3 Compound nCompoundCompound
The thermal decomposition of calcium carbonate (limestone) and other carbonates
produces two compounds, a metal oxide and carbon dioxide:
heat
CaCO 3 (s)8888nCaO(s)CO 2 (g)This is an important reaction in the production of cement. Calcium oxide is also used as
a base in industrial processes.
When some solid hydroxides are heated, they decompose to form a metal oxide and
water vapor.
heat
Mg(OH) 2 (s)8888nMgO(s)H 2 O(g)Magnesium oxide, MgO, is pressed into sheets for use as a thermal insulating material in
oven walls.
Ammonium salts lose ammonia.
heat
(NH 4 ) 2 SO 4 (s)8888n2NH 3 (g)H 2 SO 4 ()
If the ammonium salt contains an anion that is a strong oxidizing agent (e.g., nitrate,
nitrite, or dichromate), its decomposition reaction produces an oxide, water (as vapor at
high temperatures), and nitrogen gas. Such a reaction is a redox reaction.
heat
(NH 4 ) 2 Cr 2 O 7 (s)8888nCr 2 O 3 (s)4H 2 O(g)N 2 (g)DISPLACEMENT REACTIONS
Reactions in which one element displaces another from a compound are called
displacement reactions.These reactions are always redox reactions. The more readily a metal undergoes oxida-
tion, the more active we say it is.
Active metals displace less active metals or hydrogen from their compounds in
aqueous solution to form the oxidized form of the more active metal and the reduced
(free metal) form of the other metal or hydrogen.In Table 4-12, the most active metals are listed at the top of the first column. These metals
tend to react to form their oxidized forms (cations). Elements at the bottom of the activity
series (the first column of Table 4-12) tend to remain in their reduced form. They are
easily converted from their oxidized forms to their reduced forms.
(^1) More Active Metal Less Active Metal
[Salt of Less Active Metal]88n[Salt of More Active Metal]
The reaction of copper with silver nitrate that was described in detail in Section 4-3 is
typical. Please refer to it.
4-8
Solid ammonium dichromate,
[(NH 4 ) 2 Cr 2 O 7 , orange] decomposes
when heated into chromium(II)
oxide, (Cr 2 O 3 , green), nitrogen, and
steam (water vapor). This reaction
is sometimes demonstrated as the
“classroom volcano,” but it must be
done with extreme caution due to
the carcinogenic (cancer-causing)
nature of Cr 2 O 3.
4-8 Displacement Reactions 147
Alkali metal carbonates do not
decompose when heated.
A decomposition reaction may or may
not also be an oxidation-reduction
reaction. You can always identify a
redox reaction by determining the
oxidation state of each element in each
occurrence in the reaction (see the
Problem-Solving Tip in Section 4-5).