Calculating Energy Changes at Phase Changes
The heating rate is 150 J/min If substance takes 4 minutes to melt, how much heat energy was used to melt it? 150J/min x 4min = 600J
Heat of Fusion = amount energy required to change 1 gram of pure substance from solid to liquid at its mp Heat of Fusion = Hf = physical constant Hf for water = 334 J/g (Table B)
How much heat is absorbed when 10 grams of ice melts at 0oC? Heat absorbed = mass of substance x heat of fusion of substance Q = mHf = (10 g)(334 J/g) = 3340 J Where does that energy go? Particles must overcome forces of attraction to move farther apart
Heat of Vaporization = amount energy required to convert 1 gram of pure substance from liquid to gas at its bp Heat of vaporization = Hv = physical constant Hv for water = 2260 J/g
How much energy does it take to vaporize 10 g of water? Q = mHv Q = (10 g)(2260 J/g) = 22600 J It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?
The particles are spreading out a lot more!
Calculating Energy Q = mHf Q = mHv figure out which to use for given problem Look for hints in problem
Q = mHv Q = mHf Temperature Time
Q = mHf Ice Freezing Melting At 0C (for H2O) At constant temperature
Q = mHv Steam Boiling Condensation At 100C (for H2O) At constant temperature