Download Q equations.notebook

Q equations.notebook
October 01, 2015
Just to review before we start…
Topic: Calculating Energy
Changes at Phase Changes
(Hv and Hf)
What is the melting point of this
substance?
The boiling point?
Sep 23­9:51 AM
Sep 23­9:51 AM
It takes energy to heat stuff up!
• for pure substance in single phase ­ can calculate how much E needed using:
Q = mC∆T > Q = energy in Joules
> m = mass in grams
> C = specific heat capacity
∆T = change in temperature = Tf ­ Ti
• on other hand, when something cools down, energy is released!
Sep 23­9:51 AM
So, how do we calculate the amount of energy required during a phase change? • HF = Heat of Fusion (Q = mHF)
• HV = Heat of Vaporization (Q=mHV)
• We use one of these two constants instead of specific heat and delta T
Q = mC∆T Sep 23­9:54 AM
Hf = Heat of Fusion
• is amount energy required to change 1
gram
of pure substance from solid to liquid at its
MP (meaning you aren’t changing the
temperature)
• Is a physical constant
– Check out Reference Table B, what is the heat of fusion for
water?
• The Equation Q = mHf
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Sep 23­9:51 AM
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Q equations.notebook
October 01, 2015
How much heat is absorbed when 10
grams of ice melts at 0 oC?
• Heat absorbed =
mass of substance x heat of fusion of substance
• Q = mHf = (10 g)(334 J/g) = 3340 J
• Where does this energy go?
> Particles must overcome forces of attraction to
move farther apart during phase change (s → l)
Sep 23­9:55 AM
HV = Heat of Vaporization
• is the amount energy required to convert 1
gram
of pure substance from liquid to gas at its BP
(meaning you aren’t changing the temperature)
Sep 23­9:51 AM
How much energy does it take to vaporize
10 g of water?
• Q = mHv
• Q = (10 g)(2260 J/g) = 22600 J
• Is a physical constant
– Check out Reference Table B, what is the heat of vaporization
for water?
• The Equation Q = mHv
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Sep 23­9:51 AM
It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?
H2O changing from liquid to gas requires 22,600J/g
H2O changing from solid to liquid requires 3,340J/g
* greater energy required to change from liquid to gas because particles are spreading farther apart!
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Heating curve of H 2O
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Q equations.notebook
October 01, 2015
3 equations for Q
1. Q = mC∆T
Q = mC∆T
• Temperature changed • Final
2. Q = mHf
temperature
• T↑
3. Q = mHv
• T↓
• Initial temperature
• figure out which to use
• depends on section of heating curve
• Start temperature
• look for hints in word problem
• Ending
temperature
• From __ ˚C to
__ ˚C
• Water
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Sep 23­9:51 AM
Q = mHf
Q = mHv
• Ice
• Steam
• Freezing
• Boiling
• Melting
• Condensation
• Occurs at 0°C (for H2O)
• Occurs at 100°C (for H2O)
• At constant temperature
• At constant temperature
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Sep 23­9:51 AM
heating rate = 150
J/min
If the substance takes 4 minutes to melt, how much
heat energy was used to melt it?
150J/min x 4min = 600J
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