Illustrated Guide to Home Chemistry Experiments

168 DIY Science: Illustrated Guide to Home Chemistry Experiments

FIGURE 9-2: A burning splint is immediately extinguished
by the carbon dioxide gas evolved during the reaction.

TEH moST ImpoRTANT dECompoSITIoN REACTIoN

If you’re wondering what decomposition reaction could

possibly be run on a larger scale than the industrial

production of sodium carbonate, think about it the

next time you’re in an automobile. The most important

decomposition reaction is the refining of crude petroleum

to gasoline and other fuels, which dwarfs the production

of sodium carbonate both in mass and in economic

value. Most refining is done by a decomposition process

called catalytic cracking, which converts the long-chain

hydrocarbons present in crude petroleum into the

shorter-chain hydrocarbons used in gasoline and other

fuels and lubricants.

POCEDURER

CUTIOA nS

This experiment uses heat. Be careful with the heat source

and when handling hot objects, and have a fire extinguisher

readily available if your heat source uses flame. Wear

splash goggles, gloves, and protective clothing.

z This laboratory has two parts. In Part I, we’ll do qualitative

testing to determine the products of the reaction. In Part II,

we’ll do some semi-quantitative testing to verify that the mass

of the solid product (sodium carbonate) closely matches the

mass we would expect from examining the balanced equation

for the reaction.

PRTI:A

1.f you have not already done so, put on your splash I

goggles, gloves, and protective clothing.

2. Add sodium hydrogen carbonate to the test tube until it
   is about one quarter full.


3. Light your gas burner (or alcohol lamp).


4. Using the test tube holder or clamp, hold the test tube
   so that the flame heats that part of the test tube that
   contains the sodium hydrogen carbonate. Try to play
   the flame evenly over the entire sample. (Using a clamp
   mounted on a ring stand rather than a test tube holder
   makes the following steps easier, because it frees
   both hands.)


5. After you have heated the sample for at least 15 to 30
   seconds, ignite a toothpick or wood splint and insert the
   burning end into the top of the test tube. If the flame
   or ember is snuffed out (Figure 9-2), that’s a good
   indication that the reaction is producing carbon dioxide
   as expected. If the splint continues to burn, continue
   heating the sample until enough carbon dioxide is
   produced to displace the air inside the test tube and try
   using the splint again.


6. As you heat the sample, you should notice a liquid
   condensing on the cooler inside upper surface of the
   test tube, at first as fogging and later as actual droplets.
   Touch a blue cobalt chloride test strip to this liquid. Does
   it turn pink? If so, that confirms that the liquid is water.

PRTII:A

In Part II, we’ll determine the mass loss that occurs when

sodium hydrogen carbonate is heated to decompose it to

sodium carbonate. In theory, all of the mass loss is attributable

to the outgassing of carbon dioxide and water during the

heating process, and the remaining mass should represent

only sodium carbonate. In practice, things can be a bit more

complicated. Sodium carbonate exists in four hydration states:

anhydrous (Na 2 CO 3 ), the monohydrate (Na 2 CO 3 • 1H 2 O),

the heptahydrate (Na 2 CO 3 • 7H 2 O), and the decahydrate

(Na 2 CO 3 • 10H 2 O). If we don’t heat the sample strongly

enough, it’s possible that some of the water produced by the

decomposition will be retained by the sodium carbonate as

water of hydration. Accordingly, we’ll heat our sample at a

high enough temperature and for long enough to ensure that it

is completely converted to anhydrous sodium carbonate.