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 substance you 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. Examples
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 J/g°C. The molar heat of vaporization for water is 40.6 kJ/mol.

8. 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: kJ
Mass to moles to kJ

9. 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: kJ
Mass to mol to kJ

10. 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: kJ
Mass to mol to kJ
Q = m * s * change in temperature

11. 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: kJ
Q = m * s * change in temperature
Mass to mol to kJ

12. 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
Q = m * s * change in temperature
Mass to mol to kJ (twice)

13. 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 J/g°C. The molar heat of vaporization for water is 40.6 kJ/mol. (HINT: 3 #’s to add)

14. Answers
On board
16.7 kJ
225 kJ