Unit 6: Topic 5 - Energy of Phase Changes

Question 1

Provide and analyze the phase transition diagram for water.

Answer 1

This is the phase changes diagram for water. If the temperature change is such that the water keeps in its current physical state, the energy supplied or released can be calculated by q = mc∆T, as seen in the graph. However, if solid ice begins to melt or liquid water begins to boil, we must also account for the heat of fusion and the heat of vaporization.

Solids can be heated up until their melting point. At this point, extra heat is required to break the the bonds holding the solid particles together and form a liquid. This heat required is known as the heat of fusion for a compound. Similarly, as the liquid state begins to turn into the gaseous state, the analogous term is heat of vaporization.

Question 2

How is the heat is absorbed when liquid water freezes related to the heat of fusion? Similarly, how is the heat absorbed when gaseous steam condenses into water related to heat of vaporization? Explain.

Answer 2

If liquid water begins to freeze, then the heat absorbed is equivalent to the negative of the heat of fusion. Similarly, if gaseous water condenses, the heat absorbed is the negative of the heat of vaporization. This is true because by the first law of thermodynamics, the reverse process should have the opposite energy.