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AP Chemistry
Unit 6: Thermochemistry
6.4 Energy of Phase Changes
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Exothermic
phase changes
release energy.
True
What overcomes intermolecular forces during phase changes?
Energy
What is the enthalpy of fusion for water (H2O)?
6.01 kJ/mol
Order the phase changes by the type of energy required or released:
1️⃣ Melting (requires energy)
2️⃣ Boiling (requires energy)
3️⃣ Freezing (releases energy)
4️⃣ Condensation (releases energy)
5️⃣ Sublimation (requires energy)
The enthalpy of fusion represents the energy required to change a substance from solid to
liquid
The enthalpy of fusion is typically expressed in units of kJ/mol or
kcal/mol
Phase changes occur when a
substance
absorbs or releases energy.
True
What is the enthalpy of fusion for water (H2O)?
6.01 kJ/mol
Match the phase change with its energy requirement:
Melting ↔️ Requires energy (endothermic)
Freezing ↔️ Releases energy (exothermic)
Condensation is an exothermic process.
True
Common phase changes include melting, boiling, freezing, condensation, and
sublimation
The enthalpy of fusion is generally lower than the enthalpy of
vaporization
Phase changes can be either endothermic or
exothermic
The enthalpy of fusion for water is 6.01 kJ/mol, while the enthalpy of vaporization is
40.7
kJ/mol
The melting of ice is an endothermic process because it absorbs
energy
Steps to calculate energy changes during phase transitions:
1️⃣ Identify the phase change
2️⃣ Multiply the moles by the enthalpy value
3️⃣ Determine if energy is required or released
The enthalpy of vaporization for water is higher than its enthalpy of fusion.
True
What must energy overcome during phase changes to allow transitions between solid, liquid, and gas states?
Intermolecular forces
Which phase change involves a substance directly transitioning from solid to gas?
Sublimation
The enthalpy of fusion is expressed in
kJ/mol
.
True
The enthalpy of vaporization for water (H2O) is 40.7
kJ/mol
.
True
Endothermic
phase changes absorb energy.
True
Calculating energy changes during phase changes requires multiplying the moles of substance by the corresponding enthalpy value.
True
In freezer operation, what type of energy change occurs during water freezing?
Exothermic
What do phase changes refer to?
Transitions between states of matter
Phase changes occur when the energy of the system is increased or
decreased
Adding heat energy to a
solid
can cause it to melt.
True
Match the enthalpy type with its description:
Enthalpy of fusion ↔️ Energy to change solid to liquid
Enthalpy of vaporization ↔️ Energy to change liquid to gas
The enthalpy of fusion measures the energy required to change a substance from solid to
liquid
What is the enthalpy of vaporization for water (H2O)?
40.7 kJ/mol
The phase change from solid to liquid is called
melting
What is the enthalpy of fusion for water (H2O) in kJ/mol?
6.01
What does the enthalpy of vaporization depend on primarily?
Intermolecular forces
Which phase change is an example of an endothermic process?
Melting
What is the energy required to melt 2.5 mol of ice at 0°C using the enthalpy of fusion for water?
15.025 kJ
To melt 3 moles of ice,
18.03
kJ of energy is required.
True
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