Unlts ot an Equillbrlum Constant To the Editor:
Helfferich [1987,64,1069] has effectively replied toQuintero's uncom~romisingreiection of an equilibrium constant having units;but i t i s n o t only chemicalengineers who give them units; so do chemists. Thus, the IUPAC manual on Quantities, Units and Symbols in Physical Chemistry (The Green Book) allows the use of Kc a n d & havingunits, and a dimensionless or thermodynamic K, which is the ualue of either Kc or K,. Besides the ~ o i nmade t bv Helfferich, an equilibrium constant having appropriate G i t s can, under some conditions (elementarv Drocesses, or anv reaction a t equilibrium), he equated t o i h e ratio kJk-l of rate constants in forward and reverse directions. This useful relationship is lost if K is renuired to he ~- dimensionless unless. as surelv no one would advocate, we take rate constants to be dimensionless also. The best . oolicv.is to define eauilibrium constants in terms of ratios of concentrations or pressures, in which case they have units. I alwavs require my students t o state the units; thin is more convknieni than stating standard states, and also it avoids errnra that easily arise when the dimensionless quantities are used. If, for a logarithmic relationship, a dimensionless quantity is required, this is readily obtained, as with any other physical quantity, simply by dividing by the unit quantity. One can conveniently indicate that this has been done by adding a superscript to show that the quantity has been made unitless, in other words, that one is taking the ualue of the quantity. ~~
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Journal of Chemical Education
If Quintero's argument were valid, that a quantity must be dimensionless because we sometimes have to take its logarithm, we would have to require many other physical quantities to be dimensionless; examples are pressure, electric current, and rate constant. Units are valuable scientific assets: let us not relinquish them until we have to. Keith J. Laidier university of Ottawa Ottawa. ON KIN 6N5 Canada
"Redox Demonstrations and Descriptive Chemistry, Part 1. Metals" To the Editor:
The equation for the formation of the bisthiosulfateferrate(lll) ion stated immediawly under Discussion Procedure 1 of the .~-~~tested -~~demonstrations in the article "Redox Demonstrations and Descriptive Chemistry Part 1. Metals" 11987, 64(3), 716-7171 should be written as Fe3+ 2 S ~ 0 3 ~ [Fe(S203)2]- instead of Fe3+ 2S20z2[Fe(S203)2]-. AU other equations are correctly balanced. ~~
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William D. Hill, Jr. N. C. Central University Durham. NC 27707
