Download Phase Changes and latent heat

Lecture 2.5
Phase Changes and latent heat
Foundation Physics
Temperature,
p
, Internal Energy
gy and Heat
Most phase changes, or changes of a substance from one
phase of matter to another, require large amounts of energy
compared
d tto the
th energy needed
d d ffor ttemperature
t
changes.
h
Energy must be put into a Substance to cause it to melt or
boil Energy must be taken out of a Substance to cause it to
boil.
freeze or condense (gas to liquid). The energy can be heat
transfer or can be due to work done on or byy the system.
y
Energy used to cause a phase change does not cause a
temperature change. When ice melts at OoC it becomes
water
t att 0oC;
C when
h water
t b
boils
il att 100oC,i
Citb
becomes steam
t
at 100oC.The same is true in reverse: When water at 0oC
freezes it becomes ice at 0oC; when steam at 100oC
freezes,
condenses it becomes water at 100oC
Atoms,, Molecules,, Phases of Matter
• Heat required for phase changes:
 Vaporization: liquid  vapour
 Melting: liquid  solid
 Sublimation: solid  vapour
• Heat released by phase changes:
 Condensation: vapour  liquid
 Fusion: liquid  solid
 Deposition:
epos t o vapour
apou  solid
so d
Heat of Fusion,, Heat of Vaporization
p
To understand where energy goes during a phase change, first
consider melting and boiling; both require energy input. There are
attractive forces between molecules that must be overcome during
melting
g and boiling.
g For a solid to melt,, the spring-like
p g
forces that
hold molecules in place must be broken, and a certain amount of
energy is required to break each "spring." For a liquid to boil,
attractive forces between molecules must be overcome
overcome, and work
must be done to move the molecules to the larger separations
found in gases. The amount of energy required is thus proportional
to the number of molecules in the object and also to the strength of
the forces acting between molecules.
Q  m  hf
hf
solid
liquid
Q  m  hv
hv
liquid
gas
Latent heats
Latent heat: Energy associated with the phase changes
Heat of combustion
• Chemical reactions such as combustion are
analogous to phase changes in that they involve
definite quantities of heat
heat. Complete combustion
of 1 gram of gasoline produces about 46000 J.
H t off combustion
Heat
b ti hc off gasoline
li iis:
46000 J/g = 4.6x107 J/kg
Complete conversion to CO2 and H2O
Energy from food:
C6H12O6 + 6O2 ->
> 6CO2 + 6H2O + energy
1g of glucose -> 3.81 cal/g released
Evaporation
Humidity has a definite effect on the net evaporation
rate
t off water:
t the
th higher
hi h the
th h
humidity,
idit th
the llower th
the
evaporation rate.
vapor density
% Relative humidity 
 100
saturation vapor density
Scheme of the experimental setup of the micro
array sensor
Precise %r.h. control
in bacterial growth measurements
Dynamic Detection of selective Microorganism growth
Res
sonant Frequency
y (kHz)
Active micro-organism growth detection on
cantilevers
32.8
Reference lever LB
E. coli
modified Gompertz fit
32.6
32.4
32 2
32.2
32.0
31.8
31.6
31.4
31.2
0
60
120
180
240
300
360
420
480
Time (min)
Gfeller, K
Gfeller
K., et al (2005)
Biosens.Bioelectron.
21 528-533.
Dynamic mode
Micro-organism growth on
nano mechanical systems
nano-mechanical
No growth
H2O v.p.
Micro-Organism
Nutritive layer
Cantilever
Growth
w
->
f
Start: Cantilever ‚inked‘ with micro-organism
H2O v.p.
Micro Organism I Micro-Organism II
Micro-Organism
Nutritive layer
Cantilever
Dynamic mode
Nugaeva. N. et al. Biosensors & Bioelectr. (2005)
Saturation density
y of water vapor
p in air
Temperature (oC)
Water Vaport Density (g/m3)
-10
10
2 36
2.36
0
4.85
5
6.80
10
9.40
15
12.83
20
17 30
17.30
25
23.0
30
30.4
37
44.0
40
51.1
60
130 5
130.5
80
293.8
95
505
100
598
200
7840
Problem
• What is the density of water vapor in
grams per cubic meter in the desert when
relative humidity is 10%
% and air
p
is 40oC?
temperature
1g Water heated (Temperature vsvs. time)
Tem
mperatture (°°C)
120
d
100
e
80
60
c
40
20
b
0
-20
a
0
100 200 300 400 500 600 700
Time (sec)
Heat is put into the system at 1 cal/sec
Water is Weird
• Density INCREASES between 0ºC and 4 ºC
• Maximum density of water is 1000 kg/m3 at 4 ºC
C
• Density of ice = 917 kg/m3 .... Ice floats!
Explanation for the Anomalous Behaviour of Water
Problem
• One day the relative humidity is 90% and
the temperature is 25oC. (1) How many
grams off water will condense out off each
p
drops
p
cubic meter of air if the temperature
to 15oC? (2) How much energy does the
condensation from each cubic meter
release?
Gas laws
The gas laws are a set of laws
that describe the relationship
between thermodynamic
temperature (T)
(T), pressure (P) and
volume (V) of gases. They are a
loose collection of rules
p between the late
developed
Renaissance and early 19th
century.
century
Boyle’s
Boyle
s Law
pV  constant
(constant temperature)
Charles’ Law
Charles
V
 constant
T
(constant pressure)
Gay-Lussac’s
Gay
Lussac s Law
p
 constant
T
((constant volume))
Ideal Gas Law
The ideal gas law is a special form of an equation of state
state,
i.e., an equation relating the variables that characterize a gas
(pressure, volume, temperature, density, ….).
The ideal gas law is applicable to low-density gases.
pV
T
pV
 constant (fixed mass of gas)

nRT
pV

Nk BT
p

 RT
PV  nRT
RT
temperature
pressure
volume Ideal Gas Constant
One mole is NA =6.023x1023 molecules
number of moles
(number of 12C atoms in 12 g of 12C)
R=8.31 Nm/moleK
Phases repetition
•
In p
physics
y
and chemistry,
y, the triple
p
point of a substance is the
temperature and pressure at which
three phases (gas, liquid, and solid)
of that substance may coexist in
thermodynamic equilibrium.
•
For example
example, the triple point
temperature of mercury is at
−38.8344 °C, at a pressure of 0.2
MPa.
•
The triple point of water is used to
define the Kelvin, the SI base unit of
thermodynamic temperature. The
number given for the temperature of
the triple point of water is an exact
definition rather than a measured
quantity.
Next Lecture
• To Be Covered: heat transfer
• Reading: Chapter 5
 Section 5
5.4
4
 Section 5.5