w2014 experiment 1 main group cations

Experiment 1 25 Winter 2014

Experiment 1: The Main Group CationsIntroduction

The main group metals are the elements of groups 1 & 2 (s block) and the lower

elements of groups 13, 14, & 15 from the p block. For Chemistry 2210 purposes, we focus on

the elements Na, K, Mg, Ca, Al, Sn, Pb, and Zn. We can also add the one common polyatomic

cation, NH4+, to this group of elements as it largely behaves like an alkali metal. (Zn is included

because it behaves like a main group element although it is usually considered as the last of the

d block elements.)

Types of reactionsThe main group cations are not as versatile as the anions. For example, they do not

generally undergo any useful redox reactions or give off gases, although there are exceptions in

each case. Nevertheless, the following preliminary tests should give you a fairly good idea as to

which cation is present and therefore which confirmatory tests to try.

P1. Flame Test.

The s block cations all emit characteristic lines in the visible part of the spectrum when

heated to a high temperature - a really hot Bunsen flame at > 500ºC is hot enough. In

practise it isn't necessary to use a spectroscope since the different colours are easily

distinguished by eye.

P2. Insoluble Sulfates.

Some of the ions have insoluble sulfates. The most readily available source of sulfate

ion is sulfuric acid; the fact that it is an acid is irrelevant here.

P3. Insoluble Hydroxides.

Apart from Na+, K+ and NH4+, these cations have insoluble hydroxides. Some of them

are amphoteric and dissolve in excess base, so this test is done in two parts:

a. First use aqueous ammonia, which is strong enough to precipitate the hydroxide

but has too low a concentration of OH- ions to redissolve it.

NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq) (about 5% dissociated.)

e.g. Mg+2 (aq) + 2 OH- (aq) → Mg(OH)2 (s)


Types of reactions (continued)

P3. Insoluble Hydroxides.

b. Secondly use sodium hydroxide, adding it very carefully. You should see a pre-

cipitate with all the cations that gave one with ammonia, but some of these will

dissolve when excess base is added.

e.g. Zn(OH)2 (s) + 2 OH- (aq) ↔ [Zn(OH)4]-2 (aq)

This test also can be used to detect the ammonium ion since the stronger base dis-

places the weaker one:

NH4+ (aq) + OH- (aq) ↔ NH3 (g) + H2O (l)

The ammonia can be detected by smell and with moist red litmus paper - it is the only

common basic gas. Like SO2, NH3 is too soluble in water to come off as bubbles when it

is a reaction product formed in aqueous solution. The vapours above a hot solution con-

tain enough ammonia to give positive test results, however.

Before you begin any experimental work, read “Important Information for Qualitative

Labs (exp. 1, 2 and 3)” and the introduction above. Clarify anything you do not understand with

an instructor. Observations, and conclusions where required, will be recorded in the provided tables. Ensure that your observations are initialled by an instructor before you leave

C-5001 at the end of each lab period. The hand-in sheets should contain a summary of the ob-

servations you made for each test, including initial observations of the reagents used.

You may use small test tubes for all your reactions. Even though these are con-sidered disposable they should be rinsed and reused when possible. It is wasteful to do

otherwise! Always clean out these test tubes once you have made and recorded your observa-

tions. Use a little soapy tap water and then rinse with small amount of deionized water. The

longer they are allowed to stay dirty, the more difficult they are to clean. If you know what the

solid or precipitate is, you are more likely to know whether a drop of acid or base will help you to

remove it. You may use large test tubes to hold reagent solutions required for multiple tests.

You could lose subjective evaluation marks if you are unnecessarily wasteful of small test tubes

or solutions and if you leave test tubes behind when you leave the lab.


ProcedureThe Preliminary Tests

Record all observations in the provided tables at the end of this document.

NOTE: Discard ALL lead containing solutions, mixtures and compounds in a lead collection container after recording your observations.

5% solutions of the cations are provided for you. Use solutions with a

simple anion, such as chloride or iodide, whenever possible. When a saturated solution is required, you will need to prepare it yourself.

P1. Flame test. (Do all of this test in the fume hood)

Test solid salts containing:

i) Na+ ii) K+ iii) Mg2+ iv) Ca2+ and v) Pb2+ ions.

Take half of a piece of 11 cm filter paper and fold it into a strip about 1 cm wide. Dip a

strip of this filter paper into some deionized water. Hold the strip using crucible tongs

and introduce it into the side of a hot (violet blue) Bunsen flame. Note what you ob-serve. This filter paper test is a "blank".

Place a small amount of the salt to be tested (spatula end or aspirin tablet size) on a

clean watch glass. Touch a strip of wet filter paper to the solid. Introduce the strip with

solid into the side of the flame using crucible tongs.

Compare the flame colour observed for the salt tested with the flame colour resulting

from the wet filter paper alone. If there is no difference, then the cation does not pro-

duce a distinctive colour when placed in a flame. But beware: the sodium colour is very

strong and many other compounds contain traces of sodium. Repeat this procedure for

each cation to be tested.

P2. Insoluble sulfates. Use a 5% solution of each cation to be tested below:

Test: i) Mg2+ ii) Ca2+ iii) Al3+ iv) Pb2+ v) Zn2+ ions.

Add 20 drops of 6 mol.L-1 sulfuric acid to 1 mL of test solution.


Procedure

P3. Insoluble hydroxides. Use a 5% solution of each cation to be tested below:

NOTE: Add sufficient NH3 (aq) or NaOH (aq), as indicated in the test below, to

the Sn(II) solution to make it BASIC before recording any observations.

Test: i) K+ ii) Mg2+ iii) Ca2+ iv) Al3+ v) Sn2+ vi) Pb2+ vii) Zn2+ ions.

a. Add 10 drops of 3 mol.L-1 aqueous ammonia to 1 mL of the cation solution to be

tested, shaking between each drop. (Examine the tube carefully, at least one of

the precipitates is gelatinous and almost transparent.)

Test: i) NH4+ ii) Mg2+ iii) Ca2+ iv) Al3+ v) Sn2+ vi) Pb2+ vii) Zn2+ ions.

b. Add up to 20 drops of 3 mol.L-1 sodium hydroxide solution to 1 mL of the cation

solution to be tested, again shaking between each drop. Watch carefully to see if

any precipitate forms and then redissolves. If there is no precipitate at all, warm the tube in a

hot water bath (DO NOT BOIL) and carefully smell the vapours and test with MOIST red litmus.

(Use a Pasteur pipette to collect some of the vapour and gently blow it over the paper.)

Confirmatory tests (use the provided data tables)

These tests, taken in conjunction with the preliminary tests, should confirm the presence

of a particular ion or, in some instances, they will enable you to come to a definite conclusion

when the preliminary tests were ambiguous. Use 5% solutions of the cations unless indicated

otherwise.

C1 Sodium ion, Na+.

You may have learned in first year chemistry that all sodium salts are soluble in water.

This isn't quite true! Mix 1 mL volumes of a saturated test solution and the zinc uranyl

acetate solution (stored in a fume hood). Leave to stand and / or scratch the bottom of

the tube with a glass rod if no precipitate appears at once - it is sometimes a bit reluct-

ant.

Note: Some other cations may react to form a precipitate here. (K+ and NH4+ do not!)

This test should not be performed with any cation that has produced an

insoluble hydroxide.


Confirmatory tests (continued)

C2 Potassium and Ammonium ions, K+ & NH4+ (use NH4Cl for ammonium ion).

Add a very small amount of solid sodium cobaltinitrite [also known as sodium hexani-

tritocobaltate(III) ] to 1 mL of deionized water in a small test tube. Cap the test tube and

shake it to dissolve the solid. If it does not all dissolve, decant the solution into another

test tube. Add 10 drops of this fresh solution to 1 mL of a saturated solution containing

the cation to be tested.

Note: Many other ions give a positive result with this test too; it is only a reliable test to

distinguish these cations from sodium. This test should not be performed with

any cation that has produced an insoluble hydroxide.

C3 Magnesium and Calcium ions, Mg2+ & Ca2+.

To 1 mL of test solution, add 10 drops of sodium carbonate solution and mix well.

Then add a scoopula end of ammonium chloride and mix well again. The effect of the

ammonium ion is to reduce the concentration of carbonate ion by the following equilib-

rium:

NH4+ (aq) + CO3

2- (aq) ↔ NH3 (aq) + HCO3- (aq)

C4 Aluminium ion, Al3+.

Repeat test P3.a. for Al3+ and for Zn2+ in the presence of a few drops of alizarin or other

similar dye solution supplied by the stock room. Filter the precipitate for each cation us-

ing a fluted filter paper and wash each solid well with two portions of water. Compare

the colours of the washed precipitate in each case. The result obtained for Al3+ is

unique.

C5 Tin(II) ion, Sn2+.

Add 10 drops of test solution to 1 mL of mercury(II) chloride solution. (It is important to

add the tin(II) solution to the mercury(II) chloride solution to avoid an excess of tin(II)

ions.) Mercury is poisonous - dispose of the mixture in the mercury collection con-tainer after observations are made.


Confirmatory tests (continued)

C6 Lead(II) ion, Pb2+.

Dissolve one small crystal of potassium iodide in a 1 mL of deionized water and add the

solution to 10 drops of test solution in a small test tube. Heat the test tube in a water

bath that is near boiling until the precipitate dissolves (add more water if it doesn't) and

then allow to cool slowly. If no change is observed after 5 minutes (i.e. precipitate

does not dissolve), add 5 drops of the mixture to 1 mL of deionized water. Repeat the

heating and cooling cycle. The appearance of the re-crystallized precipitate is quite

unique. Dispose of the test tube contents in a lead collection container.

C7 Zinc ion, Zn2+.

a. Repeat P3.a. and test for solubility in excess 3 mol.L-1 ammonia. (Compare with

Ag+ in Experiment 2.) The complex is [Zn(NH3)4]2+ (aq).

NOTE: two equilibria are required.

b. The following test is also characteristic of zinc. Soak a small piece of filter paper

in the 5% zinc solution, add a drop of cobalt(II) nitrate solution to the wet paper

and place the paper in a clean porcelain crucible in the fume hood. Heat with a Bunsen

burner until the paper has burned away. Note the colour in the ashes. It is due to

CoZnO2, a mixed oxide.


Lab slot _______ Bench # _______ Name ____________________________________

Partner: Bench # _______ Name ____________________________________

Observations for Test P1.

Colour wet filter paper produces in the flame:

Cation Test P1. Flame test

NH4+, Al3+, Sn2+, Zn2+ No change in flame colour.

Na+ (*)

K+ (*)

Mg2+ (*)

Ca2+ (*)

Pb2+ (*)

(*) Indicate the actual solid tested (e.g. NaCl) and note its appearance (e.g. a white granular solid).



Observations for Test P2.

6 mol.L-1 H2SO4 (aq):

Cation Test P2.Sulfate solubility

Na+, K+, NH4+, Sn2+ A clear colourless solution results. No reaction occurs.

Mg2+ (*)

Ca2+ (*)

Al3+ (*)

Pb2+ (*)

Zn2+ (*)

(*) Indicate the appearance of the solution containing the cation

(e.g. a clear, colourless solution)



Observations for Test P3.a.

3 mol.L-1 NH3 (aq):

Cation Test P3.a. Weak base

Na+, NH4+ A clear, colourless solution results. No reaction.

K+ (*)

Mg2+ (*)

Ca2+ (*)

Al3+ (*)

Sn2+ (*)

Pb2+ (*)

Zn2+ (*)

(*) Indicate which salt solution was used (e.g. NaCl (aq) for Na+)



Observations for Test P3.b.

3 mol.L-1 NaOH (aq):

Cation Test P3.b. Strong base

Na+, K+ A clear, colourless solution results. No reaction.NH4

+ (*)

Mg2+ (*)

Ca2+ (*)

Al3+ (*)

Sn2+ (*)

Pb2+ (*)

Zn2+ (*)

(*) Indicate which salt solution was used (e.g. NaCl (aq) for Na+)



Conclusions for Experiment 1 Tests

1. Write the equation for the reaction that occurs when sulfuric acid is added to the

solution of a lead(II) salt.

2. Show the calculations to determine the concentration of OH- ions in and the pH

of 0.65 mol.L-1 solutions of aqueous sodium hydroxide and aqueous ammonia

(Kb = 1.8 x 10-5 mol2.L-2). Use the back of this sheet if additional space is required.

NaOH (aq) [OH-] ____________ pH ____________

NH3 (aq) [OH-] ____________ pH ____________

3. Some insoluble hydroxides dissolve because of complex formation. This is a sim-

ilar situation to the dissolution of some precipitates in Experiment 1. Write the two

competing equilibria involved in test P3.b. for Pb2+ { assume the complex ion is

[Pb(OH)4 ]2- (aq) }.

4. List the formulas of the insoluble hydroxides you have found in these tests which

are amphoteric:



Observations and Conclusions for Confirmatory Tests

zinc uranyl acetate solution:

sodium cobaltinitrite:

Cation Observations Formula of Precipitate

Na+ (C1) NaZn(UO2)3(CH3COO)9.9H2O

K+ (C2)

NH4+ (C2)




Na2CO3 (aq): NH4Cl (s):

3 mol.L-1 NH3 (aq): Alizarin indicator:

Cation Observations Formula of Precipitate(* = write equation)

Mg2+ (C3) • (Write below)

Why does the Mg precipitate dissolve?

Ca2+ (C3) (Is Ksp > or < that of the Mg2+

compound? Why?)

Al3+ (C4) (Comment!)

Zn2+ (C4) Compare results with Al3+:

Two equilibria for Mg2+:




HgCl2 (aq):

KI (aq):

KI (s):

3 mol.L-1 NH3 (aq):

Co(NO3)2 (aq):

Cation Observations Formula of Precipitate(* = write equation)

Sn2+ (C5) *

Pb2+ (C6) *

Zn2+ (C7) a.

b.

a. (write equations below)

b. Colour is due to CoZnO2,

no comment required here

Two equilibria for Zn2+:

w2014 experiment 1 main group cations

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