Heat of Fusion and Phase Change Calculations

Introduction to Phase Changes and Heat Calculations

• Understanding phase changes involves the absorption or release of energy,
  which can be quantified using specific heat capacities and enthalpy changes.

Common Substances and Their Heat of Fusion

• Water (H₂O)
  • Heat required to melt ice: 6.01 kJ/mol
• Benzene (C₆H₆)
  • Heat required for phase change: 10.59 kJ/mol
• Ethanol (C₂H₅OH)
  • Heat required for transition: 4.60 kJ/mol
• Acetone (CH₃COCH₃)
  • Heat required for melting: 5.72 kJ/mol

Heat Required for Phase Changes

• The heat required for a material to change phase can be determined with the formula: $q = n imes riangle H$ where:
  • $q$ = heat required (in kJ)
  • $n$ = number of moles (mol)
  • $riangle H$ = heat of fusion (kJ/mol)

Example Calculation: Melting Ice

• Question: How much heat is required to change 25.0 g of ice from a solid to a liquid at 0°C?

Step 1: Convert grams of ice to moles.

• Molar mass of ice (H₂O) = 18.02 g/mol
• Calculate moles of ice:
  $n = \frac{25.0 ext{ g}}{18.02 ext{ g/mol}} \approx 1.39 ext{ mol}$

Step 2: Use the heat of fusion for water.

• Heat of fusion for ice: $6.01 ext{ kJ/mol}$
• Calculate heat required:
  $q = n imes riangle H = 1.39 ext{ mol} \times 6.01 ext{ kJ/mol} \approx 8.35 ext{ kJ}$

Conclusion

• Therefore, the heat required to change 25.0 g of ice to liquid at 0°C is 8.35 kJ.