Chapter 11: Liquids, Solids, & Intermolecular Forces Flashcards
What does the state of a sample of matter depend upon?
It depends on the magnitude of intermolecular forces among the constituent particles relative to the amount of thermal energy in the sample.
When thermal energy is high compared to the intermolecular forces, what state of matter is the sample usually in? When thermal energy is low?
When thermal energy is high relative to the intermolecular forces, matter tends to be gaseous. When low, matter tends to be a liquid or solid.
What is the difference between crystalline and amorphous solids?
Crystalline solids have their atoms/molecules arranged in a well-ordered three-dimensional array. Amorphous solids have no such long-range order.
Which intermolecular force is present in all molecules and atoms?
The dispersion force.
What are dispersion forces the result of?
Fluctuations in the electron distribution within molecules or atoms. An instantaneous dipole on any one atom induces instantaneous dipoles on neighboring atoms, which then attract one another.
What does the magnitude of the dispersion force depend on?
It depends on how easily the electrons in the atom or molecule move or polarize in response to an instantaneous dipole, which in turn depends on the size (or volume) of the electron cloud.
How is the size of an atom correlated with the magnitude of its dispersion force?
A larger electron cloud results in a greater dispersion force because the electrons are held less tightly by the nucleus and therefore polarize more easily. If all other variables are held constant, the dispersion force increases with increasing molar mass because molecules or atoms of higher molar mass generally have more electrons dispersed over a greater volume.
Why are dispersion forces weaker in neopentane than in n-pentane, resulting in a lower boiling point for neopentane?
The straight shape of n-pentane molecules allows them to interact with one another along the entire length of the molecules. The nearly spherical shape of neopentane molecules allows for only a small area of interaction.
Which intermolecular force exists in all polar molecules?
The dipole-dipole force.
Why do polar molecules have higher melting and boiling points than nonpolar molecules of similar molar mass?
In addition to dispersion forces, polar molecules exhibit dipole-dipole forces. This additional attractive force raises their melting and boiling points.
What is the dipole-dipole force?
The positive end of one permanent dipole (caused by polarity) attracts the negative end of another.
Define miscibility.
The ability to mix without separating into two states.
Which type of molecules exhibit hydrogen bonding?
Polar molecules that contain hydrogen atoms bonded directly to small electronegative atoms–most importantly F, O, or N–exhibit this force.
What is a hydrogen bond?
A hydrogen bond is a sort of super dipole-dipole force. The large electronegativity difference between hydrogen and these electronegative elements causes the hydrogen atom to have a fairly large partial positive charge within the bond, while the F, O, or N atom has a fairly large partial negative charge. In addition, since these atoms are all quite small, the H atom on one molecule can approach the other atom on an adjacent molecule very closely.
Why are the boiling and melting points of ethanol and dimethyl ether different, even though they are isomers?
Ethanol is a liquid at room temperature while dimethyl ether is a gas because the structure of ethanol allows it to undergo hydrogen bonding.
When does the ion-dipole force occur?
This force occurs when an ionic compound is mixed with a polar compound; it is especially important in aqueous solutions of ionic compounds.
List the four intermolecular forces in order of increasing strength.
- Dispersion.
- Dipole-dipole.
- Hydrogen bonding.
- Ion-dipole.
Why do liquids exhibit surface tension?
Because interactions lower potential energy, the greater the number of interactions, the lower the potential energy. Consequently, molecules crowd into the interior of a liquid (and avoid the surface) in order to maximize their interactions with other molecules.
Define surface tension.
The energy required to increase the surface area by a unit amount, or the tendency of liquids to minimize their surface area.
Define viscosity.
The resistance of a liquid to flow. Viscosity is measured in a unit called the poise (P), defined as 1 g/cm x s.
What is the relationship between viscosity and temperature?
Most liquids become less viscous as temperature increases because thermal energy partially overcomes the intermolecular forces, allowing molecules to flow past each other more easily.
Define capillary action.
The ability of a liquid to flow against gravity up a narrow tube.
Capillary action results as a combination of what two forces?
The attraction between molecules in a liquid (cohesive forces) and the attraction between these molecules and the surface of the tube (adhesive forces).
What does it mean to call a liquid volatile?
It means it vaporizes easily.
