Intermolecular Forces Investigation

Intermolecular forces investigation

Assessed criteria: Criterion C - Processing and Evaluating

Objective

To relate evaporation rate to the intermolecular forces found in different chemicals.

Theoretical background

Evaporation rate: is the amount of liquid that evaporates in a certain amount of time. This is related with the boiling point due to while higher is the boiling point, the evaporation rate will be lower. As the molecular size increases, the evaporation rate will decrease as it will be more difficult to break IMF like Van der Waals.

Penman, made an experiment looking for the evaporation rate from open water, in a lake, and here below we will show a graph with his results

This man came up with a formula to reach the evaporation from water:

Here we leave an example of the evaporation rate of butyl acetate (Ilpi.com, 2015)

The main relationship is that while the intermolecular force is stronger, the boiling point is higher, this is a direct relationship, when one increases, the other increases also.

This is because when the intermolecular force is stronger, it needs to have more kinetic energy (movement) to break the intermolecular force, for this reason, we all know that the heat foment kinetic energy, it foment the movement of the particles

In this image we can see that while we increase the heat, we also increase the movement of the particles, as we can see when all the particles are moving around.

(Chemistry.bd.psu.edu, 2015)

This graph represent the relationship between temperature and intermolecular force. As we can see, there are three lines, they represent the three intermolecular forces (Van der Waals, dipole-dipole, and hydrogen bonding) The one placed at the left is the Van der Waal is force, as we can see, it need very little temperature to break the intermolecular force, however, the one right at his right, the dipole-dipole needs more temperature to break the intermolecular force, and finally, the last one, hydrogen bonding (that is the strongest intermolecular force of the three) needs the higher temperature to break the intermolecular force.

Hypothesis

As we increase the molecular size of the compound, the evaporation rate will decrease. It is an inversely proportion relationship

I make this assumption because to evaporate a substance, every intermolecular force has to be broken down. Every molecule has Van der Waal forces, which are caused by random movements of electrons, which cause temporary dipoles. They are very weak bonds, however big molecules contain more Van der Waal forces than small. For that reason, it is easier to evaporate a molecule with small molar mass than another one with bigger molar mass.

We worked with four different substances:

Methyl Acetate, which has a chemical formula of CH3COOCH3, is a permanent dipole dipole. This means that there is a permanent attraction between the poles, due to a high difference on electronegativity. However, it also has Van der Waal forces, just like all the molecules. (chemspider,2015)

Butyl Acetate, which has a formula of  C6H12O2, is also a permanent dipole dipole, due to a permanent attraction of the poles. However, it also has Van der Waals forces, like every substance

Ethyl Acetate, which has a formula of  C4H8O2, is also a permanent dipole dipole due to a permanent attraction of the poles. However, it also has Van der Waals forces, like every substance.

Propyl Acetate, which has a formula of C5H10O2  is also a permanent dipole dipole due to a permanent attraction of the poles. However, it also has Van der Waals forces, like every substance.

Results

The maximum drop of temperature in Methyl Acetate is of 14,081 (from 20,816 to 6,735)

The maximum drop of temperature in Butyl Acetate is of 10,845 (from 30,616 to 19,771)

The maximum drop of temperature in Ethyl Acetate is of 18,66  (from 30,021 to 11,361)

The maximum drop of temperature in Propyl Acetate is of 15,152 (from 30,443 to 15,291)

Relationship between the drop of temperature and the intermolecular forces. 

As we have seen before, Van der Waals forces are the weakest IMF. However, if the substance is bigger, it will have more VdW forces so it will be more difficult to evaporate. This means that more energy will be needed to evaporate the substance.

           

Here we can see Ethyl Acetate (left) and Butyl Acetate (right). Butyl Acetate is a bigger molecule, so there are more Van der Waals forces in it. This means that it´s more difficult to evaporate as every Van der Waal force has to be broken down.

The drop of temperature on Ethyl Acetate was of 18,66 ºC while on Butyl just 10,845 ºC. That shows how easy was to break down the VdW forces on Ethyl in comparison to Butyl, which just dropped 10 degrees due to the size and the Van der Waals forces.
We also see how in Ethyl the maximum drop just tool 70 seconds happen, while in Butyl a minute and a half. This shows how easy was to break down the intermolecular forces of Ethyl and the amount of energy needed to break down Butyl.
What strikes me out is that the last temperature recorded was the minimum of Butyl. This means that the drop could have continued and that not all the energy was absorbed.

Butyl has a molecular mass of 116,16 g/mol-1 as Ethyl Acetate 88,105 g/mol-1. As we explained before, the size influences evaporation rate due to intermolecular forces. If the molecular mass increases, more Van der Waals forces will be found

Conclusion

Once we have observed both graphs, we can see the relationship between the intermolecular forces and the evaporation rate. We have linked the molecular weight and the time taken to drop the temperature. As we said before, if the molecular mass is higher, it will have more Van der Waals forces, therefore it will be more difficult to break down. When we dipped the probe in the substance and then left it 4 minutes outside, the substances which had more molecular mass and therefore, more intermolecular forces, had more difficulties when evaporating. They took more time to evaporate or even they just didn’t evaporate. However, the ones which were small and had weak VdW forces evaporated quickly and took less time to absorb energy. The drop was big as they absorbed a lot of energy in few time.
On the scatter graph we can see that as the molecular mass increases, the time taken to evaporate also increases.
On the bars graph we can see the same but with different organization.

So, in brief, we can conclude that as the molecular mass increases, the rate of evaporation decreases.

Evaluation

While the computer measured the temperature and made a graph with the data, there was a pause button, and the computer could be stopped from recording data by just pressing the spacebar, which can always be accidentally pressed, causing it to ruin the experiment, as when the computer stops, the chemical’s temperature doesn’t stop dropping, which could lead to inaccurate data. This could be avoided by staying away from the laptop that is measuring the temperature.

If during the experiment anything touches of warms up the probe, the temperature drop would not be the same, therefore, it is better not to touch it and to keep objects away from it, in order to get more accurate and realistic results.

When taking the probe away from the chemical, the play button must be pressed, which means that if there are any problems with the program that records the temperature, then the computer could start recording the temperature at the wrong time, causing it to not record the temperature correctly. The same could happen if the student in charge of doing that presses the play button at the wrong time.

Another problem could be that not all the paper pieces absorbs the same quantity of chemical. Maybe, when testing ethyl it absorbs more than when testing methyl.  This would ruin the experiment as it is not the same to measure evaporation rate of a partially wet paper than a complete wet paper. For avoid this problem, we must deep the same length of probe (6 cm of probe in contact with the chemical, for example) always and keep it inside the same time.

Also, it could also happen that the piece of paper which is stuck to the probe gets impregnated of another substance and that would ruin the experiment. We must be careful and take a brand new and clean piece of paper.

References

Ilpi.com, (2015). The MSDS HyperGlossary: Evaporation Rate. [online] Available at: http://www.ilpi.com/msds/ref/evaporationrate.html [Accessed 22 Jan. 2015].

Education.com, (2015). Evaporation Rate of Water | Education.com. [online] Available at: http://www.education.com/science-fair/article/effect-surface-area-evaporation-rate/ [Accessed 22 Jan. 2015].

Chemistry.bd.psu.edu, (2015). Intermolecular forces. [online] Available at: http://chemistry.bd.psu.edu/jircitano/IMforces.html [Accessed 23 Jan. 2015].