Five white solids

1For years our first-year chemistry lab taught two set pieces in separate weeks: an acid-base titration, and a hot metal dropped into water to watch the heat move.From ICCH224 syllabi, 2018-19 · Paper, pp. 2356-2357 · Equipment sketches, p. 1

2Three things set how well heat can be measured: how big the temperature change is, how precise the thermometer is, and how much heat the calorimeter lets through.From Paper, p. 2357 (Background) · Equipment sketches, p. 3

3In the trials, my first change was the thermometer: a digital probe in place of the glass one made the readings more precise.From Equipment sketches, p. 1

4In the next trial I turned the experiment around: the metal was chilled in an ice bath, then dropped into room-temperature water.From Week 5 methods, pp. 1-2 · Equipment sketches, p. 2

5I also tried other liquids in the calorimeter, ethanol and silicone oil, and water worked best.From Week 5 methods, pp. 1-2 · Equipment sketches, p. 1

6Even so, the metal values stayed too far from the literature: in our hot-metal trials the mean absolute error ran from about 9% for aluminium to 46% for copper, too far to check the Dulong-Petit law.From Heat capacity workbook, aggregate · Paper, p. 2357

7So I tried neutralisation instead: an acid and a base react fast with a large temperature change, which is likely to produce better results, and a titration can check the answer.From Paper, p. 2357 · Calorimetry trial 3 sketch, p. 1

8The design we published uses five white solids, each with its own heat of dissolution, and a titration for the two that heat alone cannot tell apart.From Paper, abstract and p. 2358 · Paper, graphical abstract and Figure 1

9The plain beaker is a choice: heat moves in and out through the glass, and in return students see the solid dissolve and feel the beaker warm or cool.From Paper, pp. 2358-2359

10A plain 50 mL beaker took the place of an insulated calorimeter: about 4 g of solid into 40.0 mL of water, reading the starting temperature and the lowest or highest one.From SI instruction sheet, p. 2 · Paper, p. 2358, Figure 1

11Each solid becomes a point on one axis: its literature heat of dissolution per gram, warming the water to the left and cooling it to the right.From Paper, Figure 2 and Table 1

12Heat alone names three: potassium hydroxide warms the water most, urea cools it most, and table salt barely moves it.From Paper, p. 2359 and Table 1

13The two acids are too close for heat to separate: citric acid 0.100 against ascorbic acid 0.118 to 0.124 kJ per gram.From Paper, Table 1 and p. 2358

14So students titrate each acid with the potassium hydroxide solution to a phenolphthalein end point: at the same molar concentration, citric acid needs about 3-fold the titrant of ascorbic acid.From Paper, p. 2358 · Paper, Figure 2 and p. 2359

15On the map that becomes a second axis, acid or base equivalents per gram: citric acid rises well above ascorbic acid, and the only base drops below zero.From Paper, Figure 2 and p. 2358

16With help from the instructor, ranking the five on this map is enough to match them, with no calculation at all.From Paper, p. 2359

17In class, 18 results per compound gave means of ascorbic acid 0.109, citric acid 0.113, potassium hydroxide -0.505, table salt 0.048 and urea 0.211 kJ per gram.From Paper, Table 1 · SI class results, aggregate

18The titration also weighed the hidden water: the class put citric acid at 217 g per mol, above 192 for the anhydrous form, and most students answered monohydrate.From Paper, Table 1 and p. 2358

19Every student matched the five compounds.From Paper, p. 2358 (Results)

20The two set pieces I started with, calorimetry and titration, became one problem: matching five unknowns. We published it in the Journal of Chemical Education in 2020.From Paper, pp. 2356-2359