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Isabel Caro

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Chemistry Major

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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


http://answers.yahoo.com/question/index?qid=20070528000024AAn3Fo8

Answers

Lab Experience n. 5 Gay-Lussac's Law

Tuesday 12th February 2013

Lab Experience n. 5 Gay-Lussac's Law

Objective/Task:

Test Gay-Lussac's Law, see how the pressure of a gas changes as we change the temperature.

Background Information:

Pressure: The force exerted per unit area of surface, typical pressure units are ATM, mmHg and kPa.

Volume: The measurement of space taken by a substance, it is length cubed, typical units are L, mL and m3.

Temperature: A measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. Typical units are K, F and C. (1)

Suppose we double the thermodynamic temperature of a sample of gas. According to Charles’s law, the volume should double. Now, how much pressure would be required at the higher temperature to return the gas to its original volume? According to Boyle’s law, we would have to double the pressure to halve the volume. Thus, if the volume of gas is to remain the same, doubling the temperature will require doubling the pressure.This law was first stated by the Frenchman Joseph Gay-Lussac (1778 to 1850). According to Gay-Lussac’s law, for a given amount of gas held at constant volume, the pressure is proportional to the absolute temperature. Mathematically,

                                                               where kG is the appropriate proportionality constant.

Gay-Lussac’s law tells us that it may be dangerous to heat up a gas in a closed container. The increased pressure might cause the container to explode. (2)

Hypothesis:

To start with we should say that what we will say is not a hypothesis, it is a direct conclusion from Gay-Lussac's Law. 
As a direct conclusion from Gay-Lussac Law we state that there is a the pressure of a sample of gas at constant volume, is directly proportional to its temperature in Kelvin, in simpler words, as we increase the temperature of a gas with a constant volume, the pressure will also increase.

Variables:

Independent variable: Temperature (ºC)
Dependent variable: Pressure (hPa)
Controlled variables: Volume of gas. The mass, the amount of gas inside the tube, as it was closed. The nature of the gas (atmospheric gas which is mainly nitrogen). 

Material/Equipment:

- Tripod
- Stand and clamp
- Bunsen burner
- Magnetic stirrer + stir bar (Picture 1)
- Water Container
- Wood squares (Wood planks)
- Gas pressure sensor
- Laptop (including the programme Logger Pro) 

Procedure:

1. Place the water container on top of the tripod. Adjust the height of wood planks so that the magnetic stirrer is at the same level of the tripod. Make one corner of the container be on top of the tripod and the other corner on top of the wood planks.

2.  Arrange the gas tube in the stand with the clamp. Place it inside the water container. (As we want water to circulate, so it must not touch the bottom part).

3. Pour water into the container, just until the elastic band, so that the tube is completely covered except from the plastic tubule. 

4. Set up the bunsen burner and place it under the tripod.

5. Place the stir bar inside the water bath and turn on the magnetic stirrer.

6. Open in your lap top the Logger Pro 3.6.0 computer program to record the data you will obtain.

7. Connect the gas pressure sensor to the plastic tubule, to know the pressure of the gas in the tube, and to the laptop by the USB entrance. 

8. Connect the Temperature Probe (sensor) USB entrance to the laptop and place the other part inside the water bath.

(Picture 2 - how it would like)

9. Put ice inside the water bath to cool down the temperature of the water and consequently the temperature of the gas in the tube. Remove it after 5 minutes.

10. Collect/record the data you have at the moment in the computer program. 

11. Light up the bunsen burner. Once the temperature has increased 3 or 4 degrees remove the bunsen burner from the water container and allow the temperature to stabilize and just then collect the data. 

12. Repeat this last step as many times as you can to record as many data as possible. (try to obtain at least 12)

13. Make a table and a graph with your results.

Results: 

It will be the same for all gases. It is a general law in nature for gases, as you will find the same result for all the gases. It is a general property of matter.

Conversions:
1 K = 273 ºC
1hPa = 0.000986923266716 atm

Explanation of Reyes: How to make a graph using excel?

(At first it was going to be a video, but the programme she was using couldn't produce videos at all, and she couldn't find a better solution)

In Gay Lussac’s Law experiment we need to make a graph using the data we have obtained. In order to do it we have to follow these steps:

Firstly, include the values to do a table. The first column will contain the pressure values and the second column will contain the temperature values. Name the first column P (hPa), and write it at the top of the table. hPa stands for hectopascals. Below it write the values in order. Then, name the second column t (ºC), and fulfill it the same way you did it with the first column.

Secondly, copy the first column and paste it at the right side of the second column, because the temperature has to be the independent variable and the pressure the dependant one in the graph (so, when the temperature rises, the pressure rises too).

Thirdly, make the graph. In order to do it, select the two columns with the mouse, after that click the button “insert” and click in the graphs section “dispersion only with markers”. The most important thing now is to do the best fitting line. Select the points with the mouse and click the right button, and click “add line of tendency”. Click afterwards “lineal” and also “show the equation in the graph” and “show the value R squared in the graph”. These final two are needed to show the error value in the graph. It is called the regression coefficient and it can be 1 maximum. If it reaches 1, the result will be perfect.

Table 1: Table that shows how the temperature affected the pressure of a gas with a determined volume.

P (hPa)	t (ºC)
15,4745423	100,80167
20,74013	102,4353
25,5784592	104,139604
29,4939479	105,413909
33,6073767	106,824685
37,3726825	108,04899
41,0636121	109,336876
45,282456	110,734052
49,227756	112,138357
53,0649118	113,429115
57,1886965	114,889873
60,8728898	116,164178
64,3273836	117,444936

Table 2: Table that shows the conversion from the units obtained of the magnitudes studied to the S.I units of these magnitudes. 

P (hPa)	P (atm) - SI	t (ºC)	T (K) - SI
15,4745423	0,0153198	100,80167	373,80167
20,74013	0,02053273	102,4353	375,4353
25,5784592	0,02532267	104,139604	377,139604
29,4939479	0,02919901	105,413909	378,413909
33,6073767	0,0332713	106,824685	379,824685
37,3726825	0,03699896	108,04899	381,04899
41,0636121	0,04065298	109,336876	382,336876
45,282456	0,04482963	110,734052	383,734052
49,227756	0,04873548	112,138357	385,138357
53,0649118	0,05253426	113,429115	386,429115
57,1886965	0,05661681	114,889873	387,889873
60,8728898	0,06026416	116,164178	389,164178
64,3273836	0,06368411	117,444936	390,444936

Graphs to show how the pressure of the gas studied with constant volume changed as we increased the temperature.

Graph 1: Temperature (ºC) vs Pressure (hPa)

Graph 2: Temperature (K) vs Pressure (atm) - S.I Units

Conclusions:

We are proud to say that we carried out  the experiment really well because the results we obtained were really closed to what would be perfect. As you can see on the graphs there is a value expressed by R, it refers to the regression coefficient. The ideal value for R is 1, meaning that as closer your experimental R gets to one the better the results - less error. The real value we should take for our experimental R is 0,9998 and not 1, because we should take into account that in the values for the second graph there is more rounding.

We have done a very good experiment because we had 0,0002 points of difference only to reach perfection, because our regression coefficient was 0,9998. In this case we can say that our results have been both very accurated and precised.

In this case we knew that the nature of the gas atmospheric gas which is mainly nitrogen. Furthermore you should know by now that if an experience shows a patter, there is some underline, physical reason. In this case the physical reason refers to Gay-Lussac's Law which is the same for all gases, so it is a general law in nature of gases. 

Pictures:

1.

2.

Videos:

Isabel Caro 

Bibliography:

(1)The ChemEd DL (Sunday, 23 August 2009 13:07); 

Gay-Lussac's Law, Retrieved February 15, 2013 from: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Gay-Lussac-s-Law-952.html

(2)The Columbia Encyclopedia, 6th ed.. 2012."Gay-Lussac's law." Retrieved February 15, 2013 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-X-GayLussa.html