Download Heat, Enthalpy, Temperature

Heat vs. Temperature and Calorimetry
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What is the difference between heat and temperature?
o
We often talk about heat and temperature as the same thing but in
chemistry they have different meanings
o
Temperature is the average kinetic energy of molecules
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o
Heat is the amount of energy exchanged due to a temperature
difference between two substances.
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Units: Joules (J), calories (cal)
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Heat is dependent upon the AMOUNT of substance
 Example: A glass of hot water will give off less heat than a
gallon of hot water.
 Careful: In chemistry, a glass of hot water does not HAVE
heat. A glass of hot water has ENERGY which is given off in
the form of heat.
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Heat can be given off (q < 0, exothermic) or absorbed (q > 0,
endothermic) but generally, substances do not HAVE heat.
A substance loses or gains heat because of a temperature
difference.
Energy will flow in a way that evens out the difference.
 The energy lost by one system will be equal to the amount
of energy gained by another system.
 ENERGY LOST = ENERGY GAINED
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Units: K, Kelvin
Kelvin temperature = Celsius temperature + 273.15
Therefore, an exchange of heat causes a change in temperature
If we know the amount of heat we can calculate the change in temperature
and vice versa. This is called CALORIMETRY
o q = mCpT
o q = heat in J
o m = mass in g
o Cp = specific heat capacity in J/gC (see below)
o T = change in temperature in C or K
Cp = specific heat capacity (the p means constant pressure)
Specific heat capacity: physical property that gives the amount of energy it
takes to raise the temperature of one gram of substance one degree
Example:
If you drink a cold glass of water (250 g) at 0 C, how much heat is transferred to
the water as it warms to 37 C. The specific heat of water is 4.187 J/gC
q = mCpT,
m = 250 g,
Cp = 4.187 J/gC
T = 37 – 0 = 37 C
q= (250 g) (4.187 J/gC) (37C) = 1.5466 x 105 J = 150 kJ

How is the amount of energy in food measured?
o Bomb Calorimetry
o Food is burned to release energy (like burning wood for fire)
o The energy is absorbed by water
o Using specific heat of water and temperature change of known mass of
water the heat can be calculated.
o This amount of heat is the amount of energy that is available for your body. It
is what determines how many calories are in food.
Example: See example 9B on page 320 in your book
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At constant pressure, heat is equal to the change in enthalpy, H
Enthalpy (symbol H)is the total energy of a system (kinetic + potential)
From hear on out, I may interchange heat, energy and enthalpy.