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Pre-AP Chemistry
Homework: Phase Quiz Review
1.
Chloroform CHCl3, has a boiling temperature of 60° while CH2Cl2 has a boiling temperature of 40°. Explain this difference
in terms of intermolecular forces.
2.
The boiling temperatures of the hydrides in the oxygen group are shown. Most are very low, except
water. Explain why its boiling point is dramatically different from other members of the same chemical family.
Molecule
Boiling Temperature
H2Te
–2.2°C
H2Se
–42°C
H2S
–61°C
H2O
100°C
At room temperature, the halogen family ranges from a gas (fluorine and chlorine) to a liquid (bromine) to a solid (iodine).
Why do members of this chemical family exist as different phases under the same conditions? Explain the trend as you work
down the family.
4.
Based on the most recent experiment, explain why the temperature probe showed a decrease in temperature as each
liquid evaporated from the tip.
b.
Consider the liquid molecules and temperature probe. Which experienced an endothermic process and which underwent
an exothermic process?
5.
Sodium chloride, NaCl, has a melting point of 800°C and a boiling point of
approximately 1400°C.
a.
Sketch an approximate heating curve to show sodium chloride change
from the solid phase to the liquid and finally to the gas phase over time as
the temperature is increased.
b.
Explain why the phase changes occur at such high temperatures,
compared to the liquids used in the evaporation experiments.
6.
Which molecules are moving faster, on average, liquid water at 0°C or
solid water (ice) at 0°C? Explain.
7.
Why are burns by steam at 100°C generally more severe than burns by water at 100°C?
Review: Phase Changes
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Temperature
3.
Time
2/19/15
8.
Make these pressure unit conversions:
a. 8.12x104 Pa = ____________________ atm
b. 77.01 kPa = _____________________ Pa
c.
d.
0.299 atm = _____________________ Pa
209 kPa = _______________________ Pa
9.
An approximate phase diagram for carbon is drawn. Two solid forms are shown, diamond and graphite.
a.
_________________ At a pressure of 1x108 Pa, what phase change occurs as it is warmed from 3000K to 4000K.
b.
_________________ Is the process in the previous question exothermic or endothermic
for the carbon atoms?
c.
_________________ What phase change occurs if carbon is held at a temperature of
5000 K as its pressure is lowered from 109 Pa to 107 Pa?
d.
_________________ What form does carbon take under normal temperature and
pressure?
e.
What would need to be done to graphite to change it to diamond, at a constant
temperature?
f.
Describe a series of temperature-pressure stages that could change carbon from the gas,
to the liquid to diamond.
10. Use the element calcium for these questions:
a.
Draw its orbital box structure.
b.
_______________________________ Write its electron configuration, in 1s 2s…format.
c.
_________________ How many valence electrons does its atoms have?
d.
What is the most common charge for ions formed by this atom. Explain why.
11. Hypochlorous acid (HOCl) can decompose into oxygen dichloride and water.
a.
Write the balanced equation for this process.
b.
Draw the dot structure of hypochlorous acid.
c.
_________________ 17.83 grams of hypochlorous acid is decomposed into 12.33 grams of oxygen dichloride. Calculate
the percent yield.
Review: Phase Changes
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Homework: Phase Quiz Review [Answers]
(1) Chloroform CHCl3, has a boiling temperature of 60° while CH2Cl2 has a boiling
temperature of 40°. Explain this difference in terms of intermolecular forces.
Both molecules are polar. They would both have dipole-dipole
attractions. CHCl3 has one part (the hydrogen) which stands out as a
single, unique spot. The electrons would be pulled away from this
part toward the more electronegative chlorines, causing CHCl3 to
have significant positive and negative zones on the molecule. In CH2Cl2 the
electrons are also pulled away from the hydrogens, but since
there are two hydrogens (and two chlorines) there would not be a
focused, single area for the positive and negative to form.
Although CH2Cl2 is polar, it has a less dramatic +/– zone (a
weaker “dipole”). Since CH2Cl2 has a weaker attraction between
molecules, it will boil at a lower temperature
Weak attractions = low boiling temperatures
(2) The boiling temperatures of the hydrides in the oxygen group are shown. Most
are very low, except water. Explain why its
All of these molecules are polar so there will be
boiling point is dramatically different from
dipole to dipole attractions. Water has a VERY
other members of the same chemical family.
LARGE dipole because —OH sections are unusually
Molecule
Boiling Temperature
polar (there is extremely un-even sharing of
electrons between O & H). This special dipoleH2Te
–2.2°C
dipole attraction is called a hydrogen bond. This
H2Se
–42°C
extra strong attraction between water molecules
H2S
–61°C
raises its boiling point to an even higher
H2O
100°C
temperature than would be expected compared to the
other members of the oxygen family.
Water has strong attractions (hydrogen
bonds) which causes a HIGH boiling point.
(3) At room temperature, the halogen family ranges from a gas (fluorine and chlorine) to a liquid
(bromine) to a solid (iodine). Why do members of this chemical family exist as different phases
under the same conditions? Explain the trend as you work down the family.
F2(gas), Cl2(gas), Br2 (liquid), I2(solid) à as one works “down” this
chemical family, there are more and more total electrons in each
molecule. More overall electrons mean that are there more electrons in
one molecule to affect the electrons in a neighboring molecule. This
“induces” polarity in neighboring molecules causing them to stick
together. F2 and Cl2 don’t have as many electrons so there is not much
induced polarity, the force between them is weak and they fly apart to
form gases. Bromine has enough electrons to cause molecules to “stick
together” to form a liquid. Iodine is has so many electrons that there
is significant induced polarity and it sticks together rather strongly
to form a solid.
(4) Based on the most recent experiment, explain why the temperature probe showed a decrease in temperature as each
liquid evaporated from the tip.
The liquid absorbed or removed energy from the thermometer tip as it evaporated. The fast
(“hot”) molecules flew off of the end of the thermometer and the slow (cool) molecules
remained. The more easily this process occurred, the more the temperature dropped.
b.
Consider the liquid molecules and temperature probe. Which experienced an endothermic process and which underwent
an exothermic process?
The thermometer tip lost energy (exothermic); the molecules received or had energy put into
their motion (endothermic)
(5) Sodium chloride, NaCl, has a melting point of 800°C and a boiling point of approximately 1400°C.
Review: Phase Changes
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a.
Sketch an approximate heating curve to show sodium chloride change from the solid phase to the liquid and finally to the
gas phase over time as the temperature is increased.
1600
Explain why the phase changes occur at such high
temperatures, compared to the liquids used in the
evaporation experiments.
To separate NaCl “units” from each other, ionic
bond needs to be broken. Breaking bonds takes
much more energy than separating molecules held
together by molecular “attractions”. In
addition, ionic bonds are a very strong bond.
They are formed by permanent positive ions
sticking to negative ions (vs. more momentary
and weaker “dipoles.”) To break apart this very
strong + to – bond, much more energy is required
which is observed as a higher temperature.
1400
1200
Temperature
b.
1000
800
600
400
200
0
TIme
(6) Which molecules are moving faster, on average, liquid water at 0°C or solid water (ice) at 0°C?
Explain.
The liquid molecules are more free to move around, so they are generally moving faster. The
overall speed of the molecules depends on the temperature AND the phase.
(7) Why are burns by steam at 100°C generally more severe than burns by water at 100°C?
This is similar
energy of being
to a gas. These
molecules. As a
8.
to the previous question. Steam molecules (water vapor) not only have the
at 100°C, but they also has the energy that went into them to change them phase
freely moving water molecules have more energy than the more restricted water
result, steam molecules have more overall energy and cause more serious burns.
Make these pressure unit conversions:
a. 8.12x104 Pa = ____________________ atm
b. 77.01 kPa = _____________________ Pa
c.
d.
0.299 atm = _____________________ Pa
209 kPa = _______________________ Pa
9.
An approximate phase diagram for carbon is drawn. Two solid forms are shown, diamond and graphite.
a.
_________________ At a pressure of 108 Pa, what phase change occurs as it
is warmed from 3000K to 4000K.
Graphite (solid) to liquid = melting
b.
_________________ Is the process in the previous question exothermic or
endothermic for the carbon atoms?
Heat energy is put into the atoms to separate them=
endothermic
c.
_________________ What phase change occurs if carbon is held at a
temperature of 5000 K as its pressure is lowered from 109 Pa to 107 Pa?
Liquid to vapor = boiling. Basically, for this high
temperature, lowering the external pressure allows the
molecules to escape their neighbors and boil.
d.
_________________ What form does carbon take under normal
temperature and pressure?
Normal = 101.3 kPa and 273°C (really LOW temps and pressures,
compared to this graph) so carbon would be graphite.
e.
What would need to be done to graphite to change it to diamond, at a constant temperature?
Hold the temperature and increase the pressure.
f.
The
(1)
(2)
(3)
Describe a series of temperature-pressure stages that could change carbon from the gas, to the liquid to diamond.
specific numbers are only estimates:
Start with gas [5000 K, 106 kPa, for example]
Increase pressure at this same temp [from 106 kPa to 1010 kPa]
Hold this pressure and cool the carbon [1010 kPa and cool from 5000 K to 3000 K]
Review: Phase Changes
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10. Use the element calcium for these questions:
a.
Draw its orbital box structure.
b.
_______________________________ Write its electron configuration, in 1s 2s…format.
1s2 2s22p6
c.
3s23p6 4s2
_________________ How many valence electrons does its atoms have?
2
d.
What is the most common charge for ions formed by this atom. Explain why.
+2. It loses its outer electrons in the 4th shell. Since there are two, it makes a +2 ion.It
loses these two because they are the farthest from the nucleus. The protons pull on them is the
weakest.
11. Hypochlorous acid (HOCl) can decompose into oxygen dichloride and water.
a.
Write the balanced equation for this process.
2HOCl à OCl2
+ H2 O
b.
Draw the dot structure of hypochlorous acid.
c.
_________________ 17.83 grams of hypochlorous acid is decomposed into 12.33 grams of oxygen dichloride. Calculate
the percent yield.
H O Cl
17.83 g HOCl • 1 mole HOCl • 1 mole OCl2 •
52.458g
2 moles HOCl
86.90 g =14.7683 grams expected (at 100% yield)
1 mole OCl2
12.33 ÷ 14.7683 = 83.5% actual yield
Review: Phase Changes
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