Illustrated Guide to Home Chemistry Experiments

Chapter 9 Laboratory: Introduction to Chemical Reactions and Stoichiometry 169

TABLE 9-2:

Observe a decomposition reaction—observed and calculated data.

dISpoSAL: Sodium hydrogen carbonate and sodium

carbonate are not hazardous. flush them down the drain

with plenty of water.

1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.

2. Place the covered crucible in the clay triangle, and
   adjust the height of the supporting ring to put the
   bottom of the crucible in the hottest part of the flame.
   Make sure that the crucible lid is positioned to allow gases
   to be vented. Heat the empty crucible and lid strongly
   for at least a minute to drive off any water and volatile
   contaminants present.


3. After allowing them to cool completely, weigh the
   crucible and lid and record the mass to 0.01 g on line A
   of Table 9-2.


4. Add approximately 5.0 g of sodium hydrogen carbonate
   to the crucible, reweigh the crucible and lid, and record
   the mass to 0.01 g on line B of Table 9-2.


5. Subtract the empty mass of the crucible and lid from the
   combined mass of the crucible, lid, and contents, and
   record the mass of the sodium hydrogen carbonate to
   0.01 g on line C of Table 9-2.


6. Place the covered crucible in the clay triangle, make sure
   that the crucible lid is positioned to allow gases to be
   vented, and heat the crucible strongly for 15 minutes.


7. After allowing the crucible, lid, and contents to cool
   completely, reweigh the crucible, lid, and contents and
   record the mass to 0.01 g on line D of Table 9-2.


8. Determine the mass of the product remaining in the
   crucible by subtracting the mass of the empty crucible
   and lid from the combined mass of the crucible, lid, and
   contents after heating. Record the mass of the sodium
   carbonate to 0.01 g on line E of Table 9-2.


9. Calculate the mass loss attributable to outgassing of
   carbon dioxide and water vapor by subtracting the mass
   of the sodium carbonate product (line E) from the mass
   of the sodium hydrogen carbonate reactant (line C). Enter
   the mass loss to 0.01 g on line F of Table 9-2. Calculate
   the mass loss percentage by multiplying F by 100, then
   dividing by C. Enter the result on line G.

Item Data

A. Mass of crucible and lid ______.______ g

B. Mass of crucible, lid, and sample

(before heating) ______.______ g

C. Mass of sodium hydrogen carbonate

(B – A) ______.______ g

D. Mass of crucible, lid, and sample

(after heating) ______.______ g

E. Mass of sodium carbonate (D – A) ______.______ g

F. Mass loss (C – E) ______.______ g

G. Mass loss percentage (100 · F/C) ______.______ %

oALIpToN ACTIvITIES

If you have extra time, consider performing these

optional activities:

• Determine the effect of time and temperature on the
  decomposition reaction. Use four crucibles (or Pyrex
  custard cups). Weigh each crucible to 0.01g and record
  the mass. Add 5.0 g of sodium hydrogen carbonate,
  replace the lid, reweigh each crucible, and record the
  combined mass of crucible, lid, and sample. Preheat
  the kitchen oven to its lowest setting (anything
  over about 50°C or 120°F will do) and place all four
  crucibles on a baking tin into the oven. Start your
  timer. Using crucible tongs, remove one crucible after
  15 minutes, and the remaining crucibles at 30 minutes,
  one hour, and two hours. After allowing the crucibles
  to cool, determine how much mass has been lost from
  each crucible and calculate the percentage of mass
  loss. Compare the mass loss percentage to the value
  you determined in Part II. Propose an explanation for
  your findings.


• Repeat the preceding experiment, but with the oven
  preheated to 450°F or higher. Do the results differ?
  Propose an explanation.