Test for Anions, Identification of Gases

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In today’s class, we will be talking about the test for anions, identification of gases. Enjoy the class!

Test for Anions, Identification of Gases

Test for anions

Identification of gases e.g. CO₂, SO₂ and O₂.

Characteristics test for anions e.g. SO₄^2-, SO₃^2-, CO₃^2-,NO^-3, Cl^–, etc.

The anions are negatively charged ions examples of anions are SO₄^2-, SO₃^2-, CO₃²-, S^2- etc.

Identification of gases

Gases                       Identification

H₂                                       It gives a ‘pop’ sound when the gas in contact with a lighted splint.

O₂                             It rekindles a glowing splint.

CO₂                           It is colourless and odourless. It turns lime water milky.

Cl₂                            A green wish yellow gas, it turns moist iodide paper black.

SO₂                          Colourless gas with an irritating smell. It turns damp blue litmus paper to red. That is acidic gas.

Evaluation

1. State two (2) examples of anions with their gases.
2. Identify Cl₂ and NH₃

Test for anions

• Test for SO₄^2-

Test                             Observation                                    Inferences

Sample +H₂O          dissolve in water                             soluble

Soln +Bacl₂+            white ppt                                          SO₄^2-, SO₃^2-,CO₃^2-

Dil HCl                        ppt insoluble                                    SO₄^2- confirmed.

Soln + dil HCl            No visible reaction                         SO₃^2-,CO₃^2-,S^2- are absent

Soln + Bacl₂              white precipitate                            SO₄^2-, confirmed

• Test for SO₃^2-

Soln+Bacl₂                white precipitate

+ dil HCl                     dissolves in dil HCl                            SO₃^2- or CO₃^2-

soln+acidified         decolourise the purple

soln  of KMnO₄        colour to colourless                                   SO₃^2- confirmed

soln +                         the orange colour change                    SO₃^2- confirmed

acidified K₂Cr₂O₇     to green (reducing property)

• Test for CO₃^2-

Soln+dil HCl              Effervescence occurred and a Colourless gas is librated           SO₃^2-, CO₃^2-, S^2- may be present

Gas+litmus paper      it changes moist blue litmus paper to red                   acidic gas

Gas + lime water    The gas turned lime water milky.                                  gas is CO₂ from CO₃^2-

• Test for S^2-

Test                                                   Observation                                               Inferences

Soln + dil HCl            A colourless gas evolved with rotten egg smell              H₂S gas from S^2-

Gas +KMnO₄ soln     purple colour is decolourised with a deposit of sulphur       S^2- present

• Test for Cl^–

Soln + dil HNO₃                   No visible reaction             SO₃^2- or CO₃ are absent

Soln + AgNO₃                      white precipitate                Cl- present

General evaluation

1. Give the common reagents for a confirmatory test of anions
2. State how you would confirm/test for NO^3-
3. Name three gases that are colourless and acidic to litmus.
4. Sodium chloride and silver trioxonitrate(V) crystals are separately soluble in water to give colourless solutions. Explain what happens when their solutions are mixed together.

Reading assignment

School Chemstry by O.Y Ababio pages 165 – 183

Practical Chemistry by R. I Makanjuola pages 27-33.

Practical Chemistry for Schools and Colleges by Godwin O. Ojokuku pages 30 – 98.

Weekend assignment

1. Brown ring test is used to confirm (a) S^2- (b)NO^3- (c) SO₄^2- (d) SO₃^2-
2. The presence of SO₃^2- change the colour of acidified K₂Cr₂)⁷ from (a) green to red (b) purple to colourless  (c) orange to green  (d) green to orange
3. A greenish-yellow gas that changes moist iodide paper black is (a) Cl₂ (b) SO₂            (c) H₂ (d) O₂
4. Sometimes in the presence of HsSO₄, copper turning and heat NO^3- gives (a) brown ring reaction  (b) pure brown fume   (c ) reddish-brown   (d) effervescence
5. The gas that turns lime-water milky is (a) H₂ (b) SO₂ (c) NH₃ (d) CO₂.

Theory

1. State the colour of these solutions a. KMnO₄ K₂Cr₂O₃ c. HOBr d. CuSO₄.5H₂O e. Ca(OH)₂
2. Carry out the following exercises on sample K. add about 10cm3 of distilled water to K in a test tube. Divide the solution into four.
3. To the 1^st portion add NaOH dropwise and in excess.
4. To the 2^nd portion add NH₃ solution dropwise and then in excess.
5. To the 3^rd portion, add it drops of BaCl2 follow by the addition of dil HCl
6. To the 4^th portion, add dil HCl followed by the addition of BaCl₂.

Record your observation and inferences then name the salt K.

 

In our next class, we will be talking about Volumetric Analysis.  We hope you enjoyed the class.

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