Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

heat

VIEWS: 2 PAGES: 37

									               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
Radiant (light)

              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

								
To top