Chemistry

Name: __________________________ TOTAL HEAT ABSORBED OR RELEASED BY A SYSTEM

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For a system that undergoes both temperature and phase changes the total energy may be calculated.  For phase changes heat is calculated from: q = n∆H 

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For temperature changes heat is calculated from:

q = mc∆T

Remember that ∆H is specific to the phase change taking place and that c is specific to the phase of the system.

Example: Calculate the total energy needed to heat 2.5 g of H2O at 20.0oC to 120.0oC. cH2O(l) = 4.184 J/goC cH2O(g) = 2.01 J/goC ∆Hvap for H2O = 40.7 kJ/mol Steps for solving: 1. Draw the corresponding heating curve 2. For each phase change find the energy using q = n∆H 3. For each temperature change find the energy using q = mc∆T 4. Convert to a common energy unit and add your heat values to find the total energy.

Solution: