J. A. CAMPBELL Colls~a Cloremont, California 9171 1
Honay Mudd
Questions Q266. A UP1 press release in June of 1974 reported t h a t teeth could be treated, with phosphoric acid to eat the natural enamel and the outer surface of the tooth, and then painted with a zinc chloride solution. The article based on work by Dr. Neil Randol, claimed that the method works on the principal t h a t microscopic metal particles can be applied to teeth. Dr. Randol is quoted, "Seen under a n electron microscope, this wall of zinc chloride glitters like thousands of tiny diamonds." Comment on the possibility t h a t the coating material is zinc chloride, or t h a t it is made u p of metal particles as reported in the article. Q267. A moth weighing 2 g is found t o have a respiratory rate of 50 cm3 of oxygenlg of body weightthr. If sugar is the source of energy, what is the minimum amount of sugar the moth must gather from flowers in a n hour? Q268. A chemist uses the term endothermic to describe a system which absorbs heat (AH= + ) , b u t a n entomologist uses it t o describe a n insect which produces heat a t a rate sufficient to increase body temperature. For example, many insects must often go through a period of wing motion before
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thev are "warm enoueh t o take off!' On the other hand.. flieht .. motions in inserts have a mechanical efficiency of less than 20°0: so there is a serious heatine Drohlem during - flight. 1)escribe the problem and how t h i i n s e c t may be adapted t o handle it. 6269. Rate of wing vibration in lepidotera (e.g., butterflies) is proportional to ambient temperature, b u t the rate is determined in the nervous system, not in the wing muscles. Which nerve conduction or wine- muscle reaction probably has the higher activation energy? Q270. All US space flights t o date have kept cabin CO2 concentrations below 3% by using LiOH a s the absorbent, forming Li2C03. This system is essentially irreversible, and therefoie requires carrying 0 2 , usually in a pressurized system whose weight at least equals that of the 0 2 . Another possible system would abmrb the C07 on Ag20, then recycle the C and 0 2 . using a system weighing bout 150 kg. All data are based on a sinele Derson exhaline about 2.0 ke of CO, Der dav. Calculate tLe &eight requirements of the t w o options fo; a one week and a one month mission. Comments?
Answers AZfi6. Dr. Hnndol was either misquoted or misinformed by his chemistry instructcm, or muld there he another possibility? Zinc chlorrde is quite soluble in water tor sali\,a~and would require n very strong reducing agent to convert it to metal, not to mention keep it in that state. Of course, there need he no correlation between the efficaev as a tooth treatment and the hvoothesized chcmrstry. It is tkely that phusphoric acid wouldattack the ennmel rlargrly a calcium phouphaw) and possible that zinc inns might fmm a qurte stable, in%oluble"phosphate" on the resulting surface. I t isdouhtful that theaurfacp would becompletely analuyuus tn"thousendsof tiny diammds." A267. 1 mole of gas at usunl cmditions occupies about 2.5 X 10'rm'. So5Orm~~aSO/t2.5 X 1u.1 = 2 X lO-'moleofO~.With carbohydrates the optimum reaction is
CHp0 + O2 = Cop + H20 mole of CHzO per Thus 2 X 10W (mole of O z h ) require 2 X hr. 2 x 10-Vmolehr) X 30 (g CHzOImole) X 103(mglg) = 60 (mgihr) of carbohydrate must be found, or 1 (mglmin) for 1 g moth, which is 2 (mglmin) for the 2 g moth. This keeps the moth very busy. [BerndHeinrich observed rates of 1flower per second, with manv reoetitions. Science. 185.747 (1974).1This also shows why somi inskcts that fly s great deal have no mduths, never feed. and lead a s h d life, a5 well as why few inserts fly continuousl? for any appreciable period. A268. At least 8Wb of the expended energy goes into heating the wing muscles, but if their temperature rises appreciably the enzvmes will beain to decom~ose.Direct air cooling of the thorax coGd work with'small insects, but in large insects & probably not efficient enough to give good coaling because of law flight speeds ,andthe low heat capacity of air. These problems could be mini-
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716 / Journal of Chem;calEducatIon
mized if the wings were used as heat transfer surfaces, but evaporative cooling would seem to be a much simpler technique. Unfortunately, most insects cannot afford to lose Hz0 (little is stored), so the usual process is to spread the heat (vialiquid such as blood) through the body followed by transfer to air. A269. According to the Arrhenius equation: rate = k exp (-AEIRT), where AE is the activation energy. A larger activation energy leads to a more rapid change in rate as Tvaries than does a lower AE.Since the nerves are controlling the varying wing beat rate they must have the larger AE value. A270. One month (30 da) use of the LiOH system requires absorption of 103 (glkg) X 2 (kg COdda) X 30 (dal144 (g COplmole Con) = 1.5 X 103moleLiOH and 1.5 X 103mole02. 1.5 X 103(mole LiOH) X 20 (g LiOHImole LiOH) = 3.0 X lo4 g LiOH. 1.5 x 103 (mole 0 2 ) X 32 (g Oplmole 0 2 ) = 4.8 X 104g Op which will require 4.8 X lo4 g storage. Thus, the total weight for 30 da of the LiOH system is at least (3 4.8 + 4.8) X lo4 g = 13 X lo4 g, or 130 kg. There will certainly be some other ancillary equipment, so let's assume a total of 150 kg. This weight is approximately linear in days so would be about 35-40 kg for a 7 day trip. The longer the trip the greater the load of Oz LiOH to be carried. [See this Journal 48,260 (1971).] The AgxO system recycles, so its weight is essentially constant regardless of the flight time. Since a weight of 150 kg is assumed it is clear that the LiOH system is advantageous up to about one month of flight time and AgzO thereafter, assuming that both systemsare equally reliable. Note that recvcline svstems (in essentiallv all eases) become more and more.attr&t& as the time incr;ases ove; which a problem is anticipated. Single pssssystems are more economic on short time scales.
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