SCH4U
LAB # 1
Enthalpy of Fusion of Ice
When heat is added to a solid, the kinetic energy of the molecules in the solid increases and the
increased motion of the molecules is reflected by a rise in the temperature of the solid. When the melting
point of the substance is reached, however, the temperature remains constant as the change of phase
occurs. The continued addition of heat at that temperature is used to cause changes in the potential energy,
which results in the loosening of bonds between the molecules. The heat required to change a solid to a liquid
with no change in temperature is called the latent enthalpy of fusion.
In this experiment you will use calorimetry to determine the molar enthalpy of fusion (i.e. the latent
enthalpy of fusion per mole) for ice. The calorimeter used will be a simple Styrofoam cup. It will be assumed
that no heat is gained or lost by the calorimeter.
Pre-lab Questions:
1. What does the term latent mean? Why is the enthalpy of fusion considered to be latent?
2. Look up the accepted value for the enthalpy of fusion for ice in kJ/mol (molar enthalpy) and in J/g
(specific enthalpy).
3. Using the data from question 2, how much heat is required to melt a 15 g ice cube?
4. What data would you need to determine the heat required to boil 15 g of water? Look up this value
and calculate the heat required.
5. The specific heat capacity of water is 4.18 J/goC. Look up the specific heat capacity of water vapour
and ice.
Materials:
Styrofoam cup
thermometer
glass stirring rod
Balance
paper towels
ice
Procedure:
1. Determine the mass of the Styrofoam cup.
2. Half fill the cup with water at a temperature of approximately 50 oC. Determine the mass of the cup
and water together.
3. Record the temperature of the water immediately prior to the next step.
4. Obtain some small pieces of ice which have been dried with a paper towel and transfer them to the
cup.
5. Stir the mixture until all the ice has melted. Record the lowest temperature reached by the water.
6. Determine the mass of the cup again to obtain the mass of the ice added.
Observations:
Initial mass of cup:
Mass of cup with warm water:
Final mass of cup and contents:
Initial temperature of warm water:
Lowest temperature of water:
Specific heat capacity of water:
___________________g
___________________g
__________________ g
__________________ oC
__________________ oC
4.18 J/goC
Calculations:
1.
The quantity of heat gained lost or gained by a substance is given by the equation:
q = mcΔT
Use this relationship and your data to calculate the following:
a) the heat lost by the warm water
b) the heat lost by the calorimeter
c) the heat gained by the water that was formed from the melted ice
2. Hess’s law of constant heat summation indicates that the heat gained by one part of a system equals
the heat lost by another part of the system. (The whole premise of calorimetry.) Notice that the
heat gained by the cold water (i.e. the water that came from the ice cube) does not equal the heat
lost by the warm water. The difference must be heat gained by the process of melting the ice cube.
Use this law to calculate the heat gained by the melting ice.
3. Using the mass of the ice cube, calculate the specific enthalpy of fusion (in J/g) and the molar
enthalpy of fusion (in J/mol).
4. Using the data that you researched in the pre-lab, calculate your percent error.
Post-Lab Questions:
1.
Explain why:
a) you were asked to dry the ice before you put it in the cup of water. If you had not done this,
in what direction would your experimental enthalpy of fusion have been shifted (i.e. greater
or less.) Use hypothetical values to illustrate your answer.
b) you were asked to use warm water rather than room temperature water. If you had not done
this, in what direction would your experimental enthalpy of fusion have been shifted (i.e.
greater or less.) Again, justify your answer.
2. Consider a 20.0 g sample of ice. If it was heated until it turned into water vapour,
a) make a sketch of the heating curve, labelling each change on your curve.
b) Using this curve as a guide and the specific heat capacities and enthalpies you have looked up in
the pre-lab, determine the total amount of heat needed to change 20.0 g of ice at -15oC to steam at
120oC.