Heating And Cooling Curve Lab

Heating and Cooling Curve Lab

Introduction

In Part 1, stearic acid will be cooled (heat removed) at a constant rate.  Starting with the substance in its liquid phase at a temperature well above its freezing point, temperature readings will be made at regular intervals until the substance changes to its solid phase and cools to a temperature well below its freezing point.  The temperature readings will thus show the effects of removing heat from a pure substance in the liquid phase, during a phase change (liquid to solid) and in the solid phase.

In Part 2, the procedure will be reversed and stearic acid, in its solid phase, will be heated at a constant rate and temperature readings will be made until the substance is in its liquid phase at a temperature well above its melting point.

Purpose

·       Study the effects of heating and cooling a pure substance through a phase change.

·       Construct heating and cooling curves using experimental data.

·       Determine the freezing and melting point temperatures of the pure substance.

Equipment

·       Ring stand

·       Test tube clamp

·       Test tube of stearic acid

·       Iron ring

·       Bunsen burner

·       Test tube rack

·       Beaker (250 mL)

·       Wire gauze

·       Stop watch

·       Thermometer

Procedure – Part 1: Cooling Curve

1.     Fill a 250 mL beaker with cold water.  Obtain a corked test tube containing a sample of the substance (stearic acid) to be studied.  Clamp the test tube to a ring stand with a test tube clamp.

2.     Remove the cork.  Heat the sample carefully by moving the burner gently back and forth.  Caution: Before heating ensure that the mouth of the test tube is pointing away from yourself and others.

3.     As soon as the sample BEGINS to melt, remove the heat and place a thermometer in the sample.  Using the thermometer to stir the sample, resume heating gently until the sample is completely melted.  DO NOT OVERHEAT.  The final temperature of the sample should be less than 70^oC.  If it is higher, wait until it drops to this temperature before proceeding with step 4.

4.     At this point in the experiment, one group member will call out the time every 30 s.  Another group member can record temperature data in the Data Table.  Keep stirring.  The other partner will read off the temperature of the sample at each half-minute interval.

5.     Continue this procedure until the temperature of the sample reaches 40^oC.  Remove the test tube from the water and let stand in a test tube rack.

 Part 2: Heating Curve

1.     Heat the water in the beaker to 80^oC.  Remove the heat.

2.     While the water was being heated, the sample in the test tube was cooling to approximately room temperature.  Set the time to 0 minutes and record the exact temperature of the sample and immerse it below the water level in the hot water bath.  Read and record the temperature of the sample every 30 s. as in Part 1.

3.     As soon as the thermometer is free to move, it should be used to stir the solid-liquid sample.  Continue stirring and recording the temperature at half-minute intervals until the temperature of the sample reaches 70^oC.

4.     Clean up all materials as instructed.

Analysis

Plot your data from this experiment on graph paper.  Be sure to label both axes and to use as much of the graph as possible.  Use one color for cooling data and another color for the heating data.  Do not connect the dots!  Draw a smooth curve through the points.

Questions

1. Based on your graph, determine the melting and freezing point of the substance used.  How do these values compare?

The freezing point and melting point are both the same time and temperature. At 300 seconds, the melting point and cooling point were both at the temperature of 79.5 degrees Celsius. 

2.         Describe the shape of your graph during the actual changes of state (while the substance is actually melting or solidifying).

The shape of the graph when it came to the point of melting or solidifying is a line with a slight slope. The graph of the melting point has a slight slope upward, while the graph of the freezing point has a slight slope downward. Once it reaches it melting and freezing point, the amount the temperature has been decreasing or increasing at, becomes to be increasing and decreasing at larger increments. 

3.         During the heating process, heat is continually being supplied to the sample throughout the entire time of the experiment even though the temperature remains constant during the actual change of phase.  How can this be explained at the molecular level, in terms of what is happening to the chemical bonds holding the particles together in the solid state?

On a molecular level, the chemical bonds are slowly breaking apart as heat is continuously applied. The energy from the heat breaks apart the chemical bonds which causes exertion to occur, which in the end, causes the temperature to remain constant during the actual change of phase.

4.         Consider the diagonal region of the cooling curve, as the sample is being cooled.  What does the temperature change indicate about the change in kinetic energy of the particles in the sample?

As the temperature decreases, so does the kinetic energy. This happens because temperature is directly related to the pressure of a substance, causing the molecules of that substance to be less excited. The IMF are being strengthened as it reaches the freezing point.