# Just to review before we start…

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Just to review before we start…
What is the melting point of this substance? 50˚C The boiling point? 100˚C

Topic: Calculating Energy Changes at Phase Changes (Hv and Hf)

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!

Temperature Time IV I II III V Q = mCliquidT Q = mCgasT
Q = mCsolidT Temperature IV I II III V C = specific heat capacity (amount heat required to raise temp of 1g of pure substance by 1C) C is a physical constant unique for every pure substance CAN YOU FIND THE SPECIFIC HEAT OF WATER ON YOUR REFERENCE TABLE? Time

Why can’t I use Q = mCT for II and IV??
Temperature IV I II III V Because T = 0, temp isn’t changing!!!! Time

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

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

How much heat is absorbed when 10 grams of ice melts at 0oC?
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)

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

How much energy does it take to vaporize 10 g of water?
Q = mHv Q = (10 g)(2260 J/g) = J

H2O changing from liquid to gas requires 22,600J/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 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!

Temperature Time IV I II III V Q = mHV Q = mHF Q = mCsolidT
Q = mCLIQUIDT Q = mCgasT Q = mHV Q = mHF Temperature IV I II III V Time

Heating curve of H2O

3 equations for Q 1. 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

Q = mCT Temperature changed T ↑ T ↓ Initial temperature
Start temperature Final temperature Ending temperature From __ ˚C to __ ˚C Water

Q = mHf Ice Freezing Melting Occurs at 0C (for H2O)
At constant temperature

Q = mHv Steam Boiling Condensation Occurs at 100C (for H2O)
At constant temperature

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