Experiment no. 3

Qualitative analysis of an organic compound

Planning

Research problem:

Which elements build organic compound?

Hypothesis:

Carbon and hydrogen are present in organic compound. We will also prove if there is any nitrogen, sulphur or halogen elements in our compound.

Independent variable:

·  unknown organic compound

·  lime water

·  reagents

Controlled variables:

·  temperature

·  pressure

·  mass of unknown compounds

·  concentration

1. Oksid decomposition

This experiment will prove existance of carbon and hydrogen in a mixture. We build an apparatus as shown in fig. 1. Because the tube will be exposed to high temperature, you have to use special tube which is made from pyrex glass. You mix copper oxide and your unknown compound. The you insert this mixture to the bottom of the tube. Then you fix the tube horizontally to the rack. After that you insert copper sulphate with the small spatula in the middle of the tube. In the end you have to connect the tube and the beaker with lime water. Small tube will do this job. Then you start heating an organic compound.

Figure 1: An apparatus for proving existance of carbon and hydrogen in an unknown compound

2. Reducing decomposition

With this experiment we prove existance of nitrogen, sulphur and halogen elements in a mixture. We put put an organic compound and a small piece of sodium into a pyrex test tube. We heat this mixture until it start beaming with red color. Then we plunge the tube in a cold water. The tube explodes. As soon the temperature falls down, we filtrate the mixture. In the end we divide into several test tubes for further analysis. This procedure is very dangerous therefore was this part of an experiment already done for us.

Prove for nitrogen existance

We put into the filtrate 1 drop of FeSO4 and 1 drop of FeCl3. We observe the colour of the solution.

Prove for sulphur

We can prove existance of sulphur in two ways.

  1. In a test tube we drip 1 or 2 drops of Pb(CH3COOH)2. We are looking for percipitate.
  1. We put 1 – 2 drops of FeCl3 into a tube containing the filtrate. We observe the colour of the solution.

Prove for halogen elements

For proving halogens elements there is also two tests.

  1. We drip 1 or 2 drops of AgNO3(aq) into a test tube. We are looking for precipitation.
  1. Beilstein reaction
    Take a piece of copper wire, with a loop on the end. Dip this loop in the organic compound. Heat the compound in a Bunsen burner. We will observe the colour of the flame.

Data Collection and Presentation

1. Oksid decomposition

Prove for carbon

before / after
lime water / transparent / white precipitation

Table 1: Changes of lime water during the experiment; before and after heating

Proove for hydrogen

before / after
CuSO4 / white powder / green powder

Table 2: Changes of copper sulphate during the experiment; before and after heating

2. Reducing decomposition

before / after
1st test tube
FeSO4
FeCl3 / transparent / black blue
2nd test tube
Pb(CH3COOH)2 / transparent / brown solution with black particles
3rd test tube
FeCl3 / transparent / blood red
4th test tube
AgNO3 / transparent / white precipitation

Table 3: Changes of the solution during the experiment; before and after adding reagents

3. Beilstein reaction

Our organic compound burned with green flame.

Data processing and presentation

1. Oksid decomposition

White precipitation (calcium carbonate) in lime water was made. That means that carbon was present in our unknown compound.
Copper sulphate also changed colour from white to blue. Blue powder is CuSO4·5H20. That means that hydrogen was also present in our unknown compound.

organic compound CO2(g) + H2O(l)

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

5H2O(l) + CuSO4(s)CuSO4 · 5H20(s)

2. Reduction decomposition

Solution in first test tube changed colour to black blue. That black blue mixture (Fe4[Fe(CN)6] gives us typical prussian blue colour) shows the presence of nitroogen in an unknown compound.

CN- + FeSO4 Fe(CN)2 + SO42-

Fe(CN)2 + 4 CN- [Fe(CN)6]4-

3Na4[Fe(CN)6] + 4FeCl3 Fe4[Fe(CN)6] + 12NaCl

Black particles were made in second test tube. These black particles are are PbS. This is a proof for presence of sulphur in an unknown organic compound.

S2-(aq) + Pb(CH3COO)2(aq) PbS(s) + 2 CH3COO-(aq)

In a third test tube solution changed colour to blood red. Blood red is typical for [Fe(H2O)5NCS]2+.

NCS-(aq) + FeCl3(aq) [Fe(H2O)5NCS]2+

White precipitation was made in the fourth test tube. White precipitate is AgCl. We proved presence of chlorine in our unknown compound.

Cl- + AgNO3(aq) AgCl(s)

3. Beilstein reaction

Our organic compound burned with green fire. This is a proof for presence of halogen elements in our compound.


Cu + O2 CuO
CuO + X - CuX2

(X is halogen)

Conclusion and Evaluation

We proved that our compound consists of carbon, hydrogen, sulphur and chlorine.

You have to watch out for several things when doing first experiment – Oksidation decomposition. First of all you have to put copper sulphate to the right position in the tube. If you put it too close to the organic compound, there is big probability that you will be also heating copper sulphate not just the organic compound. When heating copper sulphate chemical reaction occurs. Because of that we can no longer prove the presence of the water. But if you put copper sulphate too far from organic compound, there will not come enough water vapour to it. You will not be able to detect the change of colour.
The second thing you have to be aware of is that you have to disconnect the tube and the beaker with lime water after you finish with heating and with an experiment. Otherwise the negative pressure inside of the tube will suck the lime water into the tube. As a result the test tube can explode.

Second experiment is much more dangerous than the first one. But the dangerous part was already done for us. We got already prepared filtrate. You just have to be careful, not to put to much drops of an reagent into the test tube. If you put too much of reagent into the test tube you will not get the typical colour for certain substance. Therefore you will not be able to conclude which substance was present in a tube. For exaple, if you put too much FeSO4 and FeCl3 into the tube, you will get black solution instead of typical blue.

Halogen can be detected easily by the Beilstein test. It is the simplest method for determining the presence of halogen, but it does not differentiate among chlorine, bromine, and iodine, any one of which would give a positive test – green flame. This test is very reliable.