Chemistry
Ms. Boon
Name: _________________________________
Period: ______ Date: _______________
Thermochemistry Notes 3
Catalyst: Analyze the heating curve posted on the board here.
Specific Heat Review: Use your notes to do the following specific heat problem.
3000 g of water are heated from 50 °C to 99°C. The specific heat of water is 4.18 J/g*K. How much heat was absorbed?
Write the specific heat equation here:
Set-up and Solve:
Notes: Heat Transfer During Phase Changes
We discussed the fact that temperature does not increase during a phase
change. Instead, all the energy that is released or absorbed is used to
power the phase change. On the phase change diagram this portion of
the heating curve has a slope of zero.
Today we will calculate how much heat is absorbed or released during a
phase change. We will use water as our example, but a similar calculation
can be done for other substances.
The amount of heat absorbed or released during a phase change is
generally called the latent heat of transformation. Different substances
have different latent heats of transformation. Also, different phase changes absorb or release different amounts of
heat. For example, less energy is absorbed to melt ice than is absorbed when water is vaporized. Table 1 contains the
heats of transformation for water.
Table 1: Heats of Transformation for H2O
Analyzing Data. Use the table to answer the following questions.
Change of State
Energy Term
Joules/gram
1. How do the heats of transformation for water compare to
(J/g)
the specific heat of water?
Fusion (solid to
Heat of Fusion 333.9
2. How many joules are released when 1 gram of water
freezes?
3. Which requires more energy: melting or vaporization?
4. Why is the heat of fusion the same as the heat of
crystallization?
liquid)
Freezing (liquid to
solid)
Vaporization (liquid
to gas)
Condensation (gas
to liquid)
Heat of
Crystallization
Heat of
Vaporization
Heat of
Condensation
333.9
2256.8
2256.8
Latent Heat of Transformation Calculations
If we know the latent heat of transformation of a substance, we can calculate the amount of heat energy absorbed or
released during a phase change.
The formula:
Q = m × ΔHtranformation
Chemistry
Ms. Boon
Name: _________________________________
Period: ______ Date: _______________
Example 1: How much heat is required to
melt 1000 g of ice at 0°C?
Phase Change Calculations
Example 2: How much heat is needed to
vaporize 200 g of water at 100°C?
Example 3: How much ice, at 0°C, can be
melted by 100 J of heat?
(1) How much heat is released when 500
g of water freezes at 0°C?
(2) How much heat is released when 2500
g of water condenses at 100°C?
(3) How much water, at 100°C, can be
vaporized by 1000 J of heat?
Joule and Calorie Conversions: 1 calorie (cal) = 4.187 Joules (J) and 1 Joule (J) = 0.239 calories (cal)
(4) What is the heat of fusion of water expressed in calories?
(5) What is the specific heat of aluminum expressed in Cal/g*K?
Molar Conversions: The Molar Heat of Transformation is the amount of heat absorbed or released during a phase change of 1 mole of
a substance.
(6) Calculate the molar heat of vaporization for water. (Step 1: Calculate the molar mass of water. Step 2: Multiply by the heat of
transformation given in J/g)
(7) Calculate the molar heat of fusion for water.
Draw a phase change diagram for a substance with the following characteristics: (a) specific heat: 2.0 J/g*K, (b) Heat of fusion: 100
J/g, (c) Heat of vaporization: 300 J/g.