UNIT 7: Intermolecular Forces Test

Does a liquid have a tendency to increase or decrease its surface area?

*decrease* its surface area The net force on the molecules is *toward* the center, which decreases the surface area. A decreased surface area is optimal.

Do molecules with a larger or smaller polarity do a better job at inducing dipoles?

*larger* Bigger difference = greater attraction

two key examples of covalent network solids

1) SiO₂ 2) C (graphite and diamond!)

two types of alloys

1) interstitial: smaller elements inserted in between larger element particles 2) substitutional: similar-sized elements substitute places with original element particles

How is vaporization different from boiling/evaporation?

Vaporization does not require the input of energy. It is a spontaneous process -- sometimes, a molecule gets enough kinetic energy by chance to break out of the solid network. Relate this process to vapor pressure. Boiling/evaporation is when heat is added to excite all of the molecules in a substance to eventually vaporize the entire substance.

What is the relationship between viscosity and temperature?

Viscosity decreases as temperature increases.

What is the relationship between viscosity and IMFs?

Viscosity increases as IMFs increase.

Why is water more dense than ice (the density anomaly)?

When water freezes, its hydrogen bonds form a lattice network within the ice, keeping the water molecules farther apart and leaving more space. Thus, the ice is less dense.

In general, a compound with a higher molar mass is going to have stronger or weaker LDFs?

higher molar mass (larger and thus more polarizable)

Do ionic compounds have low or high vapor pressures?

low (because of their high Coulombic attractions)

List the four types of solids in increasing order of melting point.

molecular solids ionic/metallic solids (sort of a tie) network covalent solids

London dispersion forces (LDF) basics

occur between *all molecules* (even the polar ones) Coulombic interactions between two induced dipoles

crystalline solid

particles arranged in an orderly, 3-D repeating pattern (unit cells) e.g.) quartz, ionic solids

Heats of fusion and vaporization: positive or negative?

positive, because it requires energy to loosen intermolecular forces

What is the "hierarchy" of protein structure?

primary: amino acid makeup (what functional groups, for instance) secondary: how amino acids bend and form different shapes > H bonds! tertiary: shapes made on one domain by different amino acid *functional groups* (also called R groups) interacting > can include London dispersion forces or H bonds quaternary: shapes made on multiple domains between different chains of amino acids > LDFs and H-bonds

amorphous solid

random arrangement of particles in solid formation e.g.) glass, rubber

sublimation (relate this to vapor pressure)

solid goes directly to gas (skips liquid) These solids then have a *vapor pressure* because the solids are evaporating. These vapor pressures are typically low because solids have high IMFs.

What is viscosity?

the measure of a liquid's *resistance* to flow

What is vapor pressure? How does it vary with temperature? With IMFs? With boiling point?

the pressure created by a substance evaporating; depends on rate of evaporation An increase in temperature increases vapor pressure (more stuff evaporating). Vapor pressure increases as IMF strength decreases (because molecules are more loosely held). Vapor pressure decreases as boiling point increases (less stuff evaporating).

What is surface tension?

the resistance of a liquid to spread out and increase its surface area

What is capillary action, and what are the two main forces at work? Which force must be greater for capillary action to occur?

the tendency of a liquid to rise up a capillary (small glass tube) placed in a beaker of that liquid 1) adhesive force: intermolecular attractions between liquid and glass 2) cohesive force: intermolecular attractions between liquid particles *The adhesive forces must be greater than the cohesive forces* for capillary action to occur.

What is the best molecular shape for being polarized? I.e., which shape is most polarizable?

those that allow for neighboring molecules to get close to one another (e.g., linear)

distillation

used to separate two liquids with different boiling points a condenser is used to collect the liquid that boils off first (condenser is a smaller tube surrounded by a larger tube filled with cool water) The temperature at the top of the Florence flask (containing mixture of two liquids) is equal to the boiling point of the liquid being boiled off.

How strong are H-bonds compared to regular dipole-dipole interactions? What accounts for this difference?

very strong (because)... 1) F, O, and N are very electronegative (makes compounds very polar) 2) Atoms are very small (concentrated charge differences) 3) lone pairs on F, O, and N increase partial negative charge

When does a liquid boil? (hint: has to do with pressure values)

when the atmospheric pressure is equivalent to the vapor pressure As elevation increases, the boiling point decreases (because of decreased pressure).

how to calculate Rf for paper chromatography

"substance" = particular component of the solute you are investigating

List the seven intermolecular forces of interaction from greatest to least.

1) ion-ion (could also be considered a bond/intramolecular force) 2) ion-dipole 3) H-bonds 4) dipole-dipole 5) ion-induced dipole 6) dipole-induced dipole 7) London disperson (induced dipole-induced dipole) Ions trump polar species, which in turn trump nonpolar species.

Identify and describe the four types of solids.

1) molecular solids: molecules (covalently bonded) held together with pure IMFs (no bonds involved in between the molecules themselves); very weak and soft 2) ionic solids: ions held together by ionic bonds; soluble 3) metallic solids: metals held together by metallic bonds (seas of valence electrons); great at conducting heat and electricity 4) network covalent solids: formula units of elements held together with covalent bonds; hard with very high melting point

What are the two main secondary structures that proteins make? How do they make them?

1) α-helix structure 2) β-pleated sheet *Both are held together by H-bonds.*

What are proteins? What are they made of?

> macromolecules made from amino acids > bound together with peptide bonds The picture shows an amino acid with a peptide highlighted.

What is an induced dipole?

A dipole (polar charge) can be induced in a nonpolar molecule for a brief amount of time, as electrons are fluid and can move around the molecule. This lasts for a very short time.

How does an ionic compound vaporize?

It breaks apart into its component ions. E.g.) NaCl has Na⁺ and Cl⁻ ions just break off and float around.

Describe what a cooling curve looks like for a liquid that *supercools*.

Before turning into a solid completely, the liquid supercools, forming solid crystals in liquid. This actually increases the temperature of the solution because formation of ice releases energy (molecules get closer together).

Describe graphite's structure.

Carbons are single bonded together in sheets The sheets are held together with delocalized π-bonds and LDFs. The sheets can slide past one another and deposit on the paper. Graphite is a *network covalent solid*, so it has a *very high* melting point in order to break those covalent bonds!

paper chromatography

Components of a solution are separated by running them in a solvent through a piece of paper. The solvent could be polar or nonpolar. The components that are attracted more to the solvent (nonpolar for nonpolar, polar for polar) move faster and progress farther down the paper.

What is doping? Explain in the context of semiconductors (n-type vs. p-type) and what elements are typically used.

Doping is adding an impurity to a metal to create a blend that either contains extra electrons or extra *holes* (gaps where electrons should be -- sort of positively charged areas). When you dope elements like silicon or germanium with antimony, arsenic, or phosphorus, the resulting substance has extra electrons. This results in an n-type semiconductor. When you dope elements like silicon or germanium with boron, aluminum, or gallium, the resulting substance has extra holes. This results in a p-type conductor.

column chromatography

First, the solution to be separate (analyte) is pushed through a column filled with a stationary substance (e.g., sand). Then another solute (eluent) is put through the column. Certain components in the analyte will be attracted more to the eluent, and they will come out of the bottom of the column faster. can be used with liquids or gases

What is the heat of fusion? Heat of vaporization?

Heat of fusion is the amount of energy absorbed as one mole of a solid liquefies. Heat of vaporization: ditto but for liquids evaporating

difference between intermolecular and intramolecular forces

Intramolecular forces are bonds. Intermolecular forces are forces of attraction between molecules.

What is the Fermi level?

It is the level of the highest occupied molecular bonding orbital. The Fermi level dictates where electrons are located (highest point) in a semiconductor. When the Fermi level is raised, that means it will take less energy to make electrons jump into the conducting band.

Would larger or smaller ions in ionic compounds be soluble?

LARGER because there is weaker ion-ion attraction (shorter distance) Ion-ion interactions decrease at a greater rate than ion-dipole interactions (when considering movement down a column in the periodic table).

Are larger or smaller molecules easier to polarize?

Larger molecules are easier to polarize because they have more electrons and have a weaker hold on those electrons. *larger and more polarizable* This can increase the strength of IMFs (like London dispersion forces).

Do liquids with greater or weaker IMFs have a high surface tension?

Liquids with greater IMFs have a greater surface tension (directly proportional) because they have larger differences in potential energy between molecules.

Explain molecular orbital theory in the context of the valence and conducting bands.

MO theory states that there are two "zones" or orbitals of electrons formed when two atoms bond together: bonding and antibonding. Electrons like to occupy the bonding region. In valence/conducting band theory, the bonding orbital turns into the valence band, and the antibonding orbital turns into the conducting band. *The conducting band is of higher energy than the valence band* (just like antibonding is of higher energy than bonding). Electrons typically occupy the valence band. *When energy is added to the system, electrons jump over a gap to the conducting band*. This then conducts electricity.

definition of an ideal gas

Particles have no attraction to any other particle.

synthetic polymer: polar or non-polar? (in general)

Plastics (synthetic polymers) are non-polar, so they are just held together by LDFs. *This makes them flexible*

Does the presence of π-bonds increase or decrease polarizability?

Presence of π-bonds increases polarizability because electrons have more freedom to move.

Describe what a heating curve for water looks like.

See image.

rule about proteins (shape + function)

Shape determines function. E.g.) shape of enzyme determines which substrate it can break down

Does surface tension increase or decrease with temperature?

Surface tension decreases as temperature increases because the additional thermal energy pulls molecules apart, increasing surface area and decreasing surface tension.

Do ionic substances experience intermolecular forces?

Technically, no, unless you consider ion-ion attractions to be IMFs. These ion-ion interactions, however, are ionic bonds, so realistically, ionic substances only experience the force of their bonds and no other IMFs.

Explain how the two forces involved in capillary action work together to make the liquid climb up the capillary.

The adhesive forces (the stronger forces) pull the water up the sides of the capillary. The cohesive forces (basically the surface tension of the liquid), wanting to decrease surface area, raise the center of the liquid to reduce the meniscus. Thus, both the sides and the center of the capillary become filled with liquid.

Which force (ion-dipole or ion-ion) must be stronger to make an ionic compound not soluble?

The ion-ion force must be stronger so that the water (a polar species) cannot pull the ions apart. *Use Coulomb's Law* to determine the solubility.

How do you compare two molecules that both experience hydrogen bonding for relative strength?

The molecule with more F, O, or N atoms has stronger H-bonding because it has more spots for H-bonds to form.

Relate potential energy to surface tension and surface area.

The molecules on the surface have more potential energy than those in the interior of a liquid (because those on the surface actually have a net force toward the center). Interior molecules have virtually no PE because their attractions to nearby molecules cancel out. A system wants to *reduce its PE*, and reducing surface area reduces the number of molecules with PE (i.e., molecules on the surface). Thus, *reducing surface area can reduce PE*, so reducing surface area is *favorable*.

Which is better at conducting: n-type or p-type semiconductor?

The n-type semiconductor is better at conducting because it has more electrons (so an overall negative charge). However, p-type semiconductors are still able to conduct well because the holes attract more electrons, increasing electron movement.

What does heat do to a plastic (in terms of IMFs)?

The plastic increases in flexibility and can be molded because the LDFs (type of IMF) weaken. This is also known as melting.

How are the R groups of water soluble proteins arranged?

The polar R groups face out to form H-bonds with water. The nonpolar R groups face in (they don't form as strong IMFs). *These groups actually form LDFs on the inside to help hold the shape.*

As molecules increase in polarity, what happens to the potential energy of the intermolecular forces?

The potential energy decreases (because distance between two molecules decreases as polarity increases). *inversely related*

A liquid is at equilibrium in a sealed container. What can be said about the vapor pressure (rate of vaporization) compared to the rate of condensation?

They are equivalent, and it cancels out, so the same amount of liquid remains (and the same amount of gas).

What is a p-n junction?

When you combine a p-type semiconductor with an n-type semiconductor, you can allow electricity flow in *one direction only*. This takes alternating current and takes only one direction of current in pulsating direct current.

What is an H-bond?

a special type of dipole-dipole IMF (but *NOT A BOND*) that occurs between a hydrogen covalently bonded to Fluorine, Oxygen, or Nitrogen and another F, O, or N with at least one lone pair *must be F, O, or N*

What is a dipole-dipole interaction?

attractive forces (Coulombic interactions) between two polar molecules happens in *polar covalent species*

chemical vs. physical process

chemical: breaks or forms bonds physical: changes in weak intermolecular interactions

What state of matter do polymers normally exist in?

flexible solids or viscous liquids