Heat Lecture Notes
(Chapter 12 and 15 of Glencoe Chemistry)
(Chapter 13 and 17 of Prentice Hall Chemistry Chapter 9 and 10 of Addison-Wesley 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 DT
Where: Q = the amount of heat
m = the mass of the material
C = the specific heat of the material
DT = 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
Know the temperature scales: 1oC = 1K = 180/100 oF (some people call this 9/5 oF)
Remember they all have a different origin: 0 K = -273.15 oC = -460 oF
For a change of phase (state). The heat equation is:
Q = m Hfus or Q = m Hvap
Where: 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).
(Remember K = oC; you can use them interchangeably)
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
What is the relationship between heat and temperature?
GROUP NOTES MUST INCLUDE:
Solid: fixed shape and size (volume) ionic solids have a crystallize matrix. While they are fixed in relationship to each other they still have irrational energy (as long as they are not at ABSOLUTE ZERO.
Liquid: atoms or molecules close together but not in a fixed position, they are FLUID. The molecules are moving around and some have enough energy to become gases and therefore exert a vapor pressure which increases as the temperature increases.
Gas: atoms or molecules have LOTS of room between them have no fixed shape or volume are VERY FLUID
Heat: is ENERGY! Measured in joules, BTU’s or Calories.
Temperature: is a measure of the AVERAGE KINETIC ENERGY! Measured in Fahrenheit, Celsius, or Kelvin
Name the six most common phase transitions:
melting/freezing, boiling/condensing, evaporation and sublimation.
Specific Heat Capacities Table
Substance / J/kg oCor 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
Brass / 380 / 0.0907
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 à C6H12O11
And would have a positive change in enthalpy DH = + 500 joules
(energy is “gained by the product)
Some are exothermic
C6H12O11 à Energy + CO2 + H20
And would have a negative change in enthalpy DH = - 500j (energy is “lost”)