Heat, Enthalpy, Temperature

Heat vs. Temperature and Calorimetry
What is the difference between heat and temperature?
We often talk about heat and temperature as the same thing but in
chemistry they have different meanings
Temperature is the average kinetic energy of molecules
Heat is the amount of energy exchanged due to a temperature
difference between two substances.
Units: Joules (J), calories (cal)
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.
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
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.
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
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
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.