Experiment 15b: H&T Lab 4  Phase Change

Heat Transfer Without Temperature Change:  Is it Possible?

Duh,  Of course it is, otherwise why would we be doing this lab?

Objective:  Recognize and understand conditions under which heat can be added to a mass without a temperature change.

Just in case you haven't memorized this yet, let's return to the principle of heat transfer we developed in the last unit:

"Heat is energy in transit between two systems in thermal contact due to the temperature difference only."

Consider the heating coil, if you touch it, is very hot.  It is capable of transferring a known amount of heat energy to any system having lower temperature than it has.  The question is this – how is it possible for a system to absorb heat and not change temperature?

 

Activity 1.  Changing Ice to Water and then to Steam.

 Discussion :   Predicting T vs. t  for Water

(1)  Suppose you were to add heat  at a constant rate to a container of water at 0ĄC (with no ice in it) for 10 minutes at a low enough rate that the water almost reaches its boiling point.  Sketch the predicted shape of the heating "curve" on the graph below.

 

 

(2) Suppose that the container had a mixture of ice and water at 0ĄC when you started heating it at a faster rate and that the water starts boiling after five minutes (300 seconds).  You keep adding heat energy at the same rate for five more minutes. Draw a dotted line on the graph above showing your prediction.

(3)  Determining Heats of Fusion and Vaporization for water.

By transferring a known amount of heat energy to a mixture that is originally half water and half ice until the ice melts, we can determine the amount of heat energy needed to melt a gram of ice. This energy is known as the latent heat of fusion.  By transferring a known amount of heat energy to a known amount of water at 1000C and then measuring what's left of the water, one can also determine the amount of heat energy needed to turn a gram of boiling water into steam.  This energy is known as the latent heat of vaporization.  It is often measured in joules per gram.  We will use these units in this activity.  For this activity you will need:

                                    „           A LabPro and Temperature Probe - red toolbox  PS - 5

                                    „           A heating coil- Cab 4D

                                    „           A Voltmeter &  an Ammeter- get 2 multimeters from Cab G/3 C

                                    „           10 Volts DC (approximately)  - R&B plugs under table

                                    „           SPST switch  Cab 1 D

                                    „           A large (plastic insulated)  cup

                                    „           Small styrofoam cups to put inside the large cup  Cab 4B

                                    „           Crushed ice and paper towels

                                    „           Water at 0ĄC

                                    „           An electronic balance

 

Notes:

 

1.  You should calibrate and set up the temperature sensing system to take temperature vs. time data for 20 minutes (1200 s) from one temperature sensor while the heating coil is enabled. Set the temperature scale from 0 to 120.  You do not necessarily have to run the experiment for the full 20 minutes.  You should continue adding heat to the system until all of the ice is melted and then keep going until the water reaches the boiling point and boils for 5 minutes or so but not enough to cause water level to drop below the coils.

 

2.  You should stir continuously during the experiment and keep the heating coil immersed at all times.

 

3.  WARNING!!  DO NOT TURN ON HEAT UNLESS THE HEATER IS IN THE WATER! Leave the heating coil unplugged until you are ready to start. 

 

4.  As with any experiment, you should record and clearly label all data necessary to obtain results included herein.  Each incidence of missing data will cost you 1 point.

 

Procedure:  

 

1.  You must be prepared to begin this experiment immediately after you mix the water and ice together, so first set up the apparatus and calibrate your temperature probe.

 

2.  Pre-cool the heating coil by placing it in ice-water bath.

 

2. Measure 80 g ice water in a styrofoam cup (which is inside one of the plastic cups). Make sure the styrofoam cup sits high enough in the plastic cup so that its rim is above the plastic cup's rim. 

 

3. In a separate cup measure 30 g (approximately) of fresh, dry ice.  Be sure to record the exact mass of ice.

 

4.  Take both cups over to your setup. Dump in the ice then put in the coil, then throw the switch and start Logger Pro immediately!

 

5.  Don't forget to stir constantly.  Also record the voltage and current at the end of each minute, (you'll take an average over the entire time of the lab.)

 

6.  Continue to heat the water for 5 minutes after the water begins to boil. 

 

  1. When the experiment is finished, immediately remove the coil and go weigh the cup and water combined.  Then pour out the water and re-weigh the cup to find out how much water was left .  Be sure to record the mass of the water that was vaporized.  You'll need this in part B below.

 

A.  Determining the latent heat of fusion of water.

 

1.         How long did it take the ice to melt?  ________

 

            Calculate how much energy in joules was added to the mixture by the heating coil while the ice was melting. 

 

Show your formula!  Qexp__________

 

 

2. Remembering how many grams of ice you started with and the fact that Q = mLf, calculate the experimental value of the latent heat of fusion.  Show your calculations.

 

                        Lf exp =               J/kg

 

 

 

3.   Analyze your curve after the ice has melted and figure out how many joules it takes to raise each gram of water (after the ice has melted) by 1ĄC. (You may want to go back and see how you did this in H & T Lab 2.  Just pick two points on your T vs t curve and use them).   This is of course the specific heat cw, of water.  Show data used, and the formula you used to obtain:

 

                        c =        ____

 

B. Determining the latent heat of vaporization of water.

 

1.         Record the number of seconds the water boiled. __________

 

2.         How many grams of water turned to steam in that time?________

                        How much heat energy was added in that time?  ________

                        (Show all pertinent data and calculations.)

 

 

 

 

3.  Using these number and the mass of the water vaporized,find the experimental value for the latent heat of vaporization, Lv exp.  Show your calculations.

 

 Lv exp. = __________

 

 

4. Compare the values of specific heat and latent heat of fusion to the accepted values stated in your textbook or a handbook.  What % error is there in each case?

 

 

Exp

Accepted

% error

latent heatof FusionLf (J/kg)

 

 

 

Specific Heat (c) J/(kg cĄ)

 

 

 

Latent Heat of Vaporization Lv (J/kg)

 

 

 

 

5. Are your values higher or lower than the accepted values in each case?  Can you think of any sources of systematic error to explain this?

 

 

 

 

 

.6  Obtain a print out of the graph for this experiment.

 

Homework for Changing Phase:

Ice to Water and Water to Steam

 

1.         Describe what happens to the temperature of water when heat energy is transferred to it at a constant rate.

            _________________________________________

            ___________________________________________

 

 2.        a)  Sketch a temperature history on the axes below. 

            b)  Describe what happens to the temperature of a water-ice mixture originally at 0ĄC when heat energy is transferred to it at a constant rate.   

            c)  Describe in your explanation where the ice disappears. 

d)    If the temperature is constant for part of your history, explain where the energy being transferred is going.       

 

 

 

 

3.         Convert your value for the Latent Heat of Fusion of ice from joules per gram to calories per gram.  Show your calculations.

                       

            ___________calories/gm

 

4.         Suppose you start with 250 grams of ice at 0 ĄC.  Calculate the amount of energy that must be transferred to melt the ice.  (Use 335 joules/gram for the Latent Heat of Fusion of ice.)  Show your calculations.

                       

           

_____________joules 

5.         A mixture of 150 grams of ice and 300 grams of water is at 0 ĄC.  How many joules of heat energy must be transferred to bring this mixture to a final temperature of 75 ĄC ?  Assume the heat lost to the room is very small. Show your calculations. (Use 335  joules/gram for  the Latent Heat of Fusion of Ice and 4.18 joules/gram ĄC for the Specific Heat Capacity of water.)      

                       

            __________joules

6.         Describe what would happen to the water in Question 2 if you continue to transfer heat at a constant rate even after the ice has melted.  Sketch the temperature history on the axes below.  Indicate on your sketch where the water begins to boil.  If the temperature is constant for part of your history, explain where the energy being transferred is going.

 

 

7.         Convert your value for the Latent Heat of Vaporization of water from joules per gram to calories per gram.  Show your calculation.

                       

            ___________calories/gm

 

8.         Suppose you start with 250 grams of ice at  0 ĄC.  Calculate the amount of heat energy which must be transferred to convert the ice to steam at 100 ĄC.  Show your calculations.  (Use 335 joules/gram for the Latent Heat of Fusion, 2250 joules/gram for the Latent Heat of Vaporization and 4.18 joules/gram ĄC for the Specific Heat Capacity of water.)

 

 

            __________joules  .