How do you determine what molecule will have the highest or lowest boiling points.
example: if you had CH3F, N2H4, CH3OH and O2. how would you determine what is the highest boiling points? What do you look at?
Read about intermolecular forces.
Two things in particular.
In general, the boiling point is higher for more mass which just means adding to obtain the molar mass and checking that one for the higher boiling point. For isomers with the same mass, the more compact ones tend to boil at a lower temperature. For example, n-butane you would expect to be higher boiling than tetramethyl methane.
The second thing is hydrogen bonding. Hydrogen bonding makes the boiling point much higher than would be expected, even for lower molecular mass molecules. Hydrogen bonding is always something to consider with the elements with higher electronegativity such as N, F, and O. I see three of the molecules have that but I would expect H bonding to be more of a factor with CH3OH than CH3F. The electronegativity of N is less than that of O. O2 is not polar and you would expect it to have a lower boiling point than any of the otherrs.
To determine which molecule would have the highest or lowest boiling points, you need to consider the intermolecular forces between the molecules.
The strength of intermolecular forces increases in the following order: London dispersion forces < dipole-dipole interactions < hydrogen bonding.
1. London dispersion forces: This is a temporary intermolecular force that occurs in all molecules due to the motion of electrons. Molecules with larger molecular mass tend to have stronger London forces. In your example, both CH3F and CH3OH are polar molecules, and the molecular mass of CH3OH is greater than that of CH3F. Therefore, CH3OH will have stronger London dispersion forces.
2. Dipole-dipole interactions: This force occurs between polar molecules due to the attraction between the positive end of one molecule and the negative end of another molecule. In your example, both CH3F and CH3OH have dipole-dipole interactions since they are polar molecules.
3. Hydrogen bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs between a hydrogen atom bonded to an electronegative atom (such as oxygen, nitrogen, or fluorine) and another electronegative atom in a different molecule. In your example, CH3OH has the potential for hydrogen bonding as it has a hydrogen atom bonded to an oxygen atom.
Considering the above points, the molecules from highest to lowest boiling points in your example would be:
N2H4 > CH3OH > CH3F > O2
N2H4 would have the highest boiling point since it can form hydrogen bonds between its nitrogen and hydrogen atoms. CH3OH would have the next highest boiling point due to hydrogen bonding between the hydroxyl group and the hydrogen atom. CH3F and O2 would have lower boiling points since they only have dipole-dipole interactions and London dispersion forces.
To determine which molecule will have the highest or lowest boiling points, you need to consider several factors including molecular size, molecular weight, types of intermolecular forces, and polarity.
1. Molecular Size and Weight: Generally, larger and heavier molecules tend to have higher boiling points. This is because larger molecules have more electrons and thus experience stronger intermolecular forces.
2. Intermolecular Forces: The nature and strength of intermolecular forces play a crucial role. Here are the three main types of intermolecular forces, listed from strongest to weakest:
a. Hydrogen Bonding: Occurs when a hydrogen atom is bonded to a highly electronegative element (such as oxygen, nitrogen, or fluorine) and forms a bond with another electronegative atom. Molecules that can form hydrogen bonds tend to have higher boiling points.
b. Dipole-Dipole Interactions: Occur between polar molecules due to the attraction between the positive and negative ends of the dipoles. Generally, molecules with stronger dipole-dipole interactions have higher boiling points.
c. London Dispersion Forces: Also known as Van der Waals forces, these forces occur due to temporary shifts in electron density, resulting in temporary dipoles. London dispersion forces increase with increasing molecular size and weight.
3. Polarity: Polar molecules tend to have higher boiling points compared to nonpolar molecules of similar size. This is because polar molecules can experience stronger intermolecular forces such as dipole-dipole interactions and hydrogen bonding.
Applying these concepts to the given example of CH3F, N2H4, CH3OH, and O2:
- CH3F: This molecule has dipole-dipole interactions and London dispersion forces, but no hydrogen bonding. Its boiling point will likely be lower than that of CH3OH.
- N2H4: This molecule can form hydrogen bonds due to the nitrogen-hydrogen bonds. Hydrogen bonding makes N2H4 have a higher boiling point compared to CH3OH and CH3F.
- CH3OH: This molecule can also form hydrogen bonds due to the oxygen-hydrogen bonds. CH3OH will have a higher boiling point than CH3F but lower than N2H4.
- O2: This molecule is nonpolar and lacks dipole-dipole interactions or hydrogen bonding. O2 will likely have the lowest boiling point among the given molecules.
In summary, to determine the relative boiling points of molecules, consider their size, weight, intermolecular forces (particularly hydrogen bonding), and polarity.