Hey chemists! We are sure that you will find this lab session really interesting and stimulating. Some colleagues and us worked on the same experiment, but we used different alcohols in order to obtain a range of data. This way, we could represent the data on a graph in order to describe the trend or relationship between the variables. The alcohol that we used was butanol.
Make a plastic model of a molecule of butanol.
Background information
In Chemistry, an alcohol is an organic compound in which the hydroxyl functional group (-OH) is bound to a carbon atom. Butanol is a primary alcohol with a 4 carbon structure and the molecular formula of C4H9OH.
In Chemistry, an alcohol is an organic compound in which the hydroxyl functional group (-OH) is bound to a carbon atom. Butanol is a primary alcohol with a 4 carbon structure and the molecular formula of C4H9OH.
- "Butanol - Wikipedia, la enciclopedia libre."
Wikipedia, la enciclopedia libre. N.p., n.d. Web. 21 Mar. 2013.
<http://es.wikipedia.org/wiki/Butanol>.
- "Alcohol - Wikipedia, la enciclopedia libre."
Wikipedia, la enciclopedia libre. N.p., n.d. Web. 21 Mar. 2013.
<http://es.wikipedia.org/wiki/Alcohol>.
Materials:
1- A schlenk tube.
2- Vaseline.
3- A rubber tube, in order to connect the schlenk tube to the vacuum schlenk line.
4-- A pressure sensor.
5- 2 Rubber bands.
6- A computer with Logger Pro installed, in order to record the data.
7- Plastic balls to represent the atoms making up the alcohol. Each color and size represent a different atom.
8.- Butane.
9.- Vacuum schlenk line.
10.- 1Clamp.
11.- 1 Stand.
1- A schlenk tube.
2- Vaseline.
3- A rubber tube, in order to connect the schlenk tube to the vacuum schlenk line.
4-- A pressure sensor.
5- 2 Rubber bands.
6- A computer with Logger Pro installed, in order to record the data.
7- Plastic balls to represent the atoms making up the alcohol. Each color and size represent a different atom.
8.- Butane.
9.- Vacuum schlenk line.
10.- 1Clamp.
11.- 1 Stand.
Procedure
1. Arrange the schlenk tube with its stopper in the correct way. Vaseline is used to avoid the pieces from getting stuck or blocked. The following video shows how the schlenk tube should look like:
2. Pour the butanol inside the schlenk tube.
3. Using a rubber tube, connect the schlenk tube to the vacuum schlenk line.
4. Turn the stopcock tap that allows vacuum to be selected without the need for placing the sample on a separate line. Create vacuum in the schlenk tube. The butanol will boil because the pressure is lowered in a closed container.
5. Connect the pressure sensor to the schlenk tube, and to the computer, and using the program ‘Logger Pro’, notice that there is actually some pressure. The alcohol has produced a vapour which exerts some pressure.
6. Wait until the pressure reaches equilibrium and write down the result.
6. Wait until the pressure reaches equilibrium and write down the result.
7. As different groups used different alcohols, we used all the data in order to make a graph and to figure out the relation between the alcohol used and the vapour pressure.
We observed that the alcohols produce a vapour that exert some pressure. The following video shows how
we figured that out:
Results: The pressure exerted by butanol's vapour was of 5,25 kPa.
Table: Showing the relationship between the pressure exerted by vapours of different alcohols and their molecular mass.
Graph: Showing the relationship between the pressure exerted by vapours of different alcohols and their molecular mass.
Relationship between variables
The relationship that can be found between the variables on the first graph is that as the molecular weight of an alcohol increases, the pressure exerted by the gas decreases. Therefore, there is an inverse relation between the variables. As the experiment was carried out by students and the results were not very precise and do not seem to be correct, we used a processing data computer program in order for it to draw the best fitting line.
EXPLANATION
Volatile substances tend to
evaporate easily. This means that alcohols which are more volatile than others
produce more vapour when exposed. Consequently, the vapour pressure of a
volatile alcohol inside a schlenk tube would be high.
If we know that i
fan alcohol has less carbon atoms, the vapour pressure is higher, it can be
said that volatile alcohols have more quantity of carbon atoms tan non-volatile
or less volatile ones.
As there are more
atoms, it is more difficult for the forces to break (more energy would be
needed) in order to obtain more quantity of vapour. As there is a defined space
inside the schlenk tube, if more gas is produced in the same space, then, the
pressure will be higher.
If an alcohol has more carbon
atoms, it looks more like an alkane, a non-polar compound.
When vapour pressure increases
(as well as the temperature) to the point at which it is equal to the
atmospheric pressure, the liquid will reach its boiling point. As far as
intermolecular forces are concerned, the boiling point is the temperature at
which the molecules, with enough kinetic energy, are able to ‘overcome’
intermolecular attractions between molecules. This way, the higher the
intermolecular attractive forces are, the harder it is for molecules to change
to the vapour state and, therefore, the lower the vapour pressure of that
liquid. As a conclusion, the higher the number of carbon atoms, the higher the
intermolecular attractive forces are and the higher the boiling point is.
The larger the molecule, the greater the number of electrons in it, the more polarizable it is and so the attractive force is greater.
The larger the molecule, the greater the number of electrons in it, the more polarizable it is and so the attractive force is greater.
GRAPHS
We can see that the best fitting line represented on the first graph looks like an exponential graph and we proved that using Excel as reference program. This would mean that there is not a proportional relationship between the variables. On the other hand, if we look at the graph carefully, it is possible to see that, considering that the results obtained are far from precise or correct, although they are acceptable, instead of a best fitting line with exponential appearance, a straight line could be represented. This would mean that there is an inversely proportional relationship between the two variables explained.
Therefore, this has been one of our weaknesses. All of us might have commited some errors, such as not building up the structure correctly at the beginning, or not waiting for the pressure to stabilise and reach equilibrium, which made us obtain rather strange results.
Variables
As the vapour pressure of the alcohols depend on the number of carbon atoms that the alcohols have the dependent variable is the vapour preassure exerted by each alcohol, while the independent variable is the molecular mass of the alcohols.
The structure of butanol
In the lab, we used different coloured balls in order to build up a butanol's molecule. Each ball, of a different size and colour, represented a different element. We used carbon, oxygen and hydrogen, as those are the atoms that make up alcohols.
The following images show many different shapes that the molecule can adopt.
Conclusions
We came up with many different conclusions.
-The first thing that we observed, as explained in one of the videos, was the fact that the alcohols that we used produced a vapour that exerted some pressure.
-As a different pressure was obtained when different types of alcohol were used, we also noticed that different alcohols have different vapour pressures.
-When we built the molecule up we realised that those molecules can adopt different shapes. They are not rigid.
-We also did some research on the different alcohols and we saw that all of the alcohols used were made up of the same atom types: oxygen, carbon and hydrogen, but the arrangement was not the same.
-Looking at the pressures of the vapours emitted by the different alcohols, we got to the conclusion that the bigger the molecular mass of an alcohol is, the lower the pressure exerted by the vapour emitted by this gas.
- All alcohols have a common 'template' or 'pattern'. We have a homologous series of elements with the same structure.
- All alcohols have a common 'template' or 'pattern'. We have a homologous series of elements with the same structure.
Calculations :
rmm carbon: 12
rmm hydrogen: 1
rmm oxygen: 16
rmm octanol (C8H18O):
12x8+1x18+16x1= 130
rmm hexanol (C6H14O):
12x6+1x14+16x1= 102
rmm ethanol (C2H6O):
12x2+1x6+16x1= 46
rmm butanol (C4H9OH):
12x4+1x9+16x1+1x1= 74
rmm propanol (C3H8O):
12x3+1x8+16x1= 60
rmm methanol (CH4O):
12x1+1x4+16x1= 36
So P / T = k (constant).
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