Re: “Know the limit and teach within it series”
December 2017/January 2018, March and April 2018
On reading about Rutherford’s atomic model in David C. Stone’s excellent series in the March issue, I recalled having read somewhere long ago that Rutherford did not presume to assign positive or negative charge to the nucleus.
Thanks to Dr. Google, I was able to find Rutherford’s May 1911 article in Philosophical Magazine at www.chemteam. info/Chem-History/Rutherford-1911/ Rutherford-1911.html. Early on he says, “Consider an atom which contains a charge +/-Ne at its centre surrounded by a sphere of electrification containing a charge -/+Ne…. It will be shown that the main deductions from the theory are independent of whether the central charge is supposed to be positive or negative. For convenience, the sign will be assumed to be positive.”
Then, near the end of the article Rutherford says: “…it has not so far been found possible to obtain definite evidence to determine whether [the nucleus] be positive or negative.” And he goes on to suggest some experiments for determining the sign of the nuclear charge. I don’t know whether he or someone else finally carried out those experiments.
As a challenge to your students, ask them to draw the track of a positive alpha particle, in Rutherford’s gold foil experiment, approaching a nucleus at a near miss, first for a positive nucleus, then for a negative nucleus.
Lew Brubacher, Professor Emeritus
Chemistry Department
University of Waterloo
Waterloo, Ontario
Editor’s comment
For Lew’s challenge, students might need a diagram to get them started. Ask them to predict the path of the alpha particle.
Answer
Re: David Cash's article "Calorimeter calculations with aqueous electrolytes", February, 2018
This article was excellent and extremely interesting.
I admit my own guilt, and I too calculated and would have accepted from my students the incorrect answer — which used the mass of water, not the solution. In all my years of teaching calorimetry, I ignored the mass of solutes both in problems and labs involving enthalpy of solutions and in reactions. I cannot recall (it has been 10 years since I retired) if our textbooks were consistent; however, I assume they were as I do not recall having any textbook problem questioned for that reason.
My explanation to students who asked was that a Styrofoam cup calorimeter is fraught with assumptions (100% efficiency). Since the specific heat capacities of chemical substances other than water are generally small, I felt it was a better assumption to assign them a zero value compared to the value of 4.184 J mol–1 K–1. For a high school introduction to thermodynamics via calorimetry, making these assumptions is efficient.
The presented graph of specific heat capacity as a function of NaOH concentration is an excellent reference and likely could be used by teachers to refine how they approach problems of this type. The NaOH concentration in the Canadian Chemistry Contest question was just under 1.0 M which would be the target most likely used in an enthalpy of neutralization lab.
It is interesting to note that if one uses Cash's suggested value of 4.0 for specific heat capacity of the solution, one gets an answer very close to one of the incorrect answers (A), which had not used the mass of the solute.
Lyle Sadavoy
Toronto, Ontario
