Section 10.2 The Flow of Energy

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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