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Chemistry
Calculating Heat
Transfer of Energy into a
Substance

Solid water
(ice) is heated
at a steady
rate until
some has
boiled away
Transfer of Energy into a
Substance

At the plateaus,
the particles
rearrange as the
substance
changes phase.
Energy is stored
as Ei.
Transfer of Energy into a
Substance

On the inclines,
particles are
moving faster
as the
temperature of
the substances
increases.
Energy is
stored as Eth.
Transferring Energy Out of a
Substance

Boiling
water is
cooled at a
steady rate
until it
freezes.
Transferring Energy Out of a
Substance

At the
plateaus, the
particles are
rearranging as
the Ei
decreases and
it changes
phase
Transferring Energy Out of a
Substance

On the
declines, the
particles are
slowing down
as the Eth of
the
substance
decreases.
Heat Capacity (c)



The amount of energy required to raise
the temperature of one gram of a
substance one degree Celsius.
4.18J/g°C or 1 calorie for liquid water
Thus it takes 4.18Joules of energy to
raise one gram of liquid water one
degree Celsius.
Which Requires More Energy?
Raise 10 grams of liquid water one
degree Celsius?
J
4.18
x10 g  41.8 J
or
g C
 Raise one gram of liquid water 100
degrees Celsius?

4.18
J
x100C  418 J
g C
Calculating Heat required to
change temperature (Eth)
The energy required to change the temperature
of a substance is equal the
mass of the substance X the heat capacity of
the substance X the change in Temperature.

Q  (m)(c)( T)
Determining Change in
Temperature (ΔT)

Change in temperature is the difference
from beginning temperature to final
temperature
T  T f  Ti

Change in temperature will be positive
if energy is being added and negative if
energy is being lost
Energy Transfer during a
Phase Change


We can’t use the earlier equation
because there is no temperature
change during a phase change.
This new equation is based on the
amount of energy required to rearrange
the particles during a phase change.
Heat of Fusion (ΔHf )


The amount of energy required to
rearrange the particles of one gram of
solid to one gram of liquid (or one gram
of liquid to one gram of solid)
Heat of fusion for water is 334J/g.
Heat of Fusion (ΔHf )

Heat of fusion can be positive or
negative depending on the direction of
the energy transfer
-Hf
Q
Substance
Substance
Q
+Hf
Heat of Vaporization (ΔHv)


The amount of energy required to change
one gram of liquid into one gram of gas (or
one gram of gas into one gram of liquid)
The heat of vaporization of water is 2260J/g
Heat of Vaporization (ΔHv)

Heat of vaporization can be positive or
negative depending on the direction of
energy transfer.
-Hv
Q
Substance
Substance
Q
+Hv
Calculating Heat during a
Phase Change

The heat required for a phase
change is equal to the mass of the
substance X the heat of fusion /
heat of vaporization of the
substance.
Q  mHf 
Q  mH v 
Practice Problems

A cup of coffee (140 g) cools from 75˚C down to
comfortable room temperature 20˚C. How much
energy does it release to the surroundings?
Eth
Ei
Eth
Cup of Coffee
Q  (m)(c)( T)

J 
20C  75C 
Q  140 g  4.18
g C 

Q  32,200 J
Ei
Practice Problems

Suppose during basketball practice, you lost 2.0 lbs of
water due to sweating. If all of this water evaporated,
how much energy did the water absorb from your
body? (1kg = 2.2lbs)
2.0lb 1kg
Eth
Ei
Eth
Sweat
Q  mH v 

J
Q  910 g  2260 
g

Q  2,060,000 J
Q  2,060kJ
Ei
x
1
2.2lb
.91kg  910 g
 .91kg