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CHAPTER 2
HeatTemperature and ConversionsSpecific Heat
What is Energy?
The ability to move or change matter. (Units: Joules)
All physical and chemical changes involve energy!
Examples of Energy
Kinetic – energy of motion KE = ½ mv2
Potential – stored energy/energy of position
Light Sound Electricity Heat (Thermal) Chemical
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.
Energy and mass are related
Einstein derived an equation to show this relationship in 1905.
Nuclear reactions can create energy from mass.
Energy is transferred during physical and chemical changes:
Endothermic – energy is absorbed by the system
+Exothermic – energy is
released into the surroundings
-
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.
TEMPERATUREWhat is temperature? Temperature is the
measure of the average kinetic energy of all the particles within an object.
Measured with a thermometer.
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.
The heating curve for water shows that temperature does NOT change during a phase change.
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.
Scales to Measure Temperature
Fahrenheit Scale (U.S.A.)
Celsius Scale (everyone else)
Kelvin Scale (scientists)
How do Thermometers Work?
Usually contain alcohol or mercury.
Temperature increase (particles move faster), liquids expand
Temperature decreases (particles move slower), liquids contract
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.
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.
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
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
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?
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
SPECIFIC HEAT CALCULATIONS
q =m x Cp x T
Rearrange the formula:
m= q/Cp T
Cp = q/ m T
T = q/ m Cp