Friday 3 March 2017

GCSE OCR Gateway C4:2b Carbonate and sulphate, halide and nitrate tests.

GCSE OCR Gateway C4:2b Carbonate and sulphate, halide and nitrate tests.
Learning Objectives:

C4.2b To be able to describe tests to identify various aqueous anions: carbonates using hydrochloric acid and limewater; sulfates using hydrochloric acid followed by aqueous barium chloride solution; chloride, bromide and iodide using nitric acid followed by silver nitrate solution.
Anions are negatively charged ions so named because they are attracted to and discharged from the positive anode during electrolysis. 

There are several common anions in school and college chemistry and each one comes from a simple acid.

Acid
Hydrochloric (HCl)
Sulphuric
(H2SO4)
Nitric
(HNO3)
Carbonic
(H2CO3)
Anion

Chloride 
Cl
Sulphate  SO42–
Nitrate
NO3
Carbonate CO32–


How to test for halide ions

Halide ion refers to the anions of chlorine, bromine and iodine.  The halides: chlorides, bromides and iodides require just one simple test to identify them.

First to the solution you suspect contains halide ions add a few drops of nitric acid.  The nitric acid is there to remove any other ions particularly carbonate ions that would also form a precipitate with silver nitrate since silver carbonate (Ag2CO3) is a white insoluble solid.


Second, add several drops of 0.05M silver nitrate solution (AgNO3(aq)) to the solution of halide ion.  The colour of the precipitate will tell you which halide ion is present. 

The silver ions in the silver nitrate solution form insoluble coloured precipitates with chlorides, bromides and iodides.

With Chlorides:

Silver nitrate forms a white precipitate with chlorides.

AgNO3 (aq)    +     NaCl(aq)             AgCl(s)    +    NaNO3 (aq)
                                                        White ppt
The ionic equation:
Ag+ (aq)    +     Cl(aq)                 AgCl(s)


With Bromides:

Silver nitrate forms a cream precipitate with bromides.

AgNO3 (aq)    +     NaBr(aq)           AgBr(s)       +     NaNO3 (aq)
                                                       Cream ppt
The ionic equation:
Ag+ (aq)    +     Br(aq)                 AgBr(s)


With Iodides:

Silver nitrate forms a yellow precipitate with iodides.

AgNO3(aq)    +     NaI(aq)             AgI(s)       +      NaNO3(aq)
                                                     Yellow ppt
The ionic equation:
Ag+ (aq)    +     I(aq)             AgI(s)


Further confirmatory tests:

If dilute ammonia solution is added to the white precipitate of silver chloride, the precipitate should dissolve.

If concentrated ammonia solution is added to the cream precipitate of silver bromide, the precipitate should dissolve.

The yellow precipitate of silver iodide is insoluble in ammonia solution.


Effect of bright sunlight

Lastly, put the three precipitates in bright light or sunlight and after a few minutes the white precipitate of silver chloride turns dark purple, the bromide turns dark green but the iodide remains yellow.

Summary




How to test for a sulphate SO42–

First to the solution you suspect contains sulfate ions add a few drops of hydrochloric acid.  The hydrochloric acid is there to remove any other ions particularly carbonate ions that would also form a precipitate with barium chloride since barium carbonate (BaCO3) is a white insoluble solid.

Second, add several drops of 0.1M barium chloride solution (BaCl2(aq)) to the solution of sulfate ion.

The barium ions in the barium chloride solution form an insoluble white precipitate with sulphate ions.

BaCl2 (aq)    +     Na2SO4(aq)             2NaCl(aq)    +    BaSO4(s)
                                                                                  White ppt
The ionic equation:

Ba2+(aq)    +     SO42—(aq)                 BaSO4(s)



How to test for a carbonate CO32–

First, carbonates are usually solids, only Group 1 carbonates are soluble in water.  So to the solid (or solution) you suspect contains carbonate ions add a few drops of hydrochloric acid.  The hydrochloric acid reacts to form carbon dioxide and fizzing occurs instantly. 

Second, pass the gas produced into limewater and if it is carbon dioxide you will see cloudiness or chalkiness in the limewater.

Hydrochloric acid neutralizes the carbonate ions to from carbon dioxide.

CaCO3 (s)    +    2HCl(aq)         CaCl2(aq)    +    CO2(g)   +  H2O(l)

The ionic equation:

CO32-(aq)    +     2H+(aq)           CO2(g)   +  H2O(l)  

And when the carbon dioxide gas enters limewater the following reaction takes place.

CO2(g)    +   Ca(OH)2(aq)                  CaCO3(s)  +   H2O(l)
                                                              chalkiness


How to test for the nitrate ion NO3

Now this test isn’t always included in school and college chemistry but so what it’s different and a great test with great chemistry.

Of course the reason for the absence of nitrate tests in school and college chemistry is that they are not always reliable.

I think I can agree with Wiki when it says:

Testing for the presence of nitrate via wet chemistry is generally difficult compared with testing for other anions, as almost all nitrates are soluble in water. In contrast, many common ions give insoluble salts, e.g. halides precipitate with silver, and sulphate precipitate with barium.
The nitrate anion is an oxidant, and many tests for the nitrate anion are based on this property. Unfortunately, other oxidants present may interfere and give erroneous results.

A test that is reliable uses an alloy of copper, aluminium and zinc called Devarda’s Alloy. 

If you suspect a solution contains a nitrate ion then add a few drops sodium hydroxide solution and a piece of Devarda’s Alloy to the solution. 

Heat the solution gently in a fume cupboard and test the gas given off with a piece of damp red litmus paper.  If the paper turns blue the gas is likely to be ammonia and the ion in the solution is a nitrate!!

3 NO3   +    8Al    +    5OH  +   18H2O       3NH3    +    8[Al(OH) 4]

It is unusual for any alkaline gas to evolve in any test in wet chemistry other than ammonia so you can be pretty sure that the evolution of an alkaline gas confirms the presence of the nitrate anion.


In my next posts, I’m going to discuss titration technique and develop and update some earlier work in this blog.




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