270 DIY Science: Illustrated Guide to Home Chemistry Experiments
EvRE ydAy THERmoCHEmISTRy ANd CALoRImETRy- The chemical cold packs sold in drugstores illustrate
heat of solution (as defined on the next page). These
packs have two compartments. One contains water
and the second a solid chemical (usually ammonium
nitrate or sodium thiosulfate) with a positive
(endothermic) heat of solution. When you break the
divider between the compartments, the solid chemical
dissolves, absorbing heat from the environment. - The reusable chemical heat packs sold in drugstores
illustrate a different aspect of heat of solution. These
packs contain a very concentrated solution of sodium
acetate. Placing the pack in boiling water or heating it in
a microwave oven for a few minutes dissolves the solid
sodium acetate. When the pack has cooled to room
temperature, it contains a supersaturated, supercooled
solution of sodium acetate. When the pack is initiated,
normally by bending a the small embedded metal disc
that contains tiny slots with microscopic crystals of
sodium acetic, the sodium acetate immediately begins
recrystallizing, producing heat as it does so. - If you’ve ever made ice cream in an old-fashioned ice
cream maker, you’ve used a practical example of the
heat of fusion of ice (as defined in Lab 15.2). Adding
salt to water ice reduces the freezing point, converting
some of the ice to liquid water. The heat of fusion
required to melt the ice must come from somewhere,
and the only available source of heat is the ice itself.
As the salt dissolves, the temperature of the ice/salt
bath decreases enough to freeze the milk and cream,
producing ice cream.
All you need for a homemade calorimeter are a couple of
8- or 16-ounce (250 or 500 mL) foam cups, a lid, and a beaker
or similar heavy container to provide stability. Place one
cup inside the other to provide better insulation. Insert your
thermometer or temperature probe through the center hole
in the lid, and use it both for measuring temperatures and as
a stirring rod. You can significantly improve the accuracy of
your homemade calorimeter by replacing the snap-on plastic
lid with a lid constructed of foam. Using the top (smallest
diameter part) of the plastic lid as a template, mark and cut a
circular piece of foam just large enough to fit inside the mouth
of the cup. Using the top of the cup as a template, mark and
cut a second circle of foam just a bit too large to fit inside the
mouth of the cup. Center the smaller circle of foam on the
larger one, and glue the two pieces together. Poke a small
hole through the center of the new lid assembly to allow the
insertion of a stirring rod or thermometer.
Measuring temperature changes accurately and precisely is
the foundation of thermochemistry. We used an inexpensive
digital temperature probe (thermometer) to measure
temperatures to 0.1°C. You can substitute a standard glass-
tube thermometer with some loss of accuracy and precision.
If you do use a glass-tube thermometer, try to interpolate
your temperature measurements by estimating values
between the markings. With a standard 300 mm glass-tube
lab thermometer, it’s usually possible to interpolate values
accurate to 0.2°C. With longer thermometers, or those with
narrower ranges, it may be possible to interpolate to 0.1°C.
In this chapter, we’ll use calorimetry to examine several
aspects of thermochemistry.