intermolecular Flashcards
Define intermolecular force.
Intermolecular is the electrostatic attraction between molecules. These forces are the key determinants of physical properties such as boiling point, vapour pressure, viscosity, and water solubility etc.
List the three intermolecular forces.
The three intermolecular forces are London dispersion forces, dipole-dipole forces, and hydrogen bonding.
Describe the London dispersion forces.
London dispersion forces are the attraction between molecules as a result of temporary dipoles.
The strength of such forces is related to how polarizable the electrons are, which in turn is related to how big the molecule is. We usually use molar mass as an indication of molecular sze. Therefore, we say London dispersion forces are related to a molecule’s molar mass. The greater the molar mass, the stronger the London dispersion force.
Another factor to consider when comparing molecule’s of similar molar mass is molecular shape. Molecules’ with greater surface area (i.e. long and cylinderical) will have stronger London dispersion forces.
Describe diplole-dipole forces.
Dipole-dipole forces are the attraction between molecules as a result of permanent dipoles.
That is, they are the attraction between polar molecules (or the attraction between the polar parts of the molecule). The more polar the molecule is, the stronger the dipole-dipole forces. Molecular polarity is determined by considering bond polarity and how these bonds are oriented as related to the molecular geometry.
Describe hydrogen bonding
Hydrogen bonding refers to forces of attraction between the nucleus of a hydrogen atom (which is covalently bonded to a nitrogen, oxygen, or fluorine) and a lone pair of electrons on a nitrogen, oxygen, or fluorine. Hydrogen bonding is an intermolecular force (weaker than a typical chemical bond) and can be considered as a very specific kind of dipole-dipole force.
Because the hydrogen is covalently bonded to the highly electronegative nitrogen, oxygen, or fluorine, the hydrogen has large delta positive charge. As such, it is strongly attracted to the lone pair of electrons of nitrogen, oxygen, or fluorine on a different molecule.
Which intermolecular force is common to all molecules?
London dispersion forces exist for all molecules. Some molecules (e.g. H2) only experience London dispersion forces, whereas others (e.g. H2O), in addition to London dispersion forces, have also dipole-dipole forces and hydrogen bonding.
Is hydrogen bonding always the most important (i.e. strongest) intermolecular force for a molecule?
No, while hydrogen bonding is considered to be a “strong” intermolecular force, it may not always be the most important.
For a small molecule, like H2O, London dispersion forces are considered to be relatively weak, but the hydrogen bonding is quite significant.
Whereas for a large molecule like octan-1-ol, (CH3CH2CH2CH2CH2CH2CH2CH2OH), while hydrogen bonding (and dipole-dipole forces) are present, the most important intermolecular force would be London dispersion forces.
Why is the boiling point related to the strength of intermolecular forces?
Although boiling point is defined as the temperature at which the vapour pressure is equal to atmospheric pressure, the boiling point also indicates the temperature at which molecules have sufficient energy to break from the forces of attraction between molecules.
The stronger the intermolecular forces, the more energy that is required to break these forces of attraction, and the higher will be the boiling point.
When predicting the relative boiling points of two molecules, list the factors which need to be considered.
In order - the following needs to be considered:
- a) How do the molar masses compare? b) How much branching in each molecule?
- Are there dipole-dipole interactions?
- Are there hydrogen bonding interactions?
Does a compound’s boiling point also predict its water solubility?
No - boiing point does not predict water solubility. A high boiling point indicates that there are strong intermolecular forces which hold one molecule of a compound to another of the same compound. While the intermolecular forces may include dipole-dipole forces and hydrogen bonding, they may also include London dispersion forces.
High water solubility indicates that molecules of a compound can form strong interactions with water molecules through dipole-dipole forces and especially hydrogen-bonding.
Therefore if a compound’s high boiling point is mainly due to London dispersion forces, then this compound will have poor solubility in water. Boiling point and water solubility will not correlate.
Explain the chemist rule of thumb: “Like dissolves like”.
“Like dissolves like” means that non-polar solvents (like hexane) are likely to dissolve non-polar molecules like other alkanes.
Whereas, polar solvents, like water, are likely to dissolve other polar molecules (like ethanol), especially if these other polar molecules can also engage in hydrogen bonding.
