Cooling Diagram for t-Butanol
By Chuck Roser
Retired Chemistry Instructor, North Carolina School of Science and Mathematics
National Science Education Standards
- Grades 9–12, H.U.3c Change and Measurement, H.A.2 Understanding about Scientific Inquiry, H.B.2d Physical Properties of Compounds, and H.B.5 Conservation of Energy
- To measure and record the temperature change as t-butanol (2-methyl-2-propanol) cools from its boiling temperature to 10° C and to determine its boiling and freezing points.
- To calculate the heat associated with the phase changes (potential energy) of a pure substance and raising or lowering its temperature (average kinetic energy) in the solid and liquid states.
Summary of theory
As you heat a solid to its melting temperature, the average kinetic energy (KE) of its molecules and its temperature increase. As the solid melts, a phase change occurs and the molecules store the energy being added as increased potential energy (PE). The heat is used to overcome some of the intermolecular forces between the molecules so the molecules are free to move around each other. The temperature remains constant during the phase change.
Heating the liquid to its boiling temperature increases the average KE and temperature. As the liquid boils, PE increases as the remaining intermolecular forces are overcome. The temperature remains constant during the second phase change. Once this phase change is complete, additional energy heats the vapor to a final temperature. A typical heating diagram is shown below (Fig. 1) with the y-axis representing increasing temperature and the x-axis sample-heating time. The plateau regions represent the melting and boiling temperatures.
The time required to complete each part of the heating process depends on the amount of heat that must be absorbed and the rate at which the heat source supplies energy to the sample (J/sec). The heat absorbed for each part of this transition of solid t-butanol to vapor can be calculated as follows:
|Q = heat||m = mass||c = specific heat||ΔT = change in temperature|
Heating the solid to its melting temperature:
|Q1 = (mass in g)(csolid)(ΔT)||ΔT = Tfinal – Tinitial|
Melting the solid:
|Q2 = (moles)(ΔH°fusion)||No temperature change occurs.|
Heating the liquid to its boiling point:
|Q3 = (mass in g)(cliquid)(ΔT)||ΔT = Tfinal – Tinitial|
Boiling the liquid at its boiling temperature:
|Q4 = (moles)(ΔH°vaporization)||No temperature change occurs.|
Heating the gas/vapor to its final temperature:
|Q5 = (mass in g)(cgas)(ΔT)||ΔT = Tfinal – Tinitial|
The total heat requirement for all 5 events is Qtotal:
Qtotal = Q1 + Q2 + Q3 + Q4 + Q5
The CRC Handbook of Chemistry and Physics provides the following data on t-butanol:
Density of liquid t-butanol is 0.789 g/mL at ≅25° C
ΔH°fusion = 6.71 kJ/mol
ΔH°vaporization = 39.1 kJ/mol
csolid = 2.30 J/g °C
cliquid = 3.35 J/g °C
cgas = 1.93 J/g °C
The structure of t-butanol is
In this experiment, you will take temperature readings every 30 seconds (s) from the boiling point to below the freezing point, construct a cooling diagram of temperature vs time, estimate the boiling and freezing points, and calculate the heat released for parts of the cooling diagram.
The following materials are for a class of 20 students working in pairs.
- Students and instructor wear safety glasses during data collection.
- Use care when handling hot test tubes.
- Do not use open flames in the lab. t-Butanol is flammable.
- Follow your instructor’s directions for cleaning the equipment and disposing of t-butanol.
- Obtain and wear safety goggles.
- Measure 5.0 mL of t-butanol in a graduated cylinder. Pour the t-butanol into a 15 × 120-mm test tube. Clamp the test tube to a ring stand with a small 3-prong clamp.
- Fill a 250-mL beaker with about 150 mL of hot water and place it on a hot plate set to maintain 90° C.
- Adjust the ring stand assembly and the test tube so that the test tube is immersed in the hot water above the level of the t-butanol. Make sure the test tube does not contact the beaker’s bottom and sides.
- Place the thermometer in the test tube. Hold it in the middle of the liquid during data collection. Stir the t-butanol continuously, moving the thermometer with a small up-and-down motion to keep the temperature homogeneous.
- Heat the t-butanol until it boils and the thermometer reading stabilizes. Record 3 to 4 data points while the t-butanol is boiling. Record the temperature every 30 s.
- Remove the test tube from the hot-water bath using the test tube clamp and transfer it to a 250-mL beaker containing about 150 mL of room temperature water.
- Place the test tube in the middle of the beaker and clamp it to a ring stand. Make sure that the test tube is immersed in the cooling water above the level of the t-butanol and that the test tube does not contact the beaker’s bottom and sides.
- Continue to record the temperature every 30 s and stir the t-butanol as it cools. Allow the t-butanol to cool in the room temperature bath until it starts to solidify. Stir the water bath with a stirring rod to promote even cooling.
- After the t-butanol starts to solidify, add small amounts of ice to the water bath until the t-butanol freezes and the temperature starts to drop.
- Stop stirring the t-butanol once it freezes and the thermometer resists movement. Continue stirring the water bath to promote even cooling. Add ice and record data until the temperature reaches 10° C.
- Do not attempt to pull the thermometer out of the solid t-butanol; you could break the thermometer. Instead, immerse the test tube in the hot-water bath to melt the t-butanol and then remove the thermometer. Rinse the thermometer with water into a waste beaker and dry it with a paper towel. Follow your instructor’s direction on handling the test tube containing t-butanol.
- Prepare a graph of temperature in °C vs time in s. This graph should have labeled axes with units. If you have access to a graphing calculator or graphing software, graph the data and print the graph.
- Estimate the boiling and freezing points from the graph’s plateau regions.
- For the heat calculations, assume a volume of 5.00 mL of t-butanol unless otherwise instructed.
The handbook values for t-butanol are: boiling point, 83.0° C, and melting point, 25.5° C. Compare your results to those values.