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25.3 Heat and Thermal Energy• Imagine heating a cup
of coffee to a temperature of 100°C.
• Next think about heating up 1,000 cups of coffee to 100°C.
• The final temperature is the same in both cases but the amount of energy needed is very different.
Specific Heat Capacity Table
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Does it take the same amount of energy to heat 1 cup of water as
compared to 1 gallon of water
• No…more energy to heat 1 gallon…..MORE MASS
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Do all materials change temperature at the same rate?
• No some heat faster than others.
• The SLOWER something heats, the HIGHER Heat Capacity.
• WATER has a very high HEAT CAPACITY.
• Takes a lot of energy to change the temperature.
• METALS have a LOW specific heat….
• Doesn’t take a lot of energy to change the temperature.
• WHERE IS IT BENEFICIAL FOR WATER TO HAVE A HIGH SPECIFIC HEAT?
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25.3 Specific Heat
• The temperature of gold rises quickly compared with water because its specific heat is much less than the specific heat of water.
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25.3 Specific Heat• The specific heat is the quantity of heat
it takes to raise the temperature of one kilogram of material by one degree Celsius.
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Which is going to take more energy….change the temperature
5oC or 100oC?
• Takes more energy to change it to 100oC.
• GREATER the temperature change, the more temperature needed.
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25.2 Calculate Heat
• One kilogram of water is heated in a microwave oven that delivers 500 watts of heat to the water.
• One watt is a flow of energy of one joule per second.
• If the water starts at 10°C, how much time does it take to heat up to 100°C?
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25.2 Heat Equation
Q = mc(T2-T1)
Specific heat (J/kgoC)
Mass (kg)
Heat energy (J)
Change inTemperature
(oC)
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How much energy is needed to raise 50 grams of water from 25oC
to 75oC?
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Specific Heat / Heat Capacity
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STEPS OF EQUATIONS
• Determine whether Q is going IN or OUT
• Determine if ENDOTHERMIC or EXOTHERMIC…which graph?
• Determine location of energy change
(ie. TEMP. CHANGE of S,L,G or melting/freezing or evaporating/condensing)
**use appropriate C, Hf or Hv
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endothermic = increase in temp.
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exothermic (negative heat)
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• How much heat is required to raise the temperature of water of 50 grams from 20oC to 80oC?
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• How much heat is required to REDUCE the temperature of water of 50 grams from 80oC to 20oC?
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• How much heat is required to raise the temperature of water of 50 grams from -20oC to -5.0oC?
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• How much heat is required to raise the temperature of water of 50 grams from 120oC to 180oC?
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• How much heat is required to MELT 50 grams of water?
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• How much heat is required to CONDENSE
75% of 50 grams of water?
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#6
• 130 kcal x 4.18 kJ = 543.4 kJ
1 kcal
534.4 kJ x 1000 J = 543,400 joules
1 kJ
convert 130 kcal = 543,400 joules
• 2.5 kg = 2500 grams