Experiment 6: Vapor pressure on alcohol

Objective: (Angela)

• Learn how to make an alcohol molecule (pentanol).
• Calculate the pressure of the alcohol (pentanol) while creating a vacuum and using a pressure sensor.

Background:

Video 1: Background on pentanol. (Concha)

Video 2: Background on the schlenk line (Irene)

Materials: (Angela)

- Certain alcohol (in our case is pentanol).

- Pressure sensor.

- Interphase.

- Laptop.

- Schlenk tube.

- Rubber bands.

- Clamp. 

- Stand.
- Vaseline. 

Procedure: (Ana)

1.- Cover the top of the schlenk tube with vaseline in order to avoid any possible mistakes creating the vacuum.

2.- Fill the lower part of the schlenk tube with certain alcohol (in our case, pentanol).

3.- Close the schlenk tube.
4.- Grab a rubber band and use it to ensure the schlenk tube remains closed putting it around it as shown in the images below:

5.- Join the rubber band and the gas sensor using the interphase.

6.- Open the Logger Pro in the computer.

7.- Join the rubber tube to the schlenk tube and the gas sensor to the laptop.

8.- Now, using the vacuum tube, create a vacuum inside the schlenk tube and don't stop until you see some bubbles.

9.- Collect the data using the computer.

Video 3: Procedure of the experiment (Ana)

Collecting data:

Table 1: table showing the relation between the different components depending on their number of carbon atoms and the pressure in kPa. (Ana)

Graph 1: graph showing the relation between the Pressure (in kPa) and the number of carbon atoms in each component. (Angela)

1-Pentanol molecule: (Concha)

During the lab experience we had the chance to create the pentanol molecule using small plastic balls made to model molecules. 

The black ones represent the carbon atoms of which we had 5, the white ones stand for hydrogen atoms of which we had 12 and the single red one is an oxygen atom.

The aim of this GIF is to show the different shapes in which we can find the molecule from the most streched positions to the smallest posibles obtained from twisting and turning the small plastic balls. We think that this GIF will allow us to show better one of our conclusions. As we can see in the series of images of the pentanol molecule, this model of the molecule does have flexibility meaning it is not fixed in a single position (appart from the fact that in other forms of pentanol the OH can be found in other places). As in this model, the real molecule of pentanol, does not have a fixed shape so it is also flexible.
GIF: done by Irene -->

Conclusion: (Irene)

While performing the experiment we observed that liquids exerted a pressure and that different alcohols produced different gases with different pressure, each alcohol has a different vapour pressure. We have found that this is because of the volatility of the alcohols. Alcohols are volatile compounds as they are organic compounds and volatility is defined as "the tendency of a substance to vaporize or the speed at which it vaporizes" (Epa.gov, 2000) meaning that the substances with a higher vapor pressure will be more volatile and the ones with lower vapor pressure willbe less volatile this happens because the ones with higher vapor pressure have less carbons therefore it's easier for them to brake their bonds and vaporize while the ones with lower vapor pressure have more carbons and won't be so willing to vaporize as it's harder to brake their bonds.

We also deduced from the results obtained that as more carbon atoms the molecules have the lower the vapour pressure is.

As we comented before  when talking about the pentanol molecule, the fact that we saw different shapes means that the alcohols molecules are flexible, they addopt different shapes. This can be applied to all alcohol molecules as they are all composed by hydrogen, oxygens and carbons and are joined by covalent bonds, they form an homologus series (series of elements with the same characteristics, structure). From the structures of the different alcohols we deduced that they follow a pattern, for every carbon atom there are two hydrogen ones except for the ones in the ends which have three, we could representate it as the following where the n stands for the number of carbon atoms:

video: (Angela)

Conclusion on the graph: (Concha)

As we can see the best fitting line of the graph is a potential one. Now the question is, why? Well, the anwser is simple: the potential best fitting line fits better for this values than the rectilinear one, but again, why? 

Usually, we say that in a rectilinear movement when a variable increases by a fixed amount, the other variable increases or decreases, depending if the relation between them is directly or indirectly proportional, a fixed amount too. The division of the difference within the points of each variable is called slope which is given by the formula beneath:

                                         

In potential functions this changes. The easiest way to explain this is with the uniformly rectilinear movement and the uniformly accelerated rectilinear motion equations. 

                  

As we can see, the first formula corresponds with what is said in the second paragraph of this conclusion. Meanwhile, the second formula does not. This formula is a potential function and so the best fitting line will be curved. 

As we can see the last two components of the equation resemble the uniformly rectilinear movement equation so the difference must be in the other component of the equation. The addition of this third component leads to a change in the progression of the values. With this third component, instead of increasing a fixed amount, as one variable increases a fixed amount, the corresponding values of the other variable will increase more each time. An easy example we can observe in real life is when an object falls to the ground. The velocity of this object increases more and more with each second that goes by and so the resultant best fitting line will be exponential.

Evaluation (Concha):

Now, if we look back to our performance of the experiment we could list some strong and weak points of it so, in case we have to repeat it, we will know what we did wrong and how to improve it.

First of all, I think that one of our strong points is that as the performance of the experiment was dispensed between all the members of the class, we all learned how to perfom this kind of experiment, the theory behind it and how to use the equipment involved in it. In addition, I think that we followed all the steps with great ability as our result is not far from the best fitting line shown in the graph. In general, I think that all the groups did a great job beause the points that are not in the path of the best fitting line are not far from it.

On the other hand, I think that one of our weak points and the cause of our result not being in the path of the best fitting line is that we didn't added enough vaseline to the schlenk tube and therefore we didn't created a complete vacuum. Next time we perform this experiment we will add more vaseline in order to achieve a more accurate result.

In conclusion, I don't think we performed a bad experiment due to the fact that the part of the data we collected is not far from the best fitting line.

References: (Irene)

En.wikipedia.org (1891). Amyl alcohol - Wikipedia, the free encyclopedia. [online] Retrieved from: http://en.wikipedia.org/wiki/Amyl_alcohol [Accessed: 4 Apr 2013].

Epa.gov (2000). Respirable Particles | Indoor Air | US Environmental Protection Agency. [online] Retrieved from: http://www.epa.gov/iaq/voc2.html [Accessed: 1 Jun 2013].

Zeably.com (2012). Pentanol Pictures and Images. [online] Retrieved from: http://www.zeably.com/Pentanol [Accessed: 4 Apr 2013].