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Yoko Kato (NJ CHEM-E 22-23) • 6 months, 2 weeks ago
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I am looking at the reference sheets, and I am finding that the values for water constants do not match what was given in the lecture slides. The specific heat for ice says 2.09 but the slides said 2.06 and I am finding some reference online that says 2.108 j/g-C. The specific Heat for vapor says 1.84 on the reference sheet and on the slides, it said 2.02 j/g-C. The Heat of fusion value is different too. The reference says 330j/g but the slides say 334 j/g. I remember getting some of the practice questions wrong because of these constants being slightly off. Which is better to go by?
Hi Yoko.
The problem with the specific heat capacities is that they vary somewhat with temperature. At this level, for simplicity, we usually give our students the number we want them to use, or make assumptions if no specific values are given.
At conditions in the classroom (about 25 degrees C), the specific heat for ice is around 2.09 to 2.108 J/g-C, at 0 C, it is around 2.05 to 2.06 J/g-C.
2.1 might be a safe value to use for specific heat of ice with your students.
The specific heat of water vapor has the same problem; it varies somewhat with temperature. At 0 degrees Celsius to room temperature, it is about 1.86 J/g-C, and 1.89 J/g-C at 100 degrees C (it can go up to 2 and higher when superheated by a bunsen burner);
1.86 might be a good value to use for the specific heat of water vapor/steam in general problems, if no value is given.
For the heat of fusion for water, 334 J/g is a good value (330 is slightly less precise).
The heat of vaporization should be 2260 J/g.
I hope this clears up some of the confusion for you.
Dan Reinhold
NJCTL Chemistry Instructor
Yoko Kato (NJ CHEM-E 22-23) • 6 months, 2 weeks ago
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3 Comments