Illustrated Guide to Home Chemistry Experiments

Chapter 9 Laboratory: Introduction to Chemical Reactions and Stoichiometry 167

LABORATORY 9 .2:

oBSERvE A dECompoSITIoN REACTIoN

The refractory (using heat) decomposition

of sodium hydrogen carbonate to sodium

carbonate, carbon dioxide, and water is a

typical decomposition reaction, in which a

compound reacts to form two or more other

elements and/or compounds. The balanced

equation for this reaction is:

2 NaHCo 3 (s) → Na 2 Co 3 (s) + Co 2 (g) + H 2 o(g)

REqUIREd EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing paper

£ ring stand

£ support ring

£ clay triangle

£ gas burner

£ crucible and lid

£ test tube

£ crucible tongs

£ test tube holder or clamp

£ sodium bicarbonate (~10 g)

£ cobalt chloride test strip (optional)

£ toothpicks or wood splints

I didn’t choose this example decomposition reaction arbitrarily.
The refractory decomposition of sodium bicarbonate to sodium
carbonate is the second most important decomposition
reaction used in industrial processes, and is crucial to the
world’s economy. This reaction is the final step in the Solvay
process, which is used to produce about 75% of the world’s
supply of sodium carbonate, also known as washing soda or
soda ash. (The remaining 25% is mined or extracted from
brine.) Worldwide production of sodium carbonate is more than
40 billion kilograms annually, about 7 kilograms for every person
on the planet. Sodium carbonate is an essential component of
glass, and is also used for hundreds of other purposes, including
processing wood pulp to make paper, as an ingredient in soaps
and detergents, as a buffer, and as a neutralizer.

Unlike many decomposition reactions, which are essentially
irreversible, the refractory decomposition of sodium hydrogen
carbonate is easily reversible, and in fact a considerable amount
of sodium hydrogen carbonate is produced commercially by this
reverse reaction.

Na 2 Co 3 (s) + Co 2 (g) + H 2 og) ( → 2 NaHCo 3 (s)

Carbon dioxide gas is bubbled through a saturated solution
of sodium carbonate and reacts to form sodium hydrogen
carbonate, which is less soluble than sodium carbonate. The
sodium hydrogen carbonate precipitates out, and is isolated by
filtration.

In this lab, we’ll produce sodium carbonate from sodium
hydrogen carbonate via a decomposition reaction.

SBSTITUTIU oNS ANd modIfICATIoNS

• If you do not have a crucible and lid, you can substitute
  a Pyrex dessert cup with a Pyrex saucer, watch glass,
  or similar heat-resistant item as a lid.


• If you do not have the recommended equipment
  needed to heat the samples, you can substitute a large
  tin can lid placed flat on a stove burner or hotplate.


• You can make your own cobalt chloride test strips
  by soaking ordinary paper in a saturated solution
  of cobalt chloride and then drying it thoroughly. If
  any moisture is present, the test strip is pink. In the
  absence of moisture, the test strip is blue. If you don’t
  have any cobalt chloride, look for one of those silica gel
  drying packets that changes color from blue when dry
  to pink when it needs to be recharged by heating it in
  the oven. You can use a few of the blue crystals to test
  for the presence of water, which turns them pink.


• You may substitute a propane torch for the gas burner.


• You may substitute a pair of pliers or similar gripping tool
  for the crucible tongs. (Be careful; crucibles are fragile.)