Intermolecular forces, Unit 4

Question 1

London-Dispersion Forces

Answer 1

result of uneven electron distributions within atoms or molecules that create temporary dipoles
-all atoms and molecules have this
-increases with increasing molar mass because molecules or atoms with higher mola masses have more electron dispersion over larger volume
-shape of molecules influences strength of these forces
-long molecules can interact along their length

Question 2

Dipole-Dipole

Answer 2

Exist in all polar molecules
Positive end of one polar molecule of similar molar mass
-Polar molecules have higher melting points than nonpolar molecules of similar molar mass
-Polarity of molecules determines miscibility(ability to mix without separating two states of liquids
-nonpolar and polar don’t mix

Question 3

Hydrogen Bonding

Answer 3

a type of dipole force that is stronger due to a large electronegativity difference (FON)
-not as strong as chemical bonds

Question 4

Ion-Dipole

Answer 4

occurs when an ionic compound is mixed with a polar compound and is important in aqueous solutions of ionic compounds
-strength is one of main factors that is determines solubility

Question 5

surface tension

Answer 5

tendency of liquids to minimize surface area
-molecule at surface of a liquid experiences less intermolecular attractions that molecules in interior of the liquid and so is less stable, has higher potential energy
-paper clip will floats on surface of water because it order for it to sink the water must increase its surface area and increase that is resisted by surface tension
-surface tension decreases as intermolecular forces decrease

Question 6

viscosity

Answer 6

resistance of a liquid to flow
-greater in substance with stronger intermolecular forces because if molecules are more strongly attracted to each other they do not flow around freely
-depends on molecular shape and increasing molar mass, increasing in longer molecules that can interact over a greater area and become entangled
-depends on temperature because thermal energy partially overcomes intermolecular forces(nearly all liquids become less viscous with increasing temperature)

Question 7

capillary action

Answer 7

results from combination of two forces-COHESIVE and ADHESIVE

Question 8

If adhesive forces are greater than cohesive forces…

Answer 8

(ex: water in glass tube) then the attraction to surface draws liquid up the tube and cohesive forces pull along those molecules not in direct contact with tube and cohesive forces pull along those molecules not in direct contact with tube walls
-water rises up tube until force of gravity balances out capillary action

Question 9

If adhesive forces are smaller than cohesive forces…

Answer 9

(ex: liquid mercury) then liquid does not rise up tube

Question 10

cohesive forces

Answer 10

attraction between molecules in a liquid that cause liquid to stay together

Question 11

adhesive forces

Answer 11

attraction between molecules and surface of a tube that cause molecules to spread out over surface of tube

Question 12

why are intermolecular forces weaker than bonding forces?

Answer 12

intermolecular forces are smaller charges over a larger distance

Question 13

what kinds of interactions do intermolecular forces originate?

Answer 13

IMF originate from interactions between partial charges, charges, or temporary charges between molecules, atoms or ions

Question 14

how can you predict the presence of dipole-dipole forces in a compound?

Answer 14

The σ+ end of one molecules is attracted to the σ – end of another molecule

Question 15

how is the miscibility of two liquids related to their polarity?

Answer 15

-Polar liquids are miscible with other polar liquids, but are not miscible with nonpolar liquids.
-Nonpolar liquids are miscible with other nonpolar liquids

Question 16

How does surface tension result from intermolecular forces? How is it related to the strength of intermolecular forces?

Answer 16

-Molecules at the surface have fewer neighbors than molecules in the interior of the liquid and are less stable (higher potential energy).
-Liquids minimizes their surface area so that there are less molecules at the surface creating a “skin”.

Question 17

Viscosity depends on:

Answer 17

-Temperature: thermal energy overcomes IMF
-Molecular shape: longer molecules interact over a larger area and can become entangled
-Molar mass: Increase in LDF causes increased viscosity

Question 18

vaporization

Answer 18

process by which thermal energy can overcome molecular forces and produce a state change from a liquid to a gas

Question 19

larger surface area…

Answer 19

faster rate of evaporation

Question 20

condensation

Answer 20

-opposite of vaporization
-molecules that do not have enough energy stay in liquid state

Question 21

summary of vaporization

Answer 21

-rate of vaporization increases with increasing temperature
-rate of vaporization increases with increasing surface area
-rate of vaporization increases as strength of intermolecular forces decrease

Question 22

endothermic

Answer 22

energy needed to pull molecules away from each other

Question 23

exothermic

Answer 23

-ex: condensation
-heat is released when gas becomes liquid

Question 24

heat(enthalpy) of vaporization

Answer 24

amount of heat required to vaporize one mole of a liquid to gas

Question 25

dynamic equilibrium

Answer 25

rate of condensation and rate of evaporation become equal

Question 26

vapor pressure

Answer 26

pressure of gas in dynamic equilibrium with its liquid

Question 27

boiling point of liquid

Answer 27

temperature at which liquid’s vapor pressure equal externa pressure
-thermal energy is enough for molecules in interior of liquid to break free of their neighbors and enter gas phase
-bubbles in boiling water are pockets of gaseous water that have formed within liquid water

Question 28

normal boiling point

Answer 28

temperature at which its vapor pressure=1 atm
-normal boiling point of water is 100 degrees C
-at lower pressure, water boils are lower temperature
-once boiling point is reached, additional heating causes rapid boiling, does not raise temperature of liquid above its boiling point

Question 29

Explain what happens in the processes of vaporization and condensation.

Answer 29

-Molecules are in constant motion, the greater the temperature, the greater the average energy of the collection of molecules.
-Some particles have more kinetic energy and some have less than the average. Molecules with highest energy will break free from the surface and enter the gas phase (vaporization).
-The greater the temperature, the greater the rate of vaporization as more molecules have enough energy to escape the intermolecular forces in the liquid.
-Some of the water molecules in the gas phase have less energy than the average and may reenter the liquid phase (condensation).

Question 30

Why does the rate of vaporization increase with increasing temperature and surface area?

Answer 30

The intermolecular forces at the surface of the liquid are less than those in the interior so the greater the surface area, the less intermolecular forces at the surface to prevent molecules from entering the gas phase and so increasing surface area increases rate of vaporization.

Question 31

Which evaporates more quickly; 55 mL of water in a beaker with a diameter of 4.5 cm or 55 mL of water in a dish with a diameter of 12 cm? Is the vapor pressure of the water different in the two containers?

Answer 31

-The beaker with a diameter of 12 cm as there is more surface area.
-Vapor pressure will be the same in the two containers as the evaporation rate = condensation rate at dynamic equilibrium.

Question 32

Explain the process of dynamic equilibrium. How is dynamic equilibrium related to vapor pressure?

Answer 32

-Molecules leave the liquid for the gas phase and molecules reenter the liquid from the gas phase
-Dynamic equilibrium is reached when the rate of vaporization = rate of condensation
-Vapor pressure is the pressure of a gas in dynamic equilibrium with the liquid

Question 33

What happens to a system in dynamic equilibrium when it is disturbed in some way?

Answer 33

The system responds in a way that reestablishes dynamic equilibrium

Question 34

What is fusion? Is fusion exothermic or endothermic? Why?

Answer 34

-Fusion (melting) is the phase transition from solid to liquid (we use the term fusion because if we heat several crystals of a solid, they will fuse into a continuous liquid upon melting)
-Fusion is endothermic because solids have less kinetic energy than liquids so heat must be added to increase the kinetic energy of the solid in order for it to melt.

Question 35

solids

Answer 35

at room temperature and atmospheric pressure
-two elements exist as liquids (Hg and Br2)
-eleven as gases(He, Ne, Ar, Kr, Xe, Rn, F, Cl, O, N, H)
-remaining elements are solids

Question 36

solid structures defined based on…

Answer 36

-attractive forces that hold them together
-arrangement of atoms in crystals themselves

Question 37

metallic crystals

Answer 37

delocalized, sea of electrons
-all metals are solid at 25 degrees C(except for Hg)
-rigid structure of metal nuclei and inner electrons
-valence electron are mobile and act to bond metal atoms with varying degrees of force

Question 38

melting points with metals

Answer 38

-some have melting points above 1000 degrees C
-some metals have melting points close to room temperature(Ga and Hg)

Question 39

What are malleability and ductility? Explain why metals possess these properties.

Answer 39

-Malleability: ability of a metal to be molded or shaped without breaking
-Ductility: Ability of a metal to be drawn into a wire
-The structure of the metal allows atoms to move from one position to another without a major disruption of the crystal.

Question 40

What is lattice energy?

Answer 40

Lattice energy is the amount of energy needed to separate the ions in the crystal lattice of an ionic compound.

Question 41

Explain why ionic solids are brittle

Answer 41

-Disruption of the crystal lattice causes ions with the same charge to line up with each other.
-The resulting repulsions cause the crustal to break.

Question 42

What are the properties of molecular solids? Explain these properties.

Answer 42

-Soft, low melting and boiling points
-The crystals are held together by intermolecular forces (dispersion forces, dipole-dipole or H-bonds) which are weaker than those in ionic crystals

Question 43

Substitutional alloy

Answer 43

Metal atoms of similar size and attractive forces take the place of the atoms in the metallic crystal. Properties of the alloy are between the properties of the two metals

Question 44

interstitial alloys

Answer 44

Small atoms fill the spaces between larger atoms. The resulting alloy is stronger and harder than the original metals.

Question 45

alloy

Answer 45

a solution of one metal dissolved into another

Question 46

How is the bonding in graphite different from the bonding in diamond? Explain the differences in the properties of these two substances

Answer 46

-In graphite, each C atom is covalently bonded with three other C atoms in a trigonal planar (sp2) structure (thin sheets). The extra p-electron hold the sheets together weakly and the graphite sheets can slip past each other giving graphite its slippery feel and the ability to conduct electricity.
-Diamond has each C atom covalently bonded to three other C atoms (sp3) in a tetrahedral geometry making diamond very hard.

Question 47

amorphous

Answer 47

do not have defined melting point and soften gradually over a range of temperatures(glass, plastic)