AP Chemistry : Energy of Phase Changes
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Example Question #11 : Phase Changes
In the given heating curve, when is the potential energy the highest?
The end of segment 2
The beginning of segment 4
The end of segment 4
The beginning of segment 2
The end of segment 4
The potential energy increases as the substance goes through phase changes. Therefore the potential energy is highest at the last possible instant of the most energy intensive phase change. In this case it is at the end of segment 4.
Example Question #11 : Phase Changes
The given heating curve represents a substance in phases solid, liquid, and gas. What is the melting point of the substance?
Because phase changes happen when the kinetic energy (temperature) remains constant melting will occur during a flat portion of the heating curve.
In this case it is the first flat portion because solid is the lowest energy phase represented in this diagram. Therefore the melting point is at 
Example Question #13 : Phase Changes
Suppose that a block of ice is exactly at its melting temperature, 
The temperature remains constant until the ice melts
The kinetic energy of the water molecules increases
The entropy of the system increases
The intermolecular hydrogen bonds between water molecules are broken
The kinetic energy of the water molecules increases
This question is presenting us with a scenario in which heat is being added to a block of ice at its melting temperature. It's asking us to identify a false statement.
Recall that when anything goes through a phase change, the temperature remains the same until the phase change is complete. Furthermore, since temperature is a measure of the average kinetic energy of a system, this will not change either. Moreover, as energy is added to a system, the bonds between the components of that system will gradually weaken and then break as the entropy of that system increases.
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