CHEMISTRY 11
TEMPERATURE versus TIME CURVES
TEMPERATURE:
-a measure of the average kinetic energy possessed by particles of
a substance or mixture
KE = 1/2 mv2
-in a pure substance, all particles have nearly equal mass (note that
isotopes have slightly different mass because they have a different number
of neutrons)
-Therefore a change in temperature is caused by a change in velocity for a
pure substance
CELSIUS SCALE
-based on water
at 1 atmosphere of pressure (1 atm)
or 101.3 kPa:
100oC = boiling point and condensing point
0oC = freezing point (SOLIDIFICATION OR CRYSTALLIZATION) and melting point
(FUSION)
KELVIN SCALE
-no negative numbers!
Absolute zero: a theoretical point where particles stop moving
-273oC = OK
0oC = 273K
100oC = 373K
HEATING CURVE (since temp is increasing with time)
TEMPERATURE
75 % liquid
50% solid
A
l
25 % solid
75
E
D
C
B
TIME
SEGMENT PHASE
(OR STATE)
A
solid
B
solid&liquid
VELOCITY
BONDING
PE vs. KE
particles speed up
KMT!!!
-no change in bondingKE increases
because all energy
going into speeding
PE constant
molecules up
constant velocity
-bonds start being
broken as material melts
KE constant
PE increases
until all
material melted
C
liquid
particles accelerate
-no change in bondingKE increases
PE constant
D
liquid &gas
constant velocity
-bonds broken as
material boils
E
gas
particles accelerate
- no change in bonding
Fourth state of matter: Plasma
-subatomic particles break up at hundreds, even millions of degrees.
-flow like particles of stars
KE constant
PE increases until
all material evapd
KE increases
PE constant
FROM SOLID TO LIQUID TO GAS, POTENTIAL ENERGY INCREASES
BECAUSE THE ENERGY REQUIRED TO BREAK THE INTERMOLECULAR
FORCES OF ATTRACTION IS ABSORBED AND STORED WITHIN THE
PARTICLES. THE BREAKING OF INTERMOLECULAR FORCES IS
ENDOTHERMIC.
GAS PARTICLES HAVE NO
INTERMOLECULAR FORCES
LIQUID PARTICLES
PE
HAVE INTERMOLECULAR FORCES
BUT THEY ARE NOT FIXED
SOLID PARTICLES HAVE FIXED
INTERMOLECULAR FORCES
Evaporation happens when only some liquid particles have acquired enough energy to break
their intermolecular attractions to change into gas particles.
Evaporation takes place at all temperatures.
Boiling happens when all liquid particles have acquired enough energy to break their
intermolecular attractions to change into gas particles. The boiling point of a substance is the
temperature at which the liquid and gas phase exist at equilibrium
COOLING CURVE
Superheated gas
TEMPERATURE
M
N
O
P
Q
Super cooled solid
(below freezing point)
TIME
SEGMENT PHASE
M
N
(OR STATE)
gas
gas &liquid
VELOCITY
BONDING
particles slow down
KMT!!!
-no change in bonding
because all energy
released as particles
slow down
liquid
P
liquid &solid
Q
solid
KE dec
PE constant
constant velocity
-bonds start forming
KE constant
as gas condenses
(mutual attraction of
PE dec
liquid particles - KMT)
until all
gas condensed
particles slow down
-no change in bonding
constant velocity
-bonds form as
material solidifies
particles slow down
- no change in bonding
***
O
PE vs. KE
KE dec
PE constant
KE constant
PE dec until
all matter frozen
KE dec
PE constant
*** Why a constant velocity? Because molecules slow to a certain velocity where
they are “slow enough” to join up and form liquid particles.That is the “red line” – say –
where the molecules are moving slow enough to re-from those intermolecular forces of
attraction.
FROM GAS TO LIQUID TO SOLID, POTENTIAL ENERGY DECREASES
BECAUSE THE ENERGY ABSORBED TO BREAK THE INTERMOLECULAR
FORCES OF ATTRACTION IN HEATING, IS NOW RELEASED AS
INTERMOLECULAR FORCES ARE FORMED DURING COOLING. THE
FORMING OF INTERMOLECULAR FORCES IS EXOTHERMIC.
GAS PARTICLES HAVE NO
INTERMOLECULAR FORCES
LIQUID PARTICLES
PE
HAVE INTERMOLECULAR FORCES
BUT THEY ARE NOT FIXED
SOLID PARTICLES HAVE
FIXED
INTERMOLECULAR
FORCES
TESTING YOUR UNDERSTANDING OF
TEMPERATURE-TIME STUDIES - KEY
1. As ice melts at standard pressure, its temperature remains at 0oC until it has completely melted.
Its potential energy: a) decreases b) increases c) remains the same.
2. Given the equation: H2O (s)  H2O (l). At which temperature will there be a state of
equilibrium between the solid and the liquid at 1 atmosphere of pressure?
a) 0 K b) 100 K c) 273 K d) 373 K
3. The table below show the normal boiling point of four compounds. Which compound has the
strongest intermolecular forces?
Compound
HF (l)
CH3Cl (l)
CH3F (l)
HCl (l)
Normal Boiling Point (oC)
19.4
-24.2
-78.6
-83.7
4. Which phase change is exothermic?
a) H2O (s)  H2O (l)
b) H2O (s)  H2O (g)
c) H2O (l)  H2O (g)
d) H2O (g)  H2O (l)
5. Which phase change is exothermic?
a) freezing of water
b) melting of iron
c) vapourization of ethanol
d) sublimation of iodine
6. The melting point is 60oC
At the segment at 90 oC the liquid and gas particles are at equilibrium. That is, the liquid is
evaporating, or boiling. (Vapourization). The molecules are moving at a constant velocity. KE is
constant. All of the energy absorbed by the heating of the particles between 60oC and 90 oC is being
used to break the intermolecular bonds between the liquid particles. PE increases.
The average kinetic energy is increasing from 30 to 60, from 60 to 90, from 90 to 120.
Draw at least 5 particles (as little circles) as they would appear at 120 oC.
GAS PARTICLES WITH NO INTERMOLECULAR FORCES OF ATTRACTION.