Heat Transfer and the
nd
2 Law of
Thermodynamics
Warm Up 1
• 10 A sample liquid is cooled from 150°C to
30°C, causing the liquid to change into a
solid. Which of the following has occurred?
• F Chemical composition has changed.
• G A physical change has occurred.
• H A new compound has formed.
• J Evaporation of a solvent has occurred.
Heat Transfer
• Heat flows from _______________.
• To reverse the process, work must be
done.
Heat Transfer
• The amount of heat absorbed (Q) is
equal to the amount of heat released
(-Q).
Q=-Q
mcΔT = mc-(ΔT) ; ( -Tf + Ti)
mc (Tf-Ti) = mc (Ti –Tf )
• The final temperature for both will be
the same. (Thermal Equilibrium)
Heat Transfer
Positive Q value if
-energy is absorbed, added, required,
needed
Negative Q value if
- energy is released, removed
Heat Transfer
• Example
A 50 g metal bolt (cbolt= 899 J/kg oC) is
dropped into a 150 ml of water. The initial
temperature of the water is 21 o C and
they both reached an equilibrium
temperature of 25 o C. What is the initial
temperature of the bolt?
Remember: energy absorbed by the water is
equal to energy released by the metal bolt.
Solution
Q water = - Q bolt (released
energy)
mwcw (Tf - Ti) = mbcb (Ti-Tf)
(0.15 kg)(4186 J/kgoC)(25 oC – 21 oC) = (0.05 kg)(899 J/kg oC)(Ti
- 25 oC)
(0.15 kg)(4186 J/kgoC)(4 oC) = (0.05 kg)(899 J/kg oC) (Ti 25 oC)
2511.6 J = (44.95 J/ oC) (Ti - 25 oC)
2511.6 J
= (Ti - 25 oC)
(44.95 J/ oC)
55.88 oC = Ti - 25 oC
55.88 oC + 25 oC = Ti
2nd Law of Thermodynamics
• The Law of Increased Entropy
–In a closed system the entropy of
the system will either remain
constant or increase.
Law of Heat Transfer and Entropy
• Entropy – disorderliness
of things (gas particles
have more entropy than
solids and liquids)
• Heat flows from higher
temperature to lower
temperature.
• To reverse the process,
work will be done.
Law of Heat Transfer and Entropy
Examples:
1. Ice cream easily melts but to refreeze it you
need a heat pump (fridge) to do it.
2. Coffee on the table will cool down eventually
at room temperature. What kind of work is
needed to warm it up?
Entropy
• By doing work on a system you can
decrease the system’s entropy because
the work you are doing is creating order
• It takes a lot more work to put your room in
order than to mess it up.
• You will need an AC pump to cool down
(reduce the entropy) of a room in Houston
during summer time.
Light bulb moment
• To reverse the process of Heat
Transfer work must be done to do it.
• To reverse or to minimize the entropy
(disorderliness), work must be done.
Exit Ticket
• If energy cannot be created and heat only
flows from high to low, what would happen
to the universe far, far, far in the future?
Universal Thermal Equilibrium
• “Heat Death”
• Eventually everything in the universe will
be at thermal equilibrium as energy is
dissipated (lost).
• If everything is at the same temperature,
work will be impossible.
Work