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Energy Calculations for Phase Changes Explained

Learn how to calculate energy requirements for phase changes with examples and step-by-step instructions. Understand molar heat of fusion and vaporization for water and apply constants for solving. Explore the heating and cooling curve for water.

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Energy Calculations for Phase Changes Explained

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  1. Chapter 14: Liquids and Solids Energy requirements for phase changes

  2. Learning Target • You will learn to calculate energy requirements for phase changes.

  3. Heating/Cooling curve for water

  4. Energy Requirements • It takes energy to melt and vaporize water. • Remember chemical bonds are not broken in these processes. • The energy required to melt 1 mol of a substance is called the molar heat of fusion. • The energy required to change 1 mol of liquid to vapor is called the molar heat of vaporization.

  5. Constants and How to Solve • The molar heat of fusion for ice is 6.02 kJ/mol. • The molar heat of vaporization for water is 40.6 kJ/mol. • To solve for the energy to melt or vaporize a substance mass  moles  kJ (using the molar heat of fusion or vaporization value)

  6. How to solve • To solve for the energy to change the state of a substanceyou break the process into 3 steps: • The change in physical state q = s x m x ΔT (this is from Ch. 10!!!) • The vaporization and/or fusion process mass  moles  kJ (using the molar heat of vaporization value) • The total energy sum up your steps

  7. Example • Calculate the energy released when 15.5 g of ice freezes at 0°C. The molar heat of fusion of ice is 6.02 kJ/mol • Answer: 5.18 kJ

  8. Example • Calculate the energy required to vaporize 35.0 g of water at 100°C. The molar heat of vaporization of water is 40.6 kJ/mol. • Answer: 78.9 kJ

  9. Example • Calculate the energy required to melt 12.5 g of ice at 0°C and change it to water at 25°C. The specific heat capacity of liquid water is 4.18 J/g°C and the molar heat of fusion of ice is 6.02 kJ/mol. • Answer: 5.48 kJ

  10. Example • Calculate the energy required to heat 22.5 g of liquid water at 0°C and change it to steam at 100°C. The specific heat capacity of liquid water is 4.18 J/g°C and the molar heat of vaporization of water is 40.6 kJ/mol. • Answer: 60.1 kJ

  11. Example • Calculate the total energy required to melt 15 g of ice at 0°C, heat the water to 100. °C, and vaporize it to steam at 100. °C. • Hint: Break the process into three steps and then take the sum. • 45 kJ

  12. Recap/Quiz-ANSWER ON OWN! • Create the heating/cooling curve for water • How much heat is required to melt 50.0 g of ice at 0°C? (the molar heat of fusion for water is 6.02 kJ/mol) • How much heat is necessary to completely vaporize 75.0 g of ice at 0°C? The molar heat of fusion for water is 6.02 kJ/mol. The specific heat capacity of liquid water is 4.184 J/g°C. The molar heat of vaporization for water is 40.6 kJ/mol. (HINT: 3 #’s to add)

  13. Answers • On board • 16.7 kJ • 225 kJ

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