Name: _______________________
Date: _______________________
Learning Target:
I can understand the law of conservation of energy and the
processes of heat transfer.
I can perform calculations involving heat, mass, temperature
change, and specific heat.
Performances of Understanding:
-I will listen attentively while taking written notes in
Cornell format.
Criteria for Success:
I can label and explain parts of a heating or cooling curve.
I can define and calculate molar heat of fusion or molar heat of
vaporization.
I can solve for heat, mass, temperature change (or initial or final
temp) or specific heat if given all but one of the variables.
Heating and Cooling Curves
A. A graph of the temperature of the system versus the amount of heat added (or temp vs time if the heat is added at a
constant rate) is called a ______________ __________.
Segment AB:
Segment BC:
Segment CD:
Segment DE:
Segment EF:
Energy Calculations for the Sloping Segments of the Heating (or Cooling) Curve
A. We can calculate the energy change of the system for each of the segments of a heating curve using
______________. In segments AB, CD, and EF we are heating a __________ phase from one temperature to another.
1. The ______________ the specific heat of a substance, the more heat we must add to accomplish a certain
temperature increase.
Example with H2O
(Note: H2O has a different a specific heat depending on the state of matter)
AB: 100. g • 2.10 J/gºC • 20.0ºC = 4,200 J
CD: 100. g • 4.18 J/gºC • 100.ºC = 41,800 J
EF: 100. • g 1.70 J/gºC • 25.0ºC = 4,250 J
Energy Calculations for the Plateau Segments of the Heating (or Cooling) Curve
A. In segments BC and DE we are converting one phase to another at a __________temperature. The temperature
remains constant during these phase changes, because the added energy is used to overcome the attractive forces
_______________ molecules rather than to increase their average kinetic energy.
1. The energy required to make the solid to liquid transition is called the Heat of ___________ (∆Hfus). For
segment BC, in which ice is converted to water, the enthalpy change can be calculated by using ∆Hfus.
2. The energy required to make the liquid to gas transition is called the heat of _______________ (∆Hvap). For
segment DE we can use ∆Hvap.
Q=n•∆Hfus or Q=n•∆Hvap
(where n is number of moles)
Example with H2O
(Note: ∆Hfus and ∆Hvap will be provided or must be determined from a reference)
BC: Q = (100 g • 1mole/18g) • (6.01 kJ/mole) = 33.4 kJ
DE: Q = (100 g • 1mole/18g) • (40.7 kJ/mole) = 226 kJ
a. There is no ∆T because during a phase change, the substance remains at a ____________
temperature (the plateau on the graph) and no temperature change occurs.
b. These values are ____________ to each particular substance and are measured in kJ/mole (or g/J).
Interpreting Heating and Cooling Curves
A. The ______________ of the plateau is directly related to the amount of heat required to make the phase change. If
more substance was making the phase change, the plateau would be ____________.
B. Remember the angle of the sloping portion is directly related to the ___________ __________ of the phase of the
substance.
1. The slope of the graph will be ________for substances with a lower specific heat.
2. The slope of the graph will be_______ for substances with a higher specific heat.
C. Similar information can be portrayed in a _____________ curve.
Energy Changes
A. Since temperature is a measure of _________ __________ _______, any change in temperature is a change in kinetic
energy. All of the ________ line segments on a heating or cooling curve show a temperature change and therefore a
change in kinetic energy. During these regions, a __________ state of matter exists and the sample is either getting
hotter or cooler.
B. During the ______________ line segments, there is ____ change in temperature, so kinetic energy remains constant.
However, all the energy that is absorbed or released is related to changes in potential energy.
1. Remember the 3 Ps:
Plateau, Phase change, and Potential Energy Change.
Guided/Independent Practice
Directions: Show all work, and express answers with significant figures and proper labels (units and substance).
1. Calculate the molar enthalpy of solidification (ΔHsolidification) when 10.00kJ of energy are lost as 30.00g of water are
frozen at 0ºC. Remember that this value will be negative, because energy is lost when water freezes. (Hint: start by
finding out how many moles of water that are freezing)
2. Calculate the molar enthalpy of condensation (ΔHcondensation) for ammonia when 50.0g of NH3 gas turn into a liquid at
its boiling point. 68,500J of energy are released in the process.
3. Calculate the energy absorbed when 2.0x103g of dry ice (CO2) sublimate at the normal sublimation point. The molar
enthalpy of sublimation of CO2 is 8.647kJ/mol.
4. Methane (CH4) has a normal boiling point of -161.6 ºC. At this temperature, the ΔHcondensation = -8.17kJ/mol. If 16.5g
of liquid methane vaporize, how much energy is absorbed?
5. How much energy is required to melt a 20.0 lb bag of ice at 0ºC? A pound (lb.) of ice is equivalent to 0.4536 kg. The
ΔHfusion of ice is +6.009kJ/mol.
6. When water vaporizes at its normal boiling point, its ΔHvaporization = +40.79kJ/mol. Calculate the number of moles of
water that condense if 3456kJ of energy are released.
7. What mass of aluminum metal would absorb 250.0 kJ when it melted at its melting point? The molar enthalpy of
fusion for aluminum is +10.71kJ/mol.
Directions: The heating curve shown above is a plot of temperature vs time. It represents the heating of substance X at a
constant rate of heat transfer. Answer the following questions using this heating curve:
8. In what part of the curve would substance X have a definite shape and definite volume?
9. In what part of the curve would substance X have a definite volume but no definite shape?
10. What part of the curve represents a mixed solid/liquid phase of substance X?
11. What part of the curve represents a mixed liquid/vapor phase of substance X?
12. What is the melting temperature of substance X?
13. What is the boiling temperature of substance X?
14. In what part(s) of the curve would increasing kinetic energy be displayed?
15. In what part(s) of the curve would increasing potential energy be displayed?
16. In what part of the curve would the molecules of substance X be farthest apart?
17. In what part of the curve would the molecules of X have the lowest kinetic energy?
18. In what part of the curve would the molecules of X have the greatest kinetic energy?
19. Calculate the total heat energy in Joules needed to convert 20. g of substance X from -10.°C to 70.°C using the
heating curve and the information above.