Enno Wolthuis, Arie Leegwater, and John Vander Ploegl
Heat of Reaction and H$O, concentration
Calvin College Grond Ropids, Michigan
A general chemistry experiment
T h e heat of a reaction is often observed but seldom measured in general chemistry work. Perhaps the most familiar example is that of the heat generated when sulfuric acid is added to water. The amount of heat evolved in this reaction must be related to the concentration of the acid used, and therefore the concentration of an acid srtmple can be determined quickly by reference to a concentration-heat of reaction curve derived from work with acids of known concentration. Examination of the literature revealed very few previous a t t e m ~ tto s do this. Howarde suacrested a method for determining the concentration of a t least 92% sulfuric acid and of oleum by measuring the temperature rise when definite quantities of the two were mixed in a Dewar flask. I n this case the heat evolved was primarily that from the reaction of sulfur trioxide with water. Curtis and Miles8 used essentially the same method for plant control testing, but preferred to use larger amounts of acid and oleum to measure these by volume. Richmond and Merreywether4 used a thermometer reading to 0.0l0C to measure the temperature rise on mixing 92-98% acids with water, and devised a formula with correction factors to calculate the concentrations of the acids. We proposed to devise a simple, but fairly accurate, procedure for obtaining a measure of the heat of reaction between water and sulfuric acid of various concentrations, and to apply it to determine the concentration of unknown acid samples within the range of 50-98%. Preliminary experiments showed that 5 ml of acid added to 25 ml of water gave temperature increases varying from about 11 degrees for 70% to 44 degrees for 98% acid. These effects seemed suitable for our measurements, and thereafter the volumes of acid and water were not changed. The early quantitative work was done under the most favorable conditions. A pint-size Thermos bottle was used as a calorimeter. A number of acid samples were prepared, ranging from 40-98% in strength, and were standardized by titration. 5 ml of an acid was added to 25 ml of water in the bottle, and the temperature rise was measured with a 0.2°-thermometer.
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1 This paper reports the work done by Leegwater and Vander Ploeg as part of their freshman work a t Calvin College Far 34, other reports of this continuing program see THIS JOURNAL, 133 (1957),35, 412 (1958),36, 494 (1959),37, 137 (1960). ' HOWARD, H., J. Soe. Cha.Ind., 29.3 (1910). a Cuuns.. R... AND MILES,F., I.SOC. Chem. Ind... 39.. 64 (1920). 'RICHMOND, H., AND MERREYWETHEE, J., Analyst, 42, 273 (1917).
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Journal of Chemical Educotion
The temperature increases for the various samples were plotted against their concentrations. Then several acid samples of unknown strength were similarly used, and their concentrations determined by reference to this curve. The following results were typical: Temp. me,
'=c 7.7
14.9 30.4
% from
% by
curve
titration
63.5 75.4 89.8
63.8 75.9 90.5
Encouraged by these results, experiments were continued to simplify the equipment. If was found that equally good results were achieved by substituting for the Thermos bottle a small beaker wrapped in paper towelling, or surrounded with Vermiculite contained in a larger beaker. Further experiments were performed to determine the effect of the initial temperature of the water. Using 75.8y0 acid, and water a t 13", 21°, and 27'C, the temperature increases were 15.5", 15.Z0, and 14.S0, and the acid 76.0, 75.7, and 75.3, respectively. These experiments indicated that good results can be expected by starting with water a t room temperature. Examination of the reference curve shows that a nearly linear relationship obtains in the region of 80-98% acid concentration. Within these limits, one degree rise in temperature is equivalent to 0.6% acid, and an ordinary thermometer graduated in degrees can be used without greatly reducing the accuracy of the determination. Below 80% acid the slope of the curve is less and is variable, the accuracy expected dropping to about 4y0 acid per degree rise between 50 and 60% acid concentration. These estimates were confirmed by class trials of the method given below, the students using ordinary l o thermometers. Using two unknown acids of 63.6 and 82.6y0 strength, the class averages were 64.2 and 82.9%, with maximum deviations of 3.5 and 0.9%, respectively. The Experimenl
Insulate a 100 ml beaker by wrapping i t with paper towelling. (Alternately, one may set the beaker in a larger one and surround it with Vermiculite or asbestos powder, or one may use a small Thermos bottle.) Suspend a thermometer in the beaker such that its bulb nearly touches the bottom of the beaker. Add exactly 25 ml of water from a buret or pipet, let it stand a minute or two, and record its temperature as accurately as possible. Measure exactly 5 ml of an acid of known strength in a pipet, hold the tip of the pipet below the water surface and let the acid flow into the
water while stirring with the pipet. Continue stirring until the temperature reaches a maximum and record this reading. Rinse and dry the beaker and pipet, and repeat this procedure with other samples of acid whose strengths are known to lie within the range of 50-98%. Draw a curve plotting per cent acid as abscissa against the
rise in temperature as ordinate. Then repeat the procedure with the acid samples of unknown concentration and determine their strengths by reference to the curve. From the slope of the curve a t the c~ncentrat~ions of the unknowns, and assuming one can read the thermometer to O.ZO, estimate the maximum accuracy one may expect in these determinations.
Volume 38, Number 9, September 196 1
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