Compound F (molecular formula C

2H

4O

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]


...........................................................................................................................................................................

....................................................................................................................................................................... [3] Quality of language [1]

17

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