To the Editor:
We wish to comment on Squella's suggested use of the terms "standard electrode potential" and "standard relative electrode potential" instead of "standard reduction potential," for E values. In our paper (I), we have used the name "standard reduction potential" for several reasons. First, it is the most common name used in current textbooks, especially those used a t the freshman level and, therefore, is the most familiar to students and teachers of electrochemistry. A second, and more important, reason is that the half-reaction associated with a standard electrode potential must be written in the reduction mode if we are to use the relationship between free energy and cell or electrode potential,
where the potential difference between the electrodes is given by1 = E'&hOde -E'anode
(2)
All together, the table of E values, with their corresponding half-reactions (written as reductions), and eq 1 can then be used at a glance to determine the spontaneous direction of an electrochemical reaction. Areaction is spontaneous when KG < 0; this implies that an electrochemical > 0. process is spontaneous when We would like to see teachers encourage students to take the difference between cathode and anode half-reactions, written as reductions and to subtract the corresponding and Eanode. respeccathode and anode potentials, E tively. Therefore, we have retained the label "standard reduction potentials" to remind students that the half-reactions are written in the reduction mode. In spite of our preference for "standard reduction potential". we aeree with Souellu that the this lab4 may pernetuate' soml of the col;fusion students have with electrochemistry, particularly when students are taught to associate the sign of E with the word "reduction," rather than the direction of the reaction only. This is apparently due to the misleading approach of defining ~~~~~~
~
~
~
E",d",,"
= E'&hOd,
and '= E'reduetii" + E'oridatio"
where E , i d . t i . . = EnnodeWe note that the name "standard electrode potential" is frequently used synonymouslywith "standard reduction potential". In our opinion, this is an acceptable substitute. No doubt it would lead to clarification if a new unambiguous title for these potentials were introduced. Squella has attempted to do this with the addition of the word "relative" to the label "standard electrode potential", to obtain "standard relative electrode potential". We do not perceive anv advantage - to this. First, it is important that i n y new name be simple, but descriptive. ~ e w n dwe , believe that the concept that E values are given (and measured) with respect to a reference electrode, the SHE, is well understood. If it is not, we strongly urge teachers and students of electrochemistry to keep in mind a schematic of the experimental set-up that produces the measured voltage (I)when studying tables of standard reduction potentials. Furthermore, the addition of the word "relative" could be misleading in that E values are not given relative to all other electrodes but only to the SHE. Therefore, we 'We have cited equations for standard wnditions only. For nonstandard states, remove the superscript zero, and compute the halfcell potentials using the Nernst equation, taking care to wrlte both half-reactionsas reductions.
692
Journal of Chemical Education
do not see how adding this word conveys useful information to a student. In summary, we believe that the term "standard reduction potential" is acceptable in place of "standard reduction potekial", but do not consid& this an essential change. However, the insertion of the word "relative" is uninformative and unnecessary. Literature Cited 1. Birss, V . I.: k a r , D. R. J Chorn. Educ. 1990,67,40349.
V. I. Birss D. R. Truax
The University of Calgary 2500 University Drive N.W. Calgary, Alberta, CanadapN 1N 4
Octet Counting: A Connection between "VSEPR" and "Tangent Sphere" Models To the Editor: Al-Mousawi (1)has a simole octet countine rule T = 8n + b for AB, compounds, wher; T is the total vaency electron count, for deriving the number of lone pairs (bI2) on the central atom and so obtaining molecular shapes via the VSEPR model. This is welcome to students because formulation of acceptable Lewis structures can be avoided. However. the rule can be further simplified, dispensing with octet counting, by assigning the octets to the peripheral atoms and converting them into ions. The lone pair count becomes 0.5(G - qn), where G is the central atom p u p number and q the charge on then surrounding anions, corresponding with the expression 0.5(T - 8n) from the above rule. In practice one has only to consider whether or not the central atom needs to transfer all its valency electrons to ionize the outer atoms. Thus PC15 formulated as P5+5Clhas no lone pairs, P+3 C t has one. There is no need to confer an octet artificially on hydrogen as proposed by AlMousawi if H20 is written as OZ+2H- or as OZ 2Hi with two and four lone pairs, respectively, since both give the same bent shape. The ambivalent nature of hydrogen (Group VII or I) is seen clearly in tetrahedral (B3+4 H-)and linear (FHtF-1- ions, both without lone pairs. The VSEPR model is being reduced to Bent's tangent sphere model in which lone pairs are referred to as electride ions (2).
An extreme ionic formulation is perhaps less acceptable for organic molecules especially when th;! same ele&ent is present in different oxidation states. For these the octet counting rule in conjunction with isoelectronic principles (3) seems more suitable. Thus cyanate, fulminate, and azide ions, as well as nitrous oxide and carbon dioxide with 16e, must all be linear, independent of the bonding or charge distribution in the different species. Isoelectronic replacement viz., CH3 or NH2 by F; CH2 or NH by 0 ; and CH by N is also useful for deducing local geometr? In the adduct CH3CN.HC1, which could be either a salt (CH,CNH)Cl or an imidovlchloride. the respective linear .~ ,~ or trigonal shape at the cfntral carbon is obtiinrd from the equivalent 16r (FCWIor 24e ~FCOCIIcount. Literature Cited 1. Al-Moueawi,S. M. J. Cham.Edue 1990.67,861 2. Bent, H. A. J. Chem. Educ 1968.45.766. 3. Bent, H. A J Chem Edue. 1966,43,170.
A. A. Woolf Bristol Polytechnic Coldharbour Lane, Frenchay Bristol BSI6 1QY United Kingdom
