EXPERIMENT 6: VAPOUR PRESSURE OF ALCOHOLS

Objective (Alex)

Our objective is to calculate the vapour pressure of Ethanol (C2H6O) while learning to use the vacuum to help us with our later experiments. Our other task is to study how the alcohol's (Ethanol in this case) molecular structure is constructed with the different atoms.

Theoretical Background (Vincent)

Alcohols are organic compounds containing hydroxyl groups, meaning that they contain groups of hydrogen and oxygen (hydroxides). These hydroxyl groups substitute single hydrogen atoms. Alcohols fall into different classes depending on how the -OH group is positioned on the chain of carbon atoms.

Through research, we can observe that:

The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.

The boiling points of the alcohols increase as the number of carbon atoms increases.

This pattern happens because of the intermolecular attractions the substances have with each other.

Ethanol, also called ethyl alcohol, is known for its use in alcoholic beverages as well as being known as a common solvent. It is the most important of the alcohol group. Ethanol forms when ethene and water react in the presence of sulfuric acid. It is part of the Primary alcohol group, and because of its hydroxide molecule (OH), it bonds with only one carbon atom. If it were to bond with 2 or even 3 Carbon atoms, it would belong to either the Secondary or Tertiary alcohol groups. An example of a Secondary alcohol would be butan-2-ol and an example of a Tertiary alcohol would be 2-methylpropan-2-ol.

The vapour pressure of a liquid is basically the equilibrium pressure of a vapour above its liquid in a closed container.

Materials (Carlos)

- Ethanol (C2H6O)

- Schlenk flask

- Stand and clamp

- Rubber bands

- Vaseline

- Pressure gas sensor w/ rubber tube

- Logger Pro (w/ computer)

- Vacuum line



Procedure (Guillermo)

1) Set up the station on which we perform the experiment. Do this by attaching the clamp stand in order to hold the Schlenk tube.

2) After organizing everything, pour the corresponding alcohol (ethanol) into the flask. 

3) Smear vaseline over the top part of the Schlenk flask to make the two parts of the flask fit nicely without any problem, taking care of doing it properly so the pieces don't get stuck and therefore break. 

4) Attach the two parts together and hold them with rubber bands to make sure the flask is well closed so gas doesn't come out. 

5) After getting the Schlenk tube ready, attach the gas pressure sensor to it and attach it all together to the vacuum. We plugged the other end of the gas sensor to the computer with the Logger Pro software and                calculate its pressure.                                                                                             

6) Calculate the pressure of different alcohols, all the way until Octonal, which has the lowest pressure and the highest amount of Carbon atoms. (When in groups) 

Results (Carlos)

Alcohol Experiment: Graph and observations

Pressure according to the number of carbon atoms in alcohols and values of the inverse (Log Pressure)

Pressure according to the number of Carbon atoms

Observations from the graph:

- We have collected some data from the vapour pressure experiment, and with this data we have been able to create a graph with the previous characteristics as shown.

- The graph represents a linear fitting line, which is the logarithm of the Pressure (the inverse of an exponential function is a logarithm). We have done that in order to have a linear function.

- Best fitting line: We have chosen to use a linear fitting line and, as we see, it has a fairly low relative error. In order for it to be more accurate it has to be even closer to 1. We can also observe that the relationship of the variables is inversely proportional, meaning the lower the number of carbon atoms, the higher the pressure and vice versa.

- Variables: We can observe that the vapour pressure depends on the number of Carbon atoms of the corresponding alcohol. Therefore, the independent variable will be the number of carbon atoms and the dependent variable will be the vapour pressure of the alcohols calculated. We can explain this by saying that we receive data due to the amount of Carbon atoms meaning that they affect the pressure of the substance. Along with the other elements, Carbon has the most effect on the vapour pressure since there is a similar amount of Hydrogens and Oxygens that are present.

- Trend: The trend of the function of the graph is to follow its inversely proportional relationship of the variables. The larger the amount of Carbon atoms (X) there are, the lower amount of Pressure (Y) the substance has. This is applied to any context of data and will always follow a trend, making the statement true

Our result of the vapour pressure of ethanol we obtained was: 6.46 kPa. However, since this result was not very accurate, we were told a new number of the pressure we were supposed to obtain, which was: 11.70 kPa. This meant that something may have gone wrong in our experiment.(See conclusions)

Explanation (Guillermo)

There is no doubt that when modifying one of the elements value of the vapour pressure formula will affect directly to the result or value of the others therefore, when we modify the pressure, the volume temperature and so will change our final calculations so it is something that we really need to take into account while working that is why calculations are so important in chemistry the simple fact of adding a coma could make the whole experiment wrong. Eventually it is important to remember that if somehow you reach the point of having two value missing, you don't have to give up and try to find them out by making an equation system which will finally give you the correct result. 

Pressure in atm

Volume in Litres

Number of moles

R is the gas constant : 0.082

Temperature in kelvin (273+ XºC)

 

Conclusions (Vincent)

The final result we obtained by calculating the vapour pressure of ethanol was: 6.46 kPa. However, since this result was not very accurate, we were given a new number of the pressure we were supposed to obtain, which was: 11.70 kPa. This meant that something had gone wrong in our experiment. One thing that may have affected the experiment is the gas sensor which may have read the data wrong. We also could have maybe set up the equipment wrong, making it read the data in a different way. The data that we were given means that the alcohol that we tested has a high vapour pressure compared to other alcohols, due to the fact that it only contains two carbon atoms. Other alcohols, such as heptanol, which contains seven carbon atoms, giving it a lower vapour pressure due to the inversely proportional relationship between the two. We can conclude that alcohols with a larger number of carbon atoms have a higher boiling point. This happens because of the intermolecular forces and its attractions between the two components.

Evaluation (Alex)

As an evaluation, we think that we performed the experiment correctly even though the results we obtained were not the expected. As we thought we performed the experiment correctly, we guessed there was something else that may have gone wrong, supposing that the devices we used were not as precise as they should’ve or we could have as well used them better. Next time we should as well follow strictly the correct usage of the materials and making sure there are no gaps where the air could enter the schlenk tube. If we follow strictly those things, the vapour pressure of the gas, should be approximately or exactly the pressure we expected.

References:

- Chem.purdue.edu (2010). Vapor Pressure. [online] Retrieved from: http://www.chem.purdue.edu/gchelp/liquids/vpress.html [Accessed: 9 Apr 2013].

- Hyperphysics.phy-astr.gsu.edu (2001). Alcohols. [online] Retrieved from: http://hyperphysics.phy-astr.gsu.edu/hbase/organic/alcohol.html [Accessed: 9 Apr 2013].

- Clark, J. (2003). an introduction to alcohols. [online] Retrieved from: http://www.chemguide.co.uk/organicprops/alcohols/background.html [Accessed: 9 Apr 2013].