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Thermochemistry
Chapter 10
Energy
Energy-ability to do work or
produce heat
Ex: Cars need energy to run.
People need energy to live.
Energy Classifications
• Potential energy-energy
due to position or
composition
• Kinetic energy-energy due
to motion
Measuring Energy
Changes
• calorie(c)-amount of energy required
to raise the temperature of one gram
of water by one Celsius degree.
(Food you eat is measured in
Kilocalories which is abbreviated C).
• Joule (J)-the SI unit of energy
• 1 c=4.184J
1st Law of
Thermodynamics
• The law of conservation of
energy states that energy
cannot be created or
destroyed. Therefore, the
total energy in a system is
conserved.
What’s the difference
between energy and
heat?
• Temperature-measures the
molecular motions of the
particles
• Heat(Enthalpy)-the flow of
energy due to temperature
difference
**You can have temperature
without having heat
• Exothermic reactionsenergy flows out of the
system.
• Endothermic reactionsenergy flows into a
system.
• q or H represents heat
Label as exo or endo
1) Your hand gets cold when you
touch ice.
2) Ice cream melts.
3) Propane is burning in a propane
torch.
4) Water drops on your skin
evaporate after swimming.
Answers
1) Exo hand+icewater+heat
(your hand feels cold because heat
is leaving, your hand has less
energy after it touches the ice)
2) endo
Ice cream+heat  melted ice cream
(the melted ice cream has more
energy so heat must have been
added to the system)
3) exo- propane gas  heat
(heat is released)
4) endo
water+body heatevaporation
(in order for something to evaporate
heat has to be added)
Hess’s Law
Hess’s Law-the overall
enthalpy change in a
reaction is equal to the sum
of enthalpy changes for the
individual steps in the
process
Entropy
• Entropy-a measure of the
disorder or randomness
• Abbreviated with ∆S
If you take a deck of cards and
drop them on the floor, that
would have high entropy.
If you buy a new deck, that
would have low entropy.
2nd Law of
Thermodynamics
• The entropy of the universe
is always increasing
• Processes in nature are
driven toward lowest
enthalpy and highest
entropy