Section 10.2
The Flow of Energy
1.
To understand how heat energy flows and how it is
measured
2.
To understand how substances differ in their capacity
to absorb and release heat
3.
To solve problems involving heat energy
Learning Objectives
Section 10.2
The Flow of Energy
Recall: Exothermic vs. Endothermic processes
•
Heat flows when there
is a
gradient
•
from
higher to lower
temperature
•
An
exothermic
process
gives off heat (system
loses energy)
•
An
endothermic
process
absorbs head (system
gains energy)
Section 10.2
The Flow of Energy
A.
Thermodynamics
•
Thermodynamics
–
study of energy
•
First law of thermodynamics
–
Energy of the universe is constant:
just as matter is conserved, energy is conserved
Section 10.2
The Flow of Energy
•
Internal energy, E
–
sum
of kinetic and potential
energies of all the
“particles” in a system
A.
Thermodynamics
–
Internal energy can be changed
by two types of energy flow:
•
Heat (q)
•
Work (w)
E = q + w
Section 10.2
The Flow of Energy
•
Thermodynamic quantities always consist of 2 parts:
A.
Thermodynamics
–
A number (magnitude of the change)
–
A sign (indicates the direction of flow)
•
Reflects the system’s point of view
X
X
Q =
energy
flowing
(heat)
Section 10.2
The Flow of Energy
B. Measuring Energy Changes

Units
•
Common energy units for heat
–
Calorie (c)
–
the amount of energy (heat) required to
raise the temperature of one gram of water by 1
o
C.
* Note:
Food calorie
(C) =
1000 calories
or 1kCal *
–
Joule
(J)
–
1 calorie = 4.184 joules
Section 10.2
The Flow of Energy
B. Measuring Energy Changes
The energy (heat) required to change the temperature of a
substance depends on:
1.
The
amount
of substance being heated (mass in g)
2.
The
temperature change
(no. of degrees,
∆T)
∆T =
T
final
–
T
initial
cont.
Section 10.2
The Flow of Energy
B. Measuring Energy Changes
3.
The
type of substance
, because different substances
react differently to heat:
1 calorie raises a gram of water by 1
o
C
1 calorie raises a gram of gold by 32
o
C!
Section 10.2
The Flow of Energy
Water has a relatively high specific heat
–
it absorbs a
lot of heat for a small change in temperature…
Section 10.2
The Flow of Energy
Great Britain
Chicago Latitude: 41.9
o
London Latitude: 51.5
o
Section 10.2
The Flow of Energy
Measuring Energy Changes
–
Ex.: Water
How much energy is needed to raise the temperature of
7.40 g water from 29.0
o
C to 46.0
o
C?
4.184 joules (1 cal) are needed to raise one gram of water
by one degree centigrade; therefore
7.40 g water require
7.40g x 4.184 joules
to be heated by
one degree centigrade.
Since we want an
increase of 17 degrees
,
we must also multiply by 17.0:
4.184 J / g
o
C x 7.40 g x 17.0
o
C = 526 J
Section 10.2
The Flow of Energy
B. Measuring Energy Changes
•
Specific heat capacity
is the
energy required to change
the temperature of one gram
of a substance by one
Celsius degree.
* KNOW THE VALUE FOR WATER! *
Section 10.2
The Flow of Energy
B. Measuring Energy Changes
•
To calculate the energy required for a reaction:
Q = s
m
t
Q
lost
=

Q
gained
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