CaCl 2 (aq) + Na 2 CO 3 (aq) → CaCO 3 (s) + 2NaCl(aq) (4.6.7)the Ca^2 + ion in calcium chloride simply switches places with the Na+ ions in the sodium
carbonate to produce solid calcium carbonate and NaCl in solution. This is also a
precipitation reaction in which a solid material forms from two substances dissolved in
water; the solid formed is a precipitate. The removal of calcium from water as shown
by this reaction is a common water treatment process called water softening. It is done
because excessive levels of calcium cause formation of scale that can clog water pipes
and damage plumbing apparatus.
Whenever an acid and a base react, as shown here for the reaction of hydrochloric
acid with sodium hydroxide,
HCl(aq) + NaOH(aq) → H 2 O(l) + NaCl(aq) (4.6.8)water and a salt are formed. Such a reaction is a neutralization reaction or simply an
acid-base reaction.
Exercise: Classify each of the following reactions as combination, decomposition,
substitution, metathesis, neutralization, precipitation, or evolution of a gas. In
some cases, a reaction will fit into more than one category.(a) 2Ca(s) + O 2 (g) → 2CaO(s)(b) 2KClO 3 (s)∆
2KCl(s) + 3O 2 (g)(c) SO 3 (g) + H 2 O(l) → H 2 SO 4 (aq)(d) MgCO 3 (s) + 2HCl(aq) → MgCl 2 (aq) + H 2 O(l) + CO 2 (g)(e) Zn(s) + CuCl 2 (aq) → Cu(s) + ZnCl 2 (aq)(f) KOH(aq) + HCl(aq) → KCl(aq) + H 2 O(l)(g) MgSO 4 (aq) + 2KOH(aq) → Mg(OH) 2 (s) + Na 2 SO 4 (aq)Answers: (a) Combination, (b) decomposition, evolution of a gas, (c) combination,
(d) metathesis, evolution of a gas, (e) substitution, (f) neutralization, metathesis, (g)
precipitation, metathesis.
4.7. Oxidation-Reduction Reactions and Green Chemistry
Many reactions, including some of those given in the preceding section, are
oxidation-reduction reactions, frequently called redox reactions. This name derives
from the long standing use of oxidation to describe the reaction of a substance with
oxygen. Consider the following reaction of elemental calcium with elemental oxygen:
90 Green Chemistry, 2nd ed