chapter 2 heat temperature and conversions specific heat
TRANSCRIPT
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CHAPTER 2
HeatTemperature and ConversionsSpecific Heat
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What is Energy?
The ability to move or change matter. (Units: Joules)
All physical and chemical changes involve energy!
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Examples of Energy
Kinetic – energy of motion KE = ½ mv2
Potential – stored energy/energy of position
Light Sound Electricity Heat (Thermal) Chemical
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Law of conservation of energy:
Energy cannot be created or destroyed during any
chemical or physical change.Energy may be transferred
between the system and surroundings
Energy may change forms.
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Energy and mass are related
Einstein derived an equation to show this relationship in 1905.
Nuclear reactions can create energy from mass.
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Energy is transferred during physical and chemical changes:
Endothermic – energy is absorbed by the system
+Exothermic – energy is
released into the surroundings
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What is Heat?
The transfer of energy between the particles of two objects due to a temperature difference between the two objects.
Heat always flows from hot to cold. Measured in a calorimeter. Units: Joules, Calories, or calories.
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TEMPERATUREWhat is temperature? Temperature is the
measure of the average kinetic energy of all the particles within an object.
Measured with a thermometer.
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Heat and temperature
The transfer of heat does not always result in a temperature increase. During phase changes, energy goes directly to changing the phase, not into increasing the kinetic energy of the particles.
EX. The heating curve for water.
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The heating curve for water shows that temperature does NOT change during a phase change.
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Heating curve points and definitions:
Melting point/ freezing point of water: 0º C Boiling point of water: 100 º C Heat of fusion – the amount of energy
required to melt a solid Heat of crystallization – the amount of
energy released when a solid forms from a liquid
Heat of vaporization – the amount of energy required to change a liquid into a gas.
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Scales to Measure Temperature
Fahrenheit Scale (U.S.A.)
Celsius Scale (everyone else)
Kelvin Scale (scientists)
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How do Thermometers Work?
Usually contain alcohol or mercury.
Temperature increase (particles move faster), liquids expand
Temperature decreases (particles move slower), liquids contract
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Absolute Zero
The lowest possible temperature All motion STOPS. Energy is minimal/absent. In September 2003, MIT announced a
record cold temperature of 450 pK, or 4.5 × 10-10 K in a Bose-Einstein condensate of sodium atoms. This was performed by Wolfgang Ketterle and colleagues at MIT.
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SPECIFIC HEAT CAPACITY
Transfer of heat affects substances differently.
Measuring heat transferred to and absorbed by a substance under conditions of constant pressure yields specific heat capacity.
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SPECIFIC HEAT CAPACITY
Specific heat is defined as:The quantity of heat required to raise1 gram of a substance 1°C or 1 K.
Symbol: Cp The p symbolizes that the measurements were taken under
constant pressure.
Units = Joules/ gram °C or J/gK
J/g°C
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Sample Cp values
Metals have low specific heat values which allows them to heat up with little added energy. Iron 0.449 J/g°C Copper 0.385 J/g°C Platinum 0.133 J/g°C
Water has a relatively high specific heat 4.184 J/g °C
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Questions: Which would heat up faster, 5.00
grams of iron or 5.00 grams of water? Which would cool down faster, 5.00
grams of iron or 5.00 grams of water? Which is a better thermal conductor? Which is a better insulator?
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MEASURING HEAT and SPECIFIC HEAT
Must use a calorimeter.
Find the change in temperature:
T = (delta T)change in
temperature in °C
T = T final – T initial
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SPECIFIC HEAT CALCULATIONS
q =m x Cp x T
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Rearrange the formula:
m= q/Cp T
Cp = q/ m T
T = q/ m Cp