# heat by shuifanglj

VIEWS: 2 PAGES: 37

• pg 1
```									               Energy

• Makes objects move

• Makes things stop

• Energy from sun          plants      foods we
eat    energy to live

• We need energy to “do work”

LecturePLUS Timberlake          1
Work is done when

 Go up stairs
 Play soccer
 Lift a bag of groceries
 Ride a bicycle
 Breathe
 Heart pumps blood
 Water goes over a dam
LecturePLUS Timberlake   2
Potential Energy

Is stored energy
Examples
 Water behind a dam
 Compressed spring
 Chemical bonds in gasoline or coal
 Food

LecturePLUS Timberlake   3
Kinetic Energy

Is energy of motion
Examples
Hammering a nail
Water flowing over a dam
Working out
Boxing
Burning gasoline
LecturePLUS Timberlake   4
Some Forms of Energy

Mechanical
Electrical
Thermal (heat)
Chemical

LecturePLUS Timberlake   5
Learning Check E1
A. A car uses more fuel when the air
conditioner is on.
True (1) or False (2)
B. An apple hanging on a tree has potential
energy that can turn into both mechanical
or chemical energy.
True (1) or False (2)

LecturePLUS Timberlake   6
Solution E1

A. A car uses more fuel when the air
conditioner is on. True (1)

B. An apple hanging on a tree has potential
energy that can turn into both mechanical
or chemical energy. True(1)

LecturePLUS Timberlake       7
Temperature

 Particles are always moving.
 When you heat water, the water molecules
move faster.
 When molecules move faster, the substance
gets hotter.
 When a substance gets hotter, its
temperature goes up.
LecturePLUS Timberlake   8
Learning Check E2
Suppose you place water in a freezer.
A. The water particles move
1) faster    2) slower                3) the same
B. The water will get
1) hotter    2) colder                3) stay the same
C. The temperature of the water will be
1) higher    2) lower                 3) the same
LecturePLUS Timberlake                 9
Solution E2
Suppose you place water in a freezer.
A. The water particles move
2) slower
B. The water will get
2) colder
C. The temperature of the water will be
2) lower
LecturePLUS Timberlake   10
Temperature

 Measures the hotness or coldness of an
object
 Determined by using a thermometer that
contains a liquid that expands with heat
and contracts with cooling.

LecturePLUS Timberlake      11
Temperature Scales

LecturePLUS Timberlake   12
Temperature Scales

Fahrenheit             Celsius Kelvin

Water boils   212°F                100°C       373 K

Water freezes 32°F                     0°C     273 K

LecturePLUS Timberlake           13
Units of Temperature between
Boiling and Freezing
Fahrenheit                 Celsius Kelvin

Water boils   212°F                100°C          373 K

180°                 100°C              100K

Water freezes 32°F                     0°C        273 K

LecturePLUS Timberlake              14
Learning Check E3
A. Temperature of freezing water
1) 0°F     2) 0°C     3) 0 K

B. Temperature of boiling water
1) 100°F   2) 32°F     3) 373K

C. Number of Celsius units between the
boiling and freezing points of water
1) 100       2) 180     3) 273
LecturePLUS Timberlake     15
Solution E3
A. Temperature of freezing water
2) 0°C

B. Temperature of boiling water
3) 373K

C. Number of Celsius units between the
boiling and freezing points of water
1) 100
LecturePLUS Timberlake     16
Heat

Energy that flows from something warm to
something cooler
A hotter substance gives KE to a cooler
one
When heat is transferred (lost or gained),
there is a change in the energy within the
substance
LecturePLUS Timberlake 99   17
Learning Check H1
A. When you touch ice, heat is transferred
from
1) your hand to the ice
2) the ice to your hand

B. When you drink a hot cup of coffee, heat
is transferred from
1) your mouth to the coffee
2) the coffee to your mouth
LecturePLUS Timberlake 99    18
Solution H1
A. When you touch ice, heat is transferred
from
1) your hand to the ice

B. When you drink a hot cup of coffee, heat
is transferred from
2) the coffee to your mouth

LecturePLUS Timberlake 99    19
Learning Check H2
When you heat 200 g of water for 1 minute, the
water temperature rises from 10°C to 18°C.

400 g
200 g

If you heat 400 g of water at 10°C in the same
pan with the same amount of heat for 1
minute, what would you expect the final
temperature to be?
1) 10 °C       2) 14°C          3) 18°C
LecturePLUS Timberlake 99           20
Solution H2
2)14°C

Heating twice the mass of water using the
same amount of heat will raise the
temperature only half as much.

400 g
200 g

LecturePLUS Timberlake 99           21
Some Equalities for Heat

Heat is measured in calories or joules
 1 kcal = 1000 cal
 1 calorie = 4.18J
 1 kJ = 1000 J

LecturePLUS Timberlake 99   22
Specific Heat

 Why do some foods stay hot longer than
others?

 Why is the beach sand hot, but the water is
cool on the same hot day?

LecturePLUS Timberlake 99   23
Specific Heat

Different substances have different
capacities for storing energy
It may take 20 minutes to heat water to
75°C. However, the same mass of
aluminum might require 5 minutes and
the same amount of copper may take
only 2 minutes to reach the same
temperature.
LecturePLUS Timberlake 99    24
Specific Heat Values

Specific heat is the amount of heat needed to
raise the temperature of 1 g of a substance by
1°C
cal/g°C          J/g°C
water         1.00                          4.18
aluminum      0.22                          0.90
copper        0.093                         0.39
silver        0.057                         0.24
gold          0.031                         0.13
LecturePLUS Timberlake 99          25
Learning Check H3

A. A substance with a large specific heat
1) heats up quickly    2) heats up slowly
B. When ocean water cools, the surrounding air
1) cools   2) warms 3) stays the same
C. Sand in the desert is hot in the day, and cool
at night. Sand must have a
1) high specific heat   2) low specific heat

LecturePLUS Timberlake 99    26
Solution H3

A. A substance with a large specific heat
2) heats up slowly
B. When ocean water cools, the surrounding air
2) warms
C. Sand in the desert is hot in the day, and cool
at night. Sand must have a
2) low specific heat

LecturePLUS Timberlake 99    27
Measuring Heat

Requires
 Grams of substance
 Temperature change T

 Specific heat of the substance

LecturePLUS Timberlake 99   28
Calculating Heat

mass   x temp. change x                 specific heat
grams x      T                   x     Sp. Ht.

LecturePLUS Timberlake 99              29
Heat Calculations
A hot-water bottle contains 750 g of water at
65°C. If the water cools to body temperature
(37°C), how many calories of heat could be
transferred to sore muscles?

heat = g     x T               x Sp. Ht. (H2O)
750 g x 28°C             x 1.00 cal
g°C
= 21 000
LecturePLUS Timberlake 99          30
Learning Check H4

How many kcal are needed to raise the
temperature of 120 g of water from 15°C
to 75°C?
1) 1.8 kcal
2) 7.2 kcal
3) 9.0 kcal

LecturePLUS Timberlake 99   31
Solution H4

How many kcal are needed to raise the
temperature of 120 g of water from 15°C to
75°C?
2) 7.2 kcal
120 g x (75°C - 15°C) x 1.00 cal x 1 kcal
g°C     1000 cal

LecturePLUS Timberlake 99   32
Entropy- the degree of
randomness or disorder
Entropy        Entropy
Entropy
of a            of a
of a gas
solid          liquid
• A solid has an orderly arrangement.
• A liquid has the molecules next to each
other but isn’t orderly
• A gas has molecules moving all over the
place.
Gibbs Free Energy
• The energy free to do work is the
change in Gibbs free energy.
Gº = Hº - TSº (T must be in Kelvin)
• All spontaneous reactions release free
energy.
• So G <0 for a spontaneous reaction.
G=H-TS
G H S       Spontaneous?
-   -   +      At all Temperatures
At high temperatures,
? + +
“entropy driven”
At low temperatures,
? -     -
“enthalpy driven”
Not at any temperature,
+ + -
Reverse is spontaneous
• Activation Energy- minimum energy
required to transform the reactants into
an activated complex
• Heat of reaction- the quantity of energy
released or absorbed as heat during a
chemical reaction
• Enthalpy- the heat content of a system

LecturePLUS Timberlake 99     36
• Forward reaction-
exothermic
• A- Forward activation
energy
• B- Potential Energy
of Reactants
• C- Heat of reaction
• D- Potential energy
of the activated
complex
• E- Reverse activation
energy
• F- Potential energy of
37
the products

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