Question 9.6: Calculate the enthalpy change when 240. g of ice melts. Stra......
Calculate the enthalpy change when 240. g of ice melts.
Strategy The ΔH_{\text{fus}} value in Table 9.3 is in J/mol, so the amount of ice must be converted into moles. Multiplying the number of moles by ΔH_{\text{fus}} will provide the desired quantity.
| Table 9.3 | ||||
| Standard molar enthalpies and temperatures for phase changes of water | ||||
| Phase Change |
Fusion | Freezing | Vaporization | Condensation |
| Transition temperature |
0ºC | 0ºC | 100ºC | 100ºC |
| \DeltaH (J/mol) | \Delta H_{\text{fus}}=6009.5 | \Delta H_{\text{freeze}}=–6009.5 | \Delta H_{\text{vap}}=4.07\times10^4 | \Delta H_{\text{cond}}=-4.07\times10^4 |
240\mathrm{~g~H}_{2}\mathrm{O}\times{\frac{1~\mathrm{mol}}{18.0\mathrm{~g}}}=13.3\mathrm{~mol~H}_{2}\mathrm{O}
\Delta H=n\times\Delta H_{\mathrm{fus}}
= 13.3 mol × 6009.5 J/mol
= 8.01 × 10^4 J
Analyze Your Answer The enthalpy of fusion is constant at 6 kJ/mol, so the enthalpy change depends on the size of the sample. Because the molar mass of water is 18 g/mol, a sample that is 240 grams is a bit more than 10 moles. That means we should expect an answer that is a bit larger than 60 kJ, which is consistent with our result.
Check Your Understanding Calculate the enthalpy change when 14.5 g of water vapor condenses to liquid water.