LETTERS Two Easy Chemlcal Rate Experiments To the Editor:
and
One can use the spectrophotometer to investigate the mten of colored chemical reactions in solution. The reactions ~~-~~~~~~ ~
are carried out right in the cuvette, and absorbance measurements can he made a t neriodic intervals. If one knows the absorbance(A)-concentration relationship, concentrations of a colored substance can be used to calculate the specific rate constant, but for the first-order reactions in AIAocan be used without knowing the concentrations. By storing the cuvette or test tube in a water bath, one can run reactions at different temperatures. The test tube must he dried off and quickly placed in the spectrophotometer and quickly measured and returned to the bath when a measurement is made. Two systems used with this technique are FeSCN2++ S20:-
(1)
around 10-20 OC, 447 nm, [S2082-] = 0.05 M after mixing, and I d 4 + %%-
(2)
around 30-43 OC, 525 nm, [S2081-] = 0.25 M after mixing. The concentrations of FeSCN2+and I2are low, about and 10-2 M, respectively, but do not have to be prepared exactly because they are obtainable from the absorbance if need he. The FeSCN2+is made by mixing equal volumes of 0.2 M Fe3+in 0.5 M HNOa and of 1X 1 0 - W HSCN in 0.5 M HNOa. Timing the experiment can start any time after thermal equilibrium has heen reached. If the absorbance is off scale, i t soon registers. Measurements are made about every 5 rnin for 30 min, but for reaction 1a t 20 OC, every minute. Under these conditions these reactions yield pseudo-firstorder results. Reaction 1does not give a linear graph of In A/ A. vs. time hut nearly does so, unless followed for an extensive period. Specific rate constants, second order, are (1) 6.9; (2) 0.20L mol-1 min-1 a t the higher temperatures listed.
2Cu + 6HN03
-
2Cu(N03)2 + 2HN02 + 2H20
(3)
The simple sum of these two equations gives eq 1, with a = 5,b=14,m=5,x=2,y=2,andz=6asisshownbyHill. Equation 1could just as well have been balanced by multiplying the coefficients of eq 2 and eq 3 by different integers and then adding. If the coefficients of eq 3 are multiplied by three and then added to eq 2, a balanced version of eq 1 is reached using the coefficients a = 9, b = 26, m = 9, x = 6, y = 2, and z = 10 as is shown hy El-Cheikh et al. Likewise, adding the disproportionation reaction equation of nitrous acid 3HN02
-
HNOa + 2NO + Hz0
(4)
to the coefficients shown by El-Cheikh gives yet another set of coefficients for eq 1:a = 9, b = 23, m = 9,x = 3, y = 4, andz = 11; many other possibilities also exist. As El-Cheikh et al. state, many sets of coefficients will serve to balance eq 1. The reason is that more than one inde~endentreaction is disguised within a single equation. ~ o n of k us would try to write a single balancedkquaiion for the neutralization of a mixture of acetic and formic acids by sodium hydroxide and then attach significance to the ratio of coefficients on acetic and formic acids. The ratio of the coefficients of the two acids can he varied independently. The copper plus nitric acid reaction is a less apparent version of the same difficulty. I t is certainly valid to obtain balanced equations by summing two or more sequential reactions, especially if one or more products of the first reaction is completely consumed in a second reaction. Adding two chemical equations that describe parallel reactions of a set of reactants will not, however, lead to a unique (or useful) overall equation. James D. Carr University of Nebraska Lincoln. NB 68586
General Reference Corbett. John
F.J. ChemEduc. 1972,19,663.
Emerson E. Garver University of Wiswnsln-River Falls River Falls. WI 54022
Stolchlomelry for Copper Dissolution In Nitric Acid: A Comment To the Editor:
The exchange of letters between W. D. Hill and F. M. ElCheikh et al. (1987, 64, 1069-1070) does not bring out the fundamental reason that several sets of coefficients will balance the reaction equation: aCu + bHN03
-
The True Meanlng ot Isothermal To the Editor:
Referring to a recent comment made by Fain [1988, 65, 1871 on Granville's paper [1985, 62, 8471 concerning student's misconceptions in thermodynamics, I feel that Fain brought some confusion to the issue, which needs to he clarified. Fain states that "the system temperature is not necessarily a constant" inisothermal changes, "and during the transformation the system temperature may vary and may even be inhomogeneous". Also under the assumption that the system temperature is (temporarily) different from T, the entropy change of the surrounding has been expressed as
rnCu(N0a)z + xHNOz + yNO + zHrO (1)
This equation is the sum of two equations for reactions occuring in parallel as follows: Volume 67
Number 2
February 1990
183
