The Foundations of Chemistry

Hydrogen sulfide, H 2 S, is responsible

for the smell of rotten eggs.

4-9 Metathesis Reactions 159

3 Gas-Formation Reactions

The formation of an insoluble or slightly soluble gas provides a driving force for a third
type of metathesis reaction that we call a gas-formation reaction.The only common
gases that are very soluble in water are HCl(g) and NH 3 (g). All other gases are sufficiently
insoluble to force a reaction to proceed if they are formed as a reaction product.
When an acid—for example, hydrochloric acid—is added to solid calcium carbonate,
a reaction occurs in which carbonic acid, a weak acid, is produced.

2HCl(aq)
CaCO 3 (s)88nH 2 CO 3 (aq)
CaCl 2 (aq)

2[H
(aq)
Cl(aq)]
CaCO 3 (s)88nH 2 CO 3 (aq)
[Ca^2 
(aq)
2Cl(aq)]

2H
(aq)
CaCO 3 (s)88nH 2 CO 3 (aq)
Ca^2 
(aq)

The heat generated in the reaction causes thermal decomposition of carbonic acid to
gaseous carbon dioxide and water:

H 2 CO 3 (aq)88nCO 2 (g)
H 2 O(
)

Most of the CO 2 bubbles off, and the reaction goes to completion (with respect to the
limiting reactant). The net effect is the conversion of ionic species into nonionized mole-
cules of a gas (CO 2 ) and water.

2HCl(aq)
CaCO 3 (s)88nCO 2 (g)
H 2 O(
)
CaCl 2 (aq)

Salts containing the sulfite ion, SO 32 , react with acids in a similar manner to form
sulfur dioxide gas, SO 2 (g).

SO 32 (aq)
2H
(aq)88nSO 2 (g)
H 2 O(
)

Many sulfide salts react with acids to form gaseous hydrogen sulfide, H 2 S. The low
solubility of H 2 S in water helps the reaction to proceed.

MnS(s)
2HCl(aq) 88nMnCl 2 (aq)
H 2 S(g)

Blackboard chalk is mostly calcium carbonate,

CaCO 3. Bubbles of carbon dioxide, CO 2 , are clearly

visible in this photograph of CaCO 3 reacting with

HCl in a gas-forming metathesis reaction.