Appendices: a few final lollipops

APPENDIX i: QUALITATIVE ANALYSIS
REACTIONS YOU MUST KNOW
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A1.1. INTRODUCTION

A1.1.1. Most of this book has been about working things out for yourself. In qualitative analysis, this approach will help you with inferences and conclusions.

However, under the pressure of examination practicals, there are some reactions and tests which you should simply learn about. In qualitative analysis, these particular reactions come up over and over again. If you immediately recognise one or two tests, you will find it much easier to interpret the results of the others.

In addition, if you quickly read through all the practical instructions before carrying them out, you should have some idea of what to expect. Do not go to the other extreme - record what you see, not what you think you ought to see. If you observe something unexpected, explain why you are surprised in the section for inferences.

One final point. You could learn the reactions which follow on the night before the practical exam. This would mean that most of the qualitative analysis you had carried out in classes over the previous year would have been wasted. Learning about reactions from written descriptions is only fully useful if you then observe them for youself. Learn the reactions below at the start of your class practical programme, and continuously revise them during the programme.

A1.2. INORGANIC TESTS YOU MUST KNOW

A1.2.1. Aqueous silver nitrate

Addition of aqueous silver nitrate in the presence of nitric acid is very often a test for the presence of halide ions:

Cl^- → white ppt. of AgCl, soluble in dilute NH₃ solution

Br^- → cream/white ppt. of AgBr soluble in concentrated NH₃ solution

I^- → yellow ppt. of AgI, insoluble in any concentration of NH₃ solution.

Two additional points are: i) the colours are very rarely conclusive

.....................................ii) all the ppts. are insoluble in acid, so if the ppt. dissolves in acid, it is not a halide.

A1.2.2. Aqueous barium chloride or aqueous barium nitrate.

Addition of one of these solutions is nearly always a test for the presence of sulphate ions.

SO₄^2- → white ppt. of BaSO₄ which is insoluble in acid.

Again, if the ppt. dissolves in acid, SO₄^2- ions are not present. Alternatively, nothing but sulphate ions will precipitate in the presence of acid.

A1.2.3. Aqueous sodium hydroxide

Addition of aqueous sodium hydroxide is very often a test for the presence of those metal ions which form insoluble hydroxides.

i) Various metal ions give precipitates of various colours. (Look up the colours during the practical.) Strictly speaking, many of the precipitates are not simple hydroxides, but basic salts, containing other anions as well as hydroxide ions (Question 3, chapter 18).

Also, remember that amphoteric hydroxides redissolve in excess alkali.

ii) NH₄⁺(aq) NH₃(g)↑ on warming

iii) No apparent change indicates the presence of a group I metal ion, or a low concentration of a group II metal ion below magnesium. Alternatively, there may be no metal ion present e.g. in the case of acids.

iv) Orange aqueous dichromate(VI) → (Cr₂O₇^2-) yellow aqueous chromate(VI) (CrO₄^2-)

A1.2.4. Aqueous ammonia

This gives similar reactions to the the addition of sodium hydroxide but:

i) The ^-OH concentration is lower for a given concentration of solution so, for example, the ions of metals in group II are less likely to give precipitates. Also the formulae of basic salt precipitates may be different (question 3, chapter 18).

ii) Some metal ions are complexed by ammonia, so in such cases the precipitate will redissolve in excess reagent. e.g. "Cd(OH)₂" and "Zn(OH)₂" redissolve. "Cu(OH)₂" redissolves to give a solution containing the deep blue tetraamminecopper(II) ion.

A1.2.5. Acid

On addition of dilute mineral acids:

i) Carbonates and hydrogencarbonates effervesce releasing carbon dioxide.

ii) Sulphates(IV) release sulphur dioxide.

iii) Thiosulphates (S₂O₄^2-) release sulphur dioxide and sulphur is precipitated.

iv) Yellow chromates(VI) give orange dichromates(VI)

v) If a precipitate is formed, a metal ion which forms an insoluble salt with the acid anion may be present. Alternatively, the hydrogen ions from the mineral acid may form an insoluble acid with the anions from the test substance.

A1.2.6. General comments on the addition of alkali and acid

This applies to inorganic and organic analysis.

i) Acidic compounds are more soluble in alkaline conditions and less soluble in acid conditions. Thus an acid(ic compound) may show greater solubility in in aqueous NaOH than in water. Also, it may be precipitated from aqueous solution by addition of acid.

ii) Basic compounds are more soluble in acid and less soluble in alkali.

iii) A volatile acid may be released from its salt by addition of (and warming with) a non-volatile acid.

Look out for these alternatives to the more common inferences.

A1.2.7. Tests for gases

It is incorrect to record, for example, the evolution of carbon dioxide in your observations. If you believe carbon dioxide has been produced, this is an inference. You should carry out tests on any gas or vapour produced. The tests and results are what you should record in your observations.

Sometimes the practical instructions will list tests, otherwise carry them out in the following order:

i) Colour and smell

ii) pH: Moist red and blue litmus paper are far more useful than pH paper. All you want to know is whether the the gas is acidic, neutral or alkaline. The only commonly encountered alkaline gas is ammonia, though you may occasionally come across an amine in organic analysis.

iii) Special tests. Look these up during the practical. They are listed in analysis books.

A1.3. ORGANIC TESTS YOU MUST KNOW

A1.3.1. Heat and combustion

These are two different types of test. Look up the appropriate section in your analysis book before carrying out the procedure, particularly so that you can be ready to test for gases and vapours.

Information from heating can be very confusing. As a general rule, it is best to leave heating until you have carried out enough of the other tests to have some idea of what to expect. This is equally important in the case of inorganic compounds.

One especially important example of combustion is the smokey yellow flame given by compounds with a high C:H ratio i.e. unsaturated compounds, and benzene and its derivatives.

A1.3.2. Bromine water

The two most common changes on addition of bromine water are:

i) The bromine water is decolourised - an unsaturated compound is present

ii) A white precipitate forms immediately and HBr(g) may be evolved - an activated benzene ring is present e.g. phenol or phenylamine (section 21.6.4.)

A1.3.3. Iron(III) chloride in neutral conditions

Test with pH paper to ensure that conditions are neutral. When "neutral" iron(III) chloride is added in these conditions:

i) Methanoates and ethanoates red solution (complex), then heating gives a brown precipitate of "Fe(OH)₃", or more correctly, a basic salt.

ii) Benzoates give a buff precipitate of iron(III) benzoate.

iii) The phenol group gives a purple solution (complex).

A1.3.4. Ca²⁺ or Ag⁺ ions

These are usually added as tests for various carboxylate ions. The tests are based on the relative solubilites of the various salts produced.

A1.3.5. 2,4-dinitrophenylhydrazine

On addition of 2,4-dinitrophenylhydrazine, aldehydes and ketones (carbonyls) give orange/yellow precipitates of the hydrazones. An addition-elimination reaction occurs.

A1.3.6. Fehlings test, and the silver mirror test (using Tollens reagent)

If you know already that you have a carbonyl compound, a positive test here shows that you have an aldehyde. Aldehydes are reducing agents, whereas ketones are not. Remember that methanoic acid also gives positive tests.

i) Fehlings: Remember to add Fehlings I and Fehlings II. Blue Cu(II) is reduced to rusty brown copper(I) oxide.

ii) Tollens reagent = ammoniacal silver nitrate. A spotless tube is needed to obtain a silver mirror.

A1.3.7. Iodoform test

Add either: i) Aqueous KI and NaClO

...........or: ii) I₂ dissolved in alkali

A positive test (pale yellow ppt.) indicates the presence of a methyl group attached to a carbonyl carbon - which may have come about by oxidation of an alcohol under the reaction conditions (section 22.1.12.ii.).

A1.4. FINAL TIPS

A1.4.1. Organic: If you are asked to carry out a test in a small beaker or an evaporating dish, you should probably expect the production of a volatile product with a characteristic smell:

e.g. acid + alcohol ester (smell like glue, fruity etc.)

ethanol + oxidising agent ethanal (smell variously described as being like fresh apples or fresh tomatoes)

A1.4.2. Look out for oxidising agents and reducing agents

E.g. If acidified dichromate(VI) is added to your test compound, and the colour changes from orange to green, the test compound has been oxidised and is thus acting as a reducing agent. It has reduced dichromate(VI) to chromium(III).

A1.4.3. Negative tests: A negative test still gives information. E.g. If you are asked to add aqueous barium chloride to your test solution and you obtain no ppt., the examiners may be trying to tell you that you have not got a sulphate, but that it may be another anion containing sulphur such as thiosulphate - look out for oxidising agents, or disproportionation in acidic conditions. Remember also that you may have added the barium chloride in acid conditions!

A1.4.4. Be precise! "It went light blue then dark blue" does not mean the same as, "A pale blue precipitate formed which redissolved on addition of more reagent to form a deep blue solution". "The potassium manganate(VII) went clear" certainly does not mean the same as "The purple potassium manganate(VII) solution was decolourised".

A1.4.5. Do not make up your own tests in exams which give a precise schedule. This is usually taken as a sign of a weak candidate, and it frequently confuses them and leads them to loss of marks.

A1.4.6. Read the question: Many qualitative analysis questions are set out in tabular form with columns for your observations and inferences. Above the table there is usually a question, or general instructions, or important information about the test substances. Read it!

A1.5. QUESTIONS

1) Be sure you can write balanced equations for the changes described above.

Unless otherwise stated, all materials in this web version of appendix 1 are © 2007 Adrian Faiers MA (Oxon) MCIPR

What 's the connection between a dozen eggs and a garden mole?

Answer: Not a lot, really, but see Chapter 1