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

Calculating Energy Changes at Phase Changes

There is a specific amount of energy required to melt ice and to turn liquid water into steam… Heating curve of H2O

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)

Q = mHF Q = mHV Temperature IV I II III V Time

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 Hf for water is 334J/g

Practice Problem: How much heat is absorbed when 10 grams of ice melts at 0oC? Q = mHf = (10 g)(334 J/g) = 3340 J

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 HV for water is 2260J/g

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

It takes a lot more energy to go from liquid to gas than from solid to liquid. Why? * greater energy required to change from liquid to gas because particles are spreading farther apart! Heating curve of H2O

Knowing which equation to use…hints Q = mHf Q = mHv Ice Freezing Melting Occurs at 0C (for H2O) At constant temperature Steam Boiling Condensation Occurs at 100C (for H2O) At constant temperature