Intermolecular Forces Flashcards
Review: Intramolecular Forces
Intramolecular forces are forces which physically keep substances together (e.g., ionic and molecular bonds).
Determines the chemical behaviour of a substance
Intermolecular Forces
An intermolecular force is an attraction between molecules
Note: Ionic compounds do NOT have intermolecular forces because they do not form molecules!
Ionic crystals are held together by electrostatic ionic bonds, which are much stronger than intermolecular forces!
The opposite charges at the end of molecules interact with the opposite charge of another molecule. Ionic compounds are not molecules. These forces exist only in molecules. Molecules are a lot weaker in terms of forces.
Why do we care about intermolecular forces?
The properties of molecular compounds cannot be explained simply by covalent bonds
If covalent bonds were the only forces at work, most molecular compounds would be gases, as there would be no attraction between the molecules strong enough to order the molecules into solids or liquids!!
The space between the molecules impacts the state of molecules and compounds.
The strength of intermolecular forces determines physical properties of molecular compounds
As the intermolecular forces between molecules in a molecular compound increase, the compound’s melting point, boiling point, and surface tension also increase.
The stronger the force between the molecules, tells us what state we would be working with. The 4 properties are used to separate the molecules. the stronger they are is the more force to tear them apart from being held too strongly.
Dipole-Dipole Force
Neighbouring polar molecules align themselves so that oppositely charged dipoles are directed toward each other
These dipole-dipole forces are only about 1% as strong as ionic or covalent bonds
- the attraction between opposite charges is the dipole-diapole force. dont use solid line, only use dash (shows that it is an interaction).
A Special Dipole-Dipole Force: Hydrogen Bonds
A dipole-dipole force is classified as a hydrogen bond if at least one of the dipoles involved arises from an H-F, H-O, or H-N bond (FON)
* Hydrogen bonds are much stronger than regular dipole-dipole forces
Ion-Dipole Force
There are two types of ion-dipole interactions: between a polar molecule and a cation, and between a polar molecule and an anion
The strength of this force depends on the charge and size of the ion, and on the magnitude of the dipole
The charges on cations are generally more concentrated, because cations are usually smaller than anions, so a cation interacts more strongly with dipoles than does an anion that has a charge of the same magnitude
London Dispersion Force
All molecules spontaneously form temporary dipoles due to the random motion of their electrons
One molecule’s temporary dipole induces a temporary dipole in the molecule beside it, which can induce another instantaneous dipole in another adjacent molecule
The process “disperses” through the substance, creating many temporary dipoles that attract one another
Two factors affect the magnitude of LD Forces:
- The number of electrons in the molecule. The more electrons, the greater the chances of forming a temporary dipole, and so the stronger the LD forces
- The shape of the molecule. The more linear the molecule, the greater the surface area of contact between adjacent molecules, and so the stronger the LD forces
Intermolecular Forces & States
For a pure entity, the stronger its intermolecular forces, the higher its melting point and boiling point
Why?
In liquids and (to a greater extent) gases, entities are able to freely move past one another
To achieve this, entities must have sufficient kinetic energy (as measured by its temperature) to overcome its intermolecular forces
For entities with strong intermolecular forces, this sufficient level of kinetic energy is higher – that is, they melt and boil at higher temperatures
Intermolecular Forces & Water Solubility
To dissolve in water, an entity must be capable of being hydrated by water (see below for example)
Hydration can only occur if the water molecules are sufficiently attracted to the entity
Since water has a net dipole, the entity must have a net dipole or a full charge to achieve sufficient attraction to the water molecules
*ion dipole is a force which (hydrates) other molecules. only thing that mixes with polar molecules is polar or ion.
Dipole
a pair of equal and opposite electric charges or magnetic poles of opposite sign separated especially by a small distance
Water: unique due to hydrogen bonding because…
Water is one of the only compounds that is less dense as a solid than a liquid!
As liquid water freezes, the molecules lose kinetic energy, and slow down
Hydrogen bonds start to arrange the molecules into a crystalline structure
This arrangement contains more space between the molecules than does the more random arrangement of liquid water molecules
Water’s surface tension
Water has unusually high surface tension
The hydrogen bonds in water cause water molecules to hold strongly to each other, pulling together and resisting being broken apart.
This causes water to take a shape that has the smallest possible surface area, forming beads or droplets.
Other liquids don’t do this!
