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Video 5.1Q=mcΔT
Table I
0Exothermic reactions release heat and have negative values. 0 Example: When Carbon and Oxygen react they release
393.5kJ of heat per mole reacted.0Endothermic reactions absorb heat and have positive
values.0 Example: When Nitrogen and Oxygen react they absorb
182.6kJ of heat per mole.
Table I examples
0When 2 moles of CH4 burn in oxygen, how much heat is released?
0When C2H4 is formed, is heat released or absorbed?
0Reactions that release the most energy are the most stable. Which reaction becomes the most stable?
0Where did these values come from?
Calculating Heat of reactions
0q is the symbol for heat. 0If q is positive, the heat is endo.0If q is negative, the heat is exo.0q is measure in Joules, (J) or kilojoules (kJ).
0The heat of a reaction is based on the mass of the substance, the temperature change it undergoes and specific heat.
Specific Heat0Specific heat is the heat needed to raise the temperature of
one gram of a substance one degree Celsius.
To calculate heat:
q = mcΔT1. The temperature of 95.4g of
copper increases from 25 to 48C and absorbed 849J. Calculate copper’s specific heat.
Q= (95.4) (4.18) (48-25)
Q = 0.387 J/gC
q = mcΔT
2. How much heat is needed to raise the temperature of 100g of water 50C?
Q = (100) (4.18) (50)
Q = 20900J or 20.90 KJ
q = mcΔT
3. If 600J are needed to heat 50g of water to 100C, what is the initial temperature?
97.13C
600 = (50) (4.18) (x)
X = 2.87
Video 5.2Review Gases, Liquids, and SolidsWith Phase Change Diagrams
Kinetic Molecular Theory of GasesKMT describes perfect gases:0Gases move in constant, random, straight-line paths.0Gases are separated by large distances, much larger than
their particle size. Therefore, gases volume is negligible. And gases are easily compressed.
0Gases do not have attractive or repulsive forces between molecules.
0Collisions between molecules can transfer energy but the total energy of the system is constant. This is called an elastic system.
Kinetic Molecular Theory of Gases
In summary, Perfect gases:
0Have no mass0Have no volume0Have no intermolecular forces
Kinetic Molecular Theory of Gases
0But we don’t have perfect gases. How do real gases deviate from ideal gases?0They have a volume, mass and small IMF under
high pressure and low temperature.0So, a real gas must be hot and under low pressure
to behave like an ideal gas.
Pressure0Gases exert a pressure on
surrounding substances because they are constantly moving and colliding with other surfaces.
0Only in a vacuum, where there are no molecules, there is no pressure.
0Gas pressure can be measured in atmospheres or kilopascals, according to reference table A.
Liquids
0No definite shape0Definite volume0Constant motion0No arrangement0Molecules are closer
together than a gas
Solids
0Definite shape0Definite volume0Constant vibration0Molecules are packed
tightly in a geometric (crystalline) pattern
Phase ChangesIdentify the phase change and if it’s
endothermic or exothermic:0Evaporation0Condensation0Melting 0Freezing0Sublimation0Deposition
Liquid to gas endothermic
Gas to liquid exothermic
Solid to liquid endothermic
Liquid to solid exothermic
gas to solid exothermic
Solid to gas endothermic
Thermochemistry
0The study of energy changes that occur in chemical reactions.
0 Kinetic energy refers to energy of motion. (Temperature)
0 Potential Energy refers to stored energy.
Phase Change Diagrams
Cooling Curve
B
E
D
C
A
F
Video 6.3Q=mHf Q=mHv
When can you use q=mcT?0Only on the solid,
liquid and gas only lines. (Where the temperature changes)
0So, what equations do we use if the temperature is not changing?
Two more equations from Table T
0Heat of vaporization: heat needed to change a substance from gas to liquid or liquid to gas.
q=mHv
0Heat of fusion: heat needed to change a substance from solid to liquid or liquid to solid.
q=mHf
0 If the IMF is strong, the heats of vaporization and fusion is high.
Q=mHv
1. Calculate the number of joules needed to vaporize 423g of H2O.
955, 980J or 955.98KJ
Q = (423) (2260)
Q=mHf
0How much heat is needed to melt ice at 0C if the sample weighs 255g?
85,170J or 85.17 KJ
Q = (255) (334)
For class
0 When you finish # 1-4, try these:
5. How much heat is absorbed by 550g block of ice to raise the temperature from -15 to 0C?
6. How much heat energy must be absorbed to raise the temperature of a 200 gram block of ice from -10 to 0C and then completely melt it to a liquid at the same temperature?
7. How much energy would be required to heat the same 200g of liquid water in #6 (at 0C) to the normal boiling point of water and then vaporize it?
8. If the temperature of the 200 grams of steam generated in #7 were heated to a new temperature of 120C, how much energy would be absorbed?
BONUS: What is the total amount of energy needed to heat 150g of ice at -10C to gas at 140C? (Use a heating curve to help you).
Measuring heat in the labYou can measure the heat of
physical and chemical changes in a calorimeter.
The calorimeter acts like a styrofoam cup, it insulates the reaction (doesn’t let the overall heat change).
Measuring heat in the lab0The heat released by the
reaction equals the heat absorbed by the water.
0You will measure the change in heat of the water using q=mcT.
Measuring heat in the lab0You will use a calorimeter
more like this. 0You must make sure you
always stir the solution to make the heat equal throughout the cup.
A student places a 68.4g piece of metal at 99.5C in a calorimeter filled with 103g of water at 25.2C. The temperature changes to 27.6C.
1. In terms of the metal, is the reaction endothermic or exothermic?
2. Calculate the heat absorbed by the water.3. Calculate the heat released by the metal.4. Calculate the specific heat of the metal.5. Using the following specific heats, determine the identity of
the metal and calculate the % error.Aluminum: 0.21 J/gCCopper: 0.090 J/gCGold: 0.030 J/gC
