Unit 3: Topic 1 - Intermolecular Forces Flashcards
What are London dispersion forces?
London dispersion forces (LDFs) are forces that arise from the constant formation of temporary dipoles, and they are weak intermolecular forces that hold together all compounds.
How do London Dispersion forces work?
They occur when a molecule (polar or nonpolar), by random chance, has more electrons on one side of the molecule than the other creating a temporary dipole. The electrons in this dipole cause electrons in a surrounding molecule to be repulsed, forming an opposing induced dipole in the second molecule, which is slightly attracted to the initial temporary dipole. However, the electrons in the temporary dipole, through random motion, return to a state of even distribution, causing both of the molecules to return to their nonpolar form and breaking the London dispersion force between them. Although London dispersion forces are extremely short-lived, they constantly occur in all molecules but are the weakest type of intermolecular force.
What molecules experience the most London dispersion forces and why?
Larger and heavier molecules tend to experience greater London dispersion forces because they have more electrons than smaller molecules. The more electrons a molecule has, the more polarizable its electron cloud is. Higher polarizability means there are more spatial possibilities for the electrons to arrange themselves unevenly within a molecule and form a temporary dipole. With more opportunities for a temporary dipole to form, the molecules attract more frequently due to LDFs.
Within molecules with the same number of electrons, the molecules with greater surface area tend to face more London dispersion forces because there are more points of contact between adjacent molecules.
What is the difference between London dispersion forces and van der Waals forces?
van der Waals forces are a group of 3 types of intermolecular forces. They are dipole-dipole forces
(the attraction between two opposing natural dipoles), hydrogen bonding (a special type of a dipole-dipole
force where a hydrogen from one dipole is attracted to either a fluorine, oxygen or nitrogen in another
dipole), and London dispersion forces (a force holding together all compounds originating from the
constant formation of temporary dipoles). London dispersion forces are only one type of van der Waals
force, and the terms London dispersion forces and van der Waals forces are not interchangeable.
Define a dipole-induced dipole interaction.
A dipole-induced dipole interaction occurs when a polar molecule interacts with a nonpolar molecule.
The electrons in the permanent dipole cause repulsion in the electrons from the nonpolar molecule,
causing the nonpolar molecule to become an induced dipole. The permanent dipole and induced dipole
face attraction and the magnitude of this attraction depend on the strength of the permanent dipole and
the polarizability of the nonpolar molecule. The polarizability of a nonpolar molecule is the measure of
how easily an electrical charge can distort that molecule’s electron cloud. Higher polarizability means that
it is easier for a permanent dipole to induce a dipole in a nonpolar molecule. As you increase the strength
of the permanent dipole and the polarizability of the nonpolar molecule, the strength of the dipole-induced
dipole interaction increases.
A pictures of all 3 steps of forming a dipole-induced dipole interaction
Step 1. A dipole-induced dipole interaction occurs between a dipole and a nonpolar molecule
Step 2. The electrons in the dipole repulse the electrons in the nonpolar molecule and turn the nonpolar
molecule into an induced dipole.
Step 3. The permanent dipole and induced dipole face attraction.
Define a dipole-dipole interaction.
A dipole-dipole interaction occurs when the positive end of a polar molecule is attracted to the negative
end of another polar molecule. The strength of the interaction increases as the strength of a dipole
increases and the distance between the dipoles decreases.
The positive end of a dipole is attracted to the negative side of another dipole.
Define an ion-dipole interaction.
An ion-dipole interaction occurs when an ion is attracted to the opposing side of a dipole. The strength
of an ion-dipole interaction increases as the charge of the ion increases, the strength of the dipole
increases, and the distance between the ion and dipole decreases.
Negative ions are attracted to the positive side of a dipole and positive ions are attracted to the
negative side of a dipole.
Describe how HF molecules are orientated to form a dipole-dipole interaction.
The HF molecule is polar with a slightly negative charge on the side of F since F is much more
electronegative than H. This means F attracts more of the shared electrons than H. The HF molecules
are orientated with the F of one molecule next to the H of another molecule because the slightly negative
charge of the F attracts the slightly positive charge of the H.0
The negative end of a HF dipole attracts the positive end of another HF dipole
Define hydrogen bonding and the atoms capable of hydrogen bonding.
Hydrogen bonding is a special type of dipole-dipole interaction where a hydrogen is electrostatically
attracted to one of the three most electronegative elements, fluorine, oxygen, or nitrogen. Hydrogen
bonds are much stronger than other dipole-dipole forces but still weaker than ion-dipole forces and ionic and covalent intramolecular forces.
Why is hydrogen bonding stronger than other dipole-dipole forces?
There are two main reasons why hydrogen bonding is especially strong.
- Fluorine, oxygen, and nitrogen are the three most electronegative elements. This means that when
they form a covalent bond with another atom, the shared electrons are pulled more greatly toward the
highly electronegative element. Consequently, the compound becomes a more polarized dipole, which causes stronger intermolecular attraction. - Hydrogen is a very small atom. This is important because it allows the distance between attracting dipoles to be extremely small. Since electrostatic attraction grows proportionally to the square of the distance between
the two dipoles, having the dipoles closer together significantly increase the attraction between
them.
If you look at the formula for electrostatic attraction, reason 1 increases the charge (q1 * q2)
and reason 2 decreases the distance between the dipoles (r), making the overall force (F) greater
Rank the following intermolecular forces in decreasing order of strength: London dispersion forces, ion-ion, ion-induced dipole, hydrogen bonding, ion-dipole, dipole-dipole, dipole-induced dipole.
- Ion-ion
- Ion-dipole
- Hydrogen bonding
- Dipole-dipole
- Ion-induced dipole
- London dispersion forces
In large biomolecules, molecules can fold into themselves to create complex structures. What noncovalent forces are responsible for this folding?
In large biomolecules, different sides of the molecule can form hydrogen bonds and face London
dispersion forces between each other. These forces are usually referred to as noncovalent forces
because they are a type of intermolecular force that occurs between different parts of the same molecule.
These intramolecular hydrogen bonds and London dispersion forces are responsible for the helix
structure of most proteins.
