This piece explains an activity that allows for the simple and accurate determination of the heat of vaporization, ΔHvap, of water at 100°C, and ultimately the approximate strength of a hydrogen bond in boiling water, in kJ·mol–1.
The vaporization of water is an endothermic process represented by Equation 1:
My take on this well-known determination produces a value for the ΔHvap of water at 100°C to within a few percent of the accepted value.
The manner in which this experiment is presented and carried out involves an engaging class discussion which calls on students’ critical thinking; the assignment extends their learning .
All in all, this activity offers the satisfaction of a concept well understood—and of a job well done.
heat of vaporization
- electric kettle
- ca 110°C thermometer
- electronic balance—preferably large capacity
- timing device*
*Students will likely use the timer on their mobile phones. I discourage this, given that mobile phones present a pretty much insurmountable distraction. But there is light at the end of this electronic tunnel: When students employ their phones as timers, they will inevitably report times precise to the hundredth of a second, which is ridiculous. This will invite a useful discussion of confidence level in measured quantities. See my previous post: Confidence Level: Measurement and Significant Figures Simplified.
My Approach to the Activity
I use a version of a Socratic lesson, where students work through what I call an interactive handout. A handout of this type has lots of blank spaces that students fill-in as the explanation develops, allowing for better focus on the concepts at hand—by asking questions and making comments—and less time spent writing everything down. To access the handout, see the Supporting Information. (Readers must log into their account to access. Not a member? Register for free!)
My “lever” for this lesson is units. Specifically, I use the units of ΔHvap — J·mol–1 or J·g–1, and the meaning of Power (W = J·s–1) — to guide students’ thinking.
Premise of the activity
When water in a kettle is boiling, its temperature remains at 100°C. But the kettle continues to draw electrical energy, measured in Watts, or J·s–1. Through careful questioning, students will realize that this continually added energy is used to overcome intermolecular forces: to separate molecules of liquid water, which are heavily hydrogen bonded to nearby molecules, to molecules of gaseous water—steam—which, according to the Kinetic Molecular Theory for Gases, are neither attracted to nor repelled by neighboring molecules.1
And so the ΔHvap of water is simply the energy required to overcome the intermolecular forces between H2O(l) molecules. We revisit intermolecular forces later.
The procedure for determining ΔHvap goes like this:
- boil water in a kettle
- immediately record the (initial) mass of the just-boiled water + kettle
- right away, plug the kettle back in and start the timer
- the water should re-boil pretty much right away; record the time to boil off, say, 60 g of water
- record the (final) mass of the kettle + water
Students may carry this out in small groups, with the caveat that with so many kettles plugged in, there is a risk of tripping circuit breakers in the lab. Further, you may be uncomfortable with students working with boiling water. So a teacher demonstration may be a good idea.2
Equation 2
Using equation 2 and the manufacturer’s stated power of the kettle—typically stamped on the bottom of the kettle—will provide a ΔHvap value that is too high.
Ask your students: Is the wattage (power, W = J·s–1) stamped on the bottom of the kettle the input wattage—drawn from the electrical socket, or the actual wattage delivered to the water? Stated another way, is the kettle 100% efficient in its transfer of energy?
It won’t take long for students to realize that they need to determine the effective, or actual, power of the kettle. They can do so by measuring the time required to heat—not boil—a known mass of water for a period of time, and using equation 3:
Typically, kettles are 75 – 80% efficient.
When the actual power output of a kettle is used in equation 2, an impressively accurate value for ΔHvap of water (100°C) is obtained.
Sample data, determined by the author, and calculations, are below:
Table 1. Data for the determination of the actual Power of the electric kettle used
Substituting into equation 3 gives:
Aside - if you would like to take a moment to extend the idea, you could also take a moment to calculate the percent efficiency of the kettle.
Equation 4
To determine ΔHvap, in J·g–1, we substitute the data in Table 2 into equation 3:
Table 2. Data for the determination of ΔHvap of water at 100°C
Multiplying by the molar mass of water gives
ΔHvap = (2.32x10 J·g–1) * (18.02 g·mol–1) = 41.8x103 J ·mol–1
And so 41.8 kJ·mol–1 is the empirically determined heat of vaporization of water at 100°C.
The accepted value4 for ΔHvap at 100°C is 40.8 kJ·mol–1 , giving us an error of 2.5% —not too shabby for a half an hour’s work, using an inexpensive electric kettle.
As mentioned previously, this activity can be taken an important step further—to determine the approximate strength of a hydrogen bond, in kJ·mol–1.
Water can form a maximum of four hydrogen bonds per molecule (figure 1).
Figure 1. A water molecule can form a maximum of four hydrogen bonds.5
An internet search reveals that each water molecule at 100°C is involved in anywhere from just over 2 to up to roughly 3.3 hydrogen bonds.6,7 We can use these values to find a range for the value of the strength of a hydrogen bond in 100°C water.
Equation 5
This tells us that the strength of a Hydrogen bond in boiling water is anywhere between 1.3 x 101 to 2 x 101 kJ·mol–1, or about 16 kJ·mol–1.
This is fabulous for two reasons:
- It is in the range of published data for the strength of a Hydrogen bond (4 – 50 kJ·mol–1).8
- It shows that the strength of a Hydrogen bond in water (ca 16 kJ·mol–1) is about 3.5% as strong as an O–H intramolecular bond (459 kJ·mol–1). Students may not think that this is a big deal—but it is!!! It shows that an intermolecular force is not negligible compared to an intramolecular force.
The assignment questions in the accompanying student handout end by asking students to prepare an annotated graph of Temperature vs Time for a given mass of water as it is heated from -10°C to 110°C. This will generate an interesting discussion after-the-fact.
See the Supporting Information for access to the teacher resource. Readers must log into their account to access. Not a member? Register for free!
- 1.4: The Kinetic Molecular Theory of Ideal Gases - Chemistry LibreTexts
- See supplemental interactive handout for author-obtained data.
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Taken from Heat (purdue.edu); we assume that this value is constant with temperature—this is high school chemistry, after all.
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User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia to Commons by sevela.p. Translated to english by by Michal Maňas (User:snek01). Vectorized by Magasjukur2, Public domain, via Wikimedia Commons
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“Defining and counting the hydrogen bonds [in water] is not straightforward …” ; Hydrogen bond - Wikipedia
Set up equipment and materials.
N/A
Safety
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.
Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.
Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
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Comments 2
Credit to Martin Chaplin's work
This is a very nice activity.
I suggest that a better reference than (7) is the original -- from Martin Chaplin's astounding web site about all things water.
Here's the page about H-bonds in water:
https://water.lsbu.ac.uk/water/water_hydrogen_bonding.html
Heat of Vaporization
Love this! The thinking behind efficiency is a great real-world science application.