Bruce H. Mahan University of California
Berkeley
A first experiment in thermochemistry
A Simple Ice Calorimeter
I t is becoming increasingly desirable to teach and use the first law of thermodynamics in freshman chemistry. Teachers of physical chemistry will generally agree that even the more elementary concepts involved in the first law of thermodynamics often are not fully understood or appreciated until the student has had some laboratory experience which illustrates these concepts. In teaching freshman chemistry it is imperative that the exposition of the idea of an enthalpy change, for instance, be accompanied by an experiment in which AH actually is measured. Difficulty arises, however, when a great number of unskilled individuals try to use a necessarily limited supply of delicate calorimeters. This note describes a relatively crude and simple ice calorimeter which can be supplied to each student. The heat evolved when a reaction takes place is measured by the volume change it produces in an ice-water mixture which surrounds the reaction vessel. This volume change can he conveniently measured if the ice-water mixture is enclosed in such a manner that the water meniscus level in a 1-ml pipet changes as ice is melted by the reaction. Once the volume change corresponding to a known amount of reaction has been determined, an elementary calculation involving the densities of ice and water and the heat of fusion of ice leads to a value of AH for the reaction. The operation of the calorimeter is rapid, its construction is rugged, and the precision of the lytic beaker, a 15 X 120-mm test tube, a 1-ml pipet graduated to 0.01 ml, a No. 12 nibber stopper, and short pieces of 5-mm glass tubing, glass rod, and rubber tubing. Figure 1 shows the general arrangement. Care should be taken to have the pipet, test tube, and glass tubing make air tight fits to the rubber stopper. It is important t o have the test tube well cenfl tered in the electrolytic 11 c ; ~ n s sROD beaker, with the bottom -RUBBER of the test tube well removed from the base and 111 r U walls of the beaker. The lip of the test tube should be approximately 2 cm above the level of the rubber stopper. The diameter of the glass rod and rubber tubing should be chosen such that the rod is held firmly by the ~i,.,, I. r h e assembled calorimtubing, but a t the same eter. The g l a u tubing should not extend below the rubber stopper ur time may be slid up and tho1 air bubbles may b e eerily down to act as a plunger.
The calorimeter is best suited t o measure the AH of fast exothermic reactions like the solution of magnesium metal in hydrogen ion. To determine the enthalpy of formation of Mg++, fill the electrolytic beaker to the very top with a mixture of ice and water and put the stopper assembly firmly into place with the glass rod plunger removed from the rubber tubing. It is helpful t o cool the rubber stopper to ice temperature before inserting it into the electrolytic beaker. Care should be taken that the tip of the test tube be surrounded by ice particles, and only a few air bubbles should be allowed t o remain in the beaker. Place the entire assembly in a 1000-ml or preferably 1.500-ml beaker which is also filled with an ice-water mixture. Ice should be piled over the t.op of the stopper as high as the lip of the test tube. Transfer 5 ml of 2.00 M HC1 by pipet to the test tube, and wait for 10 or 15 min. so that the entire calorimeter can cool to ice temperature. Pour water into the rubber tubing to fill the inner part of the calorimeter; then put the glass plunger in place and drive the water in the pipet as high as possible. Measure the rate of change of the pipet reading. When the heat leak has reached a value corresponding to a volume change of 0.005 ml/ min., take volume time readings for five minutes and then add a slight excess of linely divided magnesium metal. Take readings every 30 sec until the volume change no longer exceeds 0.005 ml/min.; thereafter readings may be taken every minute for 5 min. Plot the results as indicated in Figure 2, and determine AT'
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Journol o f Chemical Edumfion
0
5
10
15
T I M E (rnln) Figure 2. Volume change in milliliters as a fundion of time in minutes. Five rnl of 1 M HCI ware "red with excess magnesium.
caused by the heat evolution. The relation between the volume change ( A n and the weight of ice melted can he derived in the following manner. AV = Vi - V.
where V (and Vmare the volumes of the ice melted and water formed as a result of the reaction, W is the weight of the ice melted, and D,and D, stand for the densities of ice and water respectively at 0°C. The heat evolved in the experiment (h) is given by the expression
where I is the latent heat of fusion of ice. Figure 2 shows data obtained by the author in a run. in which only 2.5 X 10W3 moles of magnesium were reacted. Since the volume change was -0.305 ml, the heat evolved by the reaction was
Since 2.5 X moles of magnesium were dissolved, t,he A H of formation of a mole of magnesium ion is
which compares favorably with the accepted value of -110,000 calories. A class of 60 students who also used only 5 ml of 1.00 M acid and thus only 2.5 X 10W3 moles of magnesium had answers which averaged 100,000 + 5000 calories. The use of 2.00 M HCl improves the results to the extent that it is possible to obtain answers which are consistently within 5y0 of the accepted value. The heat of solution of MgO in HC1 can also be determined by this technique. The AH for this reaction is only -34,000 calories per mole, hence it is desirable to use 5 ml of 6.00 M HCl to obtain 5% precision. From the measured AH of this reaction together with the AH of formation of Mg and the AH of formation of liquid water, students can calculate the AH of formation of MgO. For the reaction MgO 2H + H30 + Mgt+ AH = AH,(Mg++) AHI(HzO)- AHAMgO)
+
+
+
Since the quantity on the left-hand side of the equation and AH,(Mg++) have been measured, AH,(MgO) can he determined if AH,(HIO) is supplied. Thus in one experiment students become acquainted with the meauing of AH and also with the procedure of measuring enthalpies of formation by direct and indirect methods.
Volume 37, Number 12, December 1960
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