AP Chemistry : Energy of Phase Changes
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Example Questions
Example Question #1 : Phase Changes
As condensation forms on a glass of ice water, the temperature of the air surrounding the glass __________.
increases
decreases
is impossible to determine
stays the same
increases
As a substance condenses from the gas phase to the liquid phase, it loses energy in the form of heat loss. Heat is transferred from the water to the air, resulting in an increase in the temperature of the air.
Example Question #2 : Phase Changes
How much energy is required to boil 9 moles of liquid water at its boiling point, and what is the temperature of the water vapor product?
The enthalpy of vaporization gives the amount of energy required to evaporate a liquid at its boiling point, in units of energy per mole. The total energy requirement to heat a given amount of steam is found by mulitplying the the number of moles to be vaporized by the energy of vaporization per mole.
The temperature remains constant throughout a phase change, thus the final temperature would still be 100°C.
Example Question #1 : Phase Changes
How much heat must be added to raise a sample of 100g of water at 270K to 280K?
The specific heat capacity of water is 
The following fomula gives the heat needed to generate a given temperature change for a substance of known specific heat capacity:
where 
However, in the event of a phase change (water melts at 273K), the heat of fusion or vaporization must be added to the total energy cost. The formula becomes:
Example Question #1 : Phase Changes
Why does water boil at a lower temperature at high elevation?
Water has a higher vapor pressure at high elevation
The atmospheric pressure is lower at high elevations
There is a lower heat of fusion at higher elevation
Hydrogen bonds are easier to disrupt at high elevation
The higher the elevation, the denser water is.
The atmospheric pressure is lower at high elevations
Increasing temperature means that vapor pressure increases as well. When vapor pressure is equal to the atmospheric pressure, water boils. The atmospheric pressure is lower at high elevation, so water boils at a lower temperature.
Example Question #2 : Energy Of Phase Changes
Using the heat curve, define the segment time(s) that the kinetic energy of the substance is increasing.
1, 2, and 5
2, 3, and 4
1, 3, and 5
2 and 4
1, 3, and 5
Remember, temperature is a measure of the average kinetic energy of molecules. Therefore the kinetic energy increases whenever the temperature is increasing. So, the kinetic energy is increasing during segments 1, 3, and 5.
Example Question #1 : Phase Changes
Using the heating curve, determine which segment(s) relate to an increase in potential energy.
2, 3, and 4
1, 2, 3, 4, and 5
1, 3, and 5
2 and 4
2 and 4
Remember, temperature is a measure of the average kinetic energy of molecules. So, the potential energy of the molecules will increase anytime energy is being supplied to the system but the temperature is not increasing. Therefore the potential energy is increasing during segments 2 and 4.
Example Question #1 : Phase Changes
The heating curve represents a substance in phases solid, liquid, and gas. Which segment represents only the liquid phase?
2
3
4
1
3
When the kinetic energy is increasing (the temperature is also increasing) the substance is not going through a phase change. Therefore only the segments that are at an incline will have the substance in just one phase. The flat areas of the graph represent areas in which heat is being added, but there is no corresponding increase in temperature. Rather, this added heat energy is used to break the intermolecular forces between molecules/atoms and drive phase changes.
Finally, because liquids are higher in energy than solids, and lower in energy than gasses the middle slanted line must be the liquid phase. In this case it is labeled as segment 3.
Example Question #5 : Energy Of Phase Changes
In the given heating curve, which segment(s) correlate to a mixture of phases?
1, 2, 3, 4, 5
1, 3, 5
2, 4
2, 3, 4
2, 4
When kinetic energy is increasing molecules are simply moving faster. However, when the potential energy is increasing molecules are changing phases. Therefore, when the potential energy is increasing is when the molecule is changing phases. Therefore there is a mix of molecules during segments 2 and 4.
Example Question #6 : Energy Of Phase Changes
The given heating curve represents a substance in phases solid, liquid, and gas. Which segment represents the substance as it is boiling?
4
2
3
5
4
Boiling is a phase change from liquids to gas. Therefore we are looking for a segment that is flat (because the potential energy is increasing) and that is between the liquid and gas phases.
In this case, gas phase is the highest energy phase, and liquids is the next highest. Therefore the substance is boiling during segment 4.
Example Question #7 : Energy Of Phase Changes
In the heating curve shown above, at what point do the molecules have the highest kinetic energy?
The beginning of segment 1
The end of segment 1
The beginning of segment 5
The end of segment 5
The end of segment 5
Remember, temperature is a measure of the average kinetic energy. Therefore the kinetic energy will be the highest when the temperature is the highest.
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