Heat of Vaporization
15. Heat of Vaporization*
You will learn about latent heat of vaporization
Learning Objectives:
st
1. Learn that during a 1 order phase change, liquid and gas phases coexist, at a fixed
boiling temperature, and heat must be supplied to convert a given amount of material
from liquid to gas, even though the temperature does not change.
2. Learn about power transfer to heat in an electrical circuit element.
Reading Assignment:
Before lab, read the section on phase change and heat of vaporization in your textbook. Also you MUST
read the section on power in electric circuits. This goes beyond what was covered in class, but you must
read it anyway. Recall too that power is rate of transfer of energy by heat or work.
Knight, Jones & Field (161): 12.5 Specific Heat and Heat of Transformation
Serway and Vuille 11.4 Latent Heat and Phase Change
Serway and Jewett (251): 20,3 Latent Heat
The Apollo 13 mission to the moon
was cut short and almost ended in disaster
due to the failure of a liquid hydrogen fuel
cell.
A heater malfunctioned, causing
excessive boiling of the hydrogen and an
increase in pressure that finally caused an
explosion. This damaged the space ship
and left it without enough power to perform
essential control and navigation functions.
Volts
In this scenario, your team at NASA
will study the rate of boiling of a liquid gas
under standard atmospheric pressure. For
obvious safety reasons, liquid nitrogen will
be used rather than liquid hydrogen.
( Nevertheless, you should be aware that
the liquid is very cold (cryogenic) and can
cause injury similar to the way boiling water
can cause burns.)
Amps
Equipment: In the illustration you see an insulated flask (called a Dewar flask, or just a Dewar) similar to
a thermos bottle that can contain liquefied gases. In this will be some liquid nitrogen, with a boiling point
of around 77 Kelvin (  321 F ). In the nitrogen is an emersion heater, similar to one you could use to
boil water, but operating at a much lower temperature. Outside the flask are an ammeter to measure
current, and a volt meter to measure voltage. An electric current can be provided by a D.C. power
supply, represented in the diagram as a battery. You will also have a triple beam balance and some
masses.
o
Pre-lab assignment: As always, you need to bring solutions with you when you come to lab. The
questions depend on your reading assignment.
______________________________________________________________________
*© William A Schwalm 2012
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Heat of Vaporization
PRE-LAB QUESTIONS
1. Water has a specific heat of about 1 calorie per gram, and a heat of vaporization of about 539
calories per gram. Suppose 3.5 kilograms of water is sitting on the stove, just at its boiling
temperature. If you add 30 kilocalories of heat to the water, will its temperature go up? If yes, then
how much will it go up? If no, then what will happen if 30 kilocalories of heat is added to the water?
Give a numerical result as part of your answer.
2. An electric motor operates at 6 volts. In order to lift a mass of 1 kilogram a distance of 2 meter in 3
seconds, how much current in amps must the motor draw? (Assume the motor is 100% efficient.)
3. An emersion heater is used to boil water. Assuming no heat escapes from the system, how much
power must the emersion heater dissipate in order to boil five grams of water into five grams of steam
in ten minutes at standard atmospheric pressure?
Problem 1
Your group is working on a method for determining the rate of vaporization of a liquid gas such as
hydrogen or helium. In the experiment, you use nitrogen. (NASA might be interested in liquid oxygen or
liquid hydrogen.) Now you will do more or less the following: First off, suppose that in your set up (figure
above) no current flows through the heater. Nitrogen still boils slightly because heat transfers in from the
room. Work out a simple way to measure how much liquid nitrogen boils away per minute from the
Dewar due only to heat coming in from the environment. Check this by making some actual
measurements.
1. Method question: How can you tell how much liquid nitrogen has boiled away in 15 minutes with no
current running in the heater?
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Heat of Vaporization
2. Consider liquid nitrogen at its boiling point, sitting in an open dewar, and suppose it absorbed heat
energy from the room at a constant rate (constant power, that is)
a. During this experiment, what would a graph of temperature versus time for the liquid nitrogen look
like? Would it be a straight line? Explain.
b. What would a graph of the mass of liquid nitrogen remaining in the Dewar look like as a function
of time? Would it be curved? Explain.
Plan: It will be convenient to perform the experiment with the Dewar resting on a triple beam balance, if
possible. Write a brief plan for taking data over the space of 15 minutes to estimate the rate of nitrogen
loss due to heat coming in from the room. By now, you know what kinds of things need to be in the plan.
You should consider making a simple graph.
Implementation: While you are implementing this plan, taking data periodically etc., your group should
also begin working on the next problem, since you need to make a seamless transition from one to the
other, and you don’t want to waste time.
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Heat of Vaporization
Analysis: Find the rate of loss. Explain what you’re doing.
Conclusion: Here you need to draw a brief conclusion about the meaning of what you found. Write at
least one sentence about the rate of loss of nitrogen from environmental heating.
Problem 2
If a heater provides additional heat (heat flow in excess of heat entering from the environment) how is the
rate of vaporization of nitrogen (due only to the additional heat) related to the power input from the
immersion heater? Calculate the heat of vaporization from the rate of vaporization and the power input.
1. Prediction question: Suppose you perform the following experiment with a Dewar containing liquid
nitrogen. You place the Dewar on a triple beam balance. You measure the mass of the apparatus
plus nitrogen. 15 minutes later you measure the mass of the apparatus plus nitrogen again, and then
right away start a current flowing in the heater. You let the current flow for 30 minutes, then stop the
current and immediately record the mass. Finally 15 minutes later you record the mass again, and
record the value. If you plot the four data points on a mass versus time graph and connect the four
points with three straight line segments, what do you predict the graph would look like? Draw it using
a ruler with enough detail so that another student, not taking this course, could read and understand
it.
2. Method question: If you measure the amount of liquid nitrogen, then apply a current to the heater for
30 minutes, and then measure the amount of nitrogen again, some part of the change will be due to
heat produced by the current flow, and some part will be due to heat entering from the environment.
How can you estimate how much nitrogen was boiled off due to environmental heating during the 30
minute period?
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Heat of Vaporization
3. Prediction question: If 2.0 amps of current flows through a 5.0 Ohm resistor for 30 minutes,
a. how many Joules of heat would be produced?
b. If 18 grams of liquid were vaporized due to the heating from this current flow, what would be
the heat of vaporization?
c.
How much would the temperature of the liquid change due to heat added?
4. Method question: Why do you want to have two different periods with no current flow, one before and
one after the period of heating? Does something in the experiment have to have time to “equilibrate,”
or might there be something different before and after? In either case, what is it and how will your
method of taking data help cancel this effect?
Plan: Write out a brief plan to determine the heat of vaporization of nitrogen. It turns out to be a good
idea to measure the rate of loss for 15 minutes with no current, then for 30 minutes with the heater on,
then another 15 minutes with no current.
Implementation: Carry out your plan, taking data periodically. Record the data you need here.
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Heat of Vaporization
Analysis: Carry out the analysis according to your plan. You need to make at least one graph. Attach it.
Find the heat of vaporization and explain the analysis in sufficient detail so that your supervisor will be
able to follow easily.
Conclusions: What did you learn?
1. Describe two significant things that do or that don’t happen during vaporization. Use at least one
complete sentence each.
2. If you had to recommend procedures for measuring the rates of vaporization for liquid oxygen or liquid
hydrogen for NASA, which aspects of your procedures could be carried over and applied to these other
substances and what changes would be necessary for safety reasons?
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