What is the melting point of this substance?

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

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

3. 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!

4. Temperature Time Q = mCliquidT Q = mCgasT Q = mCsolidT 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?

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

6. 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

7. 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

8. 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)

9. 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

10. How much energy does it take to vaporize 10 g of water? Q = mHv Q = (10 g)(2260 J/g) = 22600 J

11. 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!

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

13. Heating curve of H2O

14. 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

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

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

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

18. 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