Monday, February 7, 2011

Unconventional Wisdom: General Chemistry

There are some differences in style that we as chemistry instructors accept, or even embrace. For example, many of us point out to students that the groups of the periodic table can be numbered in multiple ways. Sure, we may have a personal preference, but we acknowledge that students are better off knowing all systems in the long run. During my time developing Sapling Learning's general chemistry library and accommodating more than 20 different textbooks, I've learned a few conventions that most of us assume are universal.

1. STP
In 1982 [PDF], IUPAC recommended that standard pressure be 1 bar (not 1 atm) and redefined STP as such in 1990 [PDF], which means standard molar volume is 22.7 L, not 22.4 L. A few books have adopted this convention (namely McQuarrie et al.), but most do not [PDF]. Since the difference would cause students to get different answers, we've maintained two different sets of questions on this topic in our library.

2. Units of K
IUPAC distinguishes between Kc, Kp, and the thermodynamic equilibrium constant. Whereas thermodynamic K must be unitless, some textbooks (namely McQuarrie et al.) assert that Kc and Kp need not be. Rather, they can have units such as M, M2, atm-1, or even unitless, depending on how the terms cancel. Since this would not affect answers, I figured it would be fine to expose students to both conventions (if they even notice). You may notice that equilibrium questions with the McQuarrie banner allow Kc and Kp to have units, whereas those with the Sapling banner always show K as unitless. Of course, if we hear from you that you'd prefer not to mix the conventions, we'd consider making two separate sets of questions like we did for STP.
Update: We've gone ahead and made alternate versions of the McQuarrie questions so that users of traditional textbooks can have a perfectly unitless K experience. If you're interested in learning more about McQuarrie's approach, please take a look at this preface [PDF] for the 4th edition of his textbook.

3. Units of ΔHrxn (and similarly ΔSrxn and ΔGrxn)
Version A: kJ/molVersion B: kJ
Averill
Burdge
Burrows
Chang
Laird
McQuarrie
Petrucci
Spencer
Whitten
Brown
Brady
Ebbing
Gilbert
Kotz
McMurry
Moore
Olmsted
Oxtoby
Silberberg
Tro
Zumdahl
In some books (we call them "Version A" books) the coefficients from the chemical reaction are considered unitless, so ΔHrxn comes out in units of kJ/mol (where the "per mole" is assumed to mean per mole of reaction). In other books (we call them "Version B" books) the coefficients from the chemical reaction have units of moles, so ΔHrxn comes out in units of kJ, and that value is specific to the reaction as written (but also understood to be scalable per mole of reaction). Although this difference does not affect the students' answer in any way, some instructors strongly prefer one method over the other for pedagogical reasons. Thus, we went ahead and made separate sets of questions, one for each convention.

The table to the right is a non-comprehensive list of popular General Chemistry textbooks (first author only) classified by which version they use.


4. The Rydberg formula
The Rydberg constant is expressed differently from book to book.

If RH = 2.18 × 10–18 J, then the Rydberg formula is E = –RH(1/n2) and 1/λ = –(RH)/(hc) × (1/n2)
If RH = 1.097 × 107 m–1, then the formula is 1/λ = –RH(1/n2) and E = –RHhc(1/n2)
If RH = 3.29 × 1015 Hz, then the formula is ν = –RH(1/n2).

RH in JRH in m–1RH in HzRH not defined
(value shown in formulas)
Chang
Ebbing
Moore
Petrucci
Brown
Gilbert
Kotz
McMurry
McQuarrie
Silberberg
Spencer
Burrows
Laird
Tro
Zumdahl
Obviously students should know how to interconvert between energy, frequency, and wavelength, but it would be dangerous for a student to take RH = 1.097 × 107 and plug it into the equation E = –RH(1/n2). So in Sapling Learning questions, we've been careful to put the value directly into all equations rather than using the symbol RH. For example, a hint might show E = –(2.18 × 10–18 J)(1/n2). Some textbooks also use this style for presenting equations and thus, refrain from defining RH. The table above is the non-comprehensive breakdown of textbooks (first author only).

Do you have any concerns about these not-quite-convention conventions, or the way we handle them (particularly the units of K)? Let us know in the comments.

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