Transcript
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Qualitative AnalysisQualitative AnalysisChapter 21

LEARNING OUTCOMES

Identify gases – H2, O2, CO2, NH3, SO2, Cl2 and H2O

Identify cations – Pb2+, Al3+, Ca2+, Zn2+, Fe3+, Fe2+, NH4+, Cu2+

Identify anions – CO32-, SO4

2-, SO32-, NO3

-, Br-, I-, Cl-

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What is QA? QA stands for Qualitative Analysis.

It is a process that a chemist carries out

to identify an unknown substance. The analysis will identify the cation(s)

and anion(s) present in the unknown sample.

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The QA Process

1. Make preliminary observations about the unknown substance.

2. Carry out tests in the order given.

3. Make observations and record them in a suitable format.

4. Draw conclusions from the observations

5. Identify the unknown substance.

Five important steps

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The colour tells us about the possible cations

present in the substance.

What is the colour of the substance?

- Is it white, colourless, blue, yellow or some other colour?

What is the nature of the substance?

- Is it crystalline, metallic or in powder form?

Preliminary Observations

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Colour Possible identity of substance

White (colourless) Sodium (Na+), potassium(K+), ammonium (NH4

+)

Black Copper(II) oxide (CuO),Iron(II) oxide (FeO)

Blue/green Copper(II), Cu2+ or Iron(II), Fe2+

Yellowish/brown Iron(III), Fe3+

Greyish, metallic Metals: Zinc, Iron, Magnesium,Aluminium

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Preliminary Observations

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Is it soluble in water?

The solubility tells us about the

possible anions present in the

substance.

Insoluble

Soluble

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Preliminary Observations

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Salt Solubility in water

Carbonates Insoluble except those of sodium, potassium and ammonium

Chlorides Soluble except those of lead(II) and silver.

Nitrates All are soluble.

Sulphates Soluble except those of lead(II) and Barium

Table of Solubility

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Preliminary Observations

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Solid : 1 small spatula, just enough to cover the bottom of a test tube. Liquid : About 2 cm-depth in a test tube. Reagent : Pour slowly and carefully down the side of the test tube.

Or

use a dropper and drop it slowly into the test tube.

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Quantity of Substance

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Using a Bunsen burner

Heating Liquids Use a gentle non-luminous flame (air-hole half open) for

heating a liquid in a test tube. Move the test tube up and down the flame and stop the

heating as soon as the liquid starts to boil.Heating Solids When heating a solid, place it in a dry, clean test tube. Heat

gently at first, then strongly. For your own safety, wear protective goggles and point the

test tube away from yourself and others when heating.

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Recording Results

Record all your observations immediately or you may forget.

Record the change in colour

before and after the test.

Record the formation of a precipitate - its colour and whether it is soluble in excess (extra amount) of the reagent.

Before After

Ppt. is soluble

White ppt. formed

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Recording Results

formulae for names of chemicals

ppt. for precipitate

aq. for aqueous solution

You may use phrases and short forms:

Record the liberation of a gas by noting if any bubbles (effervescence) are seen. State the test you carried out to identify the gas, and the result you obtained.

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Quick Check 11. State the colour of the following

substances:(a) Sodium carbonate: __________(b) Potassium sulphate: _________(c) Calcium chloride: ___________(d) Iron(II) sulphate: ___________(e) Iron(III) sulphate: ___________(f) Copper(II) sulphate: _________(g) Copper(II) carbonate: ________(h) Copper(II) oxide: ___________

Solution

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Quick Check 1 (cont’d)

2. State whether the following substances are soluble or insoluble in water:(a) Sodium carbonate: ________________(b) Magnesium carbonate: _____________(c) Calcium chloride: _________________(d) Zinc sulphate: ____________________(e) Calcium carbonate: ________________(f) Lead(II) chloride(g) Lead(II) nitrate: ___________________

Solution

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1. State the colour of the following substances:(a) Sodium carbonate: white(b) Potassium sulphate: white(c) Calcium chloride: white(d) Iron(II) sulphate: greenish-yellow(e) Iron(III) sulphate: reddish-brown(f) Copper(II) sulphate: blue(g) Copper(II) carbonate: green(h) Copper(II) oxide: black

Return

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Solution to Quick Check 1

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2. State whether the following substances are soluble orinsoluble in water:(a) Sodium carbonate: soluble(b) Magnesium carbonate: insoluble(c) Calcium chloride: soluble(d) Zinc sulphate: soluble(e) Calcium carbonate: insoluble(f) Lead(II) chloride: insoluble(g) Lead(II) nitrate: soluble

Return

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Solution to Quick Check 1 (cont’d)

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Detection of gases How do we tell if a gas is given off?

(a) Effervescence - Bubbles of gas are seen in the solution. (b) Fumes - White fumes are seen when a substance is heated.

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Testing for gases

Look here

Not here

Note the smell of the gas by holding the mouth of the tube a few cm from the nose and fanning the gas towards you.

Note the colour of the gas by looking through the side of the test tube above the substance or solution.

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Testing for gases Testing with litmus paper

Wet the litmus papers (one piece of blue and one piece of red) with distilled water, then hold it near the mouth of the test tube.Do not allow the litmus paper to touch the sides of the test tube or drop it into the reagent, as it will react with the reagent and give a false result.

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Table: Testing for gases Gas Colour Odour Litmus paper

(moist)

Oxygen Colourless Odourless No effect

Hydrogen Colourless Odourless No effect

Carbon dioxide Colourless Odourless Blue to red

Ammonia Colourless Pungent (like urine) Red to blue

Chlorine Light greenish-yellow

Pungent(Like swimming pool water)

Blue to red then white

Sulphurdioxide

Colourless Pungent (like burning match)

Blue to red

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Test for gases with lighted splint

POP

Place a lighted (burning) splint near the mouth of the test tube. (A) If the lighted splintextinguishes with a “pop” sound, then the gas is hydrogen.

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Test for gases with lighted splint

(B) If the lighted splint burns more brightly, then the gas may be oxygen. In this case, you can confirm its identity by using a glowing splint.

(D) If the burning splint is extinguished, then the gas may be carbon dioxide, water vapour, or some other gases.

(C) Insert a glowing splint (no fire) into the mouth of the test tube. If the glowing splint is relighted (rekindled) and burns brightly, then the gas is oxygen.

flame extinguished

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Testing for carbon dioxide

NoteThe carbon dioxide should not be passed for too long, as the limewater will turn clear again (due to the formation of soluble calcium hydrogen carbonate), giving a false impression of a negative test.

• Pour limewater (calcium hydroxide solution) into a test tube until it reaches a depth of 2 cm from the bottom of the test tube.• Stopper the reacting test tube with a delivery tube and pass the gas into the limewater.• If a white ppt. is formed, the gas is carbon dioxide.

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Quick Check 2

Solution

Complete the table below.

Test Observation Inference

1. Heat a little of solid X strongly in a test tube.

Gas evolved extinguishes a burning splint. It turns limewater chalky.

2. Heat the solution P gently and test for any gas evolved.

A pungent gas was given off. It turned moist red litmus to blue.

3. Heat solid Q strongly in a test tube. Test for any gas evolved.

A colourless and odourless gas is given off. The gas relights a glowing splint.

4. Add solid X to the solution Z and warm.Test for any gas evolved.

Bubbles of gas were given off. The burning splint is extinguished with a “pop” sound.

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Test Observation Inference

1. Heat a little of solid X strongly in a test tube.

Gas evolved extinguished a burning splint. It turned limewater chalky.

Gas produced is carbon dioxide. CO3

2- may be present.

2. Heat the solution P gently and test for any gas evolved.

A pungent gas was given off. It turned moist red litmus to blue.

Gas evolved is NH3.

NH4+ may be present.

3. Heat solid Q strongly in a test tube. Test for any gas evolved.

A colourless and odourless gas is given off. The gas relighted a glowing splint.

Gas evolved is oxygen.

4. Add solid X to the solution Z and warm.Test for any gas evolved.

Bubbles of gas were given off. Gas extinguished a lighted splint with a ‘pop’ sound.

Gas evolved is hydrogen.Solid X may be a metal and solution Z may be an acid.

Return

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Solution to Quick Check 2

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Identification of cations Aluminium, Al 3+ Iron(II), Fe2+

Calcium, Ca2+ Iron(III), Fe3+

Zinc, Zn2+ Ammonium, NH4+

Copper(II), Cu2+ Lead(II), Pb2+

Qualitative AnalysisQualitative AnalysisChapter 21

The cations can be identified using sodium hydroxide solution and aqueous ammonia (ammonium hydroxide).The cations react with the hydroxide ions in the alkali to form insoluble metal hydroxides. These hydroxides appear as precipitates.The cations can be identified from: (a) the colour of the precipitate,(b) the solubility of the precipitate in excess of the reagent.

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Identification of cations The unknown substance must be in solution form.

If not, a solution must be made as follows: - soluble salt : dissolve in distilled water. - insoluble salt : dissolve in a little dilute nitric acid.

Test using sodium hydroxide solution: (a) Add sodium hydroxide solution drop by drop to the unknown solution. (b) Observe the precipitate formed and note its colour. (c) Add more sodium hydroxide solution (excess). Shake the test tube and examine whether the precipitate dissolves in excess sodium hydroxide solution.

Ppt. formed

Colourless/ soluble

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Identification of cations

Ammonia gas has a pungent smell (like urine) and turns moist red litmus paper blue.

This confirms the presence of ammonium ion (NH4

+) in the unknown.

d) If no precipitate is formed, warm the mixture and test for ammonia gas.

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Test for cations (Table 1)Cation Add a little NaOH (aq) Add excess NaOH (aq)

Al 3+ White ppt ppt. dissolves to form colourless solution

Ca 2+ White ppt ppt. remains insoluble

Zn 2+ White ppt ppt. dissolves to form colourless solution

Pb 2+ White ppt ppt. dissolves to form colourless solution

Cu 2+ Blue ppt ppt. remains insoluble

Fe 2+ Dirty-green ppt ppt. remains insoluble (turns brown on standing)

Fe 3+ Reddish-brown ppt ppt. remains insoluble

NH4 + No ppt. When warmed,NH3 gas given off.

No change

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Identification of cations (using aq. NH3)

Test using aqueous ammonia:(a) Add ammonia solution drop by drop to the unknown solution.(b) Observe the precipitate formed and note its colour.(c) Add more ammonia solution (excess). Shake the test tube and examine whether the precipitate dissolves in excess ammonia solution.

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Ppt. formed

Colourless/ soluble

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Test for cations (Table 2)Cation Add a little NH3 (aq) Add excess NH3(aq)

Al3+ White ppt. Ppt. remains insoluble

Ca2+ No ppt (or very slight ppt.) No change

Zn2+ White ppt. Ppt. dissolves to form colourless solution

Pb2+ White ppt. Ppt. remains insoluble

Cu2+ Blue ppt. Ppt. dissolves to form dark blue solution.

Fe2+ Dirty-green ppt. Ppt. remains insoluble (turns brown on standing)

Fe3+ Reddish-brown ppt. Ppt. remains insoluble

NH4+ No ppt. No change

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Quick Check 31. What are some of the preliminary observations that you can make on an

unknown sample, before carrying out any test?2. When a solid is insoluble in water, what acid is used? Why?3. Which cations will produce a white precipitate with sodium hydroxide solution

and aqueous ammonia?4. Which cation will produce a white precipitate which is soluble in both aqueous

ammonia and sodium hydroxide?5. Which cations will produce a white precipitate which is soluble in excess

sodium hydroxide, but not in excess aqueous ammonia?6. Which cation will produce a white precipitate with sodium hydroxide solution

but not with aqueous ammonia?7. Which cation will produce a blue precipitate with sodium hydroxide?8. Which cation will produce a deep blue solution with aqueous ammonia?

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Solution

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Return

1. Observe the colour and texture of the unknown sample, and determine if it is soluble in water.

2. Dilute nitric acid is used to dissolve the solid because nitrates of all metals are soluble in water.

3. Aluminium, lead and zinc ions4. Zinc ions5. Aluminium and lead ions6. Calcium ion7. Copper(II) ion8. Copper(II) ion

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Solution to Quick Check 3

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Identification of AnionsCarbonate, CO3

2- Chloride, Cl-

Iodide, I- Sulphate, SO42 -

Nitrate, NO3-

Each anion has a different specific test. For carbonate, you need not make a solution to carry out the test.For other anions, you must make a solution of the unknown in distilled water only.

*If you use dilute nitric acid to dissolve it, you are introducing the nitrate ions into the unknown.

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Test for carbonate

Test Observation Inference Add some dilute hydrochloric acid (or nitric acid) to the unknown. Pass the gas into limewater.

Bubbles of gas are given off. The gas turns limewater chalky.

The gas is CO2 .

Therefore, CO32-

is present in the unknown.

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Test for chloride

Test Observation Inference

Add a little silver nitrate solution to the unknown. Add dilute nitric acid to the above, then shake.

A white ppt is formed. White ppt is insoluble in nitric acid.

White ppt is AgCl.

Therefore, Cl- is present.

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Test for iodideTest Observation Inference

Add a little lead(II) nitrate solution to the unknown. Add dilute nitric acid to the above, then shake.

A yellow ppt is formed. Yellow ppt is insoluble in nitric acid.

Yellow ppt is PbI2.

Therefore, I- is present in the unknown.

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Test for sulphate

Test Observation Inference To the unknown solution, add a little barium chloride (or barium nitrate) solution. Then add some dilute hydrochloric acid (or nitric acid) and shake.

A white ppt is formed. White ppt is insoluble in hydrochloric acid(or nitric acid).

White ppt is BaSO4.

Therefore, SO42-

is present in the unknown.

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Test for nitrate

Test Observation Inference To the unknown, add a little sodium hydroxide solution.Then add a few pieces of aluminium foil (or a spatula of aluminium powder). Warm the mixture gently. Test for ammonia gas evolved.

A pungent gas is given out. The gas turns moist red litmus paper blue.

NH3 gas is produced.NO3

- is present in the unknown.

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Summary of Tests for Anions (Table 3) Anion Test Observation

Carbonate, CO3

2-

Add dilute HCl. Test for gas evolved.

Effervescence. Gas evolved turned limewater chalky.

Chloride, Cl-

Add AgNO3 solution,followed by dilute HNO3

White ppt. insoluble in HNO3

Iodide, I-

Add Pb(NO3)2 solution,followed by dilute HNO3

Yellow ppt. insoluble in HNO3

Sulphate, SO4

2-

Add BaCl2 solution,followed by dilute HCl.

White ppt. insoluble in HCl

Nitrate, NO3

-

Add NaOH solution, then Al foil. Warm gently.

NH3 gas evolved turned red litmus blue

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Quick Check 4

Solution

1. When we add aqueous silver nitrate to an unknown, what anion(s) are we looking for?

2. When we add aqueous barium chloride or barium nitrate to an unknown, what anion(s) are we looking for?

3. When we are told to add dilute hydrochloric acid to an unknown, what anion are we testing for and what should we get ready?

4. When we are told to add sodium hydroxide and aluminium powder to an unknown solution, what anion are we testing for and what should we get ready?

5. When we add silver nitrate or barium chloride solution to an unknown, what reagent should we also add? What is the reason for this?

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Return

1. Chloride, Cl- or iodide, I-

2. Sulphate, SO42-

3. Carbonate, CO32-. We should get a delivery tube and test tube

containing some limewater ready. 4. Nitrate, NO3

-. We should get a piece of moist red litmus paper ready.

5. We should add some dilute nitric acid. This is to exclude any carbonate present from forming a precipitate.

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Solution to Quick Check 4

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Quick Check 5

Test Observation Inference

1. Dissolve a sample of S1 in distilled water and divide it into 3 portions.

S1 dissolved to form a pale blue solution.

2. To the first portion, add aqueous sodium hydroxide slowly until in excess.

A blue ppt was formed.Ppt was insoluble in excess NaOH.

3. To the second portion, add aqueous ammonia slowly until in excess.

A blue ppt was formed.Ppt dissolved to form a deep blue solution.

4. To the third portion, add barium chloride solution, then dilute HCl.

A white ppt was formed.Ppt was insoluble in HCl.

Conclusion: S1 is _________ .

Complete the table below and identify the substance S1.

Solution

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Solution to Quick Check 5

Test Observation Inference

1. Dissolve a sample of S1 in distilled water and divide it into 3 portions.

S1 dissolved to form a pale blue solution.

Cu2+ may be present

2. To the first portion, add aqueous sodium hydroxide slowly until in excess.

A blue ppt was formed.Ppt was insoluble in excess NaOH.

Cu2+ is present.

3. To the second portion, add aqueous ammonia slowly until in excess.

A blue ppt was formed.Ppt dissolved to form a deep blue solution.

Cu2+ is confirmed.

4. To the third portion, add barium chloride solution, then dilute HCl.

A white ppt was formed.Ppt was insoluble in HCl.

SO42- is

present.

Conclusion: S1 is copper(II) sulphate.

Complete the table below and identify the substance S1.

Solution

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Qualitative AnalysisQualitative AnalysisChapter 21

References

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver


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