Why dry ice solid

1
Why dry ice (solid CO2) sublimates
2
Biodiesel Lab Bomb Calorimeter
Combustion reaction heat flows from the system
to the surroundings exothermic
Heat is lost to (1) water in the calorimeter
(2) the calorimeter itself
3
Section 12.4 Zooming in on Liquids
Liquids are least understood at the molecular
level.
Randomness of particles ? any region is pretty
much identical to any other
Orderly random at different times
Orderliness of particles ? Different regions
identical
Macroscopic properties of liquids are well
understood Surface tension Capillarity
Viscosoty
4
Section 12.4 Surface Tension
Intermolecular forces exert different effects on
a molecule at the surface of a liquid than at the
interior A liquid tends to minimize the of
molecules at the surface.
Interior molecules attracted by water molecules
on all sides Surface molecules attracted to
water molecules below and on sides ? Experience a
net downward attraction
How insects walk on water. (water strider)
At the surface of a liquid, water molecules
behave as a thin, elastic membrane or skin ?
surface tension energy required to increase the
surface area (J/m2 of surface area increased)
5
Section 12.4 Surface Tension
The stronger the forces are between the particles
in a liquid, the greater the surface tension.
6
Section 12.4 Capillarity
Capillary action the rising of a liquid through
a narrow space against the pull of gravity ? due
to competition between intermolecular forces in a
liquid (cohesive forces) And those between the
liquid and the tube walls (adhesive forces)
TLC and plant pigment lab
Meniscus on a test tube Glass SiO2
Mercury (Metallic bonds stronger than any
interaction with SiO2)
Water (H-bonds with SiO2)
7
Section 12.4 Viscosity
A liquids resistance to flow ? resistance
decreases as Temp increases
Molecular shape plays a role Biodiesel lab
Smaller molecules make less contact lower
viscosity
Larger molecules make more contact higher
viscosity
8
Section 12.5 Uniqueness of Water
The water molecules is bent and highly polar
? due to this structure and charge distribution,
water can engage in four H bonds with its
neighbors.
(1) Water is the universal solvent (solvent
the compound that does the dissolving)
Dissolves a range of solutes ( the compounds
that are dissolved)
Polar Covalent substances
Nonpolar Covalent substances
Ionic substances
CH3CH2OH C6H12O6
N2 gas
9
Section 12.5 Uniqueness of Water
(2) Water has a high specific heat capacity (the
measure of the heat absorbed by a substance
for a given rise in temperature Section 6.3)
In other words, water can absorb a lot of heat
with relatively small changes in temp. Earth
Daily temperature changes 40 ºC (in deserts
most extreme) Waterless Moon 250 ºC daily
fluctuations
10
Section 12.5 Uniqueness of Water
(3) Surface properties are crucial to living
things
Trees get water due to capillary action in soils
and in xylem (veins of trees)
Plant debris floating on water surface provides
shelter and nutrients
11
Section 12.5 Uniqueness of Water
(4) Density of solid and liquid water
Large spaces in the ice due to the hexagonal
crystal structure result in solid water being
more dense than liquid ? lake surfaces freeze in
winter (organisms live below)
12
Section 12.5 Uniqueness of Water Summarized
13
Heating-Cooling Curve Practice
How much heat would need to be added to heat 50.0
g of water ice at -50.0 ºC to water vapor at 135
ºC? Given Cice 37.6 J/mol ºC Cliquid
75.4J/mol ºC Cgas 33.1 j/mol ºC ?Hfusion
6.02 kJ/mol ?Hvaporization 40.7
kJ/mol Answer 1.59 x 105 J