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Chemistry Crunch #4.1 : Name: KEY
Phase Changes
Why? Most substances will eventually go through a phase change when heated or cooled (sometimes they chemically react instead). Molecules of a substance are held together in either the solidi liquid , or gaseous phase by particle attractions. Though we are very familiar with seeing things like snow melting or water boiling, it is necessary to investigate what is occurring at the particle level in order to truly be able to explain how and why snow melts or water boils.
Success Criteria:
- Construct a heating or cooling curve when given the temperature at which phase changes occur.
- Interpret a heating or cooling curve by being able to identify: o the melting/freezing point temperature o the boiling/condensing point temperature o the phase(s) present at a particular temperature o the regions where Kinetic Energy is constant OR changing o the changes that are exothermic or endothermic
Prerequisites:
- Temperature = measure of average kinetic energy of particles in a substance
- Phases of matter: solidi liquid , gas
- Particles in a particular phase have are arranged and attracted to each other in certain ways
- Endothermic
- Exothermic
Vocabulary:
- Fusion (melting)
- Solidification
- Condensation
- Boiling
- Vaporization
- Kinetic energy
Modell: Representations of Molecules in Three Phases
fusion! melting
A
Solid
..
B
Liquid
boiling
F 0 0 °
...
go ~
G
condensation C Gas
Exercises:
Label each arrow (D, E, F, G) with the appropriate phase change (fusion/melting,
solidification/freezing, boiling, condensation).
Key Questions:
1. Which arrows in Modell indicate the addition of energy? --=E __ and ~F
2. Which term, endothermic or exothermic, is used to describe the situation when energy is
added into a system from the surroundings? endothermic
3. Which arrows in Modell indicate the release of energy? -:D^ and-:G__-
4. What are the names of the phase changes that involve a release of energy to the
surroundings by the system?
condensation and freezing/solidification
If you're interested, ask your teacher about why melting is called fusion.
--
r
Information:
MELTING, or "FUSION": We can represent phase changes with particle diagrams. The
following particle diagram represents melting. Melting is also known as fusion, so chemists
refer to the amount of energy needed to make a substance melt as the "heat of fusion".
melting/fusion
~
The added energy decreases the attractions between particles, causing them to be less orderly. but
still attracted to one another.
BOILING: We can represent phase changes with particle diagrams. The following particle
diagram represents boiling. Boiling is a form of vaporization, so chemists refer to the amount
of energy needed to make a substance boil as the "heat of vaporization".
o o o o o (j (^) o
In this case, the added energy completely overcomes the attractions between particles, causing them
to become totally dis- orderly in their conduct, with no attractions to one another.
1. On a particle level, why is energy needed in order to complete an endothermic phase
change, such as melting?
Energy is needed to weaken attractions between particles (need E to separate theml)
- Energy is measured in Joules. Look on Reference Table B... how much energy is needed to melt one gram of solid water?
334 Joules per gram (334 J/g) so ... 334 J
- More energy is needed to make a substance boil than to make it melt. a. How can you tell '~his from Model 2 (the graph)?
It takes more time to boil a substance than to melt it (segment DE is longer than Be).
b. Why do you think this is so, in terms of particle attractions? (Hint: Are liquid particles attracted to each other? Are gas particles attracted to each other?)
Because gas particles have absolutely NO attraction for one another (More energy is
therefore needed to completely overcome attractions rather than just decrease them)
- Based on your Reference Table B, how much energy is needed to boil one gram of liquid
water? 2260 J
Exercise: Constructing a Cooling Curve
A sample of a mysterious substance that is in the gaseous phase is cooled at a constant rate, starting from a temperature of 320°C. After 5 minutes, it has cooled enough to reach its
boiling/condensing point temperature of 275°C. The condensing process takes 20 minutes.
Then the temperature decreases for another 10 minutes. The substance reaches its freezing point (melting point) at 120°C and remains at this temperature for 10 minutes. Then the solid
is cooled for another 15 minutes until it reaches a temperature of 100°C.
Using this information, draw a cooling curve for the sample.
'3CO
'2.():.)
@k
(0 0
D
o 10 J.c; '0 '19 50 (pO ITtv'IQ. Cl"~(') On the curve clearly label the following items in the appropriate locations (use arrows as needed to indicate direction or exact location on the curve. Some terms may be used more than once, as needed.):
(
,
'"
~ ~ (^) ""'" ,
~Il
...... ~
v<.~
...... ,/ &\ t'.~r-
'"
-- ~'lC1^ h~^ _,rlV_ ~ w (^) ~ ~,(I (^) L (^) ''''{> 1
C A^ ,"''' I (^) Do'" 7"^ ^ --..^ i'-... -0^ - Ilv t:: (^) r"" l~l-.^ .....!,,^ \
~ (^) '" I~ (^) 11'< A't i~\ '"
....
"'-
''1,(. (^) U -.,;:oa ......... (^) C l.~ 'L' \
"""
~ I^ I
~ 'I:~^ ~.lIr ^ - , (^) 1<1 (^) ~ 1J"U,s^ 1°f.'I"':. (^) o I I
"-I'--... .....,^ "'-J ~- ~ i
'"
~ ~\o~^ k~ ."....^ "r.~^ .:^ ./ i
............. ~ ft;'-; :>~tl^ -r<r)
i I
- Solid •^ Freezing^ Point^ (fp)^ •^ Melting^ Point^ (mp)
- Liquid •^ Fusion^ •^ Direction^ of
- Gas •^ SOlidification(fc~(l~^0 Endothermic^ Changes
- Condensation (^) • KE cl1aA'ging(o, ~) • Direction of
- Boiling • constant KE (~\ ~~~\ Exothermic^ Changes
