Calculations in Chapter 10 Molar Enthalpy of Fusion
- Slides: 8
Calculations in Chapter 10
Molar Enthalpy of Fusion • Used when melting or freezing = ___energy ____ mol of substance • Can be arranged to find any of the three • Units for molar enthalpy of fusion will be k. J/mol and units for energy is k. J or J, (kilojoules or joules) •
Example Problem • The molar heat of fusion for water is 6. 009 k. J/mol. How much energy is needed to convert 60. 0 g of ice at 0°C to liquid water at 0°C? • Rearrange the formula for energy • Energy = mol of substance
Have to convert the 60. 0 g of ice to mol Divide by the molar mass of water 60. 0 g/ 18. 016 g/mol = 3. 330373002 mol H 2 O(s) Energy = 6. 009 k. J/mol 3. 330373002 mol H 2 O(s) = • 20. 01221137 k. J • 20. 0 k. J needed to convert 60. 0 g of ice at 0°C to liquid water at 0°C • • •
Molar Enthalpy of Vaporization • Used when boiling or condensing energy mol of substance • Can be arranged to find any of the three • Boiling has energy required, condensing gives off energy • =
Example Problem • How much heat is evolved when 275 g of ammonia gas condenses to a liquid at its boiling point? The molar enthalpy of vaporization for ammonia is 23. 3 k. J/mol • Have to convert the g of ammonia to mol of ammonia • 275 g/ 17. 034 g/mol = 16. 14418222 mol
• • • Rearrange the formula for energy evolved Energy = mol of substance Energy = 23. 3 kj/mol 16. 14418222 mol Energy = 376. 1594458 k. J Energy = 376 k. J
Another Problem • What mass of steam is required to release 4. 97 x 105 k. J of heat energy on condensation? The molar enthalpy of vaporization for water is 40. 79 k. J/mol • First rearrange to solve for mol • Solve for mol • Multiply by the molar mass to find grams • Answer: 2. 20 x 105 g H 2 O
