Experiment n. 6 Vapour pressure of alcohols

6th March 2013

Lab Experiment n. 6 Vapour Pressure of alcohols 

Objective: 

Determine the vapour pressure of the given alcohol and see if there is a pattern in the results obtained.

Background:

The vapour pressure, is the pressure exerted by a vapour when the vapour is in equilibrium with the liquid or solid form, or both, of the same substance — i.e., when conditions are such that the substance can exist in both or in all three phases. Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature. The temperature at which the vapour pressure at the surface of a liquid becomes equal to the pressure exerted by the surroundings is called the boiling point of the liquid. (Encyclopædia Britannica, Inc., 2013)                                                                                                                                                                                     What is an Alcohol? Alcohols are organic molecules which form an homologous series with the general formula CnH2n+1OH.  Alcohols (like hydrocarbons) are named according to the number of carbon atoms in the molecule.

Methanol, CH3OH, has n = 1.	Butanol, C4H9OH, has n = 4
Ethanol, C2H5OH, has n = 2	Pentanol, C5H11OH, has n = 5
Propanol, C3H7OH, has n = 3	Hexanol, C6H13OH, has n = 6

(FRANCE, Dr. Colin, 2013) 

The Schlenk line (also vacuum gas manifold) is a commonly used chemistry apparatus developed by Wilhelm Schlenk. It consists of a dual manifold with several ports. One manifold is connected to a source of purified inert gas, while the other is connected to a high-vacuum pump. (SELLA, Andrea; 2008)

- Class notes:

Different alcohols have different vapour pressures. Depending on the carbon atoms they have there are different vapour pressures. The alcohols have the same types of atoms, which are carbon, hydrogen and oxygen. Different combination of these atoms create different alcohols. Molecules are named after the number of carbons. The bigger is the molecule, the lower is the vapour pressure. 

Elasticity is the possibility of molecules to move and it depends on temperature: The colder the material is, the less flexible/elastic.

Alcohols are an homologous series which are a series of elements which have the same structure and characteristics. 
Vapour pressure is a set of molecules that are continuously moving and hit the wall. 

Materials:

- Schlenk line

- Schlenk tube

- Vaseline

- Alcohols (Methanol/Hexanol/Propanol/Butanol/Pentanol/Ethanol/Octanol/Heptanol) 

- Rubber Bands 

Procedure:

1. Look for the molecular structure of the alcohol given. Our alcohol is methanol.

2. Get the small pieces of plastic and build up the molecular structure of your alcohol in 3D: Black (carbon. C) - White (hydrogen. H) - Red (oxygen. O)

3. Take a picture of each of the shapes the molecular structure can adopt, for this you will need to try if the structure can turn, and make it bigger or smaller but maintaining all of its components. Make sure that the background you use is always the same, and it has to have a plain colour, for example white. 

4. Add methanol (or the given alcohol) to the Schlenk tube, there is no exact amount just consider that about two fingers is enough.

5. Spread vaseline around the Schlenk tube to seal the joint. 

6.  Knot the joints of the Schlenk tube and the stopper with the rubber band.

7. Connect the Gas Pressure Sensor to a rubber tubule which will be connected to the Schlenk tube. Use an interface to connect the sensor to the laptop, which should have opened the logger pro computer program.

8. Arrange everything in the stand with the clamp.

9. Connect the equipment you have arranged to the Schlenk line by connecting one of the rubber tubes from the Schlenk line to one of the tubules of the Schlenk tube. 

10. Open the tap from the Schlenk tube and make vacuum.

11. Wait for the pressure to stabilize and write down that exact value for the pressure.

12. Share your results with your classmates to create a table and a graph out of the experimental data obtained.

* We highly recommend you to enter this website in order to perform a safety experiment.: Schlenk+Line+Safety.pdf

Results:

TABLE: A table to show the number of carbons and the molecular masses of alcohols and the experimental value for their vapour pressure. 

Nº of Carbon atoms	Alcohols	Molecular mass (g/mol)	Vapour Pressure (kPa)
1	Methanol	32.04	16.9
2	Ethanol	46.07	11.7
3	Propanol	60.10	9.8
4	Butanol	74.12	5.25
5	Pentanol	88.15	4.10
6	Hexanol	102,16	4.27
7	Heptanol	116.20	2.02
8	Octanol	130.23	2.04

GRAPH 1: Graph to show how the vapour pressure (kPa) changes with the molecular mass of the molecules - alcohols- (g/mol)

GRAPH 2: Graph to show how the vapour pressure changes with the number of carbon atoms of each molecule.

Conclusions: (which are drawn from what we saw)

Although we made vacuum there was still pressure inside the tube, which was the pressure of the substance inside, methanol. 
Different alcohols, have different vapour pressures and there seems to be a relationship between the vapour pressure and the carbon atoms, as you can see both in the table and in the graphs. As the number of carbon increases the vapour pressure decreases. Having more carbon atoms also means having a higher molecular mass, so the same relationship is shown in the second graph.

Different shapes means that they can be moved, they are not rigid, molecules can be bent. Furthermore our molecule couldn't change of shape which means that it is the smallest structure meaning that it starts the homologous series. The other alcohols are formed out of  this one.                                                                               

The fact that we see that the molecules could change their shapes, means that they can adopt different shapes. 
As we can see in the regression coefficient our results can be improved. We think that the main source of error comes from the experimental value of propanol and hexanol as they are the values which match the least with the best fitting line. We are going to pay no heed to them as we see that they are very different to the trend line/best fitting line and because we have more values to study. If we don't take care of them for example in Graph 2:

The R (regression coefficient) is closer to one in this one than in the other, meaning that the other values where sources of error. We think that the group responsible for those alcohols didn't let it enough time to stabilized and they took higher values.

The best graph would be this one:

This would be the best graph to show how the vapour pressure of alcohols decreased as the number of carbon atoms increased, because it shows all the values but the best fitting line does not take into account those that are clearly wrong. It is also better than the others because with the vertical and horizontal lines you see clearly what the points represent.

Explanation:

Volatile substances are those that evaporate easily. In order to measure how easily a substance enters the gas phase we should determine its vapor pressure. Volatile substances have high vapor pressures. Weight is a factor that affects vapor pressure (and volatility). Usually, chemicals with lower molecular masses can enter more easily the gas phase. The reason is that, since given equal amounts of energy, the smaller molecule will travel faster, and can escape into the air (or vacuum) more easily. This is why the vapour pressure decreased as the number of carbons or molecular weight increases. The smaller the molecule the easier to get into the gas phase.

Alcohols have intermolecular forces - Hydrogen bonding, van der Waals dispersion forces and dipole-dipole interactions-. The hydrogen bonding and the dipole-dipole interactions are mostly the same for all the alcohols, but the dispersion forces will increase as the alcohols molecules get bigger. As the molecules get larger and have more electrons, the attractions get stronger. The size of the temporary dipoles increases. This is why the vapour pressures decrease as the number of carbon atoms in the chains increases. It takes more energy to overcome the dispersion forces, it is not so easy to gets to the gas state and so the vapour pressure decreases.

Pictures:

                                         

This gif shows the structure of the molecule of methanol. As we can see, the molecule is so small that it cannot be bent. However here there are shown different positions of the molecule, so that it can be seen clearly.

Videos:

Isabel Caro

Carlos Rico

Reyes Machuca

Work division: Reyes was in charge of writing the procedure for the experiment. Carlos had to do the objective and the material. Isabel did the title, the conclusions and the explanation of the experiment.

Bibliography:

(1) FRANCE, Dr. Colin (2013); Products from Oil; Retrieved (7th April 2013) from: http://www.gcsescience.com/o35.htm

(2) Encyclopædia Britannica, Inc. (2013); Vapour pressure - definition; Retrieved (3rd April 2013) from: http://global.britannica.com/EBchecked/topic/623199/vapour-pressure

(3) SELLA, Andrea (January 2008). "Schlenk Apparatus". Chemistry World: 69. Retrieved (7th April 2013) from: http://www.rsc.org/chemistryworld/Issues/2008/January/ClassicKitSchlenkApparatus.asp 
Images: 

(1)MORA, J.R (5th June 2008), Fabricando Juguetes IV, Retrieved (19th May 2013) from: http://www.jrmora.com/blog/wp-content/uploads/2008/06/gomas-gomillas.jpg

(2) Sigma-Aldrich.Co.LLC (2013), Schlenk reaction and storage tube, Retrieved (18th May) from: https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTA6DbbsTi27_JP6DQkRa5eA9DWSongKLziJ4AUaDCxMfXguThHGA


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