Chemistry 11
THE DECOMPOSITION REACTION OF BAKING SODA, NaHCO3
Introduction
The object of this experiment is to find the actual chemical equation which governs the manner in which baking
soda (sodium hydrogen carbonate, NaHCO3) decomposes. You must devise a method of your own and decide
what calculations will be needed. You will, however, be given a few guidelines to help you.
When NaHCO3 is heated, it decomposes to produce a gas and leave behind a solid. Two possible equations for
the decomposition are:
NaHCO3(s) + heat  NaOH(s) + CO2(g)
and
NaHCO3(s) + heat  1/2 Na2CO3(s) + 1/2 H2O(g) + 1/2 CO2(g)
Your experimental results should allow you to decide which of the above equations correctly describes the
decomposition of NaHCO3.
Experimental
You should be aware of the following when designing your experimental procedure.
• Crucibles are usually heated in CLAY TRIANGLES, which in turn are set on top of rings attached to ring
stands.
• Hot crucibles look like just like cold crucibles, so that CRUCIBLE TONGS were invented to move or carry
hot crucibles.
• Hot crucibles can blister your bench top, so that hot crucibles should be placed on top a pile of 3 to 4
ceramic pads to protect the bench top.
• Crucibles very slowly adsorb small amounts of water vapour from the atmosphere and must be heated for a
minute or two to get rid of adsorbed water molecules before you can get an accurate "initial mass" for a
crucible.
• In order to decompose, baking soda must heated until the crucible containing it glows at “orange-red heat",
and must be heated for at least five minutes.
• If baking soda is heated without a crucible cover, some of the baking soda "pops" out of the crucible and is
lost.
• If a VERY hot crucible is put on a balance, the balance can be ruined (a new balance costs over $1000!).
• If a crucible is weighed before it has cooled sufficiently, small convection currents will be set up in the air
surrounding the balance pan and crucible. These air currents will cause an incorrect reading (the reading
will be too low). Hot crucibles generally require about 5 minutes to cool sufficiently.
This is the actual experimental problem you must solve:
You must heat about 5 g of baking soda until it has finished decomposing. You must decide how you can
experimentally determine when all the baking soda has decomposed, so that more heating will cause no
more decomposition.
Your Report
You should be able to predict the mass of solid product produced by your experimental mass of NaHCO 3 in
each of the two possible decomposition equations. Your report must include the following:
1. A suitable data table listing all the data taken.
2. A brief description of the procedure followed. You must make it clear how you solved the
problem of experimentally determining whether the baking soda had finished decomposing.
3. The calculations you used to determine the final result. All calculations must be
sufficiently described to let the reader know what you are doing at each step.
4. Your final conclusion.