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 K_c, K_p, and the thermodynamic equilibrium constant. Whereas thermodynamic K must be unitless, some textbooks (namely McQuarrie et al.) assert that K_c and K_p 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 K_c and K_p 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 ΔH_rxn (and similarly ΔS_rxn and ΔG_rxn)

Version A: kJ/mol	Version 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 ΔH_rxn 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 ΔH_rxn 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 R_H = 2.18 × 10^–18 J, then the Rydberg formula is E = –R_H(1/n²) and 1/λ = –(R_H)/(hc) × (1/n²)

If R_H = 1.097 × 10⁷ m^–1, then the formula is 1/λ = –R_H(1/n²) and E = –R_Hhc(1/n²)

If R_H = 3.29 × 10¹⁵ Hz, then the formula is ν = –R_H(1/n²).

RH in J	RH in m–1	RH in Hz	RH 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 R_H = 1.097 × 10⁷ and plug it into the equation E = –R_H(1/n²). So in Sapling Learning questions, we've been careful to put the value directly into all equations rather than using the symbol R_H. 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 R_H. 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.