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What is the melting point of this substance?

Learn how to calculate energy changes during phase transitions like melting ice and vaporizing water. Understand the heating curve of H2O and the heat of fusion and vaporization. Practice problems included.

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What is the melting point of this substance?

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

  2. Calculating Energy Changes at Phase Changes

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

  4. 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)

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

  6. 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

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

  8. 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

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

  10. 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

  11. 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

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