Q equations.notebook
1
October 01, 2015
Sep 239:51 AM
What is the melting point of this substance?
The boiling point?
Just to review before we start…
Sep 239:51 AM
Topic: Calculating Energy Changes at Phase Changes
(Hv and Hf)
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 239:54 AM
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 239:51 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
Q equations.notebook
2
October 01, 2015
Sep 239:55 AM Sep 239:51 AM
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 239:51 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)
• Is a physical constant– Check out Reference Table B, what is the heat of vaporization
for water?
• The Equation Q = mHv
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
Sep 239:51 AM
H2O changing from solid to liquid requires 3,340J/g
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
* greater energy required to change from liquid to gas because particles are spreading farther apart!
Sep 239:51 AM
Heating curve of H 2O
Q equations.notebook
3
October 01, 2015
Sep 239:51 AM
3 equations for Q1. Q = mC∆T
2. Q = mHf
3. Q = mHv
• figure out which to use
• depends on section of heating curve
• look for hints in word problem
Sep 239:51 AM
Q = mC∆T• Temperature changed
• T ↑
• T ↓
• Initial temperature
• Start temperature
• Final temperature
• Ending temperature
• From __ ˚C to __ ˚C
• Water
Sep 239:51 AM
Q = mHf• Ice
• Freezing
• Melting
• Occurs at 0°C (for H2O)
• At constant temperature
Sep 239:51 AM
Q = mHv• Steam
• Boiling
• Condensation
• Occurs at 100°C (for H2O)
• At constant temperature
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