Chapter 15 Laboratory: Thermochemistry and Calorimetry 277dEGREES CELSIUS vERSUS kELvINS
The SI unit of temperature is the kelvin—not the degree
Kelvin, just the kelvin. For all practical purposes one kelvin,
abbreviated K, equals one degree Celsius, or 1°C. (I’m so
old that I still think of the “C” as meaning centigrade rather
than Celsius, so please excuse me if “centigrade” slips in
occasionally.) For our purposes, J/g · K and J/g · °C can be
considered equivalent.CUTIOA nS
Take care with the hot water and hotplate to avoid burns.
Wear splash goggles, gloves, and protective clothing.z
heat of the metal samples. We can calculate that value by
rearranging the preceding formula to:
cmetal = [(mcΔT)water ÷ –(mΔT)metal]
Plugging in the known and experimentally determined values on
the right side of the equation gives us the unknown value for the
specific heat of the metal on the left side. This works both ways.
If we happened to have a known value for the metal but not a
known value for the specific heat of water, we could rearrange the
equation slightly and solve for the specific heat of water.
If you look up published values for specific heat, keep in mind that
specific heat can be specified on a mass basis, as we are doing
in this laboratory, or on a mole basis. Mass-based specific heat
values are represented by a lowercase c; mole-based specific
heat values are represented by an uppercase C. These values
can differ greatly. For example, the C (mole-based) specific
heats of lead and iron are quite similar, at 26.4 and 25.1 J/mol · K,
respectively, while the mass-based specific heat of lead, at 0.127
J/g · K, is less than one third the mass-based specific heat of iron,
at 0.450 J/g · K.
You may also see specific heat values represented as cp, cv, Cp,
or Cv. The subscript indicates whether the specific heat value
is specified at constant pressure or constant volume, which is
particularly important if you are measuring the specific heat of
a gas. The difference between specific heat values at constant
pressure versus constant volume is small for solids and liquids,
and can be ignored for our purposes.
POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.- Fill the 600 mL beaker nearly full of hot tap water, add
a boiling chip, place the beaker on the hotplate, and
bring the water to a boil. Once the water boils, turn
down the temperature of the hotplate until the water
just maintains a gentle boil. - Pour lead shot into one test tube until it is about half
full. We’ll be immersing the test tube in the boiling
water, and it’s important that the level of the lead shot
in the test tube be well below the surface of the boiling
water bath. - Place the weighing boat on the balance and tare the
balance to read 0.00 g. - Transfer the lead shot from the test tube into the
weighing boat and determine its mass. Record the
mass of the lead shot on line A of Table 15-3. - Transfer the lead shot back into the test tube and
immerse the test tube in the boiling water bath, making
sure that none of the water is transferred into the
test tube. If necessary, clamp the test tube in place to
secure it. - Repeat steps 3 through 6 with the iron shot.
- Measure the temperature of the boiling water bath to
0.1°C and record that value on line B of Table 15-3. - Keep the test tubes in the boiling water bath for at least
10 to 15 minutes, by which time the lead and iron shot
will have reached the temperature of the boiling water
bath. While you wait for the shot in the two test tubes to
equilibrate to the temperature of the boiling water bath,
set up your calorimeter.
Use the graduated cylinder to measure about 50.0 mL
of cold tap water to 0.1 mL, and add the water to the
calorimeter. Record the volume on line C of Table 15-3.
Measure the temperature of the water in the
calorimeter to 0.1°C and record that value on line D of
Table 15-3.
Using the test tube holder, remove the test tube of
lead shot from the boiling water bath, transfer the lead
shot into the calorimeter, and replace the cover of the
calorimeter.
Swirl or stir the contents of the calorimeter, and
measure the temperature of the water inside it. When
the temperature peaks, record that value to 0.1°C on
line E of Table 15-3.
Empty the calorimeter, and repeat steps 10 through 13
with the steel shot.
10.
11.
12.
13.
14.
dISpoSAL: dry the lead and iron shot and save them for future use.