Qualitative Detection of Elements

Pre-lab assignment

Explain briefly, how carbon is detected qualitatively› Carbon compounds can be detected

by heating. The evolution of combustible Gases or charring or both indicate the presence of carbon.

What safety precautions should be observed in this experiment› Don’t touch freshly cut sodium with

your bare hands› Use a very low flame when heating

sodium

Calculate the percentage of nitrogen in acetamide (C2H5ON). Draw it’s structural formulaMolecular wt. of C2H5ON = 59.07 g/molMolar mass of N = 14.01 g/mol%N = 14.01 g/mol x 100 59.07 g/mol

= 23.72%

When 2.5g of a substance was dissolved in 125 g of water, the freezing point was lowered by .310. Calculate the molecular weight of the compound. (The molecular freezing-point –lowering constant for water is 1.86)

Solution

)0.310 = 1.86 ()

= 0.021 mol

Given:2.5 g of substance125 g H20 = 0.310Kf water = 1.86

= 0.021 mol =

= 119.05 g/mol

Since most organic compounds do not form ions when dissolved in water due to their non-polar nature, another way is needed to identify the components of an unknown substance

This experiment shows a way of detecting the presence of Nitrogen, Sulfur and Halogens wherein

SODIUM SALTS are formed These salts are ionic compounds

that are detected qualitatively by the tests

Purpose of this experiment

Sodium Fusion

Clamp the 3-ince test tube

Place the sodium metal

Heat with a very low flame

Add 5 mg of the substance

Heat for 30 seconds

Cool to room temperature

Add 1 mL ethanol

Transfer contents to a beaker

Label filtrate “TEST SOLUTION”

Observations

After the ethanol was added, bubbles started to form

Ethanol destroys the excess sodium that did not participate in the fusion with the organic compound

This was also an exothermic reaction

Discussion- Organic compounds present are

decomposed - Soluble sodium salts of the above elements

form- Resulting ionic compounds are detected

qualitatively by common tests.

(C), (H), (O), (N), (S), (X) + Na NaCN,NaOH,NaS,NaX

Fusion

∆

Due to the non-polar nature of organic compounds, N,X and S detection is difficult.

Organic compounds don’t ionize in solution into organic ions before making the qualitative tests.

Get 1 mL of the test substance

Add DILUTE ACETIC ACID to make it acidic

Add 1-2 drops of LEAD ACETATE

Black precipitate formed

(LEAD SULFIDE)Nothing happened

WHAT HAPPENED NEXT?

Observations

After the lead acetate was added, nothing happened (black precipitate did not form)

Sulfur was not present in the sample

Discussion

Sodium sulfide is treated with Lead acetate, in the presence of acetic acid, and produces a brownish black precipitate (Lead sulfide)

Acetic acid prevents formation of other insoluble lead salts

NaS + (CH3COO)2Pb PbS + 2 CH3COONa

Get 2 mL of the test solution

Add 4 drops of ferrous sulfate

solutionIs the solution

basic?

Heat to boiling and filter

Acidify – add sulfuric acid

Add 2 drops of ferric chloride

Leave for 10 minutes

A precipitate of Prussion blue

formedNothing happened

Add NaOH

Nitrogen(Lassaigne Method)Prussian Blue Test YES NO

Observations

After the procedure was followed, nothing happened. Prussian blue precipitate did not form

Nitrogen was not present in the sample

Discussion 2 NaCN + FeSO4 Fe(CN)2 + Na2SO4

Fe(CN)2 + 4NaCN Na4Fe(CN)6

3 NaFe(CN)6 + 4FeCl3 Fe4[Fe(CN)6]3 + 12NaCl

- Sodium cyanide is converted to sodium ferrocyanide Na4Fe(CN)6

- When combined with ferric chloride in acid solution, it produces Prussian blue ferric ferrocyanide Fe4[Fe(CN)6]3

Get 2 mL of the test solution

Acidify with dilute nitric

acid

Is nitrogen or Sulfur is present?

Add 1 drop of 0.1 M silver

nitrate solution

Did precipitate

form?

What color was the

precipitate?

Chlorine is present

Bromine is present

Iodine is present

No halogen present

Boil until the volume is

about 1 mL

NO YES

YES NO

WHITE PALE YELLOW

BRIGHT YELLOW

Observations

Heavy white precipitate formed indicating the presence of CHLORINE (silver chloride)

SILVER HALIDE TEST

NaX + AgNO3 AgX + NaNO3

This test is done in order to detect presence of halides

Sodium halide (NaCl, NaBr, NaI) will form an insoluble silver halide upon treatment with silver nitrate solution in the presence of dilute nitric acid.

Dilute HNO3

Boiling the original test solution with dilute nitric acid is done in order to remove the cyanide and sulfide ions.

These ions form precipitates that can interfere with the detection of the halogens.

Silver halide gives a white to yellow color

Silver chloride is color white Bromide is pale yellow Iodine is bright yellow

Results show that only a heavy, white precipitate formed.

From this, it can be inferred that AgCl is present in the test solution

Get 1 mL of the original test

solution

Is nitrogen/sulfur present

Add 1 mL of 3M sulfuric acid

Add 1 mL of methylene

chloride

Add one drop of stabilized NaOCl

solution

IODINE IS PRESENT

BROMINE IS PRESENT

Use 2 mL and follow procedure for removing N/S

How do distinguish bromine from chlorine

COLOR OF PRECIPITATE

PURPLE

REDDISH-BROWN

Observations

No precipitate formed

BEILSTEIN TEST

Heat copper wire until flame is no longer colored

While hot, dip in some copper oxide powder and reheat until CuO adheres to the loop

Place small amount of the original sample Heat in a non-luminous flame Blue green flame indicates the presence

of Halogen

BEILSTEIN TEST(C),(H),(O),(N),(X),(S) + CuO Cu2X2 + H2O + CO2 +N2

-This test detects the presence of halogens in the test sample- The copper oxide formed from the copper wire reacts with the halogen in the compound to form cuprous halide, which burns with a green flame.

Results show that a characteristic green flame was observed

From this, it can be inferred that a cuprous halide is present in the solution.

Post Lab Questions

Why should a low flame be used in heating metallic sodium

Sodium should be heated with a low flame because it is a highly reactive substance. Therefore, heating it with an intense flame would increase reactivity and would burn or cause combustion easily. Also, it has a lower melting point and would cause charring of the substance.

The test solution should be acidified with nitric acid before silver nitrate is added to detect the presence of halides

While testing the Lassaigne’s extract for the presence of halogens, it is first boiled with dilute nitric acid. This is done to decompose NaCN to HCN and Na2S to H2S and to expel these gases. That is, if any nitrogen and sulphur are present in the form of NaCN and Na2S, then they are removed. The chemical equations involved in the reaction are represented as

Nitric acid is added before the silver nitrate and the solution is boiled so as to remove interfering ions such as sulfide, cyanide and carbonate ions. Otherwise, these will form precipitates (such as silver sulfide, silver cyanide and silver carbonate) that may be confused with the silver halides.

Write equations for the sodium fusion reaction and the test for nitrogen, halogens and sulfur on the solution from the sodium fusion

Sodium Fusion (C), (H), (O), (N), (S), (X) + Na

NaCN,NaOH,NaS,NaX

Nitrogen

Nitrogen 2 NaCN + FeSO4 Fe(CN)2 + Na2SO4

Fe(CN)2 + 4NaCN Na4Fe(CN)6

3 NaFe(CN)6 + 4FeCl3 Fe4[Fe(CN)6]3 + 12NaCl

Halogens

NaX + AgNO3 AgX + NaNO3

Dilute HNO3

Beilstein Test(C),(H),(O),(N),(X),(S) + CuO Cu2X2 + H2O + CO2 +N2

Sulfur

NaS + (CH3COO)2Pb PbS +2CH3COONa

What is the purpose of adding acetic acid in the lead acetate?› Acetic acid is added because through it,

formation of other insoluble lead salts can be prevented.

› Formation of these other insoluble lead salts can cause confusion when identifying the brownish black lead sulfide precipitate