Heat Lecture Notes
(Chapter 13 and 17 of Prentice Hall Chemistry)
What Is Heat? Heat is a form of ENERGY
Remember that ENERGY is the ability to cause change or do WORK (we will talk more about
work later on in Physics)
Temperature is just a measure of the AVERAGE kinetic energy of a sample. (that’s why all
liquids evaporate (have a vapor pressure); some molecules will have enough energy to leave the
system as a gas)
(Now is a good time to look at the diagram on page 388 of the prentice hall chemistry book)
The units of heat are calories or Joules ; or kcal or kJ
(we still sometimes use B.T.U.’s here in the USA)
One calorie is the energy necessary to change one gram of water one degree Celsius
1 calorie = 4.184 joules
Heat capacity is different for different things. Larger things have more heat capacity than
smaller pieces of the same stuff.
Water is a material that can hold a lot of heat, it has a very large heat capacity.
The specific heat of water is 1 calorie per gram oC
The heat Equation is: (page 508 and 509 of text)
Q = m C T
Where:
Q = the amount of heat
m = the mass of the material
C = the specific heat of the material
T = the change in temperature of the material
So….
The amount of heat is equal to the mass times the specific heat times the change in
temperature
For a change of phase (state). The heat equation is:
Q = m Hfus
Where:
or
Q = m Hvap
Q = the amount of heat
m = the mass of the material
Hfus = the heat of fusion of the material
Hvap = the heat of vaporization of the material
You need to know these constants for water
Hfus = the heat of fusion of water is 80 calories/gram or 6.01 kJ/mol
Hvap = the heat of vaporization of water is 540 calories/gram or 40.7 kJ/mol
You will need to know (or look up).
C = the specific heat of the material
Cwater = 4.18 J/g K or 1.00 cal. /g K This one
MUST be memorized
Cice = 2.1 J/g K or 0.50 cal. /g K
Csteam = 1.7 J/g K or 0.40 cal. /g K
Specific Heat Practice problems (w/data chart on front)
Remember on the front answers are given (check your answers)
Problem # 7 and #8 on the back are multi stepped ad you will need more
calculating space than is given:
#7 you will need to use the equations (heat ice, melt ice, heat water, boil water,
heat steam)
Q = mCT three times (ice, water and steam) see above
Q = mH once for the ice/water change, Hfus = 80 calories/gram
and Q = mH once for the water/vapor change; Hvap = 540 calories/gram
Specific Heat Capacities Table
J/kg oC
or J/kg K
cal/g oC
or cal/g K
Water (0 oC to 100 oC)
4186
1.000
Methyl Alcohol
2549
0.609
Ice (below 0 oC)
2093
0.500
Steam (above 100 oC)
2009
0.480
Benzene
1750
0.418
Wood (typical)
1674
0.400
Air ( @ 50 oC)
1046
0.250
Aluminum
900
0.215
Marble
858
0.205
Glass (typical)
837
0.200
Iron/Steel
452
0.108
Copper
387
0.0924
Silver
236
0.0564
Mercury
138
0.0330
Gold
130
0.0310
Lead
128
0.0305
Zinc
388
0.0926
Substance
Thermodynamics: (Chapter 17 of text)
Enthalpy (H) is kind of like heat at constant pressure.
By definition:
H = q + w (enthalpy equals heat and work, often heat expands a gas which drives
a piston in an engine)
Some reactions are endothermic and can be written like:
Energy + CO2 + H20  C6H12O11
And would have a negative change in enthalpy H = -
Some are exothermic
C6H12O11  Energy + CO2 + H20
And would have a positive change in enthalpy H = +