Download Heat is energy

Thermochemistry
(The study of energy transfers)
Mr. Forte
Atascadero High School
Units of Energy
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The most common energy unit is the joule (J).
1000 J = 1 kilojoule (kJ)
The calorie (cal) is also used.
1.000 cal = 4.184 J
The nutritional Calorie is actually 1000 cal or
one kilocalorie (kcal)
Kinetic vs. Potential Energy
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The energy of a phase change is potential
energy, energy of position.
During a phase change there is no change in
the average kinetic energy (temperature) of
the particles.
In general, the energy change from solidliquid phase is much less than liquid-gas
phase.
Heat vs. Temperature
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Temperature is the measure of the average
kinetic energy of particles (related to particle
motion).
Heat is energy that transfers between
substances at different temperatures (hotter to
colder).
A cup of water has less heat energy to transfer
than a bucket of water at the same temperature.
Temperature Change (T)
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To calculate the amount of heat energy lost or
gained during a change in temperature, the
specific heat formula is used: Q = m · s · T
Where Q = heat energy; m = mass; and s = the
object’s specific heat capacity.
Specific Heat Capacity (s) is the quantity of
energy needed to change one gram of the
substance by 1.0 C.
Common Specific Heat Capacities
See pg.297 ( J / g · C)
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Water (l)
Water (s)
Aluminum (s)
Iron (s)
Copper (s)
Silver (s)
Gold (s)
4.184
2.03
0.89
0.45
0.38
0.24
0.13
Calorimetry
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Calorimetry is the study of heat flow and
measurement during physical and chemical
changes.
A coffee cup calorimeter is often used in high
school chemistry labs to conduct calorimetry
experiments.
Coffee cup Calorimeter
Thermometer
Coffee cup
Stirrer
Practice Problem
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How much heat energy is needed to raise the
temperature of 5.0 grams of liquid water by
12 degrees Celsius? (specific heat of water is
4.184 J/g · C)
Solution:
Q = (5.0 g) · (4.184 J/g · C) · (12  C) = 251 J
Phase Changes and Energy
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The quantity of energy needed to change one mole of a
solid to liquid is called its molar heat of fusion. Energy is
absorbed (endothermic). Reversing the process releases
this same quantity of energy (exothermic).
The quantity of energy needed to change one mole of a
liquid to gas is called its molar heat of vaporization. This
is also an endothermic process.
Change of Phase
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To calculate the amount of heat energy lost or
gained during a change in phase, the heat of
fusion or vaporization formula is used:
Q = mol · Hfus or Q = mol · Hvap
Where Q = heat energy; mol = moles; and H
represents the latent heat of fusion or
vaporization of the substance per mole.
Change of Phase
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Heats of Fusion or vaporization can also be
given as per gram quantities. The equations
then become:
Q = m · Hfus or Q = m · Hvap
Where Q = heat energy; m = mass; and H
represents the latent heat of fusion or
vaporization of the substance on a per gram
basis.