Key Mixon CH 132 4 p.m.
Name: _KEY Mixon CH 132 4 p.m.______________
Homework 4 OPTIONAL ONLY – not for credit
1.) Explain why the formation of solid and/or liquid phase(s) would be favored under
conditions of: Explain using the ideas of kinetic energy (molecular motion), how close together
molecules are/could be, ability to be attracted to its neighbor:
high pressures: High pressures will force molecules to be closer together. The closer
together species are, the more likely they will be able to feel attraction with/to a
neighbor. If a molecule feels and attraction with its neighbor, it will form a liquid if
the attractions are there but intermediate or a solid, if the attractions are great.
low temperatures: Low temperatures slow down the motion of molecules/atoms. This
reduces their kinetic energy. Slower moving molecules/atoms are more likely to be
able to be attracted to a neighbor (when KE is high, they are moving too fast and with
too much energy to be able to stay attracted or even feel an attraction to another
species as they go whirring past!). Since the molecules/atoms are moving more
slowing, the attraction with the neighbor(s) will increase. If the attraction is
intermediate, the species will be a liquid. If there is a lot of attraction then the species
will be a solid.
2.) Will the boiling point of water be higher or lower than 100oC on top of Mt. Everest? Explain
why/how you made your choice.
The species will boil at a lower temperature. Boiling occurs when the atmospheric
pressure = the vapor pressure of the liquid (the molecules pushing up wanting to
leave the liquid phase). If the atmospheric pressure is less, then the vapor pressure
will equal the atmospheric pressure much more easily and therefore will not need as
much heat energy to boil (thus a lower temperature).
3.) Therefore what effect does increasing the atmospheric pressure have on the boiling point of
a liquid – explain your choice?
Increasing the atmospheric pressure has the opposite effect than in the question
above. Now the atmospheric pressure is pushing down harder on the molecules in
the liquid phase (higher pressure like in Question 1!!). The molecules are "forced" to
stay in the liquid phase. In order for the vapor pressure to equal the atmospheric
pressure more heat must be applied so the molecules in the liquid phase have
enough KE to overcome the attractions they are feeling from being shoved so close
1
Key Mixon CH 132 4 p.m.
together. The increase in temperature raises the vapor pressure of the liquid so now
the vapor pressure will equal the atmospheric pressure and the sample will boil. But
adding more heat means raising the temperature!!
Triple point:
0.0060 atm
0.01oC
90oC
0.69atm
4.) Use the phase diagram for water (above). It is not drawn to scale. The triple point
occurs at 0.0060 atm and 0.01oC. Using this knowledge, answer the following questions:
a. Label the triple point – use this as a reference point for the following questions (put a
dot there!)
b. The vapor pressure of liquid water at 90oC is 0.69 atm locate this point on the phase
diagram and label it
c. What is the most stable state of water at 90oC and 0.5 atm?
most likely a gas. (The point could be on the vapor-liquid equilibirum line)
d. What will happen if the pressure is increased from 0.006 atm to 1.5 atm while the
temperature remains at 90oC?
the sample will turn into a liquid
2
Key Mixon CH 132 4 p.m.
5.)
Distinguish the differences between intermolecular forces and intramolecular forces.
Which type of force is stronger? Would you expect a bond dissociating to take more
energy than the energy needed to vaporize a sample? EXPLAIN
Intramolecular forces are the bonds that hold atoms together WITHIN a
molecule/compound.
Intermolecular forces are the attractions that occur BETWEEN separate
molecules/compounds/ions or atoms.
For example: solid water melts at 0oC and turns into liquid water which
boils at 100oC. For example, to disrupt the IMFs in water (dipole dipole or
H-bonding) it takes between 10-40 kJ/mole energy
But in order to BREAK the O-H bond in water is takes 463 kJ/mole!!! (bond
dissociation)
6.) Would you expect to find a significant degree of H bonding between a pure sample of: If a
molecule exhibits H-bonding, please draw an example of this
NH3? Explain. Yes: there is and N-H bond (actually 3 of them!) and the nitrogen has
a lone pair of electrons available for H-bonding
H
N
H
H
CH3OH? Explain. Yes: there is an O-H bond in the molecule and the oxygen has 2 sets
of lone pair electrons available for H-bonding
H
H
C
O
H
H
HBr? Explain. NO!! There is no O-H. N-H, or F-H bond in the molecule
H
Br
3
Key Mixon CH 132 4 p.m.
7.) You have two containers. One contains only dimethyl ether, the other only contains ethanol.
Which substance would you expect to have the higher vapor pressure (put more molecules into
the gas phase), dimethyl ether or ethanol? EXPLAIN citing the types of IMFs that each sample
has.
CH3 – O – CH3
vs.
CH3CH2-O-H
Dimethyl ether is polar
Ethanol is polar
Therefore it has dipole-dipole IMFs Therefore it has dipole-dipole IMFs
DME also has London forces
EtOH also has London forces
EtOH has the ability to H-bond!!
Since EtOH has the ability to H-bond on top of the other IMFs, it has an extra
attraction that needs to be overcome in order for the sample to change phases.
The extra IMF needs more energy in order to change phases which means that it
would need more heat (a higher temperature). Since DME has fewer IMFs it will
be easier for the species to change from the liquid to gas phase compared to EtOH.
Easier means that more molecules will go into the gas phase. If more molecules
are in the gas phase then the measured pressure (in this case the vapor pressure)
will be HIGHER.
8.) Explain why the boiling point of pure liquid NaCl would be higher than NaCl dissolved
in water (HINT: think about the type of IMFs there would be for each sample!!!)
Pure liquid NaCl has ion-ion attraction which are the strongest type of attraction a
set of species can have. In solution NaCl dissolves into ion but the IMF present is
ion-dipole (the ions from Na and Cl with the dipole of water). Since ion-ion is a
MUCH stronger IMF, it will cause the pure liquid to have a higher boiling point.
Ion-ion attractions are very strong (even in the liquid phase! For example, the
boiling point of pure NaCl is 1413 oC!!
4
Key Mixon CH 132 4 p.m.
9.) Which of the following would have the greatest polarizability – briefly explain your choices
(one explanation is good for all if done correctly!)?
N
Al+3
Cl
P-3
or
F
Na+1
or
or
or
I
S-2
Polarizability depends on size. Remember size increases going down a column (top
to bottom) but DECREASES going left to right across a row. Cations (species that
lose electrons and become positively charged) get smaller when they lose their
electrons. Cations will be smaller than their parent atom. Cations compared to
cations must be compared in terms of number of protons to number of electrons. The
more protons there are to electrons, the more those protons can pull on the electrons
and the smaller the species becomes. Anions (species that gain electrons and become
negative) get larger when they gain electrons. The electrons have mass and take up
space and more of them mean more space (larger size!). Anions are larger than their
parent atoms. Anions compared to anions again will depend on the number of
protons present. The fewer the number of protons compared to the number of
electrons the larger the species (the less “pull” the protons will be able to have on the
extra electrons).
THINK about the types of IMFs each sample would have and then answer the question!
10.) The boiling point of phosphine (PH3) is -88oC while the boiling point of ammonia (NH3) is
-33oC. Thus, the boiling point of phosphine is LOWER than that of ammonia even though PH 3
has twice the molar mass. Why?
PH3 has stronger London forces (because of its larger size) but apparently that does
not matter enough! NH3 is a substance that has N-H bonds and a nitrogen with lone
pair electrons, therefore this species can participate in H-bonding!! This extra IMF
gives ammonia its higher boiling point (both species are also polar by the way –
which means that both species have dipole-dipole IMFs ammonia happens to have
stronger dipole-dipole attractions but that is not enough to explain a 55 degree
boiling point difference!!)
5
Key Mixon CH 132 4 p.m.
11.) Which substance in each pair would you expect to have the higher boiling point?
EXPLAIN your choice:
H-F or H-Br: we would pick H-F because it has the ability to H-bond which is a
stronger force (most of the time!!) than London. And both species are polar so both
have dipole –dipole forces. Chances are – since F is the most electronegative atom on
the periodic table so it would also be the most polar and have stronger dipole-dipole
forces than H-Br
CH3CH2OH or HO-CH2CH2-OH: we would pick 1,2-ethanediol. Both species are polar
so they both experience dipole-dipole attractions. Both have O-H groups so they can
H-bond, but the second alcohol can H-bond twice as much (it has the ability to form 6
possible H-bonds: one on each H and then remember the lone pairs on the oxygens,
so each O can H-bond twice. For ethanol, it can only form 3 H-bonds: one on the H,
and two from each set of lone pairs on the oxygen)!
12.) Phosphine (PH3) and hydrogen sulfide (H2S) each contain 18 electrons. What is the molar
mass of each? The dipole moment of PH3 is 0.58 D and the dipole moment of H2S is 0.97D.
Which would have the higher boiling point? Boiling is a “derivation” of vaporization, so which
would have the greater heat of vaporization (Hvap)? (boiling is when all the molecules in a
sample turn to the gas phase (based on vapor pressure and atmospheric pressure) while
vaporization is when the molecules on the surface of a liquid turn into the gas phase)
Both species have very similar molar masses (H2S = 34.08 g/mole PH3 = 32.99 g/mol).
But H2S is more polar (the larger dipole moment!). Therefore H2S would have the
higher boiling point (more attraction/stronger attractions to overcome and therefore
more heat is needed). And it would have the higher heat of evaporation as it is more
attracted to itself and less likely to turn into a gas!
6