There are different types of intermolecular forces that attract molecules. First, there’s London Dispersion, the weakest one. Generally, when molecules have more atoms, they increase in size and have more electrons. Therefore, their electrons are more likely to drift into one direction making one side of the molecule more negative and the other more positive. So, the London Dispersion makes the molecules more polarized and the oppositely charged ends between molecules would be attracted to each other.
In dipole-dipole forces, the intramolecular forces naturally create an uneven distribution of electrons due to the location of the bonds between atoms and the differences in electronegativity of different atoms. This leads to permanent dipoles in the molecules which cause greater attraction between molecules. An example of this type of force is hydrogen bonding. This is the strongest type of intermolecular force when hydrogen is bonded to the most electronegative elements: nitrogen, oxygen and fluorine. So, molecules with permanent dipoles usually have higher melting and boiling points than non-polar molecules of similar size.
Rankings:
1: Acetone
Acetone was labeled as the molecule with the weakest intermolecular forces because of several reasons. First of all, it doesn’t have any hydrogen bond whereas all the other molecules do. This is because acetone’s oxygen is not bonded to hydrogen to create a hydrogen bond. Since hydrogen bonds are the strongest of the types of intermolecular forces, at least among the molecules from this lab, acetone is immediately marked as the weakest one. The only type of forces it does have is London Dispersion since the electrons can shift towards a particular direction and create a little polarity. From our results, we also found that acetone evaporated at a high rate and had the greatest temperature difference upon evaporation. This proves that acetone has the weakest intermolecular forces because it doesn’t have that many attractions between the molecules and requires less energy to jump off into the air. Also, acetone has a boiling point of 56.3˚C which is lower than boiling points of all the other molecules. This again proves that acetone requires little heat from its surroundings, meaning a lower temperature to evaporate.
2: Methanol
Even though methanol has fewer atoms in the molecule compared to acetone, and therefore less London Dispersion from fewer electrons, it is still stronger than acetone because it has a hydrogen bond. However, it was ranked #2 and not any higher number because it is a smaller molecule than the other ones, so it has less London Dispersion. In our results, we found that methanol had a relatively fast evaporation rate with the second highest temperature difference. This proves that methanol’s hydrogen bond is stronger than the London Dispersion forces in acetone. Methanol’s boiling point is 64.7˚C. This value is also higher than the boiling point of acetone, but it is lower than the boiling points of the following molecules in rank.
3: Ethanol
Ethanol has greater London Dispersion than methanol because it is a larger molecule. Ethanol has an extra carbon in its carbon skeleton which gives room for many more atoms to bond and increase the total number of electrons in the molecule. Since both methanol and ethanol have a hydrogen bond, that doesn’t create a difference in the intermolecular forces when comparing the molecules. It equally adds stronger forces to the molecules instead to give them a higher rank than actone. But, ethanol is still a smaller molecule than the next molecule in rank, which also has a hydrogen bond, so it has lower London Dispersion forces and is ranked as #3. From our observations, we found that ethanol had a temperature difference of 8.8˚C compared to 15.1˚C for methanol. It also evaporated at a slower rate. That means that it would take more time and energy for ethanol to break the intermolecular forces compared to methanol, so it has stronger London Dispersion forces. Ethanol’s boiling point is 78.4˚C. This is higher than the value for methanol and therefore reassures that our results are correct.
4: Isopropyl alcohol
Isopropyl alcohol was ranked #4 because it’s a larger molecule than ethanol with a larger molar mass. This increases its amount of London Dispersion forces acting on it. Since isopropyl alcohol also has a hydrogen bond, that doesn’t make a difference when comparing the molecule to ethanol. However, the molecule ranked after isopropyl alcohol is not ranked due to more London Dispersion or more hydrogen bonds. That molecule fits a unique scenario explained in the next paragraph. However, as we found from our observations, isopropyl alcohol’s temperature difference is lower than ethanol. And it has a boiling point of 82˚C which is higher than ethanol. This all proves that more energy is required for isopropyl alcohol to break its intermolecular forces than ethanol and therefore has stronger forces.
5: Water
Water is the most unique molecule of all in the list. Normally, one would consider water to have one of the weakest forces since it has barely any London Dispersion from being a tiny molecule and also has one hydrogen bond. But, water was ranked #5 because its polarity results from intramolecular forces that cause half of the molecule to be positive and the other half to be negative. In the molecules previously analyzed, only one section of the molecule had hydrogen bonding and was usually one end of the molecule. Even though the dipole faced in that direction, the molecule is not structured to set out an equally positive side. So, only one end of the molecule would have strong attractions to other molecules. Here, the entire molecule has those strong attractions since the molecule itself is based on covalently bonded oxygen and hydrogens. Water can thus have intermolecular forces to four other molecules and creates a neat organization of molecules creating a net of extremely strong attractions. So, water can easily stack on itself causing adhesion and cohesion. Since intermolecular forces between water molecules is based purely hydrogen bonding, the result is much stronger than any of the forces of the molecules previously mentioned. Our results prove this explanation as the evaporation rate of water was much slower than most of the other molecules and has very little temperature difference. Water’s boiling point is 100˚C which is much higher than most of the other molecules proving that a lot more energy is required to break the intermolecular forces that hold water molecules together.
6: Glycerine
Glycerine has by far the strongest intermolecular forces. This is because of two reasons. First, glycerine is the biggest molecule with the highest molar mass. Therefore, it has the most electrons to increase London Dispersion. Secondly, it has three hydrogen bonds because there are three oxygens each bonded to a hydrogen molecule. This is a very important factor when comparing with the other molecules because they only have one hydrogen bond. So the strength of the intermolecular forces is increased by a lot in this molecule. Also, like water, the polar distribution in glycerine is pretty symmetrical. With three oxygens on the bottom and three carbons bonded to hydrogens on the top, it is clearly identified which region is positive and negative. So, it is much easier for glycerine molecules to position themselves to minimize electron repulsion between molecules. If there was only one spot where there was a hydrogen bond on the molecule, the molecules would have more difficulty organizing themselves since there is an uneven ratio of positive and negative regions on each molecule. With gylcerine in its form shown in the picture above, it has the strongest intermolecular forces, which caused it to have the lowest temperature difference in the observations and slow evaporation rate. The low temperature difference means that the solution wasn’t cooled that much as the most energetic molecules evaporated. So, not too many molecules did evaporate to give these results. Also, glycerine’s boiling point is 297˚C which is much higher than any of other molecules. That means that much more energy is required to get the molecules to evaporate and break the forces.
Conclusion:
From our results, we can conclude that molecules with high intermolecular forces have lower vapor pressures because more energy is required for them to break those forces. We ranked the molecules in the lab from lowest forces to highest. These were our results: acetone, methanol, ethanol, isopropyl alcohol, water and glycerine.
